BiolOgialJoumal of thc Linnean So&
(1997),61: 559-584. With 6 figures
Genetic differentiation in the Bladder campions,
Silene vuZgaris and S. unzflora (Caryophyllaceae),
in Sweden
HELENA RUNYEON’-3AND HONOR C. PRENTICE3
’Department of Ecological Botuny, Uppsala Universip, Ww@en 14, S-752 36 Uppsala,
Sweden; “Department of Ghetics, Uppsala University, Box 7003, S-750 07 Uppsala, Sweden;
3Departmentof &jutematicBohny, Lund Universip, Ostra Elkatan 18-20,
9223 61 Lund, Sweden
Received 7 October 1996; acceptedforpublicaEion 23 Januay 1997
Allozyme variation was studied in Swedish populations of S i h vulgaris (a widespread weed),
S. unjffora ssp. ungora (restricted to coastal habitats) and S. un@ra ssp. petram (endemic to
Sweden and confined to open limestone habitats). The taxa are diploid, gynodioecious,
perennial herbs and showed high levels of withhtaxon and within-population gene diversity
at four polymorphic loci. Within-taxon diversity was highest (H,=O.52) in S. vulg& and
lowest (H,=0.36) in S.unjffora ssp. unjfforu. The weedy S.vulgaris has more alleles than either
of the other two taxa and 5 out of a total of 27 alleles are unique to S. vulgaris. Most of the
gene diversity within each of the taxa is accounted for by within-population diversity. The
between-populationcomponentofdivenity is 10%in S.vulgaris, and 24% and 5%, respectively,
in S.unjffora ssp. unjffora and ssp. petraea. Hybrids may occur between S.vulgaris and S. unjffora,
but introgression is limited by the species’ ecology. Neither allozyme nor distributional data
support the suggestion that ssp. petraea is a recent hybrid between S. vulgarti and S. unjffora
ssp. unjffora, although an older hybrid origin for ssp. petraca is possible. Patterns of allele
frequency variation suggest that there has been some historical gene flow between taxa,
outside their present areas of sympatry. It is likely that the two subspecies of S.unjffora, which
occur in naturally open habitats, were able to colonize Sweden during the Late Glacial or
early post-glacial, whereas S. vulgaris followed the spread of agriculture into Sweden.
&, 1997
The Linnean Society of London
ADDITIONAL KEY WORDS:-allozymes - gene diversity
immigration history - allele frequencies - closely related taxa.
-
geographic variation -
CONTENTS
Introduction . . . . . . . . . . . . . . . . . . . . .
Material and methods . . . . . . . . . . . . . . . .
The taxa . . . . . . . . . . . . . . . . . . . .
The ecology and distribution of S. vulgaris and S.unjffora in Sweden
Hybridization . . . . . . . . . . . . . . . . . . .
. .
. . .
. .
. . .
. .
560
56 1
561
562
563
Correspondence to: H. Runyeon, Department ofEcological Botany, Villavagen 14, S-752 36 Uppsala,
Sweden. Email: [email protected]
0024-4066/97/080559+ 26 $25.00/0/bj970140
559
Q 1997 The Linnean Society of London
560
H. RUNYEON AND H. C. PRENTICE
Sites and sampling . . . . . .
Enzyme electrophoresis . . . .
Analysis of allozyme variation . ,
Results . . . . . . . . . . .
Alleles and allele kequencies . . .
Partitioning of gene diversity . . .
Genetic differentiation withii taxa .
Genetic differentiation between taxa
Discussion . . . . . . . . . .
S i h vu&aat;r . . . . . . . .
S i h unjffora ssp. unjpma . . . .
S i h unjPora ssp.pe&aea . . . . .
Differentiation between taxa . . .
History and evolution . . . . .
Acknowledgements . . . . . . .
References . . . . . . . . . .
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INTRODUCTION
The development of the flora and fauna of Fennoscandia has been strongly
influenced by the region’s glacial history. Virtually the whole of Fennoscandia was
ice-covered during the Weichselian glacial maximum (c. 20 000-18 000 yr BP) and
plant and animal species began to recolonize Fennoscandia towards the end of the
Weichselian, around 13 000 yr BP. As the ice-sheet retreated successivelynorthwards
and inwards towards the Scandinavian mountains, immigration routes for plant and
animal species were opened up both to the west and the east of the Baltic. Plant
species characteristic of pioneer vegetation were the first to colonize the open habitats
surrounding the contracting Fennoscandian ice sheet (e.g. Berglund, 1966; Prentice,
1982; Berglund et al., 1994) but the continuous establishment of forest trees in
southern Sweden did not begin until the start of the Holocene period, around
10 000 yr BP (e.g. Berglund et aL, 1994).
The structure of genetic variation within many Fennoscandian species may reflect
their history of immigration. A series of elegant studies of geographic variation in
mitochondrial DNA in animal species support this prediction and trace separate
western and eastern routes of immigration into Fennoscandia (e.g. Jaarola &
Tegelstrom, 1995; Taberlet et al., 1995). Fewer studies have been carried out on
the large-scale geographic structure of plant species in Fennoscandia. A large-scale
study of genetic variation in the wind-pollinated conifer, picea ubies, detected little
geographic differentiation within Sweden (Lagercrantz & Ryman, 1990). However,
a study of the shrub, Hi@oCrepis emeru.s, revealed pronounced differences between
disjunct Norwegian and Swedish regional populations (Lonn, Prentice & Tegelstrom,
1995))suggesting separate western and eastern origins. The geographic structure of
intraspecific differentiation within the sedge, Carex lepidocurpa, also appears to have
a historical origin (Hedrkn & Prentice, 1996).
The Werent origins of migrating populations may influence the overall patterns
of geographic differentiation within species. The size of immigrating populations
may also play a role in shaping the structure of infraspecific variation. Episodes
of small population size during phases of migration or colonization-population
bottlenecks or founder effects-may have lead to a loss of allelic variation and
divergence between migrating populations (e.g. Nei, Maruyama & Chakraborty,
1975; Loveless & Hamrick, 1988; Barrett & Kohn, 1991). Such historical events
GENETIC DIFFERENTIATION IN SILENE
56 1
may have a lasting impact on levels of allelic variation and genetic diversity within
regional populations and on the degree of divergence between populations or
geographic races in different parts of species’ ranges (Stebbins, 1950; Ricklefs, 1989;
Barrett & Kohn, 1991).The reciprocal fixation of allozymes and low levels of DNA
diversity in the native Norwegian and Swedish populations of Hippocrepis m
suggest a history of bottlenecks and founder events during the species’ immigration
into Scandinavia (Lonn et al., 1995).
Successive glacial and interglacial cycles are associated with repeated and major
range-shifts for the majority of organisms. Migrating species may hybridize and new
taxa may evolve. In animals, interspecific gene flow has been shown to occur during
species’ immigration into Fennoscandia (e.g. Tegelstrom, 1987).The fact that western
and eastern chloroplast DNA races cut across the species-boundaryin the European
oaks, Querczls robur and Q petraea, also suggests interspecific gene flow in association
with migrational range-shifts (Ferris et al., 1993).
The present study investigates the structure of allozyme variation in three closelyrelated taxa within the genus S i h e in Sweden. The taxa have partially sympatric
distributions, but occur in different habitats and are expected to have had different
post-glacial histories. Two of the taxa, S i h e unjrrora ssp. unjrrora and S. unjrrora ssp.
petraea, are restricted to coastal habitats or to open, frost-disturbed, ‘alvar’ habitats
which are thought to have been available throughout the post-glacial period (Iversen,
1958; Fries, 1965; L.-K. Konigsson, pers. comm.). The third taxon, Silene uulgaris, is
a widely but locally distributed anthropogenic weed, occurring in disturbed habitats
and on roadsides. Silene vulgaris and S. unjrrora are interfertile but hybridize relatively
infrequently in Sweden at the present day (Runyeon & Prentice, 1996; H. Runyeon,
pers. obs.). Nevertheless, the distinction between S. unjrrora and S. vulgaris is less clearcut in Scandinavia than in the rest of Europe (cf. Marsden-Jones & Turrill, 1957;
Chater & Walters, 1990)’ and the Swedish endemic, S. ungora ssp. petraea, is
morphologically somewhat intermediate between S. vulgaris and S. ungora ssp. ungora.
