High levels of gene flow in the surf bivalve Donax deltoides (Bivalvia: Donacidae) on the east coast of Australia

We surveyed patterns of allelic variation within twelve samples of the pipi Donax deltoides Lamarck from beaches separated by up to 1200 km but connected to varying degrees by the East Australian Current. We used these data to test the prediction that the irregular patterns of water movement would cause genetic differentiation in pipis, so that there would be more genetic variation within and among the more southern regions than the northern regions. We found that six loci were at least moderately variable within all samples, and there were no clear geographic patterns in allelic frequencies. In general, genotype frequencies within samples were consistent with predictions for an outcrossed, sexually reproducing species, and we detected no evidence of population subdivision. Within samples, with the exception of the peptidase loci, single-locus genotype frequencies were in close agreement with expectations for Hardy–Weinberg equilibrium. We observed no significant linkage disequilibrium for any pairwise comparison of loci in any sample. Our hierarchical analysis of genetic variation revealed little variation among all samples (F st = 0.009). Loci showed consistently low levels of subdivision (F st from 0.003 to 0.018). We found almost no variation among the four geographic regions sampled (F rt = 0.001). All variation was therefore attributable to variation among samples within regions (F sr = 0.010). These data imply that larvae are moving between regions and that levels of present or recent gene flow are high, and support the conclusions of other studies which have inferred widespread gene flow for animals dispersing via planktonic, outcrossed larvae in parts of this region. This implies that the East Australian Current is sufficient to produce strong larval connections despite its intermittent nature. If existing levels of population subdivision reflect current levels of gene flow, then these data imply that D. deltoides represents a single fishery on the east coast of Australia. Received: 16 September 1996 / Accepted: 25 September 1996     

Genetic differentiation of Texas Gulf Coast populations of the blue crab Callinectes sapidus

Allelic frequencies at three polymorphic, enzyme-encoding gene loci (GOT-2, EST-1, EST-2) were determined for Callinectes sapidus (Rathbun) megalopae and adults sampled along the Texas coast of the Gulf of Mexico. Significant temporal and spatial variation was observed at all three loci. Primary findings included: (1) megalopal allelic frequencies often differed significantly from those observed among neighboring adult populations; (2) larval allelic frequencies appeared to vary seasonally, with populations showing sharp differences in the summer months but tending to be more homogeneous in winter; (3) allelic frequencies among adult populations were significantly heterogeneous, but only one locus (EST-2) showed significant temporal variation; (4) juvenile and adult crabs sampled within one bay showed no size-specific differences in allelic frequencies. The spatial heterogeneity in allelic frequencies suggests that interpopulation gene flow is not sufficient to overcome population differentiation resulting from genetic drift and/or natural selection. Temporal variation in larval allelic frequencies suggests seasonal changes in larval source populations which may result from population differences in spawning season or developmental times or from seasonal changes in coastal current patterns.     

Phenotypic assortative mating and genetic population structure in the crab Trapezia digitalis

Colonies of Pocillopora spp. containing the crab Trapezia digitalis were collected from Hawaii in summer of 1979 and of 1980. Individuals of T. digitalis in Hawaii vary extensively in coloration of the carapace: crabs were separated into phenotypic classes by their degree of carapace reticulation. When presented with two females of different phenotype, males preferentially paired with the female which was similar to them in reticulation. The genetic population structure of T. digitalis was unusual by comparison with other species of Trapezia. Previously reported homozygote excesses at esterase-4, hexokinase-1, phosphoglucoseisomerase, and general protein-2 cannot be explained by the single-locus Wahlund effect, since there was not significant genetic differentiation among phenotypic classes, as measured by Wright's F _ST statistic. There was significant variation among classes in allele frequencies at a fifth locus, hexokinase-2. Two pairs of loci were in significant gametic disequilibrium. Gametic disequilibrium was not detected in seven other species of Trapezia. It is suggested that multi-locus genetic analysis and an increased geographic sampling range are needed to determine the genetic population structure of T. digitalis in relation to observed variation in coloration.     

