Biological Ljournal ofihe Linnean Sociely (1988), 34: 81-92. With 4 figures
Temporal patterns of reproduction and
outcrossing in weedy populations
of Echium plantagineum
J. J. BURDON, A. M. JAROSZ AND A. H. D. BROWN
CSIRO Diuision oJ Plant Industry, G.P.O. Box 1600, Canberra,
A.C. T. 2601, Australia
Received 6 Julj, 1987, accepted,for publication 26 November 1987
Single and multilocus estimates of outcrossing rates were made in three populations of Echium
plantagineum. Despite spatial separation, variations in population size (though not density) and
reproductive output, no statistically significant difference was detected in outcrossing rates between
the populations. Similarly, only slight differences in outcrossing rates were detected within
populations when estimates were based on seed collected from flowers open at different times in the
flowering season. The earliest flowers tended to have a lower estimated rate of outcrossing. In all
cases, multilocus outcrossing rates were high, ranging from 0.81 to 1.05.
In all three populations flower production extended over a period of more than 2 months but thr
majority of seed was produced by flowers that opened during the first third of the flowering season.
This was largely caused by a high rate of flower production during the early part of the season and
not changes in the number of seeds set per flower. Thc average number of seeds produced per
flower varied both between individuals within populations, and between the different populations,
but neither of these differences were significant.
Controlled pollination of flowers with self or outcross pollen, applied either singly or together,
failed to detect any differences in the likelihood that either type of pollen would give rise to fertile
seeds.
K E Y WORDS:
-
Echium - outcrossing estimates
-
reproductive biology
-
temporal variation
CONTENTS
Introduction . . . . .
Material and methods . . .
Results..
. . . .
Reproductive performance.
Outcrossing rates . . .
Controlled pollination . .
Discussion. .
. . . .
Acknowledgements
. . .
Referrncrs.
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81
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84
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86
87
87
91
91
INTRODUCTION
Estimates of outcrossing rates have been made for a wide range of plant
species (see Schemske & Lande, 1985, for review). Somewhat less information is
available concerning intra-specific variation in this key component of the
0024-4066/88/05008 1
+ 12 $03.00/0
81
0 1988 The Linnean
Socicty of London
82
J . J . BURDON E T A L .
breeding system. However, in some species little variation has been observed
between outcrossing rates in different populations while, in others, extreme
variability has been recorded (Brown & Albrecht, 1980). Thus three studies of
Avena barbata (Marshall & Allard, 1970; Allard, Babbel, Clegg & Kahler, 1972;
Hamrick & Allard, 1972) and two of Pinus ponderosa (Mitton, Linhart, Hamrick
& Beckman, 1977; Mitton, Linhart, Davies & Sturgeon, 1981) found relatively
little intra-specific variation in outcrossing rates. In contrast, Schoen ( 1982)
found outcrossing rates that ranged from 0.15 to 1.06 among nine populations of
Gilia achilleiflia. Temporal variation in outcrossing within a species has usually
been studied between different years (Moran & Brown, 1980; Hamrick, 1982;
Brown, Barrett & Moran, 1985; Cheliak, Morgan, Dancik, Strobeck & Yeh,
1984; Cheliak, Dancik, Morgan, Yeh & Strobeck, 1985). Few studies have
tested the uniformity of outcrossing rates in natural populations within a single
season. yariation could arise when different individuals or genotypes with
different propensity to outcross, flowered for restricted periods within the season.
Alternatively, temporal variation in pollinator bchaviour and activity may
result in different levels of outcrossing.
