Developmental stability is not disrupted by
extensive hybridization and introgression among
populations of the marine bivalve molluscs
Mytilus edulis (L.) and M . galloprovincialis
(Lmk.)from south-west England
J. P. A. GARDNER
Ocean Sciences Centre, Murine Sciences Research Labomtoy, izleinoriul Uniziersib of
.Newfoundlund, St. John’s, .New$kdlund, A1 C‘ 5S7, Cunudu. *
Keceii,ed 22 December 199.3’: acceptrdfor publicaiiorl 29 dfurch I991
‘Ihe eft’ects of hybridization and introgression were assessed among two naturally h y b r i d i h g
bivalve molluscs (the mussels hfvtilus edirlis and itl. gal1oproUinciali.r) from western Europe to estimate
how disruptivr these processes are to developmental stability (measured in terms of morphological
variability). Ten shell traits were measured for 392 mussels from four allopatric populations (two
each of Af. e d r h and Af. galloprooincialis) and two hybrid populations. An index of variability
(defined as Z, = Iyl-y,( where 7) is the population mran of the length-standardized trait, and y,
is the individual’s length-standardized trait value) was constructrd for each trait. and for the sum
of the traits. The hybrid populations did not exhibit greater mean variability than the allopatric
populations for any of the indices. Upon pooling, the hybrid populations had significantly lower
populations in
variability than the pooled ‘if. edulzs populations and the pooled ‘21. gullopr~~z~incialis
two analyses, and had similar means in the remaining ninr analyses. Where significant differences
existed, the pooled XI. gulloprouineialis had lower levels of mean trait variability than the pooled
,\f. edulb. Among the two hybrid populations, the putative F1 hybrids and backcross individuals
exhibited means of trait variability which were similar to those of the parental types. Thus,
extensive hybridization and introgression do not adversely affect developmental stability among
these musscl populations. There was a strong significant correlation between the ranking of indices
(based on the amount of variability) across all six populations, indicating that a large genetic
component determines the measured morphological variability. It is concluded that the genes or
gene complexes which control morphological development in Af. edu1i.i and Af. galloprouirzcialiJ arc’
very similar, probiding further evidence of the close evolutionary relatedness of these mussel t a u .
ADDITIONAL KEY WORDS:-Bivalvia
developmental instability.
Mollusca
marinr mussels
-
hybrid zone
-
CONTENTS
.
,
,
.
.
,
,
.
Introduction
hlatrrial and methods ,
.
,
,
.
.
Populations sampled
.
,
,
,
.
Electrophoresis. ,
.
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,
. ,
Mcasurenient of developmental instability
Statistical analyses ,
. . . . .
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76
*Present address: Island Bay Marine Laboratory and School of Biological Sciences, Victoria University of
\Vellington, P.O. Box 600, Wellington, New Zealand.
0024 4066/9Y00107 1 + 1 G $08.00/0
71
0 1995 T h r Linnean Society of London
72
J. P. A. GARDNER
Results.
. . . . . . . . . . . . .
Comparison of individual populations . . . .
Comparison of pooled populations . . . . .
Intra-hybrid population comparisons.
. . . .
Ranking of traits for developmental instability.
. .
Discussion . . . . . . . . . . . . .
Developmental instability: hybridization and introgression
Developmental stability as a fitness component . .
. . .
Genetic basis of developmental instability.
Acknowledgements.
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. . . . . . . . . . .
References.
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77
77
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84
84
84
INTRODUCTION
One of the most immediately noticeable aspects of hybridization (defined as
the interbreeding of races, sub-, semi- or full species which are characterized
by differences in allele frequencies) is that hybrids are usually, and on average,
morphologically intermediate between the parental taxa (e.g. Skibinski, 1983;
Wolf & Mort, 1986; Campton, 1987; Lamb & Avise, 1987). From theoretical
considerations, individuals of mixed ancestry are expected to exhibit greater
variability ( = developmental instability) than pure individuals (Lerner, 1954)
which results from the break-up by hybridization and recombination of the
parental co-adapted gene complexes (Falconer, 1989). The observed levels of
morphological variability are considered to be directly proportional to the
amount of genetic differentiation between the two hybridizing taxa. Thus,
hybrids derived from closely related taxa are expected to display lower levels
of developmental instability than those derived from distantly related taxa.
In the present study, levels of morphological variability for ten shell characters
were compared among four allopatric and two hybrid populations of marine
mussels, Mytilus edulis (the blue or common mussel) and M. galloprovincialis (the
Mediterranean mussel). The aim was to determine if extensive hybridization
and introgression result in greater developmental instability, expressed in terms
of increased morphological variability. Examination of genetically determined
developmental instability within and between hybridizing taxa, in conjunction
with a knowledge about the origin and history of the hybrid zone, permits an
assessment of the evolutionary relatedness of the taxa, since increased
developmental instability is expected to be associated with increased genetic
divergence. Furthermore, because developmental instability is a component of
individual fitness, estimates of relative developmental instability for mussels from
the hybrid zone in south-west England provide information about fitness
differences between the two taxa, their hybrids and individuals of mixed ancestry
which effect the structure and maintenance of the zone.