Ecological genetics of the barnacle,<Emphasis Type="Italic"> Hexaminius foliorum</Emphasis>, within mangrove forests near Sydney,Australia

Spatial and temporal distribution of allozyme variation at three loci in a cohort of the barnacle, Hexaminius foliorum, living on leaves of Avicennia marina was studied from recruitment to adulthood at three geographical scales. Analysis of populations shortly after recruitment showed that there were significant divergences in allele frequencies at the coarsest geographical scale studied (between estuaries, 50 km apart, Wright's F statistic=0.016) and at the finest geographic scale (between sites, 50–100 m apart, Wright's F statistic=0.018). There was, however, no significant genetic divergence at an intermediate scale (between bays, 3–4 km apart, Wright's F statistic=0.002). The genetic differences between populations decreased over time due to the selection against the null homozygotes originally present at high frequency at two loci. There was sufficient mortality (ranging from 35.5 to 80%) between seasons to account for the deaths needed for the observed changes in allele frequencies. Differences in the genetic structure between estuaries may be the result of isolation and limited mixing of cyprid larvae among estuaries. Differences in the genetic structure between sites may be due to pre- and post-settlement mortality acting on H. foliorum.Communicated by G.F. Humphrey, Sydney     

Genetic relationships within and between clonal and solitary forms of the sea anemone Anthopleura elegantissima revisited: evidence for the existence of two species

Along the temperate Pacific coast of North America, the actiniarian sea anemone Anthopleura elegantissima exhibits two discrete life-history phenotypes. Although both forms sexually produce planula larvae, the clonal morph can also asexually propagate by fission, whereas the solitary morph does not. Whether the two forms constitute one or two species has long been contested. Hand originally designated the two forms as conspecifics, whereas Francis – on the basis of differences in microhabitat, biogeographic range and phenotypic frequencies – argued that the two forms constituted a sibling-species pair. From the results of an electrophoretic survey in which they pooled allelic frequencies across several geographic locations, Smith and Potts subsequently argued that the two forms were not genetically differentiated, and therefore represented a single species. We re-examined the relationship between the forms electrophoretically, substantially extending the geographic range and doubling the sample sizes beyond those used by Smith and Potts, and not pooling allelic frequencies in our analyses. Our analysis of patterns of genetic variation at ten highly polymorphic allozyme loci shows that although no fixed genetic differences distinguish the two forms, there are significant differences in allele frequencies between clonal and solitary A.␣elegantissima at every site we sampled throughout their range of sympatry (over 1000 km); within each form, however, there is little detectable genetic differentiation among populations. We therefore conclude that the two forms represent recently reproductively isolated taxa, and propose that the clonal form retain the binomial A. elegantissima (Brandt, 1835), whereas the solitary form be described and named a new species, Anthopleura sp. Received: 28 August 1996 / Accepted: 25 September 1996     

Population structure in the sexually reproducing sea anemone Oulactis muscosa

The intertidal sea anemone Oulactis muscosa (Drayton) is dioecious and most individuals are sexually mature throughout the year. Biochemical genetic evidence was used to determine the genetic structure of populations and to infer the relative contributions of sexual and asexual reproduction to recruitment. Data were collected for six enzyme-encoding loci from local populations spread along 735 km of the south east coast of Australia. The genetic structure of each of the nine local populations studied was consistent with recruitment by sexually produced individuals. In almost all cases, the observed single-locus genotypic frequencies closely matched those expected for hardy-Weinberg equilibria, however, consistent deficits of heterozygotes were detected for all loci. No apparent subdivision of the population was detected within the sampling area. Low levels of genetic differentiation were found between local populations and standardised variance (F _ST ) values were similar to those for other species with widespread planktonic dispersal of larvae.Contribution No. 60 from the Ecology and Genetics Group of the University of Wollongong     

Chaotic genetic patchiness in an intertidal limpet,Siphonaria sp.