Inter- and intra-populational differences in outcrossing rates of the magnitude
observed by Schoen (1982) and others (for example, Harding, Mankinen &
Elliott, 1974; Jain, 1978; Rick, Fobes & Holle, 1978) are likely to have a
substantial effect on both the gcnetic structure of different populations of such
species and on the speed at which they may respond to changes in selective
pressures. The present study examined outcrossing rates in three populations or
Echium ~ ~ a n ~ a g i nL.,e uan
~ annual weedy species of Mediterranean origin that
has spread extensively through southern Australia. T h e aim of this work was to
test for variation in outcrossing rates within a single generation. T o determine
the effect of any temporal differences in outcrossing on the overall breeding
system, the reproductive biology and, in particular, the number of seed
produced by each cohort of flowers, was monitored simultaneously. While
previous work has shown populations of E. plantagineum to be highly
polymorphic (Brown & Burdon, 1983; Burdon & Brown, 1986), relatively little
is known about its reproductive biology and, in particular, the level and
seasonal pattern of outcrossing it exhibits. The flowers of this species are
protandrous (Piggin, 1982). However, the very rapid rate of floral development
may limit the degree to which protandry can prevent selfing particularly at
peak season (Corbet & Delfosse, 1984; personal observation). In addition,
protandry does not preclude crossing to other open flowers on the same cyme or
other cymes on the same individual.
MATERIAL AND MKI'HODS
Reproductive biology of populations of Echium plantagineum
Three populations of E. planlagineurn were monitored during this study. In
each, a number of plants was selected at random and their reproductive
performance followed throughout a n entire flowering season. Early in the season
the main flowering cyme of each plant was tagged and a t this and each
subsequent visit (approximately every 4 days), the following characters were
recorded: (i) the number of open flowers on each tagged cyme; (ii) the
cumulative number offlowers on those cymes; and (iii) the total number ofopen
OU’ICROSSING IN E PLANTAGINEIJM
83
flowers on tagged plants. In addition, the density of E . plantaginturn plants in
each population was determined in eight randomly placed 0.25 m 2 quadrats. In
each quadrat, the percentage of plants flowering and the total number of open
flowers that were present were recorded. Later in the season as seed began to
mature, the number of seeds produced per flower (maximum of 4) on tagged
cymes was recorded. Ripe seed from each of these flowers was collected
separately and used later in estimating outcrossing rates.
The number of plants tagged and the average number of open or finished
flowers on marked cymes at the time of initial tagging varied between the three
populations (Table 1 ) . I n population 1, the presence of a noticeable number of
later flowering individuals allowed a second subset of the population to be
monitored when it started flowering approximately 19 days after the main
sample.
Mating gutem estimation
I n order to assess variation in outcrossing rates in B. plantagineum through the
course of a single flowering season, batches of seed from discrete groups of five
flowers that opened within seven days of each other were analysed. As a result,
the flowers within each group are likely to have shared the same pollinators. I n
populations 1 and 2, these seed sets were taken at three times representative of
early, peak and late season flowering, respectively. Similarly timed early and
peak season analyses were made for population 3. However, there was an
abrupt end to flowering in that population, which prevented the determination
of a late season estimate (Fig. 1 ) . The sample size per plant (10 or 15) was
usually sufficient to ascertain the maternal genotype (Brown, Matheson &
Eldridge, 1975). I n ambiguous cases a further sample of progeny was tested.
Data for the estimation of outcrossing rates were obtained by starch-gel
electrophoresis, of enzyme extracts of germinating seeds. Extracts prepared by
grinding germinating seeds in a few drops of buffer solution (0.05 M phosphate,
pH 7.0, containing 1.5 mg ml-’ dithiothreitol) were subjected to 5 h
electrophoresis, in a buffer system consisting of 0.4 M sodium citrate pH 8.0
(electrode buffer) and 5.0 mM histidine p H 8.0 (gel buffer). Following
electrophoresis, gels were cut horizontally into three slices and the anodal
portion assayed for the following enzymes: aconitate hydratase (ACO; EC
4.2.1.3), glucophosphate isomerase (PGI; EC 5.3.1.9), phosphoglucomutase
(PGM; EC 2.7.5.1) and shikimate dehydrogenase (SDH; EC 1.1.1.25). Enzyme
assays were those cited in Collins, Rossiter, Haynes, Brown & Marshall (1984).