We investigate whether patterns of allelic differentiation can be used to clarify
the relationships between the three taxa. We also explore the extent to which the
structure of genetic variation within each of the taxa can be interpreted in terms of
patterns of post-glacial immigration or population history.
MATERIAL AND METHODS
The Silae vukaris complex (Caryophyllaceae)is represented by two diploid (2n =
24) species in Scandinavia. S i h e vulgaris (Moench) Garcke is widespread in Europe
(Jalas & Suominen, 1986)’ and can be subdivided into a number of subspecies
(Williams, 1908; Aeschimann & Bocquet, 1980), of which only the weed ssp. vulgaris
is present in Sweden. The second species, Silene unjrrora Roth (= S. maritima With.),
is restricted to northern and western Europe, with two allopatric subspecies in
Sweden. S i k unijlora ssp. unjrrora is mainly found in coastal habitats in northern
Europe (Jalas & Suominen, 1986)while ssp. petraeu (Harm.)Jonsell & H.C. Prentice,
is endemic to the Baltic islands of Oland and Gotland in Sweden.
The taxonomic status of S i h e vulgaris and S. ungora has been under debate for
562
H.RUNYEON AND H.C. PRENTICE
the last two centuries. The two taxa have been treated variously as species (Fries,
1842; Hartman, 1861; Neuman, 1901; Marsden-Jones & Tunill, 1957), semispecies
(Valentine & Uve, 1958), subspecies (Runemark, 1961; Love & Love, 1961; Chater
& Walters, 1964; Clapham, Tutin & Moore, 1989) or varieties (Hartman, 1832,
1843). At present S. vulgaris and S. unjflora are usually regarded as two separate
species (Jalas & Souminen, 1986; Chater, Walters & Akeroyd, 1993).
The status of S. unjflora ssp. petraea has been particularly problematic. This taxon
was described by Linnaeus in 1745 from the Baltic island of Gotland (Linnaeus,
1745), as a small-leaved, few-flowered variety of Behen album ( =S. vulgaris). The same
taxon was later found on oland, and Fries (1842) again treated it as a variety of S.
vukaris (under the name S. iry7ata var. petraea). Hartman (1832, 1843) argued that
‘var. petraea’ was one of three equally-distinct varieties within S. iry7ata ( = S. vulgaris),
together with var. vulgaris and var. maritima. However Hartman (1879)later considered
‘var. petraea’ to be a variety within S.maritima (= S. unjflora).The status of ‘var.petraea’
as a variety within S.unijora was more or less accepted during the following decades.
However, it has recently been suggested that ‘var. petrma’ might be included in S.
unjflora ssp. gkznosa, a montane, central European subspecies (Chater et al., 1993). In
Flora Nordica (Jonsell, 1996;Jonsell et al., in prep.), ‘var. petraea’ will be treated as
a subspecies within S. unjflora (S,unjflora ssp. petraea), and this treatment will be
followed in the present paper.
l 3 e ecology and distribution OfS. vulgaris and S. uniflora in Sweden
Silene vulgaris and S. unjflora are both long-lived perennials (Marsden-Jones &
Turrill, 1957) and are pollinated by crepuscular moths and solitary bees (Pettersson,
1991, 1992a). Both species are gynodioecious, and the proportions of male-sterile,
gynomonoecious and hermaphrodite individuals vary among populations. The
flowering phenology of the two species differs considerably. S i h vulgaris flowers for
approximately a month, from late June to late July, while S. unjflora ssp. unjflora and
S. unjflora ssp. petraeu start flowering in May and flower throughout the growing
season until the f i s t frosts in September or October.
S i h e vulgaris is a locally common weed of disturbed and arable habitats in southern
and central Sweden. The flowering shoots are relatively long (up to 100 cm), erect
to ascending, and the inflorescences contain 5-80 flowers (Clapham et al., 1989).
Populations are usually large, often consisting of several thousand individuals.
The two subspecies of S. unjflora that occur in Sweden are both sensitive to
competition and grow in open habitats (Marsden-Jones & Turrill, 1957).Silme unjflora
ssp. ungora grows on cliffs and shingle beaches along the northern Baltic coast and
on the west coast of Sweden. The flowering shoots are numerous, more-or-less
prostrate and 1-3 flowered. Populations often contain hundreds of individuals,
especially along the Swedish west coast, and old individuals may form extensive
mats. In contrast, the populations of Silene unjflora ssp. petraea are usually small (seldom
containing more than 50 plants), discrete and separated from each other by 0.5 to
5km (Runyeon & Prentice, 1996). Shoots are procumbent to ascending and the
inflorescences contain 1-3 flowers. Silene unjflora ssp. petraea is restricted to extreme,
open and frost-disturbed habitats within the areas of the steppe-like ‘alvar’grasslands
on limestone bedrock on Oland and Gotland (Ekstam et al., 1984) and is also found
on limestone gravel along sea shores on Gotland.
GENETIC DIFFERENTIATION IN SIL&W
563
Hybridization
In Sweden S. vulgaris and S. ungora ssp. petraea occasionally co-occur in locations
where anthropogenic disturbance has brought S. vulgaris into contact with alvar
habitats. In such situations, the two taxa may hybridize. The hybrids are fertile and
can persist for several years (H.C. Prentice, pers. obs.). In contrast, hybrids between
S. vulgaris and S. ungora ssp. ungora are seldom observed in the wild (Marsden-Jones
& Turrill, 1930; Walters, 1975).
Sites and sampling
Plants from 24 populations of S. vulgaris (550 individuals), 25 populations of S.
ungora ssp. ungora (456 individuals), and 30 populations of S.ungora ssp. petraea (763
individuals) were sampled for analysis of isozyme variation. Some population
samples were collected directly from wild populations, while other populations were
greenhouse-grown from wild-collected seed. Fresh leaf material from 5-30 individuals
from each sampling site was used for the isozyme analyses. The sampled individuals
were separated from each other by at least five metres and the sampled populations
were separated from each other by at least 1 km. A map of the sampling sites is
shown in Figure 1 and a list of the sites is given in Table 1.
E n q m electr0phore.k
Electrophoretic procedures generally followed those of Prentice & Giles (1993).
The starch concentration of the horizontal starch gels ranged from 9% to 11O/O. Three
enzyme systems, phosphoglucose isomerase (PGI), phosphoglucomutase (PGM) and
triosephosphate isomerase (TPI) were used. A total of 1769 individuals was screened
for variation at the polymorphic loci P&-2, Pgm-1,P p - 2 and Tpi-1. The genetic
control of the variation at these four loci has been investigated by Prentice & Giles
(1993). In addition to the alleles included in the studies by Prentice & Giles (1993)
and Runyeon & Prentice (1996), the present study includes allele 3s at Pgi-2 (S.
unjRora ssp. unjRora), alleles 1 +(S.ungora ssp. unjffora) and 4 (S. ungora ssp. petraea) at
Pgm-2 and alleles 1* (S. vulgaris) and 3 (S. vulgaris and S. ungora ssp. ungora) at Tpi1. Alleles that occurred in only one of the three taxa in this study will be referred
to as ‘unique alleles’.
Anahsis
of allogme variation
Allelic diversity was estimated from allele frequencies using Nei’s (1973) diversity
statistic, H. Standard errors and unbiased values for H (cf. Prentice & White, 1988)
were obtained by using pseudovalues, calculated using Tukey’s jackknife (Sokal &
Rohlf, 1981).
Partitioning of allelic diversity at each of the four polymorphic loci into its withinand between-population components was carried out using a similar procedure to
that of Chakraborty et al. (1982). Firstly, analyses of the hierarchical partitioning of
diversity were carried out separately for S. vulgaris, S. ungora ssp. ungora, and S.
10°E
15'E
15"E
57"N
16"E
Figure 1. Location map for 24 populations of S h vulgaris (m), 25 populations of S. @a
(0)in Sweden (and Noway). See Table 1 for details of the sampled populations.