The systematic status of the two mussel types is unclear. Nei’s (1972) genetic
values calculated over 16 allozyme loci for allopatric
identity (I) and distance (0)
populations are I = 0.85 and D = 0.172 (Skibinski et al., 1980). Such values are
normally considered to be within the range of subspecies for molluscs (Hoagland,
1984). No single character of any type is diagnostic (defined as showing >99%
dissimilarity between taxa) for differences between the two mussel types, or a
third type called M. trossulus. All estimates of genetic similarity (allozymes and
mitochondria1 DNA (mtDNA) restriction fragment length polymorphisms:
reviewed by Gardner, 1992; Gosling, 1992; Seed, 1992) and mtDNA sequencing
(Geller et al., 1993) indicate that M. edulis and M. galloprovincialis are very closely
DEVELOPMENTAL STABILITY IN HYBRIDIZING MARINE MUSSELS (MUILLIS)
73
related. In the present paper, binomial nomenclature is used, but without
implying specific status.
Regardless of their systematic status, M. edulis and M. gallopovincialis exhibit
a zone of overlap in western Europe where they hybridize extensively (Seed,
1971, 1972, 1974; Gosling & Wilkins, 1981; Skibinski, 1983; Skibinski et al.,
1983; Gardner & Skibinski, 1988; Coustau et al., 1991a). At some sites within
the hybrid zone, the percentage of backcross individuals is high. Based upon
dilocus genotype variation it is estimated to be 29% and 46% at Croyde and
Whitsand (both sites in S.W. England), respectively, and upon including putative
F1 hybrids, percentages rise to 37% and 57% respectively (Gardner et al.,
1993). When electrophoretic variation at five polymorphic allozyme loci is
considered, 86% and 92% of mussels at Croyde and Whitsand respectively are
individuals of mixed ancestry, that is, are not ‘pure’ types (Gardner, unpublished
data). Thus, the two taxonspecific genomes which occur in allopatry are
considered to have been well mixed by hybridization and introgression over a
period of many generations at these sites in south-west England.
I tested the null hypothesis that hybridization and introgression between M.
edulis and M. galloprovincialis have no effect upon levels of morphological
variability. Variability in ten shell traits was compared between individuals of
four allopatric and two hybrid populations. Within each of two hybrid
populations morphological variability was compared between five groups of
electrophoretically distinct mussels: the most M, edulis-like, M. edulis backcrosses,
putative F1 hybrids, the most M. galloprovincialis-like, and M. galloprovincialis
backcrosses. The results indicate that the limited genetic differences which exist
between the two taxa apparently do not include the genes or gene complexes
which control developmental stability, since large scale genetic differences at
these genes are expected to result in increased developmental instability (i.e.
greater morphological variability among hybrids and backcrosses) which was
not observed.
MATERIAL AND METHODS
Populations sampled
Mussels were collected in large natural clumps from four allopatric and two
hybrid populations (Fig. 1). A total of 392 individuals was analysed (see Table
1 for sample sizes). Caswell (south Wales) and Lowestoft (east England) support
pure M. edulis populations, while Bude (south-west England) and Grau d’Age
(French Mediterranean) support pure M. galloprovincialis populations. Individuals
from these populations were examined for mantle edge colouration which is
partially diagnostic for the two mussel types (Seed, 1971; Skibinski, 1983) and
were found to be consistent with the above interpretation.
Electrophoresis
Mussels from hybrid populations (Croyde and Whitsand, south-west England)
were subject to horizontal starch gel electrophoresis and assayed for two partially
diagnostic allozyme loci, esterase-D (Est-D) and octopine dehydrogenase (Odh),
which have genetic identity values (Nei, 1972) between the two forms of 0.039
J. P. A. GARDNER
NORTHSEA
%itsay
ATLANTIC
OCEAN
MEDITERRANEAN
SEA
Figure 1, Location of six collection sites. Allopatric A l . edulis populations: Caswell (south Wales)
and Lowestoft, (east England). Allopatric Af. go0Nuprmirzciulis populations: Bude (south-west England)
and Grau d’Age (French Mrditcrranean). Hybrid populations: Croyde and Whitsand (both southwest Erigland).
and 0.232, respectively (Gardner & Skibinski, 1988). A synthetic allele system
(Skibinski, 1983) was employed, in which alleles at highest frequency in allopatric
M. edulis populations are classified as compound E alleles, and those at highest
frequency in allopatric Ad. galloprovincialis populations are classified as compound
G alleles. There are therefore nine dilocus compound genotypes, ranging from
E / E E / E (the most M , edulis-like), via E/G EIG (putative F1 hybrids), to GIG
GIG (the most M. galloprovincialis-like). The EIE E/G and EIG EIE compound
genotypes are regarded as hf. edulis backcrosses, and the GIG EIG and EIG
GIG genotypes as hf. galloprovincialis backcrosses. Approximately 30 mussels of
each of these five compound genotypes were examined per hybrid population.