TABLE
1. The number of individuals studied and their reproductive status in the three populations
of Echium planlagineum
Population
1. Black Mountain Peninsula, Canberra
Early Cohort
Late Cohort
2. Mount Mugga, North, Canbrrra
3. Mount Mugga, South, Canberra
No. of plants
Average no. of flowrrs/
tagged
cyme at tagging
24
13
20
19
0.7
0.8
1.7
3.9
Zero time
11 October
30 October
21 October
28 October
1985
1985
1985
1985
84
J. J. BUKDON E T AI.
Allele and loci designations were the same as used previously (Brown & Burdon,
1983).
Estimates of outcrossing rates were made using the procedure described by
Brown el al. (1975) for single locus data. Genotypic classifications were
converted to a diallelic basis by combining all but the commonest allele into a
single class. Outcrossing estimates were also made on a multilocus basis using
the procedure of Green, Brown & Oram (1980). The model assumes that
mating results either from self-fertilization with fixed probability (1 - t ) , or from
outcrossing ( 1 ) defined as fertilization with a pollen grain chosen at random
from the population (Brown et al., 1985). T o avoid the bias and complexities
arising from related sources of outcrossing pollen within a progeny array
(Schoen & Clegg, 1986), only one seed per fruit was tested and sufficient fruit
per maternal parent were sampled to determine each maternal genotype.
Controlled pollination
Because the estimates of the outcrossing rate were high, the relative success of
self- and outcrossing pollination events in producing seed was determined
through controlled, glasshouse, pollination of five pairs of E. plantagineum plants.
Individual plants were combined in pairs in such a way as to ensure that the
paternity of all offspring could be determined by starch-gel electrophoresis
unambiguously. O n each plant, groups of three consecutive flowers were,
respectively, selfed, both selfed and outcrossed, and outcrossed only. This
procedure was repeated four times although the order of crossing varied
between crossing cycles. Self and outcrossing of the same flower was achieved by
touching dehiscent anthers from each member of the pair of plants to one of the
two arms of the bifid receptive stigma. The paternity of any resultant seed was
determined by electrophoretic analysis of variation at the Pgil locus (Burdon &
Brown, 1986).
RESULTS
Reproductive perfrmance
All three populations of E. plantagineum showed a generally similar pattern of
flowering behaviour (Fig. 1). In the most intensively studied population (no. l ) ,
the proportion of plants flowering rose from 5% at the initial census to 100(yo
over a 30-day period. All three populations were in full flower by 12 November
1985. Simultaneously the total number of open flowers per unit area in each
population rose rapidly. However, in all populations this reached a peak before
all individuals had begun to flower. Thereafter the number of open flowers
declined (Fig. 1). T h e rate of decline differed between the populations, being
most precipitous in population 3 (situated on shallow soil) and least in
population 2 (situated in a seepage area). A late burst of flowering occurred in
populations 1 and 3 one week after 25 mm of rain.
The overall timing of flower production on tagged individuals in each of the
three populations reflected that of the population as a whole. A sharp peak in
the number of flowers occurring on tagged stems per 5-day period was followed
by a decline broken only by a short late burst. This uneven production of
flowers was reflected in the skewed distribution of the timing of seed production
OUTCKOSSING IN E. PLANTAGINEUM
85
100
50
L
01
3
0
0
-
-
0
+
U
W
c
100 $
c
VI
mz
c
u
+
u
50
f
c
ln
c
U
-
a
‘c
0
3
=
t
%
c
100
L
“1
m
a
!
Population 3
40
50
\
0
,
‘0
n
”
i\.
\
Eorly
Mid
H
F--l
I1
II
11
II
II
I1
I
1
10
20
30
40
50
60
70
II
II
80 90
I1
3
100
-Oc t ----I -N ov --+---Dee-I-Jon-I
D a y s f r o m s t a r t of monitoring
Figure 1. Temporal changes in the reproductive behaviour of three populations of Echzum
plantagzneum 0 , Total number of open flowers per Inz; 0,
percentage of plants that have begun to
flower. Horizontal bars show the timing of the outcross estimates in each population.