Variation of 4 polymorphic enzymes was studied for 2 yr (1978 and 1979) in an undescribed species of Siphonaria, a pulmonate limpet, from a rocky shore at Rottnest Island, Western Australia. Depending on the locus, significant genetic differences were found among sites along 50 m of shore, between high and low portions of the shore within sites, between adults and recruits, and between recruits in the 2 yr. This genetic heterogeneity does not follow a simple, consistent pattern, but forms a shifting, ephemeral genetic patchiness best described as chaotic. This patchiness may result from temporal variation of numbers and genotypes of recruits, which leads to the proposal that planktonic dispersal, although causing uniformity on a large scale, can give rise to fine-scale genetic patchiness.     

Non-random mating and population genetic subdivision of two broadcasting corals at Ningaloo Reef,Western Australia

Allozyme electrophoresis of two corals was used to assess whether populations at Ningaloo Reef, Western Australia are primarily self-seeding or whether recruitment is from a broader geographic pool. Significant genetic subdivision across a range of spatial scales (between 6.5 km and 155 km) was found for both Acropora digitifera and A. aspera, with mean F _ST values of 0.010 and 0.067 respectively. Large departures from Hardy-Weinberg expectations were found for both species. Without exception these were due to deficits of heterozygotes; mean D values were –0.341 for A. digitifera and –0.455 for A. aspera. The magnitude of the deficits was consistent both across loci for all sites and across all sites for each locus. Some loci were found to be in linkage disequilibrium but no consistent pattern was observed. Also, multi-locus genotypic diversity values were generally high (between 0.83 and 1.00) and so departures from equilibria cannot be attributed to asexual reproduction. The most plausible explanation for the patterns observed is restricted gene flow at both the planktonic and gametic stages, with mating between close relatives.Communicated by G.F. Humphrey, Sydney     

Genetic structure of local populations and divergence between growth forms in a clonal invertebrate,the Caribbean octocoral Briareum asbestinum

Although the genetic structure of many populations of marine organisms show little deviation from panmixia, in those marine species with limited larval dispersal, patterns of microgeographic genetic differentiation may be common. The octocoral Briareum asbestinum should show local population differentiation because colonies reproduce asexually by fragmentation, most matings occur between colonies in very close proximity, and the sexually produced larvae and sperm appear to disperse only short distances. Variability in secondary chemistry of individual B. asbestinum colonies from different populations in close proximity also suggests local population differentiation. We determined the genetic composition of local populations by surveying allozyme variation of three shallow and two deep populations within a 300 m² area at San Salvador Island, Bahamas and at a site 161 km away on Little San Salvador, Bahamas in July 1990. As B. asbestinum occurs as either an erect branching form or an encrusting mat often at the same sites, we sampled both morphs to examine the extent of genetic exchange between them. Five of 21 loci were polymorphic and most populations showed a deficit of heterozygotes. Allele frequencies differed significantly between morphs at each site where they occurred together. The mean genetic distance (D=0.065) between morphs is consistent with the interpretation that the two morphs are genetically isolated. Despite the close spatial proximity of the San Salvador populations, both the branching and encrusting morphs showed significant genetic heterogeneity among neighboring populations. Similarly, pooled allelic frequencies for samples collected from the islands of San Salvador and Little San Salvador differed significantly at 1 locus for the branching morph and at 3 out of 5 loci for the encrusting morph.     