Measurement
of developmental instabilig
The left valve of each mussel was measured for shell length and ten
morphometric traits (Fig. 2) which are routinely used in studies of morphometric
variation between the taxa (e.g. Hepper, 1957; Lewis & Powell, 1961; Lewis
DEVELOPMENI'AI, STABKITY IN HYBRIDIZING h1ARINE hlUSSEI,S [Af?TILG14)
7.5
TABLE
1. Simple statistics for the allopatric and sympatric mussel populations
Index
Caswell
~
71
Shell Length (rnm)
5 SD
XLPK
5 SD
XWPR
5 SD
XPAD
- SD
XPADV
5 SD
X\VID
SD
+
XH'I
SD
W
l
5 SD
SHP
5 SD
XLAR
5 SD
?(\TAR
+ SD
XIOTAL
5 SD
Imwestofi
Bude
Grau #age
Croyde
~~
25
27.1
9.4
1.658
~
21
3+.5
20
27.7
8.8
2.516
1.890
4.5
0.603
0.329
2.775
2.629
0.894
0.640
1.333
1.285
1.669
2.057
1.936
1.201
1.397
1.106
1.010
0.98+
2.8$2
2.02 1
1.239
0.930
3.055
1.692
2.365
1.782
1.115
1.415
0.891
2.100
1.392
3.930
2.8+5
I ,032
0.534
1.929
1 .+++
1.528
1.155
3.985
2.1 16
29
33.8
-1.5
1.7.59
1.1+7
0.68.5
0.636
1.457
1.085
1.393
0.915
0.8++
0.767
1.387
113
35.3
5.7
1.962
1.353
1.168
2.001
1.556
1.820
1.4-16
4.085
2.586
1.768
1.128
1.628
1.236
+.+69
2.775
+.I18
4.026
2.437
1.903
3.340
2.400
1.272
1.603
1.270
1.275
21.74.5
5.216
0.832
0.702
14.572
+.I61
0.98 I
0.749
20.112
1.228
1.027
21.27-1
6.50+
100. n
=
2.022
1.614
1.981
1.612
X
35.+
8.9
0.786
0.63 I
1.281
0.963
2.257
1.673
1.+43
0.790
All index values of trait variability are
~~~
151
0.77-1
0.655
1.181
+.+4
I
~~~~~~
1.6-1.1
2.948
2.32+
2.268
1.775
3.599
2.63-1
Whitsand
20.892
6.232
1.970
1.508
1.522
l.l8+
3.32-1.
2.8 I9
4.378
3.95 I
1.818
1.379
3.160
2.827
1.167
0.942
21.729
7.295
number of musscls: SD = standard deviation
& Seed, 1969; Seed, 1971, 1972, 1974; Verduin, 1979; Skibinski, 1983;
Beaumont et al., 1989; McDonald et al., 1991). Characters were measured using
electronic digital calipers accurate to 0.01 mm or by using a stereomicroscope
fitted with a calibrated eyepiece. Each of the ten traits is partially diagnostic
for differences between the two taxa on a worldwide scale (McDonald et al.,
1991) which indicates a pronounced genetic component to such variation. Thus,
trait variation is considered to be largely independent of (a) individual effects
such as age and growth rate, (b) population effects such as crowding and the
mixing of size or age classes, and (c) environmental effects such as wave
exposure and salinity variation. Because of this, these shell characters are good
markers to use to investigate the genetic basis of developmental instability. In
the present study I investigated the variability of the trait means (not the means
themselves) in an analysis which is loosely analagous to testing for homogeneity
of variances, because this provides information about how much (if at all)
hybridization and introgression affect developmental stability. To ensure trait
uniformity between populations (despite the caveats listed above) each trait was
standardized by dividing by shell length. Based upon a statistic proposed by
Levene (1960), an index of developmental instability for each trait was defined
as Zi = lyi--p,1, where pi is the population mean for a given length-standardized
trait, and yj is the individual's length-standardized measure for that trait. Thus,
76
J. P. A. GARDNER
9
Figure 2. Shell traits that were measured: (1) AAM = length of anterior adductor muscle scar: (2)
H P = length of hinge plate: (3) LAR = length of anterior retractor muscle scar: (4) WAR = width
of anterior adductor muscle scar: (5) WPR = width of posterior retractor muscle scar: (6)
LPR = posterior retractor muscle scar length: (7) PAD = length of posterior adductor muscle scar:
(8) PADV = distance between ventral edge of posterior adductor muscle scar and ventral shell
margin: (9) H T = shell height: (10) WID = shell width. (Modified from McDonald el al., 1991)
the index measures the absolute variability of each individual about the
population mean. This is a robust and easily applied test, and may be
considered to be the statistic of choice (Palmer & Strobeck, 1992; Yezerinac et
al., 1992).