(Fig. 2). I n all three populations the majority of seed was produced by flowers
that opened during the first third of the reproductive period (60, 52, 50 and
60% of all seed produced in populations I,, I , , I1 and 111, respectively). This
was due mainly to high early season flower production and much less to
temporal changes in the number of seeds produced per flower (Fig. 3 ) . While
there was a clear decline in the number of seeds per fruit with time in
population 3 and the suggestion of a similar trend in population 1, in population
2 the fecundity of flowers was unrelated to the time of season.
T h e average number of seeds produced per flower varied both between
individuals within populations and between the three populations (Fig. 4 ) .
Although neither of these differences was significant, the average seed
production per flower was 25% greater in population 1 than population 3 (2.05
versus 1.52) while on occasion differences between individuals in the same
population exceeded 100yo.
J. J . BUKDON 87 A L .
86
3001
I
I
I
Population I
YJ
n
W
Population 2
yl
c
0
i
W
n
5
z
,oat
I
Population 3
I
I
i-l
5
10
15
25
35
45
55
65
75
Ti m e of f l o w e r i n g ( d a y s f r o m Il/10/86)
Figure 2 . Temporal changes in seed production in threc populations of Echium planta,@nt~n. ‘lhc
shaded area of‘the graph of population 1 rcpresents seed production by the late flowering cohort.
Outcrossing rates
Table 2 presents the adult major allele frequency and Wright’s fixation index
values (F; calculated on a diallelic basis) for adult plants and seed for each locus
and time period (seed only). A few of the individual locus F values differed
significantly from zero, but they showed no systematic pattern. However, there
was a consistent change in the value of F between that of the adult plants and
those of their offspring. In all but one case adult F values were markedly more
negative (mean F = - 0.22) than those of the seed (mean F = 0.02).
Single locus estimates of the rate of outcrossing occurring in the three
populations of E. plantagineum are given in Table 3. (At sites 2 and 3, the Acol
locus was monomorphic which precluded a n estimate of outcrosring for that
locus.) The standard errors for the single-locus estimates are large, and indicate
that most differences between the estimates are not significant. However the
early estimate in population 1 based on the Pgi locus was significantly less than
t = 1.O. I n population 2, the outcrossing estimates also based on Pgz significantly
increased with time. Thus the only apparent departure from random mating
occurred in the earliest cohort, when a smaller fraction of the population was
flowering.
The multilocus outcrossing estimates (Table 4) suggested a similar pattern in
that only the earliest estimate for population 1 showed statistically significant
inbreeding. The overall estimates for population 2 were consistently higher than
population 1, although not significantly so.
+
OUTCROSSING IN E. P L A N T A G I N E U M
I
I
I
I
20
40
60
87
I
Time (days)
Figure 3. ‘l‘cmporal patterns of flower fecundity in three populations of Echium plantqineum. I n all
cases values were calculated only if more than half of the tagged plants flowered at the given time;
0,
the late cohort of population 1.
Controlled pollination
When pollination of stigmas was effected with only self or non-self pollen, 68
and 83 seeds were produced respectively. Of the 79 seeds produced as a result of
simultaneous self and outcrossing of flowers, 58(y0 were the result of selfing
events and 42% the result of outcrossing events (Table 5). A x 2 analysis found
no significant difference (x‘ = 1.49; 0.25 > P > 0.10) in the success rate of selfor outcross pollination events, or that neither type of pollen had a significantly
greater chance of producing viable seed when applied simultaneously to the
same stigma (2‘ = 1.53; 0.25 > P > 0.10).