High levels of genetic subdivision in peripherally isolated populations of the atherinid fish Craterocephalus capreoli in the Houtman Abrolhos Islands,Western Australia

The atherinid fish Craterocephalus capreoli Rendahl is abundant in the Houtman Abrolhos Islands, 70 km off the Western Australia coast and 250 km south of the southern limit of the range of the species along the mainland. Electrophoretic examination of 7 allozyme loci at 17 sites in the Houtman Abrolhos revealed a substantially lower level of polymorphism than found in an earlier study of the species in its mainland distribution, with many of the uncommon alleles and some common ones missing. There is a very high degree of genetic subdivision among the populations in the Houtman Abrolhos, measured by a mean F _ST of 0.437 over a distance of 35 km. This F _ST (standardized variance in allelic frequencies) is six times that found previously among populations along the mainland coast over distances up to 850 km. The subdivision of populations in the Houtman Abrolhos is similar within one island group on a scale up to 12 km, and between two groups that are separated by 15 km of deep water. Significant differences in allelic frequencies were found between populations from the open shore and enclosed lagoons less than 800 m apart, but the overall levels of subdivision were similar for the two types of environment. Previous work had shown high levels of genetic subdivision in the Houtman Abrolhos for a gastropod with direct development. The results for C. capreoli demonstrate that the archipelago favours subdivision even for a species with potentially much greater mobility and different life history.     

Genetic Structure of Fragmented Populations of the Endangered Daisy Rutidosis leptorrhynchoides

Abstract: The endangered grassland daisy Rutidosis leptorrhynchoides has been subject to severe habitat destruction and fragmentation over the past century. Using allozyme markers, we examined the genetic diversity and structure of 16 fragmented populations. The species had high genetic variation compared to other plant species, and both polymorphism and allelic richness showed strong positive relationships with log reproductive population size, reflecting a loss of rare alleles (frequency of q < 0.1) in smaller populations. Fixation coefficients were positively related to size, due either to a lack of rare homozygotes in small populations or to Wahlund effects (owing to spatial genetic structure) in large ones. Neither gene diversity nor heterozygosity was related to population size, and other population parameters such as density, spatial contagion, and isolation had no apparent effect on genetic variation. Genetic divergence among populations was low , despite a large north-to-south break in the species' current distribution. To preserve maximum genetic variation, conservation strategies should aim to maintain the five populations larger than 5000 reproductive plants, all of which occur in the north of the range, as well as the largest southern population of 626 plants at Truganina. Only one of these is currently under formal protection. High heterozygosity in smaller populations suggests that they are unlikely to be suffering from inbreeding depression and so are also valuable for conservation. Erosion of allelic richness at self-incompatibility loci, however, may limit the reproductive capacity of populations numbering less than 20 flowering plants.     

Genetic heterogeneity among adult and recruit red sea urchins, Strongylocentrotus franciscanus

Allozyme electrophoresis was used to characterize genetic variation within and among natural populations of the red sea urchin Strongylocentrotus franciscanus. In 1995 to 1996, adult urchins were sampled from twelve geographically separated populations, seven from northern California and five from southern California (including Santa Rosa Island). Significant population heterogeneity in allelic frequencies was observed at five of six polymorphic loci. No geographic pattern of differentiation was evident; neighboring populations were often more genetically differentiated than distant populations. Northern and southern populations were not consistently distinguishable at any of the six loci. In order to assess within-population genetic variation and patterns of recruitment, large samples were collected from several northern California populations in 1996 and 1997, and were divided into three size classes, roughly representing large adults (>60 mm), medium-sized individuals (31 to 60 mm, “subadults”) and individuals <2 yr of age (≤30 mm test diam, referred to as “recruits”). Comparisons of allelic counts revealed significant spatial and temporal differentiation among size-stratified population samples. Recruit samples differed significantly from adult samples collected at the same locale, and showed extensive between-year variation. Genetic differentiation among recruit samples was much higher in 1997 than in 1996. Between-year differences within populations were always greater for recruits than for adults. Potential explanations for the differentiation of recruit samples include pre- and post-settlement natural selection and high interfamily variance in reproductive success or “sweepstakes” recruitment. Unless recruit differentiation can be attributed to an improbable combination of strong and spatially diverse selection, such differentiation across northern California populations indicates that the larval pool is not well mixed geographically (even on spatial scales <20 km), despite long planktonic larval duration. Received: 6 July 1999 / Accepted: 25 January 2000     