55°K
60.9
5"E
ssp. u@ra
17'E
ssp. petraea
19"N
(A)and 30 populations of Silene @a
57"N
58"N
18"E
GENETIC DIFFERENTIATION IN S Z U N E
565
TABLE
1. Sampling locations for 24 populations of S i h v u k a k , 25 populations of S. ungora ssp.
ungora and 30 populations of S i h ungora ssp. petraea in Sweden (and Norway). n=number of
individuals sampled at each site
~~
Taxon
S i h untj¶o~a
ssp. pelraca
Code
Province
Locality
Habitat
n
I*
22
3*
4*
5*
6*
l*
8*
9*
Bland
Oland
Oland
Oland
Oland
Oland
Oland
Oland
Oland
Bland
Oland
Oland
Oland
Oland
Oland
Gotland
Gotland
Gotland
Gotland
Gotland
Gotland
Gotland
Gotland
Gotland
Gotland
Gotland
Gotland
Gotland
Gotland
Gotland
Mockelmossen
Stenha
Runbacken
Solberga
Skiirlov
Vickleby, W
Vickleby, SW
Gosslunda
Pr%stgropen
Jutas stubbe
Ventbinge church
Resmo
Penha
Albrunna
Mellby
Hamarshage h a a r
Holmhflar
m
y
r
Harudden
Sysneudd
Hunnstiideviken
Smiss
Geivalds
Tob
Alskog
Hoburgens fyr
Sigsarve alvar
Hoburg, Sandvik
Hejnum
Tore alvar
alVX
alvar
alVX
alVX
alVX
alvar
alvar
alvar
alVX
alVW
alVar
alVX
alvar
alvar
alVX
coast
coast
alVX
coast
coast
coast
coast
alvar
alVX
alVX
alVX
alvar
coast
alVW
alVX
29
23
31
23
29
27
27
28
29
28
28
28
25
29
27
28
24
27
29
30
30
30
1
24
9
21
24
24
25
20
S h e
Skane
S h e
S h e
Halland
Halland
Halland
Halland
Halland
Halland
Halland
Halland
Bohush
Bohusltin
Bohusltin
Bohusltin
Bohusb
Bohusltin
UPPhd
Htdsiingland
H&ingland
Uppland
UPPhd
Uppland
Uppland
fiaPPeNP
Shet
Skepparkroken
Olastorp
N. Haverdal
Ugglarp
Steninge
Morups T h g e
Galtaback
Tr%shsUge
Getteron
Tjoloholm
Kovikshamn
Brevikskile
Pinneviken
Hamburgsund
Reso
K&ringdn
SikkjiiLna
Steno
Enskhoren
Klubben
Lingsand
Rossholm
Skatudden
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
coast
24
18
25
19
26
12
5
21
1
18
23
19
21
21
22
18
13
21
15
8
8
15
25
26
25
10*
I I*
12.
13*
14*
I5*
16
11
18
19
20
21
22
23
24
25
26
21
28
29
30
S i h untj¶ora
ssp. un$ora
31
32
33
34
35
36
31
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
H. RUNYEON AND H. C. PRENTICE
566
TABLE
1.-continued
S i h Mckark
56*
57*
58*
59*
60*
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
Oland
Oland
Oland
Oland
Oland
Gotland
Gotland
Gotland
Dalarna
Dalarna
Vbtmanland
Viistmanland
Dalarna
Dalarna
Skane
Halland
Oppland,
Norway
Sogn, Norway
Uppland
Smlland
Ostergotland
Skane
Sklne
Blekinge
Penba
Grasgird
Girdby
Lundegard
Byxelkrok
Hamra
Trosings
Fide
yierbo
Aberga
Stddalen
Yxsjoberg
Nb
Vansbro
Forslev, Bdgarden
Varberg
Riste
old railway
field margin
road side
road side
road side
road side
road side
road side
road side
field margin
road side
road side
road side
road side
road side
road side
road side
25
27
26
25
27
27
27
28
26
25
25
26
21
25
17
25
17
Eggum
Glunten
Kalmar
Vadstena
Nobbelov
Fjaestad
Norje
road side
disturbed
disturbed
road side
road side
road side
road side
23
23
25
20
10
19
11
* Data from Runyeon & Prenhce ( 1996).
ungora ssp. petraea respectively. In each of these three analyses, the total diversity
within each taxon is given by H, the mean diversity within populations (for each
taxon) is given by RPT,the proportion of the within-taxon diversity that is due to
= (Htax-Rpop)/
between-population differentiation is given by Gpop.,,(where Gpop.tax
HI=) and the within-population component of diversity is given by Hpp/Htax.
Secondly, we carried out a fourth hierarchical analysis, including all three taxa, and
partitioned the total allelic diversity into its within- and between-taxa and -population
components (a three level partition). In this pooled analysis, the total diversity is
given by H,,,, the mean within-taxon diversity by R,,, and the mean within
population diversityby RPp.The proportion of the total diversity due to differentiation
between taxa is given by G,,, where Gt,,t,t =(H,,,-~,)/H,,, the between-population
= @tax-Rpp)/Htot),
and the
component of diversity is given by Gpop.t,(where Gpop.tax
Allozyme diversity
within-population component of diversity is given by Rpop/Htot.
statistics were calculated using the program MULE written by RJ. White.
Between-population and -locus differences in HPOP
(within species), and differences
in H, (in the total data set), were tested for using a two-way analysis of variance
(using the jackknifed pseudovalues for H).
Differences between the observed genotype frequencies and those expected under
Hardy-Weinberg equilibrium were determined by performing x2 tests for all loci for
each population. Where necessary, rare alleles were pooled to give expected genotype
cell values of not less than one.
Rogers’ (1972) genetic distance (based on within-population allele frequencies)
was calculated for all pairs of populations, both within and between taxa. The mean
GENETIC DIFFERENTIATION IN S Z W E
567
Rogers’ genetic distance between populations within each of the three taxa, and the
mean Rogers’ distance between pairs of taxa were also calculated. A cluster analysis
(Ward’s (1963) method) was carried out on the matrix of genetic distances (procedure
CLUSTER: SAS, 1990). Patterns of variation between populations within taxa and
the relationship between taxa were also analysed using correspondence analysis
(CA) (ter Braak, 1987a), based on within-population allele frequencies. Four CA
ordinations were produced; one for each of the taxa analysed separately and one
for the three taxa analysed together. The correspondence analyses were carried out
using the program CANOCO (ter Braak, 1987b).
Associations between matrices of genetic and geographic distances between
populations (within taxa and within the total data set) were tested for using the
generalized regression method of Mantel ( 1967) and the relative neighbourhood
graph (RNG) approach of Lefkovitch (1984), in which a likelihood ratio statistic
(referred to a x2 distribution with 1 df) for marginal independence is used to compare
the number of coinciding neighbours in the RNGs with random expectations. Matrix
comparisons were carried out using the program MATCOM, written by Mark
Fulton.
x2 tests (Workman & Niswander, 1970) were used to test for between-population
differences in total allele counts (at all loci) within each taxon, and also to test for
differences in overall counts of alleles between taxa. Rare alleles were pooled where
necessary, to obtain expected values of not less than five.
RESULTS
Alleles and allelejequencies
The mean allele frequencies for each of the three taxa at the four loci are given
in Table 2. A total of 27 alleles was present in S i h e vukani, while 20 and 21 alleles
were found in S. ungora ssp. ungora and S. ungora ssp. petraea respectively. Sixteen
out of a total of 30 alleles were shared by all three taxa. Alleles unique to one or
the other taxon were few and usually occurred in low frequencies. Five alleles were
unique to S. vu&a&, allele 3s at Pgi-2,alleles 3s and 5 at Pgm-I,allele 2s at Pgm-2
and allele 1* at Tpi-I. Allele 1 at Pgm-2was unique to S. ungora ssp. ungora and
alleles If at Pgi-2 and 4 at Pgm-2were unique to S. unjRora ssp. petraea. Four alleles
were found in both S. vu&uni and S. unjRora
(allele 2f at Pgm-2,lf, 3 and 4 at Tpi-1)
ssp. ungora but not in S. ungora ssp. petraea, while S. vu&ani and S. ungora ssp. petraea
shared two alleles (allele 6 at Pgi-2 and 2f at Pgm-I)that did not occur in S. ungora
ssp. ungora. There were no alleles that were unique to S. unjRora, occurring in both
S. ungora ssp. ungora and S. ungora ssp. petraea. The three taxa shared the same most
common allele only at the Pgm-1locus.