DISCUSSION
The reproductive performance of E. plantagineum varied markedly from site to
site and time to time during the course of the flowering season. This temporal
variation in fruiting within a season emphasizes the need to link estimates of
outcrossing of specific cohorts with their contribution to the seed bank. Thus
high selfing rates very early and late in the season may have a trivial effect if
these cohorts account for only a tiny fraction of the total seed output. However,
in the present study, estimates of outcrossing within and among the three
populations of E. plantagineum were surprisingly similar. I n part, this result is due
J. J. BURDON E 7 A L .
88
0
0.99
1-99
2.99
0
0.99
1.99
2.99
Average number s e e d s per f l o w e r
Figure 4. The season-long average number of seeds produced per flower (maximum possible = 4)
on marked plants in three populations of Echiumplantagineum. A & B are the distribution patterns for
the carly and late flowering cohorts, respectively, of population 1; C, population 2; D, population 3.
Arrows refer to the mean number of seeds produced per flower for the population as a whole.
to the large sampling errors of outcrossing estimates in highly outcrossing
populations. The standard errors of Table 4 indicate that differences in
outcrossing rates between estimation times of about 0.2 would have been
detectable in population 1, and of about 0.3 in population 2.
TABLE
2. Adult major allele frequency a n d Wright’s fixation index values (F; calculated on a
diallelic basis) for each locus a n d time period of each a d u l t a n d offspring population of Echium
plantagineum
Population
Locus
____________
I (carly)
Acol
AcoP
Pgil
Sdh
I (late)
Acol
Aco2
Pgil
Sdh
2
3
Offspring (seed) F
Adult major
allele
frequency
Adult
F
0.60
0.60
0.48
0.48
-0.13
-0.31
-0.25
-0.42*
0.65
0.54
0.65
0.58
-0.19
-0.24
-0.19
-0.10
-Early
Mid
Late
f0.08
+0.14
-0.01
fO.01
-0.04
+0.23*
-0.05
0.00
f0.01
+0.01
f0.07
+0.13
+0.19
+0.12
-0.05
f0.05
+0.05
+0.07
f0.08
-0.06
-0.13
-0.30*
c
-~
+0.03
+0.01
+0.05
+0.08
f0.05
+0.10
-0.22
-0.08
-0.04
+0.07
I .oo
t
-0.01
-0.05
t
- 0.02
Pxil
Sdh
0.48
0.55
0.70
fO.10
-0.41*
-0.43*
+0.05
-0.06
+0.06
-0.18
+0.12
0.00
-0.16
-0.02
+0.02
-0.13
f0.02
Aco 1
Aco2
Pgil
Sdh
1 .oo
0.68
0.40
0.60
t
-0.01
t
- 0.02
-0.25
+0.14
t
-0.03
-0.25
f0.07
+0.08
-0.01
Acol
Aco2
-0.05
-0.05
-0.01
fO.O1
-0.03
-0.02
t
t
*Significantly different from zero, P < 0.05.
TI: values could not be calculated as only one (homozygous) genotype was encountered.
:No samples taken.
-0.01
OUTCROSSING IN E . PLAXTAGINEUM
89
TABLE
3. Single locus estimates of outcrossing rates ( t ) in three populations of Echium plantagineum
Population
(total no. of
progeny tested)
Outcrossing estimate ( ~ s . E . )
Locus
1 . (345)
Acol
Aco2
(Early cohort)
Pgil
Early?