Genetic structuring and gene flow in Cerastoderma glaucum (Bivalvia: Cardiidae): evidence from allozyme variation at different geographic scales

Genetic variation at 16 enzyme loci was analysed in 20 Mediterranean and 1 Baltic population of Cerastoderma glaucum. Spatial genetic variation at different geographic scales was investigated. In general, this species was fairly genetically structured (over all loci Š=0.088), according to the fragmentary nature of its habitat. Spatial structuring of genetic diversity was shown to follow different models, depending on the geographic scale considered: a stepping-stone model provided a good fit at a wide scale, with gene flow inversely related to geographic distance, whereas at small scale, genetic relationships among samples could not be interpreted as simply the effect of physical distance among populations. Results are discussed taking into account the intrinsic and extrinsic factors which are most likely to affect the patterns of genetic structuring in coastal marine animals. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-001-0753-x     

Molecular and morphological analyses of the cuttlefish<Emphasis Type="Italic"> Sepia apama</Emphasis> indicate a complex population structure

The giant Australian cuttlefish Sepia apama Gray, 1849 annually forms a massive and unique spawning aggregation in northern Spencer Gulf, South Australia, which has attracted commercial fishing interests in recent years. However, many basic life-history characteristics of S. apama are unknown, and anecdotal evidence suggests that there is more than one species. The present study assessed the population structure and species status of S. apama using data from allozyme electrophoresis, microsatellite loci, nucleotide sequences of the mitochondrial COXIII gene, multivariate morphometrics and colour patterns. Analyses of allozyme and microsatellite allele frequencies revealed two very divergent but geographically separated populations consisting of specimens from the east coast and southern Australia. However, the presence of a heterozygote in a putative contact zone between the east coast and southern Australia suggested that these populations were not reproductively isolated. Mitochondrial haplotypes seem to have introgressed further north into the contact zone than have nuclear alleles. Differences in colour patterns that previously had been attributed anecdotally to different geographic populations were, in fact, correlated with sexual dimorphism. These data are most consistent with S. apama being one species the populations of which were geographically isolated in the past (historical vicariance) and have come into secondary contact. Comparison of microsatellite allele frequencies among four South Australian samples indicated significant deviations from panmixia. South Australian samples were also reliably diagnosed by means of multivariate morphometrics. Significant differences in mantle length were observed among populations.Communicated by M.S. Johnson, Crawley     

Genetic structure and allozyme variation of sea bass (Dicentrarchus labrax and D. punctatus) in the Mediterranean Sea

This paper reports data on 28 allozyme loci in wild and artificially reared sea bass (Dicentrarchus labrax) samples, originating from either coastal lagoon or marine sites in the Mediterranean Sea. F _ST analysis (θ estimator) indicated strong genetic structuring among populations; around 34% of the overall genetic variation is due to interpopulation variation. Pairwise θ estimates showed that, on average, the degree of genetic structuring was much higher between marine populations than between samples from lagoons. Six polymorphic loci showed differences in allele frequencies between marine and lagoon samples. Multivariate analyses of individual allozymic profiles and of allele frequencies suggested that different arrays of genotypes prevail in lagoons compared to marine samples, particularly at those loci that, on the basis of previous acclimation experiments, had been implicated in adaptation to freshwater. On the other hand, variation at “neutral” allozyme loci reflects to a greater extent the geographic location of populations. Allozyme differentiation was also studied in a D. labrax population from the Portuguese coast. Average genetic distance between this population and the Mediterranean populations was quite high (Nei's D = 0.236) and calls into question the taxonomic status of the Portuguese population. Finally, genetic relationships between D. labrax and D. punctatus were evaluated. Average Nei's D was 0.648, revealing high genetic differentiation between the two species, even for two sympatric populations of these species in Egypt; thus gene flow was not indicated between species. Received: 24 October 1996 / Accepted: 27 November 1996     