Statistical anaQses
Trait-specific indices were compared among the four allopatric and two
hybrid populations, and among the five compound genotypes at each of the
two hybrid populations. Comparison was carried out using the SAS statistical
package (SAS Institute, 1987) by non-parametric univariate ANOVA (e.g.
Conover & Iman, 1981) because the data were non-normally distributed even
after transformation. The Ryan-Einot-Gabriel-Welsch ( R E G W Q a posteriori
multiple range test was employed to test for the location of differences between
indices when the ANOVA indicated that significant differences existed. The
R E G W Q test (also called Ryan’s Q test) is the most powerful multiple range
test to control the experimentwise type I error rate (Day & Quinn, 1989). For
all ANOVAs, the table-wide sequential Bonferroni procedure was employed to
correct for multiple testing (Rice, 1989). The names given to the traits are
those used by McDonald et al. (1991): the prefix X has been added to each
to indicate where the measurement of variability (the index), rather than the
trait itself, is referred to. An eleventh index of variability, a multivariate statistic
called XTOTAL (the sum of the ten measures of variability) was also calculated,
and tested as described above.
DEVELOPMENTAL STABILITY IN HYBRIDIZING MARINE MUSSELS (izIY77LUs)
77
RESULI’S
Comparison
of individual populations
Simple statistics for shell length and the 11 indices of variability are presented
in Table 1. After table-wide sequential Bonferroni adjustment (Rice, 1989) five
significant results were observed for the ANOVAs comparing index means for
the six populations (Table 2). In all five cases the hybrid populations had
similar means of Variability to the allopatric populations. Among the significant
results, the only constant placement was that of the Grau d’Age population
with the lowest level of mean variability: excluding XTOTAL this was nonsignificantly different from at least one other population on each occasion.
There was no trend in the placement of populations, the sequence changing
with each index.
Comparison of pooled populations
Upon pooling, Caswell and Lowestoft formed a single M. edulis population,
Bude and Grau d’Age a single M. galloprovincialis population, and Croyde and
Whitsand a single hybrid population. ANOVA of index means revealed six
significant between-subject effects among these three populations (Table 3). In
not one of these analyses did the hybrid population exhibit significantly greater
mean trait variability than both pure populations, but in two of the analyses
exhibited significantly lower index means than both pure populations. Where
significant differences existed between the M. edulis and M. galloprovincialis
populations, in all cases the M. galloprovincialis population had lower mean values
of trait variability than the M. edulis population.
Intra-hybrid population comparisons
ANOVA of index means was carried out for the five compound genotypes
from the hybrid Croyde population (Table 4). For all three significant results
neither the putative F1 hybrids nor the two backcross compound genotypes
exhibited significantly greater means than the parental compound genotypes.
For all three analyses the most M. galloprovincialis-like mussels and the M.
galloprovincialis backcross individuals had higher levels of mean variability than
the most M. edulis-like and M. edulis backcross mussels. For the analysis at
Whitsand (Table 5), only one of the eleven ANOVAs was significant. For X H T
(variability of shell height) the putative F 1 hybrids exhibited mean variability
between the other four compound genotypes. The most M. galloprovincialis-like
mussels and M. galloprovincialis backcross individuals had higher levels of mean
variability than the most M. edulis-like mussels and the M. edulis backcross
individuals.
Ranking of traits f o r developmental instabilig
Means of the variability of each of the ten traits have been ranked from
highest to lowest for each population. The non-parametric Kendall rank
correlation test was performed to estimate how consistent the rankings of the
traits were among populations. In all cases, trait rankings were highly correlated:
J. P. A. GARDNER
78
T A ~ I4.