There are three alleles that are common in S. vu&uri.Y but rare in both subspecies
of S. ungora: allele 4 at Pgi-2 and alleles 2s and 4 at Pgm-1 (Table 2). One or more
of these ‘vdgani alleles’ occur at low frequencies in 16% of the S. unjRora ssp. ungora
populations, and in 80% of the S. unjRora ssp. petraea populations. At least one of the
alleles characteristic for S. ungora ssp. petraea (allele 3 at Pgi-2,allele 1 at Pgm-1 and
allele 3 at Pgm-2)can be found at low frequencies in 68% of the S. ungora ssp.
unjRora populations and in 100% of the S. vukaris populations. Two alleles at Pgi-2
+
H. RUNYEON AM) H. C. PRENTICE
568
TABLE
2. Mean d e l e frequencies (YO)within populations of S h e vulgaris, S. unjrlora ssp. unjflma, and
S. ungora ssp. pelraea. Data from 24 populations of S. vulgap.ariC (550 individuals), 25 populations of S.
untj7ora ssp. unjP,a (456 individuals) and 30 populations of S. unjPora ssp. petraea (763 individuals)
*
Silmc
Locus
Allele
Pgi-2
I+
If
1
2
3
3s
4
5
6
Total no. alleles
1
pgm-1
2f
2
29
3
3s
4
5
Pgm-I
Total no. alleles
Pgm-2
I+
1
2f
2
2s
3
4
Total no. alleles
Tpi-1
If
I+
1
2
3
4
Total no. alleles
Total no. alleles
all loci
+
2.9
28.6
1.4
27.5
0.2
35.4
0.5
3.4
8
5.8
2.2
79.1
5.1
3.2
1.2
3.2
0.2
8
8.6
0.2
69.5
7.8
14.0
5
S.u
m a ssp.
S. unijora ssp.
u@wa
pctraca
13.0
1.2
77.0
7.7
1.O
0.1
0. I
0.1
18.9
15.3
58.8
-
6
7.5
89.0
0.2
3.1
0.2
5
0.1
8.6
0. I
79.2
-
6.6
0.1
0.3
8
16.9
2.0
80.5
0.3
0.3
0.1
6
7.9
-
-
42.3
49.8
0. I
0.7
5
0.5
-
0.2
57.1
39.2
0.1
2.7
6
50.6
46.7
0.4
1.8
5
27
21
-
12.1
-
4
40.9
59.1
-
2
20
(alleles 1 and 2) are more common in the S. ungora ssp. ungora populations than
in the populations of the other taxa (Table 2), and at least one of these alleles occurs
in 93% of the S. ungora ssp. petraea populations, and in 50% of the S. uukaris
populations.
Between-population differences in allele frequencies were statistically significant
at all four polymorphic loci within all three taxa (2'tests RO.001, data not shown).
The differences in allele frequencies between taxa were also significant (x' test;
P<O.OOl) at all loci (data not shown).
All four loci were polymorphic in' all S. ungora ssp. petraea populations, and in all
populations except one (population 67, monomorphic at P p - 1 ) in S. uu&aris. In S.
ungora ssp. ungora, several populations were monomorphic at at least one locus; six
populations were monomorphic at Pgi-2,eight at Pgm-1,five at Pgm-2 and two
GENETIC DIFFERENTIATION IN S Z W
569
TABLE
3. Gene diversity statistics for Silene vukani, S. unjtlora ssp.
unjtlora and S. un&a ssp. petma. Separate analyses were carried
out for each taxon. H,=total gene diversity for each taxon.
A PPP =mean within-population diversity. G,,,, =between-popu=within-population comlation component of variation. H,/H,
ponent of diversity. Significance for within-taxon differences in
H, at each of the four loci were obtained using analysis of
variance based on jackknifed pseudovalues for H,
Locus
Silene uulgaqarir
Pgi-2
Pgm-I
Pgm-2
Tpi-1
Mean
Silene un&a
Pg-2
H,
0.716
0.367
0.484
0.520
0.522
Gpp.,
0.610***
0.317***
0.451***
0.490***
0.467
0.148
0.134
0.069
0.058
0.105
0.852
0.866
0.931
0.942
0.895
0.252***
0.179***
0.282***
0.403***
0.279
0.346
0.109
0.198
0.229
0.236
0.654
0.89 I
0.802
0.771
0.764
0.555***
0.296***
0.538***
0.469 n.s.
0.464
0.062
0.082
0.053
0.031
0.055
0.938
0.918
0.947
0.969
0.945
ssp. un&a
0.385
Pgm-I
0.201
Pgm-2
0.352
0.523
0.365
Tpi-1
R,/H,
%w
Mean
Silene untj7ora ssp. pehm
Pg-2
0.591
0.323
pgm-1
Pgm-2
0.568
Tpi-1
0.484
Mean
0.491
***=p<O.OOl;n.s. =not significant.
populations were monomorphic at Tpi-1.One population (population 39, represented
by only seven individuals) was monomorphic at all loci.
Twelve populations (five in S. vulgaris, two in S. unjtlora ssp. unjtlora and five in S.
unjtlora ssp. petraea), showed significant (R0.05) within-population deviations of
genotype frequencies from Hardy-Weinberg expectations at single loci (expected
number of deviations caused by ‘type 1 error’ with 95% probability= 15.8 for 79
populations and four loci).
Partitioning ofgene diversip
Nei’s (1973)gene diversity statistics, based on allele frequencies, are presented in
Table 3. Each of the three taxa of Silene showed the same hierarchical organization
of diversity, with the majority of the gene diversity explained by within-population
variation. However, S. unjtlora ssp. unjtlora showed slightly lower overall levels of
diversity than did S. vulgaris and S. unjffora ssp. petraea. The total gene diversities (H-,
mean over loci) were 0.52 for S. vukaris, 0.37 for S. unjtlora ssp. unjtlora and 0.49 for
S. unjtlora ssp. petraea. The mean (over loci) within-population component of diversity
(Hpp/Hta)was 90%, 76% and 95%, respectively for the three taxa.
Analysis of variance revealed significant between-population differences in withinpopulation diversity (Hpp)at all loci in S. vulgaris and S. ungora ssp. ungora (RO.00l),
and at all loci except Tpz-1 in S. unjtlora ssp. petraea (Table 3).
H.RUNYEON AND H. C . PRENTICE
570
TABLE
4. Overall analysis of gene diversity in Silene Mc&aris, S.unjflora ssp. unjflora and S. unjflom ssp.
petraea. H, =total diversity within all three taxa. R
, =mean within-taxon diversity, R, =mean withinpopulation diversity. G,,,,, =component of between-taxon variation. G,,, =between-population
diversity, and R,/H,, = component of within-population diversity. Sigmficance probabilities for
between-species differences in H, and overall differences between populations (H,,) for each of the
four loci were obtained using analysis of variance (based on the jackknifed pseudovalues for H,, and
H,) (not calculated for the means)
Pgi-2
Pgm-1
Pgm-2
Tpi-2
0.747
0.310
0.546
0.517
0.530
Mean
0.576***
0.305***
0.486***
0.504***
0.468
0.493***
0.272***
0.445***
0.458***
0.417
0.110
0.105
0.076
0.090
0.096
0.229
0.017
0.110
0.024
0.117
0.661
0.878
0.815
0.886
0.787
***=P<O.OOl.e
TABLE
5. Mean Rogers’ genetic distances between populations within taxa (diagonal) and between
taxa of S i b (below diagonal). Ranges within parentheses. n =number of populations
Sih
S. unjflora ssp.
Taxon
n
vu&a7i.s
ungora
S. unjflora ssp.
pelraea
Silcne uulgagarir
24
S.unspMa ssp. un3flora
25
S.ungora ssp. petraea
30
0.219
(0.057-0.448)
0.291
(0.0484.609)
0.243
(0.046-0.565)
0.242
(0.048-0.534)
0.288
(0.04s-O.700)
0.162
(0.04G0.370)
The hierarchical analysis of gene diversity including all three taxa (Table 9,
showed that 11.7% of the total genetic diversity is the result of differentiation
between taxa (Gta,tot),
9.6% is the result of differentiation between populations within
taxa (G,,,,), and the remaining 78.7% represents the proportion of the total
diversity due to variation within populations PpP/Htot).The differences between
taxa in within-taxon diversity
were significant at all loci, and the differences
in within-population diversity (Hp,,) in the total data set were also significant at all
loci (one-way analysis of variance).