Sdh
I . (180)
Acol
Aco2
(Late cohort)
Pgil
2. (271)
Aco2
Sdh
Pgil
Sdh
3. (185)
ACO2
Pgil
Sdh
Mid
c
Late
0.88
0.92
0.67
0.87
(0.13)
(0.16)
(0.14)*
(0.17)
0.78
1.18
0.98
0.74
(0.13)
(0.16)
(0.15)
(0.16)
0.82
1.09
0.80
1.27
(0.15)
(0.16)
(0.15)
(0.18)
0.82
1.06
0.82
0.95
(0.08)
(0.09)
(0.09)
(0.10)
1.05
1.01
1.11
0.95
(0.20)
(0.20)
(0.21)
(0.24)
0.92
0.68
0.85
1.02
(0.21)
(0.19)
(0.19)
(0.24)
1.28
0.62
1.32
1.04
(0.21)
(0.21)
(0.19)
(0.26)
1.08
0.81
1.00
1.00
(0.12)
(0.12)
(0.12)
(0.14)
1.03 (0.13)
0.67 (0.18)
0.98 (0.22)
1.00 (0.13)
0.80 (0.27)
0.90 (0.20)
1.26 (0.16j
1.40 (0.22)
0.95 (0.30)
1.07 (0.08)
0.97 (0.12)
0.95 (0.13)
0.94 (0.17)
1.01 (0.15)
1.44 (0.17)
0.69 (0.19)
1.22 (0.12)
0.97 (0.07)
t
0.84 (0.13)
1.11 (0.10)
1.05 (0.12)
t
t
*Significantly different from t = 1.0, P < 0.05.
?For interrelationship of the timing of outcrossing estimates within and between the different populations
scc Fig. 1.
$No late estimate was possible in population 3 due to a n abrupt end to the srason.
The mean values of t (both single- and multilocus estimates) differed only
marginally among the three populations despite marked differences in a number
of factors that were expected to affect the pattern of pollinator behaviour. T h e
number of open flowerslunit area although similar in all populations early in the
season was noticeably higher in population 2 during the middle of the
reproductive season. Population 1 was isolated (1 km from the nearest stand)
while populations 2 and 3 were both part of a patchy distribution of
E. plantagineum that spread over many hectares.
Similarly, within individual populations, factors that might have been
expected to affect outcrossing had no major effect. Such factors included,
differences in the timing of outcrossing estimates (early, mid, late season) with
its associated variation in the number of open flowers per unit area and likely
variations in pollinator activity or the division of the host population into two
TABLE
4. Multilocus cstimates of outcrossing rates (tm) in three populations of Echium plantagineum
Multilocus outcrossing estimate ( + s . E . )
Population
1. Early cohort
Late cohort
2.
3.
Mid
Late
CS
0.85 (0.07)*
0.98 (0.12)
0.87 (0.07)
0.81 (0.10)
0.90 (0.08)
0.90 (0.13)
0.87 (0.04)
0.88 (0.07)
0.95 (0.11)
0.88 (0.09)
0.97 (0.10)
1.15 (0.12)
1.05 (0.12)
0.97 (0.06)
0.97 (0.07)
Early
t
*Significantly different from t = 1 .O, P < 0.05.
?For interr~lationshipof thc timing of outcrossing estimates within and between the different populations
see Fig. 1.
$No late estimate was possible in population 3 due to a n abrupt end to the season.
§Means arc calrulated on the whole data set/population.
J. J. BURDON 87 A L .
90
'I'ABLE
5. 'I'hc succcss of sclf- and outcross pollination events in producing sccd in a series of
controlled, glasshousc-based, pollinations of Echium llantagineum plants. Parentage of individual
sccds from flowcis dustcd with both self and non-self pollen was determined by clcctrophoretic
analysis (see text)
Pollination treatment
Plant
110.
*A1
A3
B2
R4
c5
c7
D6
D10
E9
El2
'Iota1 no of seed
of total possible
Selfed
Outcrossed
Selfed/outcrossed
7
1
10
0
20
2
8
5
0
1l / l
5/4
112
15
2
3
68
14.2
1
7
13
7
11
6
19
14
83
17.3
1 /o
7/9
3/ I
4/3
3/3
7/8
4/2
46/33
9.6j6.9
*Plants with sanic prefix Icttrr werc used in intercrossing.
different cohorts (population 1 ) . How much the protandrous nature of flower
development in E. plantagirmm promotes outcrossing is unclear. T h e results from
the controlled pollinations suggested that the high level of outcrossing was due
to a very high frequency of inter-plant movements by pollinating insects, rather
than due to any competitive inferiority of self pollen or any process of
differential embryo abortion. The latter processes are commonly found in
pollination experiments with mixtures of pollen (e.g. Glover & Barrett, 1986;
Marshall & Ellstrand, 1986; Griffin, Moraii & Fripp, 1987).