I ANOVA and Ryan-Einot-Gabricl-Welsch test results of the indices of trait Variability
for the six mussel populations
Index
Population
Twe
~
X1,PR
XWPR
MG
ME
H
H
MG
RIE
MG
hlE
XPAD
XPADV
X\VID
XHT
H
H
RIE
MG
MG
MG
R.1 E
H
H
ME
ME
H
H
MC
MG
ME
H
H
ME
MG
ME
MG
MG
H
ME
H
ME
MC
Mean r d i
I1
229.2
223.5
196.7
191.3
189.8
180.7
257.9
2 17.6
196.8
192.7
177.2
170.8
244.4
208.9
194.2
193.8
192.1
187.6
208.8
205.1
201.8
174.2
162.4
158.8
219.4
199.1
198.0
175.2
143.3
127.4
255.5
207.6
198.4
197.6
163.4
119.6
20
‘4
151
~~~~~~~~
*
Budr
Idowestoft
Whitsand
Croydr
Grau d’Age
Caswell
Bude
Lowestoft
iYhitsand
Croyde
Caswell
Grau d’Age
Budr
Grau d’Age
Lowestott
Whitsand
Croydc
Caswrll
Caswell
Croyde
\Vhitsand
Bude
Grau d’Age
Lowestoft
Croyde
Whitsand
Caswell
Bude
Lowrstoft
Grau d’Age
Bude
\Vhitsand
Lowcstoft
Croyde
Caswell
GIXI d’Age
0.5650NS
143
29
25
20
24
151
1-13
25
29
20
29
24
151
143
25
25
143
151
20
29
24
I43
151
25
20
24
29
20
151
24
I43
25
29
0.1052NS
0.4970NS
0.1856NS
0.0003*
A
AB
AB
AB
0.0004*
B
B
A
AB
AB
AB
BC
C
for the pooled M. edulis versus pooled M. galloprovincialis populations, tau = 0.867,
P = 0.0005; for Croyde versus Whitsand, tau = 0.822, P = 0.0009; for the four
allopatric versus two hybrid populations, tau = 0.768, P = 0.002 (corrected for
ties).
DISCUSS10N
Developmental instobilip hvbridization and introgression
The results presented in this paper clearly indicate that there is no reason
to reject the null hypothesis, i.e. there is no evidence of increased developmental
instability (elevated levels of morphological variability) associated with extensive
hybridization and introgression, either among four allopatric and two hybrid
mussel populations from western Europe, or within either of the hybrid
populations from south-west England.
When indices of trait variability for the six populations are analysed, there
DEVELOPhlENTAL STABIIJTY IN HYBRIDIZING hfARINE MUSSKIS (AflTILCK)
‘ ~ A I H k 2.
Index
W h l
NHP
XIAR
XWAR
XTOTAL
Populatiou
Type
~~~~~~
~
H
H
ME
ME
MG
MG
ME
ME
MG
H
H
MG
ME
ME
H
hlG
H
MG
H
hlG
hlE
ME
H
MG
ME
H
hlG
H
ME
MG
~~~~
~~~~
Croyde
\Yhitsand
Caswell
Lowestoft
Bude
Grau $Age
Lowestoft
Caswell
Grau d’Age
\$’hitsand
Croyde
Bude
Lowestoft
Caswcll
Croyde
Bude
LVhitsand
Grau d’Agc
\Yhitsand
Bude
Lowestoft
Caswell
Croyde
Grau d’Age
Lowestolt
Whitsand
Bude
Croyde
Caswell
Grau d’Age
c’ontznued
Mean rank
~
~~~~
79
~~
21 1.0
210.0
200.2
174.4
130.1
115.5
233.5
222.4
203.7
198.6
184.5
179.5
248.0
210.2
204. 1
199.9
184.3
166.3
231.8
200.0
191.8
174.8
172.8
150.0
2245
21 1.0
205.2
198.9
190.8
84.7
ANOVA P
N
~~~~~~
I13
151
25
24
20
29
24
25
29
151
143
20
24
25
143
20
151
29
151
20
24
25
143
29
24
0.000 I *
20
25
29
~
A
A
AB
ABC
BC
c
0.2987NS
0.0801NS
A
0.0001*
151
143
REGLYQ
~~~~~~~~~~~
0.0001*
AB
AB
AB
AB
B
A
A
A
A
A
B
ME = 121. e d u b population: MG = W guNupruz:incin/L population: H = hybrid population: .h’= number of
individuals: ANOVA P = probability value of analysis: NS = non-significant result aft<-r tablewide sequential
Bonferroni procedure: * = significant result after tablewide Bonferroni procedure: REG\YQ = Ryan Einot
Gabriel \Vclsch multiplr rangr Q test result (means with thr same letter arr not significantly ditfrrent).