Two-way analysis of variance showed that there were significant differences in
H, between loci in all three taxa (P<O.OOl, data not shown). The locus x population
interactions and between-population differences in H,, were also significant
(P<O.OOl)in the three taxa (data not shown).
Genetic dzfmtiation within h a
Mean painvise genetic distances within and between taxa are presented in Table
5. Both the mean Rogers’ genetic distance between populations and the range of
the genetic distances within each taxon were somewhat lower in S.ungora ssp. petraea
compared to the other two taxa.
Comparisons between matrices of genetic (pooled over four loci) and geographic
GENETIC DIFFERENTIATION IN SILiNE
57 I
TABLE
6. Tests of association between matrices of genetic and geographic distances using Mantel
tests (1000 permutations) and relative neighbourhood graphs (RNG, 1 df)
Mantel test
Taxon
Silme vukaris
S. wzjpora ssp. unjpora
S. unfira ssp. petraea
Probability
0.69 n.s.
-0.038
0.5 12
0.40I
*** =R0.001;n.s. =not
RNG
test statistic)
Likelihood ratio
(x'
Correlation
coefficient
26.05***
8.17***
29.97***
o.oo***
o.oo***
significant.
0.5 -
163
74
61 62
70
1
76
.
I
71h66
169
cv
68
4
88
60 +75
671
1 69
78 77 6
6.
O.O
66
I
64
1 1
-0.5
79#73
72
0.0
67
0.5
1.0
Axis 1
Figure 2. Plot of the first two axes of a correspondence analysis (based on allele frequencies at four
polymorphic loci) of 22 populations of Silme mlgu9-i~from Sweden and two populations from Norway.
h i s 1 accounts for 13% and axis 2 for 7% of the total variance between populations. The different
sizes of symbols in Figs 2-5 represent six levels of within-population gene diversity (R mean over
four loci) in steps of 0.10. In S.uu~uris,the smallest symbols indicate populations WithPqib, values of
0.200-0.299 and the largest symbols indicate populations with R,, values of 0.500-0.599. The
population numbers are the same as in Table 1.
distances between populations, using relative neighborhood graphs (Leflcovitch, 1984)
and Mantel tests (Mantel, 1967),revealed significant correlations between geographic
distance and genetic distance for S i h e ungora ssp. ungora and S. ungora ssp. petraea
(Table 6). In S. vulgaris, however, the correlations between genetic and geographic
distances were significant (RO.00 1) using the relative neighborhood graph test, but
not significant in the Mantel test (Table 6).
Four different correspondence analyses (CA), based on within-population allele
frequencies, were carried out. The results of the analyses for the three separate taxa
are shown in Figure 2 (S. vulgaris), Figure 3 (S. un@ora ssp. ungora) and Figure 4 (S.
ungora ssp. petraea).
In the ordination plot of the S. vulgaris populations (Fig. 2) there is a tendency for
values to group together on the left side of the plot.
the populations with high HPOP
All the populations from southeastern Sweden (56-63 and 75) are included in this
group, together with one population from southern Sweden (78) and one of the
populations from western Sweden (71). The six populations from central Sweden
(64-69) are grouped in the lower right of the diagram and have high scores on axis 1.
H. RUNYEON AND H. C. PRENTICE
572
A
1.0
62
A
63
55
A
54
A
49
A
KO
A
I
I
I
I
-1.0
-05
0.0
0.6
1.0
Axis 1
Figure 3. Plot of the first two axes of a correspondence analysis (based on allele frequencies at four
polymorphic loci) of 25 populations of S i h unjffwa ssp. unyba from Sweden. Axis 1 accounts for 16%
and axis 2 for 11% of the total variance between populations. Symbol size indicates levels of withinpopulation gene diversity (see Fig. 2). In S. unyba ssp. unflom, the smallest symbols indicate populations
, values of 0.000-0.099 and the largest symbols indicate populations with R, values of
with R
0.400-0.499. The population numbers are the same as in Table 1.
0.6
t
0
20
21
0
18
h
cu
4
0.1
-0.4
-0.6
0.0
0.6
Axis 1
Figure 4. Plot of the first two axes of a correspondence analysis (based on allele frequencies at four
polymorphic loci) of 30 populations of S i h un$ora ssp. petrma from Sweden. Axis 1 accounts for 6%
and axis 2 for 4% of the total variance between populations. Symbol size indicates levels of withinpopulation gene diversity (see Fig. 2). In S.un#ha ssp. petraea, the smallest symbols indicate populations
with R
, values of 0.300-0.399 and the largest symbols indicate populations with R, values of
0.500-0.599. The population numbers are the same as in Table 1.
The central Swedish populations have fewer alleles at all loci and slightly lower
than the other S. vu,!guris populations. Population
within-population diversities (€-Ipop)
74, which is separated from the other populations, is from eastern central Sweden.
In the CA for S. ungora ssp. ungoru (Fig. 3), the populations from the Baltic coast
(populations 49-55) all have high scores on axis 1, and are also more heterogeneous
than the populations from the west coast. Alleles 1 and 3 at &-2 occur at higher
frequencies in the populations from the Baltic coast (population 49-55) while allele
+
.
GENETIC DIFFERENTIATION IN S I W E
B
I
I
64
A
49
A
3 0.0
-0.5
-0.5
A'
37AA
&
A
33
53
A
A
44
52
A
573
36
A
AArY
A
I
I
0.5
0.0
1.0
Axis 1
Figure 5. Plot of the first two axes of a correspondence analysis (based on d e l e frequencies at four
polymorphic loci) of all populations of S i h w.$pi (m), S. unjfloru ssp. unjflora (A) and S. unjfloru ssp.
petraeu (0).
Axis 1 accounts for 21% and axis 2 for 15% of the total variance between populations.
Symbol size indicates levels of within-population gene diversity. The smallest symbols indicate
populations with H,, values of O.OO(M.099 and the largest symbols indicate populations with H,,,
values of 0.50W.599 (symbol size categories as in Fig. 2). The population numbers are the same as
in Table 1.
1 at P p - 2 has lower frequencies in the Baltic populations than in the populations
from western Sweden (31-48).
Within the west coast group of populations, there is a tendency for the northern
populations to appear in the upper half of the group, while the southern populations
appear in the lower half of the group. At Pgi-2, Pgm-I and P p - 2 it is possible to
distinguish between a northern (populations 39-48) and a southern (populations
31-38) group of S. ungora ssp. ungora populations from the west coast. The two
groups of populations differ in terms of allele frequencies as well as the presence or
absence of particular alleles. Allele 1 at Pgi-2, allele 1 at P p - 1 and allele 3 at
P p - 2 are more common in the northern group of populations, whereas allele 3 at
Pp-I is more common in the southern group. Populations with low H,, values
tend to have low scores on axis 1.
Figure 4 shows the CA ordination plot of the 30 S. unzj?ora ssp. petraea populations.
The populations from Gotland (1 6-30) show more between-population variation
than the populations from Oland (1-15). The Oland populations (1-15) form a
relatively tight group in the lower centre of the plot. Ten of the 13 populations with
positive scores on axis 1 are from Gotland. Populations on the right-hand side of
the diagram bositive scores on axis 1) tend to have low frequencies of allele 2 at
Pgi-2 (&17%), whereas populations on the left of the diagram tend to have higher
frequencies (7-50%) of this allele.
+
Genetic dafuentiation between taxa
The mean Rogers' genetic distance between pairs of populations from different
taxa was not much higher than the mean distances within taxa (Table 5). The
greatest divergence was found between S. vulgaris and S. ungora ssp. ungora (0.291)
and the smallest divergence was between S. ungora ssp. petraea and S. vulganS (0.243).
The correspondence analysis (CA) including all taxa shows that the three taxa
form largely separate groups on the first two axes (Fig. 5). The S. ungora ssp. petraea
574
H. RUNYEON AND H. C. PRENTICE
population (20) that falls within the group of S. vurIpani populations, is from the coast
of Gotland and contains high frequencies of allele 4 at Pgi-2 (a ‘ v u ~ u n iallele’, cf.