Fixed allele frequency differences among Palauan and Okinawan populations of the damselfishes Chrysiptera cyanea and Pomacentrus coelestis

A survey of intraspecific allozymic variation among samples of the damselfishes Chrysiptera cyanea and Pomacentrus coelestis collected from Palau and Okinawa revealed levels of genetic divergence far in excess of estimates reported previously for populations of coral reef fishes. Absolute or nearly fixed differences in allele frequencies were detected at ADA ^*, sMDH-2 ^*, MEP-1 ^*, PEPB ^*, PEP-LT ^* and sSOD ^*, and at sAAT ^*, GPI-A ^*, LDH-1 ^*, and PEPB ^*, among the geographic samples of C. cyanea and P. coelestis, respectively. Examination of allele frequencies at most other loci (26 for C. cyanea; 24 for P. coelestis) revealed slight differentiation or identical fixation among the geographic samples. The observed patterns of allele frequency distribution suggest the existence of localized demes of these fishes: these demes may be cryptic and/or incipient species. Whether or not speciation is incipient, the observed patterns of allozymic and isozymic variability suggest that natural selection is a factor in the maintenance of population substructuring of the study species. Pronounced allelic differences were highly concentrated at a small number of loci: strongly bimodal frequency distributions of loci relative to their genetic identities (I) were observed for among-population comparisons in both study species. Allozymes encoded by the PEPB ^* locus in samples of the noncongeners from Palau exhibited identical electrophoretic mobilities. In Palauan P. coelestis, mSOD ^* is expressed but sSOD ^* apparently is not, whereas in Okinawan P. coelestis, both mSOD ^* and sSOD ^* are expressed.     

Microsatellite variation, effective population size, and population genetic structure of vermilion snapper, Rhomboplites aurorubens, off the southeastern USA

Vermilion snapper (Rhomboplites aurorubens) were collected from four sites off the Atlantic coast of the USA and one site in the Gulf of Mexico to evaluate effective population size and genetic stock structure. Previous studies had suggested geographic variation in the ratio of males to females, so this population characteristic was explored in conjunction with the genetic analysis. Sex ratio varied greatly among the five sample sites; males comprised 57% of samples in the Gulf of Mexico, while within the South Atlantic Bight they comprised between 36% (Morehead City, North Carolina) and 53% (Carolina Beach, North Carolina) of samples. No clear geographic trends in the sex ratio emerged; instead, it was found to vary with fish length, the percentage of males decreasing with increasing size. Allelic variation assessed at seven dinucleotide microsatellite loci was large; gene diversities ranged from 0.43 to 0.95 and allelic counts from 7 to 39. Estimates of the effective population size ranged from 24 500 (based on the infinite-alleles model) to 150 500 (based on the stepwise-mutation model). There was evidence for excess homozygosity within samples: estimates of F _IS (the correlation of alleles within individuals) ranged from 0.01 to 0.03 among the seven loci, and three estimates were significantly greater than zero. Differentiation among localities was very weak, as estimates of F _ST (the correlation of alleles within populations) were on the order of 0.001 to 0.002 and genetic distance estimates between localities were not related to geographic distances. This suggested that vermilion snapper in the South Atlantic Bight (Cape Hatteras, North Carolina to Cape Canaveral, Florida) and Gulf of Mexico are likely to consist of one genetic stock. Despite the overall homogeneity, there were indications of a temporally dynamic local structure that would bear further examination. Received: 6 July 1998 / Accepted: 9 February 1999     

Population genetics of the blue crabCallinectes sapidus: modest population structuring in a background of high gene flow