are no consistent patterns which emerge: that is, the level of morphological
variability observed in each population for each trait is independent of population
type (taxon/taxa present). When the populations are pooled according to mussel
type to give one M. edulis, one icI. gallojwouincialis and one hybrid population,
extent of trait variability is no longer independent of population type. However,
in none of the analyses does the hybrid population have significantly higher
mean values of trait variability, but in two of the analyses (XPAD and XLAR)
the hybrid population has significantly lower mean variability than both ‘pure’
populations. These findings are supported by analysis of compound genotype
trait variability among the hybrid populations. The putative F 1 hybrids and
the backcrosses have mean index values which are similar to those of the
parental compound genotypes, suggesting that there has been minimal divergence
between regulatory genes of the parental types which are responsible for the
successful coordination of development. A number of possibilities exist which
can explain these findings. First, the A@tilus zone of sympatry is thought to
date from the Pleistocene (Barsotti & Meluzzi, 1968; Gardner, 1994), which
means that it is probably old enough for the establishment of secondary
genomic coadaptation. Second, the considerable genetic similarity that is known
J. P. A. GARDNER
80
TABLE
3. ANOVA and Ryan-Einot-Gabriel-Welsch test results of the indices of trait variability
for the pooled M. edulis (Caswell & Lowestoft), pooled M. gulloprovincialir (Bude & Grau d'Age),
and pooled hybrid (Croyde & Whitsand) populations
Population
Index
~
YLPR
XWPR
XPAD
XPADV
XWID
XHI'
XMM
XHP
XIhR
YWAR
XTOTAL
~
Mean rank
n
231.7
200.8
189.9
263.7
198.8
184.9
24 I .4
230.0
183.4
203.0
195.3
158.8
213.7
203.4
137.9
197.0
195.8
194.0
210.6
185.4
123.1
221.2
193.1
192.2
242.0
224.7
184.2
205.9
170.3
166.5
238.8
195.5
160.1
49
49
294
49
49
294
49
49
294
2 94
49
49
49
294
49
294
49
49
294
49
49
49
294
49
49
49
294
294
49
49
49
294
49
-~
~~
ME
MG
H
ME
MG
H
ME
hlG
H
H
ME
MG
ME
H
MG
H
MG
ME
H
ME
MG
ME
H
MG
MG
ME
H
H
ME
MG
ME
H
MG
0.0548NS
0.0001*
0.0003*
0.0403NS
0.0004*
A
A
B
0.9837NS
0.0001*
A
A
B
0.2633NS
0.0007*
A
A
B
0.0173NS
A
0.0024*
B
B
ME = pooled M edulis populations: MG = pooled Af. gulloprouznciulis populations: H = pooled hybrid populations:
n = number of individuals: ANOVA P = probability value of analysis: NS = non-si~gnificant result after
tablewide sequential Ronferroni procedure: * = significant result after tablewide sequential Bonferroni procedure:
REGWQ = Ryan-Einot-Gabriel-Welsch multiple range Q test result (means with the same lrtter are not
significantly different)
to exist between M. edulis and M. galloprovincialis (reviewed by Gardner, 1992;
Gosling, 1992; Seed, 1992) permits the formation of hybrid and backcross
individuals without increasing developmental instability. Of the two alternatives,
the second is the most plausible because of the probable nature of origin of
M. galloprovincialis which mitigates against the first alternative (Gardner, 1994).
In western Europe, M. galloprovincialis is thought to have arisen in the
Mediterranean from common stock with M. edulis during the Pleistocene (Barsotti
& Meluzzi, 1968). During this time the Mediterranean Sea and North Atlantic
Ocean were only completely isolated for short periods, if they were isolated at
all (Vermeij, 1978; Bertolani-Marchetti, 1985; Sara, 1985). The opportunity for
genetic divergence between the two partially isolated mussel stocks was therefore
limited. Subsequently, the many sea level oscillations that occurred during
periods of glaciation (PCrks, 1967; Vermeij, 1978) allowed genetic mixing