Table 2). The three S. unfiru ssp. petrueu populations closest to the group of S. unijloru
ssp. unijloru populations (16, 17 and 28) are also from the coast of Gotland. Two of
the Baltic populations of S. un@oru ssp. ungoru (52 and 53) are close to the group of
S. unjfloru ssp. petrueu populations in the diagram. Allele 3 at Pgi-2,an allele common
in S. ungoru ssp. petrueu and usually rare in S. untjloru ssp. unjfloru, has frequencies of
33% and 44%, respectively, in populations 52 and 53. The same allele also occurs
in populations 33 and 44, also close to S. unjfloru ssp. petrueu in the plot. Allele 3 at
Psi-,?is also present in the rest of the Baltic S. un$loru ssp. unijloru populations, except
for population 50, with frequencies of 2-35%.
The first two a x e s in the CA in Figure 5 only explained 36% of the total variance,
so a Ward‘s cluster analysis, based on Rogers’ genetic distances of all populations,
was carried out to provide an overall summary of the variation pattern in the three
taxa. The dendrogram from this analysis is shown in Figure 6. The relationships
between taxa in Figure 6 are more complicated than those revealed by the first two
CA axes (Fig. 5). Three major groups of populations emerge from the cluster
analysis. The first group (1) includes all the Silene unjfloru ssp. petraea populations,
together with two populations of S. un@oru ssp. unijloru (52 and 53, from the Baltic
coast) and one S. vu,$pis population (70, from southern Sweden).The 15 populations
of S. unjfloru ssp. petrueu from & n d form a subgroup to the far left, together with
three populations from the alvar habitats on Gotland and one S. vulguni population
(70). The second cluster (2) consists of all the western (31-48) S. unijloru ssp. unjfloru
populations plus one population from the Baltic coast (51).Within the third cluster
of populations (3), there are two main groups: one group comprising the four
remaining S. ungoru ssp. un@oru populations from the Baltic coast (49, 50, 54 and
55), and one group containing the remaining 23 S. vulguni populations. Only one
of the seven populations from the Baltic coast clustered with the main group of S.
un@oru ssp. unjfloru populations (2).
DISCUSSION
Most of the gene diversity within each of the three taxa is accounted for by
within-population diversity (Rpop/Htax).
The within-population component of diversity
is highest in S. unjfloru ssp. petrueu, with 95% of the diversity stored within populations,
while 90% and 76%, respectively, of the total gene diversity in S. vu~&ni and S.
unyl’wu ssp. un@oru is explained by within-population variation. This structuring of
variation, with relatively low levels of gene diversity explained by differences between
populations, is consistent with what is usually found in largely outbreeding, insect
pollinated and perennial species (Hamrick, Linhart & Mitton, 1979; Hamrick &
Godt, 1989; Hamrick et ul., 1991). An earlier study of S. uu~&uri.s in the Netherlands
(verkleij, Bast-Cramer & Levering, 1985) revealed similar levels of inter-population
differentiation to those found in the present study, with 11YOof the allozyme variation
accounted for by differences between populations. On a more restricted geographic
scale, Runyeon & Prentice (1996) reported lower between-population differences in
both 8. uu&uris (2%) and S. unjRora ssp. petrueu (<1%) in a study based on material
from the Baltic island of Oland.
GENETIC DIFFERENTIATION IN SLLENE
575
0 1
0 4
0 9
0 11
0 12
o 13
0 2
0 6
0 8
o 18
o 26
o 30
0 3
0 6
0 10
o 14
o 16
0 7
70
o 16
b
63
b
62
0
20
21
22
23
26
27
29
31
39
34
41
37
36
42
47
33
61
32
36
40
38
43
o 17
o 28
o iS
o 24
0
0
o
o
o
o
-
1
..
.
..
b
b
b
b
b
b
.48
w 46
.46
.
.44
49
b 64
b 60
b
1
66
66
69
61
68
60
67
62
71
76
63
76
66
73
78
67
69
m 6 8
72
77
79
m 6 4
66
74
1
J
+
7
1
Figure 6. Dendrogram showing between-population variation in Silme vu.&;S (I),
S. un$ora ssp.
un$ora (b)and S.unjflora ssp. petraea (0).
The dendrogram was produced using Ward’s cluster analysis
of Rogers’genetic distances between pairs of populations. The bars below the figure show the mean
(over loci) within-population gene diversities (Rw). The population codes are given in Table 1.
576
H.RUNYEON AND H.C . PRENTICE
S i h e vulgaris
S i h e vulgaris is the most widespread of the three taxa in this study, both within
Sweden and Europe and, in general, widespread taxa show high levels of total gene
diversity (Hamrick & Godt, 1989). Silene vulgaris is characterized by high levels of
both total and within-population diversity in the present study (Table 3). This species
has more alleles than either of the other two taxa, and five of the 27 alleles at the
four loci are unique to S. vulgaris, with frequencies of 0.2-7.8% (Table 2).
In northern and central Europe, S. vulgaris is characteristically a weed of disturbed
roadsides and field margins. In southern Europe and in the Mediterranean region,
however, there are a number of distinct geographic races which are found in natural
or semi-natural montane habitats (Jalas & Souminen, 1986).The weedy populations
are likely to have spread northwards following the expansion of agriculture, in a
similar fashon to the weedy subspecies of S.latgolia (MarsdenJones & Turrill, 1957;
Prentice, 1986). High levels of overall and within-population gene diversity have
also been recorded from weedy populations of Tnjolium hirtum (Jain 8z Martins,
1979),Echiumplantagineum (Burdon & Brown, 1986),Helianthus annuus (Dry & Burdon,
1986) and Carduus spp. (Warwick et al., 1989).
The relatively small amount of between-population variation (c. 10%) in S.
vulgaris may be taken as a general and indirect indication of extensive-recent or
historical-gene flow between populations (cf. Govindaraju, 1988; Ellstrand, 1992).
Even though S. vulgaris is a weedy taxon with transient populations, it flowers
intensively during a short period (3-4 weeks), with up to 80 flowers per inflorescence
(Clapham et al., 1989).This mass-floweringis likely to attract pollinators and enhance
gene flow between populations (Andersson, 1988; Runyeon & Prentice, 1996).
The S. vulgaris populations from Oland and Gotland show the highest withinpopulation diversities (€Ipop)(Fig. 2). Open semi-natural and marginal arable habitats
are still relatively widespread on Oland and Gotland, and are likely to have been
available on both islands throughout the post-glacial period (Konigsson, 1968). In
contrast, the extent of such habitats has been successively reduced in many other
areas of Sweden, mainly as a result of changing agricultural practices. The central
Swedish populations of S. vulgaris occur in a part of Sweden where most of the land
is forest-covered, and areas with open habitats are often small and fragmented.
Because of the transient nature of many open habitats in central Sweden, the
populations of S. vulgaris are often short-lived and small in these areas. The populations
sampled from central Sweden (64-69) show lower levels of internal diversity and
have fewer alleles per population than those from Oland and Gotland (56-63). The
central Swedish populations have a mean H,, of 0.407 whereas the populations
from Oland and Gotland have a significantly higher mean H, of 0.525 (P=0.002;
t-test, 12 df). The mean number of alleles in the two groups of populations are 12.3
and 15.9 respectively (P=0.008; t-test, 12 df). The relatively lower levels of allelic
variation and diversity in the central Swedish populations may reflect the effects of
genetic drift in small populations or of stochastic processes during founder events.
Genetic drift and the random loss of genetic variation are expected to occur more
rapidly in small populations (Nei et al., 1975; ShafTer, 1987; Bijlsma, Ouborg & van
Treuren, 1994) and a number of recent studies have shown an association between
population size and levels of allelic variation and genetic diversity in natural plant
populations (e.g. Barrett & Kohn, 1991; van Treuren et al., 1991;Jain, 1994).
S i h e vulgaris and S. ungora ssp. petma are sympatric on Oland and Gotland, and
GENETIC DIFFERENTIATION IN SILENE
577
fertile hybrids may occur if the two taxa come into contact. Past hybridization may
have contributed to the higher levels of genetic diversity in S. vulgaris on Oland and
Gotland, whereas S. vukaris from central Sweden is unlikely to have been in recent
contact with any populations of S.unijlora. However, a detailed study of the structure
of genetic variation in S. vulgaris and S.unijlora ssp. petraea on Oland suggests that,
at present, gene flow between the two taxa is restricted by a combination of ecological
and phenological factors (Runyeon & Prentice, 1996).