This result contrasts with that obtained by Stephenson (1982) in a study of
the self-incompatible tree Catalpa rpeciosa. In that species selfed, outcrossed and
non-pollinated flowers could be visually distinguished and Stephenson found a
marked increase in the frequency of outcrossing (as opposed to unsuccessful
selfing events) as the season progressed.
The results presented here for E. plantagineum differ from other insectpollinated species in which outcrossing estimates have been shown to vary both
spatially (e.g. Lupinus nanus, Harding el al., 1974; Giliu achilleijolia, Schoen,
1982), temporally ( c g . Eucalyptus delegatensis, Moran & Brown, 1980) and even
in relation to the distribution of individual plants within a population (Ipomoea
purpurea, Ennos & Clegg, 1982). Yet substantial departures from complete
outcrossing can sometimes occur in E. planlagineum. A related study in 1984
obtained an estimate of outcrossing of t = 0.73+0.04 in a single population
(Burdon & Brown, 1986).
In the earlier study, single- and multilocus outcrossing estimates were made in
a very low density stand of E. plantagineum ( < < l0jm') in which most
individual plants were quite separate from their nearest neighbours. Such plants
often have many flowering stems and each presents several open flowers
simultaneously to visiting insects. I n such circumstances a marked level of
geitonogamous selfing could occur and depress the estimates of outcrossing.
OUTCROSSING IN E P L A N T A G I N E Z J M
91
In contrast, the high outcrossing estimates reported here may be reasonably
attributed to the high density of E. plantagineum plants in the three populations
(233, 167 and 465 plants r n p 2 in populations 1, 2 and 3, respectively).
Certainly, most of the tagged individuals in populations 1 and 2 possessed more
than one flowering cyme (mean values of 13.2, 9.2 and 9.0 in populations I,, 1
and 2, respectively) and carried sufficient open flowers, at least during the peak
flowering period (mean values of 9.1, 5.3 and 5.8 in populations I,, I , and 2,
respectively ( 14/11/85)), to make geitonogamous selfing possible. I n population
3 both these values were much lower (4.3 and 2.2 respectively). However, the
relatively high density of plants and open flowers in all three populations
ensured that all the flowering cymes of any one individual plant were
intermingled with many flowers from entirely separate plants. It is postulated
that as a result of this structure the chances of pollinators moving from flower to
flower on the one plant were low and outcrossing rates are likely to be maximal.
This argument apparently runs counter to that developed by Levin & Kerster
( 1969), who argued that the restricted seed movement and density-dependent
pollinator behaviour that is typical of many plant populations should result in a
higher frequency of mating between related plants in high density stands than in
lower density ones. This in turn should be reflected in reduced rates of apparent
outcrossing at high plant densities. Indeed, in a study of outcrossing in a
number of populations of Helianthus annuus, Ellstrand, Torres & Levin (1978)
obtained data that supported this view.
Several features of weedy populations of E. plantagineum militate against such
inverse relationships between outcrossing rate and plant density. First, the
extremely high level of genetic variability in these populations ensures that a
high level of all outcross events are observable. Second, E. plantagineum, unlike
H. annuus, is a self-compatible species, and at low densities individual plants
typically carry a relatively large number of open flowers at any given time. For
insect pollinators visiting a flower on such a plant, the next nearest open flower
will almost certainly be another flower on a different cyme of the same plant.
Thus sparse populations of E. plantagineum and even the earliest cohorts of dense
populations, can show significant inbreeding.
ACKNOWLEDGEMEN’IS
The authors wish to thank M r S. Speer and Mrs E. Gregory for their
assistance in the collection of data.
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