DEVELOPMENTAL STABILITY IN HYBRIDIZING MARINE MUSSELS ( A l l TILL'S)
81
TABLE
4. ANOVA and Ryan-Einot-Gabriel-Welsch test results of the indices of trait variability
for thc five compound genotypes of the hybrid Croyde population
Index
Compound
genotype
Mean
rank
n
ANOVA P
~~~~~~
XI,PR
EjE back
1: 1
C;jC back
EjE EjE
GjG cjc
XWPR
(;I(;
back
CjC C/C
FI
E/E back
EjE E/E
XPAD
cjc c1c
FI
GjC back
XPADV
E/E back
EIE EjE
FI
CjC hack
c;jc cjc
X\VID
EIE back
EjE EjE
cjc C/C
GjC back
FI
EjE bark
E/E E/E
XH'I'
W h 4
GIG back
cjc c;/c
FI
E/E
EjE
F1
EjE
EjE
back
EjE
back
E/E
cjc cjc
Cj(; back
XHP
Ell< EjE
F1
c;jc
c;jc
(;jC hack
,5115 back
XLAR
GIG back
EjE back
EIE EjE
(;I(; q c
XWAR
FI
EIE back
EjE EIE
cjc c;jc
GjG back
XI'OTAI,
FI
GjC back
FI
cjc c;ic
EIE back
EIE EIE
REGWQ
~
84.8
80.5
69.3
62.0
G1.7
91.2
83.5
72.7
58.6
55.8
91.0
80.6
72.9
58.8
58.1
87.8
75.8
72.6
63.6
59.3
87.7
8.5.9
68.8
63.6
56.3
92.6
86.3
75.0
63.2
44.9
91.4
77.3
68.8
65. I
54.8
76.6
75.2
72.6
68.3
67. I
77.9
72.9
72.6
70.8
66.4
82.2
76.9
71.5
65.4
64.3
82.3
82.2
80.0
70.3
45.7
29
31
28
29
26
28
26
31
29
29
26
31
28
29
29
31
28
26
29
29
26
28
31
29
29
28
26
31
29
29
31
29
29
26
28
29
31
26
28
29
28
29
29
26
31
29
29
26
28
31
28
31
26
29
29
~
~~~~~
0 IOR9NS
A
0 0030*
AB
ABC
BC
C
0 0096NS
0 0667NS
0 0 I24NS
A
AB
0 0001*
AB
BC
C
0 00'97NS
0 8850NS
0 8862NS
0 4I39NS
0 0019*
A
A
A
A
B
EjE EIE = most A4. edulis-like: E/E back = Af. rdulis backcross: F1 = putative FI hybrid: GjC CjC = most A l .
.~uaNoprouinciuliJ-like:G/G back = Af. gu//oprouznciulk backcross: n = number of individuals: ANOVA P = probability
value of analysis: NS = non-significant result after tablewide srquential Bonferroni procedure: * = si<gnificant
result after tablewide sequential Bonferroni procedure: REGWQ= Ryan-Einot-Gabriel-Wrlsch multiple range
Q test result (means with the same letter are not significantly diffcrmt).
.J. P. A. GARDNEK
82
TABLE
5. ANOVA and Ryan-Einot-Gabriel-Welsch test results of the indices of trait variability
for the five c o m p o u n d genotypes of the hybrid Whitsand population
~
~~~~~
XLPR
Mean
rank
Compound
genotype
Index
83.7
81.5
80.5
67.7
65.9
88.5
82.2
80. I
70.2
57.9
83.0
80.8
74.0
CjC hack
E/E back
F1
C/C c;/c
qs EjE
X\Z'PK
C;/C
back
FI
GIG CjC
EIB back
EjE E/E
XPN>
GjG GIG
E/E E/E
I1
E / E hack
XPADV
71.2
CljC back
c;jc GIG
C/C back
71.1
9.5.3
84.7
74.0
62.7
62.1
85.3
76.8
75.7
73.3
68.7
92.4
87.7
86.7
FI
qe
EjE
EIE back
XWID
G'jc C:jC
E/E E/E
E/E back
G/C hack
FI
XHT
CjC back
c;/c c;jc
F1
EIE E/E
EIE back
FI
61.6
50.3
87.6
79.0
77.1
73.0
63.5
92.1
76.7
72.6
C/C back
EIE back
EIE E/E
c:jc cjc
XHP
F1
EjE back
72.0
66.6
88.8
80.2
71.6
69.9
69.4
83.3
81.0
74.6
70.9
70.0
90. I
83.6
81.3
62.5
61.4
XLAR
~~~
n
~~
~~
~~~~
~
31
30
31
30
29
31
30
31
30
29
31
29
30
30
31
31
31
30
29
30
31
29
30
31
30
31
31
30
29
30
30
31
30
0.359 1NS
0.06 IONS
0.7464NS
0.0096NS
0.6730NS
A
0.000 I*
AB
AB
BC
c
0.2922NS
29
31
30
30
31
31
29
0.2075NS
31
29
30
30
31
31
30
30
29
31
31
31
30
0.3468NS
0.6949NS
0.028-1.NS
29
30
~
E/E E / E = most Ai. edidir-like: E/E back
= 121. edzrlzs backcross: F1 = putative FI hybrid: GIG GjC = ninst l\f.
gaaNopruz,lnrialis-like: GIG back = h f .galloproc~inrialisbackcross: )I = number of individuals: ANOVA P = probability
value of analysis: NS = non-significant result after tablewide sequential Bonferroni procedure: = significant
rrsult aftrr tablrwide srquential Bonferroni procedure: REGWQ = Ryan-Einot-Gabriel-Welsch multiple range
*
Q test result (means with the same letter are noii-significantly different).
DEVELOPMEN1 AL STABILITY IN HYBRIDIZING MARINE MUSSELS (MITILL‘S)
83
between the two mussel types. Thus, the accumulated genetic differences
between the two mussel types are thought to be insufficient to result in
increased developmental instability upon hybridization.