S i h unijlora ssp. unijlora
S i h e unijlora ssp. unijlora has fewer alleles per locus and lower total (Hta) and
variation than the other two taxa (Tables 2 and 3). There
within-population (Hpop)
is only one allele unique to S. unijlora ssp. unijlora (allele 1 at Pgm-2,cf. Table 2),
and several S.unijlora ssp. unijlora populations are monomorphic at one or more loci.
The sample sizes for the populations that have one or more monomorphic loci are
not generally smaller than those of the more variable populations.
The component of between-population diversity is 24% in S. ungora ssp. unijlora
(Table 3). The higher levels of differentiation between populations in S. unijlora ssp.
unijlora suggests that gene flow may be--or have been-more restricted than in the
other two taxa. Experiments carried out by Marsden-Jones & Turrill(l957) suggest
that autodeposition of pollen in hermaphrodite individuals is likely to occur more
often in S.unijlora (ssp. unijlora) than in S. vulgaris, and S. unjAora produces a higher
proportion of viable seeds after selfing (Pettersson, 1992b). Because of the character
of their habitat, coastal populations of S.unijlora ssp. unijlora are likely to have always
been more or less linear and limited to a rather narrow strip of shoreline (cf.
Marsden-Jones & TurriU, 1957).This linear population structure may have restricted
gene flow by pollen and promoted population differentiation (cf. Levin, 1979).
Marsden-Jones & Turrill (1957) also point out that the development of towns and
seaside resorts has led to the disjunction of previously more continuous coastal
populations of S.unijlora.
The differences between the Baltic and the western populations of S.unijlora ssp.
w@?ora (Fig. 3) may have a historical explanation. Most of the southern parts of the
Swedish west coast were free from ice and water at least 12 000 I4Cyr BP (Bjorck,
1995) while the central parts of the Baltic coast (where populations 49-55 were
sampled; see Fig. 1) emerged from the sea more than 5000 years later (Ericsson,
Konigsson, & Larsson, 1978). There is also a difference in land uplift rates between
the west coast and the Baltic coast (1-3 mm/year and 6-7 mmlyear, respectively).
It is thus reasonable to assume that the Baltic populations are younger than the
west coast populations.
On the east coast of Sweden, the distribution of S. unijlora ssp. unijlora extends
northwards from Sodermanland (south of Stockholm) and continues round the Gulf
of Bothnia and southwards along the west coast of Finland. There are a few old
records of populations of S. unijlora from the Baltic coast S. of Vastervik (from
SmAland and Blekinge). Herbarium material from these populations is referable to
ssp) petraea, not ssp. ungora (H.C. Prentice, pers. obs.).
Xt is tempting to speculate that the absence of S. ungora ssp. ungora from the
southern Baltic coast of Sweden might be related to the coastal geography around
the time of the Yoldia Sea between about 10 000 and 9600 BP (cf. Bjorck, 1995).
+
578
H. RUNYEON AND H.C. PRENTICE
The northern Baltic populations could then have been derived from west coast
populations that migrated eastwards, via the straits in the central Swedish Lowlands,
to the central Baltic coast.
An alternative scenario for the origin of the Baltic populations of S. unjflora ssp.
unjflora is that they were derived from an eastern or southeastern source. Historical
contact between S. unjAma ssp. ungora and ssp. petraea populations could explain the
fact that some of the Baltic populations of ssp. unjflora are genetically similar to ssp.
petraea. For example, allele 3 at P&*-.?,
which is common in S. unjflora ssp. petraea
populations and rare in the ssp. unjflora populations from the west coast, occurs in
six of the seven Baltic populations of S.unjflora ssp. unjflora.
The N-S differentiation pattern within the west coast populations of S. ungora
ssp. unjflora may also reflect the taxon’s colonization history during the early postglacial period. Whereas the southern part of the west coast was ice- and water-free
at least 12 000 yr BP, it took approximately another 2500 years until the northern
parts of the present-day west coast were above sea-level (cf. Bjorck, 1995).
Silene ungora ssp. petraea
Although the endemic subspecies, S i h unjflora ssp. petraea, has a restricted
distribution and a patchy population structure, it is characterized by high levels of
both total and within-population variation. Past hybridization between S. mkak
and S. unjflora ssp. petraea may have contributed to the variation in this subspecies.
Extensive gene flow by pollen (cf. Runyeon & Prentice, 1996) and a continuous
history of habitat-availability are also likely to have promoted the maintenance of
high levels of genetic diversity, despite the patchy local population structure. During
the late-glacial and early post-glacial periods, there were many open and base-rich
habitats in Sweden (Iversen, 1958; Fries, 1965).Such open habitats have disappeared
in most areas of Sweden, as a result of forest succession and soil acidification (Iversen,
1958; Fries, 1965). On Oland and Gotland, however, open and calcareous alvar
habitats have been available-although with a variable extent-since early postglacial times (Konigsson, 1968). In particular, the extreme, frost-disturbed habitats
occupied by ssp. petraea on Oland are unlikely ever to have supported forest vegetation
(L.-K. Kbnigsson, pers. comm.). Large parts of Oland and Gotland were above the
level of the Yoldia Sea (c. 10 OOOyr BP) and some areas of Oland and Gotland
were, at least temporarily, above sea-level during the period 12 000 to 10 000 BP
(L.-K. Kbnigsson, pers. comm.).
The habitat requirements and distribution of S i k unjflora ssp. petraea suggest that
this taxon immigrated into Sweden at an early stage during the post-glacial, and
that it may have been widespread in open habitats round the retreating ice margin.
The Baltic islands of Oland and Gotland have been shown to have acted as postglacial refuges for a number of taxa that were widespread during Late Glacial times
(Witte, 1906; KiSnigsson, 1968) and there are several taxa endemic to these islands.
The populations on Oland and Gotland may thus be remnants of more extensive
populations of S. unjflora ssp. petraea.
The between-population component of diversity is small in S. unjflora ssp. petraea
(Gpop,tar=~.
5%, Table 3). The flowering season of S. ungora ssp. petraea spans the
entire summer (3-4 months), the pollinators (moths) are long-distance flyers, and
the habitat where it occurs is extremely open; all factors that will promote gene
GENETIC DIFFERENTIATION IN S I W E
579
flow by pollen (cf. Runyeon & Prentice, 1996) and maintain similarity between
populations.
There are genetic differences (mainly in allele frequencies) between the S. ungora
ssp. p e t m a populations from Oland and the populations from Gotland (cf. Fig. 4).
If the 15 Oland populations and the 15 Gotland populations are analysed separately,
we find that the Oland group of populations is more homogeneous, with only 0.8%
of the total variation accounted for by between-population differences (Runyeon &
Prentice, 1996). The between-population component of diversity on Gotland is 10
times greater than that on Oland, i.e. 8% of the total variation. The difference in
genetic structure between Oland and Gotland may reflect the differences in the
geographic distribution of the alvar habitats on the two islands. O n Oland S i h e
ungora ssp. petraea only occurs on the ‘Great Alvar’ on the southern part of the
island. The Great Alvar is continuously open and treeless, covering an area of c.
300 km‘,and there are no physical barriers to pollinator flight in any direction. O n
Gotland, in contrast, the alvar habitats are scattered throughout the island, are
smaller and more fragmented and are separated by areas of scrub or woodland.
This habitat fragmentation, is expected to reduce gene flow between the populations
and to increase the level of between-population differentiation.
Bfirentiation between taxa
AUozyme evidence from the present study indicates that S. VukanS, S.unjrZora ssp.
untjlora and S. ungora ssp. petraea are distinct taxa. The taxa form separate groups in
the ordination plot (Fig. 5), and share the most common allele only at one (Pgm-2)
of the four loci surveyed. There are also significant differences in allele frequencies
at all loci between all pairs of taxa. Each taxon has at least one unique d e l e , even
though only one of them has a frequency of >2% (allele 2s at Pgm-2,private to S.
uulgark with a frequency of 7.8%). Finally, these unique alleles are not restricted to
geographic areas where the taxa are not in contact at the present-day.
There is morphological evidence for hybridization between S i h e vulganS and S.
ungora ssp. petraea (H.C. Prentice & H. Runyeon, pers. obs.) as well as between S.
uukaris and S. unijlora ssp. ungora (Marsden-Jones & Turrill, 1957; Walters, 1975).
The hybrids are highly fertile and may persist for several years (Marsden-Jones &
Turrill, 1957; H.C. Prentice, pers. obs). However, because the habitat requirements
of the three taxa differ considerably, hybridization is infrequent and hybrids only
occur in intermediate habitats.
Although the present study shows a clear distinction between the three taxa, the
study also suggests that there is, or has been, gene flow between taxa. There are a
few alleles which are common in one of the taxa but rare in the other two taxa.
There are three alleles that are characteristic of S. vulgaris, two S.unjrZora ssp. ungora
alleles, and two ssp. petraea alleles which occur as foreign alleles in the other taxa.
The presence of foreign alleles cannot be explained solely in terms of present day
hybridization and inter-taxon gene flow (cf. Runyeon & Prentice, 1996).The foreign
alleles are spread throughout the ranges of the other two taxa and are not restricted
to the areas where the taxa are sympatric at present. There is only a slight geographic
pattern in the occurrence of the foreign alleles. For example, the Gotland populations
of S.ungora ssp. petraea tend to contain more foreign alleles than do the populations
from Oland.
580
H. RUNYEON AND H. C. PRENTICE
Sixteen, out of the total of 30 alleles that were detected in the study are present
in all three taxa (Table 2). Genetic distances between taxa are small, and comparable
to distances within taxa (Table 5). Multivariate analysis (CA) indicates that Silene
unjrlora ssp. unjrlora and S. unjpora ssp. petraea are closer to each other evolutionarily
than either taxon is to S.vulgaris (Fig. 5). Yet there are no alleles unique to S.unjflora
(i.e. alleles that are shared by S. unjrlora ssp. unjrlora and S. unjrlora ssp. petraea but not
present in S. vulgak, cf. Table 2). A somewhat different pattern of relationships is
seen in the dendrogram (Fig. 6), where all S.unjpora ssp. unjrlora populations, except
populations 52 and 53 (from the Baltic coast), cluster together with the group of S.
vulgaris populations, and the S.unijlora ssp. petraea populations form a clearly separated
group.
S i h e vulgaris and S. unjrlora are closely related species (Marsden-Jones & Turrill,
1957; Chater & Walters, 1964, 1990; Chater et al., 1993). S i h unjrlora ssp. petraea
occupies a morphologically somewhat intermediate position between S.vulgaris and
S. unjrlora ssp. unfira, but ssp. petraea also has some unique features such as its
germination requirements (Prentice & Giles, 1993).
Marsden-Jones & Turrill (1957) carried out a detailed study of morphological
variation in S. vulgaris and S. unjrlora, and concluded that these two species are not
as clearly differentiated in Scandinavia as they are in the rest of Europe. Two
hypotheses were presented to explain why the species boundaries are apparently
less distinct in Scandinavia. Firstly, it was suggested that the habitats of S. vulgaris
and S.unjrlora may overlap more often in Scandinavia, and that this habitat overlap
provides more opportunities for hybridization as well as a greater chance of hybrid
persistence. Secondly, it was suggested that the populations of S. vulgaris (including
‘S.maritima var. petraea’) in Scandinavia might represent the remnants of an ancient
population from which S. vulgah and S.unjrlora are s t i l l diverging. Turesson (1925)
regarded ‘var. petraea’ as an ecotype of S. maritima (S. unjrlora), and Neuman (1901)
argued that ‘var. petraea’ was a transition stage between S. vulgaris and S.unjrlora.
The allozyme data presented here confirm that the three taxa are indeed closely
related. The pattern of allele frequency variation and the geographic distribution of
foreign alleles also suggest that there has been some historical gene flow between
the taxa outside the areas where their ranges overlap at present. However, the
structure of genetic variation in Oland populations of S. vulgaris and S. unjrlora ssp.
petraea, and the persistence of unique alleles, indicate that present day gene flow
between the two taxa is limited, despite their ability to hybridize (Runyeon &
Prentice, 1996).
G. Bocquet (D. Aeschimann, pers. c o r n . ) proposed that S. ungora ssp. petraea
originated as a hybrid between S. vukaris and S. unjrlora, south of the boundary of
the Fennoscandian ice sheet, during the Weichselian. The results from the present
study and from Runyeon & Prentice (1996) are consistent with Bocquet’s suggestion
that the origin of S. unjrlora ssp. petraea predated the post-glacial colonization of the
Baltic region, and that it does not have a recent hybrid origin. Further discussion
of a possible hybrid origin for S. unjrlora ssp. petraea should be postponed until data
are available from populations of S. unjrlora and S. vulgaris further south in Europe
GENETIC DIFFERENTIATION IN SlLEArE
581
and, particularly, from montane taxa such as S. unijlora ssp. glanosa (cf. Aeschimann,
1983-85; Chater & Walters, 1990).
We can tentatively suggest immigration scenarios for the three Silene taxa, on the
basis of their present day habitat requirements and the available reconstructions of
the Quaternary history of southern Sweden and the Baltic Sea. The three taxa occupy
clearly different habitats. Both S. unijlora subspecies are intolerant of competition, and
are restricted to habitats that remain open even in the absence of human management
or disturbance (cf. Marsdenjones & Turrill, 1957). Such naturally open habitats
will have been available for a longer time than the anthropogenic habitats of the
weedy S. vukaris.
It is likely that the two subspecies of S. unjflora were able to colonize open habitats
that existed in southern Sweden and on oland and Gotland, at the end of the Late
Weichselian and during the early Flandrian. The allozyme data from this study are
consistent with a separate eastern origin for S. unjflora ssp. petraea. It is still unclear
when areas of land became continuously available on oland and Gotland (Bjorck,
1995; L.-K. Konigsson, pers. comm.), but it seems probable that these two large
Baltic islands would have been available for colonization later than the coastal areas
in southwestern Sweden. Possibilities for the establishment of S. unijlora ssp. petraea
on Oland and Gotland (and on the S.E. mainland coast of Sweden) may thus postdate those for ssp. unijlora on the west coast.
S. unjflora ssp. unijlora may have been derived from a western source, or from both
eastern and western sources. The southernmost parts of the west coast of Sweden
were available for colonization earlier than the northern parts of the west coast and
it is likely that the first populations of ssp. unijlora entered Sweden from the south
west, whereas the northern Baltic coast was colonized considerably later than the
west coast. Allozyme data (for example, frequencies of allele 3 at Pgi-2) indicate that
eastern populations of ssp. unjflora may have been in limited contact with ssp. petraea
during some phase of this eastern wave of migration.
Weedy populations of S, vukaris are expected to have followed the spread of
agriculture into Sweden (cf, Marsdenjones & Turrill, 1957).Populations of S. vukaris
are often transient and the structure of the genetic variation in present day populations
of this species probably reflects multiple origins. Opportunities for hybridization
between S. vukaris and the two S. unjflora subspecies arise (and must also have arisen
historically) in areas of sympatry when alvar or coastal habitats are subjected to
anthropogenic disturbance. However, although the allozyme data in the present
study suggest that episodes of interspecific gene flow may have occurred, the integrity
of the three taxa in Sweden appears to have been maintained by a combination of
their distributional histories and distinct habitat preferences.
ACKNOWLEDGEMENTS
We are grateful to Lars-Konig Konigsson for discussions on the Quaternary
history of the southern Baltic area and to Mikael Hedrtn for his constructive
comments on the manuscript. We would also like to thank Bo Goran Johansson
and Bengt Jonsell for providing information on localities of S i h e populations, Bertil
Blohm for taking care of S i h e plants in the greenhouse, Robert Lager and H a a n
Tegelstrom for practical help with fieldwork. The Uppsala University Ecological
582
H. RUNYEON AND H. C. PRENTICE
Research Station at Olands Skogsby provided a base for much of the field and
laboratory work. The study was funded by grants from the Swedish Natural Science
Research Council and the Swedish Council for Agricultural and Forestry Research
(to HCP), by grants from the Uppsala University Ecological Research Station (to
both authors), and by grants from ‘Lennanders stipendium’, ‘Th. Kroks donation’
and the ‘Hierta-Retzius stipendiefond’ (to HR).
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