A third possible explanation for the absence of greater hybrid and backcross
morphological variability compared to parental genotypes also exists. At the
larval or settlement stage, selective mortality of hybrid and backcross individuals
with highest levels of variability would result in a subset of these genotypes
which is pre-selected with low levels of variability. Thus, only those genotypes
which were minimally disrupted by hybridization would survive, resulting in lower
mean variability for the surviving mussels. Inter-specific breeding experiments have
been carried out by Lubet et al. (1984), who reported no significant genotypedependent mortality between crosses and fertile F2 individuals. More recently,
although Beaumont et al. (1993) reported higher larval mortality rates among
F1 hybrids than among ‘pure’ types, they found no evidence of increased larval
abnormalities among F 1 hybrids compared with ‘pure’ crosses. These findings
argue against the explanation of differential mortality of the most morphologically
variable individuals.
Developmental stabilio as a fitness component
From the analyses of trait variability among the pooled populations (Table
3) there is evidence that h.1. edulis mussels have significantly greater mean trait
variability than M. galloprovincialis mussels (significant differences were observed
for four indices, XWPR, XTOTAL, XWID and XAAM), although this difference
is not apparent from the analysis of the original six populations (Table 2). In
contrast, the most hl. galloprovi~icialis-like and Al. galloproziincialis backcross
individuals have higher levels of trait variability than the most hl. edulis-like
and hl. edulis backcross mussels in both hybrid populations (Tables 4 & 5)
(XWPR and X H T at Croyde, X H T at Whitsand). Why one trend should be
observed in allopatry and the opposite seen in the hybrid zone is unknown.
Ad hoc explanations can be advanced (e.g. it is a pooling artefact among the
allopatric populations, or it is in some way linked to introgression in the hybrid
zone, etc.) but at the moment the reason remains unknown. An increased
sample size of allopatric populations might help answer the question by
determining if the observed trend in allopatry is real or not.
The finding of higher levels of developmental instability among the most hf.
galloprovincialis-like individuals compared with the lower levels among the most
M. edulis-like mussels is the first report of an apparent advantage in a fitness
component for the A4. edulis-like individuals of this hybrid zone. In all other
cases investigated to date, the hf.galloprovincialis-like mussels have a pronounced
fitness advantage compared with the A[. edulis-like individuals. Fitness differences
exist in distributional limits up the intertidal region (Gosling & Wilkins, 1981;
Skibinski, 1983; Gardner & Skibinski, 1988), fecundity (Gardner & Skibinski,
1990), parasite resistance (Coustau et al., 1991b), strength of byssal attachment
to the substrate (Gardner & Skibinski, 1991; Willis & Skibinski, 1992), and
growth and longevity (Gardner et al., 1993). In conjunction with the postulated
greater influx of M. edulis spat into the hybrid zone (Gardner & Skibinski,
1988), any advantage in developmental stability experienced by the hl. edulis
8.1
J. P. A. GARDNER
mussels would help to counterbalance the directional selection in favour of the
M. galloprovincialis mussels in all other fitness parameters so far investigated.
Genetic basis
of
developmental instabilib
Based upon knowledge of geography, tidal movement, currents and the
allozyme frequencies of each population, uni- or bidirectional gene flow is
possible between the Bude (hl.galloprovincialis), Croyde (hybrid) and Caswell (M.
edulis) populations, while the Lowestoft (M. edulis), Grau d'Age (M. galloprovincialis)
and Whitsand (hybrid) populations are each genetically isolated from the five
other populations. However, a strong significant association between trait ranking
(based on level of variability) across all populations has been observed-that is,
traits with the highest levels of variability in one population also have high
levels of variability in all other populations. For this pattern of trait ranking to
exist across all surveyed populations, morphological variability must be under
tight genetic control and largely uninfluenced by environmental conditions. This
contrasts with general shell shape (phenotypic variation) which is notoriously
plastic and strongly influenced by environmental conditions (Seed, 1968). That
the index rankings are so consistent across all populations indicates that the
underlying genetic factors controlling variability in the two mussel taxa are very
similar. The close evolutionary relatedness of the two mussel taxa is emphasized
by the absence of disruption to developmental stability upon hybridization and
introgression, and by the ranking of shell traits according to the level of
variability which is consistent across all surveyed populations.
ACKNO\t'LEDGEMENTS
I thank Dr A. R. Palmer for helpful comments on an earlier draft of this
manuscript. Electrophoresis was carried out in Dr David Skibinski's laboratory
(University College of Swansea, University of Wales, U.K.). From Memorial
University of Newfoundland, Canada, I thank the following: Donna Gardner,
Melanie Saunders and Leslie Turner for measuring the shells, and Craig Tuck
for statistical discussion and drafting of the figures.
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