To determine the genetic population structure of blue crabs (Callinectes sapidus Rathbun), electrophoretic allozyme analysis was performed on 750 individuals collected from 16 nearshore locations ranging from New York to Texas, USA. Twenty enzymes and non-enzymatic proteins coded by 31 presumptive loci were examined. Twenty-two loci were either monomorphic or polymorphic at less than theP ₉₅ level; alleles for these polymorphic loci were geographically dispersed. Allele frequencies for three of the remaining polymorphic loci were homogeneous over all populations, as were levels of polymorphism and heterozygosity. Phenograms generated by the UPGMA (unweighted pair-group method using arithmetic averages) and distance Wagner methods exhibited no geographic pattern in the clustering of populations. Estimates ofN _em (effective number of migrants per generation between populations) indicated substantial gene flow, with aalues sufficiently high to infer panmixia between all blue crab populations from New York to Texas. However, despite this high level of gene flow, two striking patterns of geographic differentiation occurred: genetic patchiness and clinal variation. Allele frequencies atEST-2, GP-1, IDHP-2, DPEP-1, DPEP-2, andTPEP exhibited genetic patchiness on local and range-wide geographic scales, and allele frequencies atEST-2 varied temporally. Genetic patchiness in blue crabs is likely to be the result of the pre-settlement formation and subsequent settlement of genetically heterogeneous patches of larvae; allele frequencies of those larval patches may then be further modified through ontogeny by localized selection. In the Atlantic Ocean, a regional latitudinal cline ofEST-2 allele frequencies was superimposed on the range-wide genetic patchiness exhibited by that locus. This pattern against a background of high gene flow is highly likely to be maintained by selection. In estuaries along the Atlantic Ocean coast, a combination of low adult long-distance migration and a high retention rate of locally spawned larvae could serve to segregate populations and allow for the development of the geographic cline inEST-2. The Gulf of Mexico showed no apparent cline, perhaps due to long-distance migration of females in some regions of the Gulf, or to masking by genetic patchiness. These results emphasize the importance of both ecological and evolutionary time scales and structuring mechanisms in determining genetic population structure.     

Genetic variance in Ophiomusium lymani (Ophiuroidea: Echinodermata) from lower bathyal depths in the Rocklall Trough (northeast Atlantic)

Specimens of the deep-sea brittle-star Ophiomusium lymani were collected from six sites in the Rockall Trough (northeast Atlantic_. Four monomorphic and four polymorphic loci were detected, with up to 75 individuals screened at any one locus. The results showed little difference in allele frequencies between sites and, consequently, estimates of genetic identity indicated no significant genetic differentiation between the populations sampled. Deficits of heterozygotes were observed at all polymorphic loci, but the deficiencies were only significant at the phosphoglucose isomerase locus from two locations. The deficit of heterozygotes was not the same across loci, suggesting that inbreeding is not the cause of the excess homozygosity. No relationship between heterozygosity and depth was observed.     

Geographic variation,niche breadth and genetic differentiation at different geographic scales in the mussels Mytilus californianus and M. edulis

Genetic differentiation was investigated in the marine mussels Mytilus californianus Conrad and M. edulis Linn. from the west coast of North America. In allopatry with M. californianus, M. edulis occurs throughout the intertidal zone; however, in microgeographic sympatry its ecological range is restricted to above the M. californianus mussel bed and to patches of substratum opened by natural disturbances within the bed. Over the same geographic scale, the broader-niched M. edulis shows greater among-locality genetic difference and greater levels of polymorphism than M. californianus at two enzyme loci. Genetic differentiation on a geographic basis was investigated in M. californianus at a single rock (on a scale of meters), on an island (on a scale of kilometers), throughout a strait (on a scale of 10² kilometers), and along the west coast of North America (on a scale of 10³ km). Differentiation was minimal over the west coast, and could be explained by microhabitat differences in a local area. The minimal differentiation in west coast mussels relative to strong geographic differentiation of M. edulis on the east coast of North America may be related to the steeper latitudinal thermal gradient on the east coast. Local ecologically related microgeographic variation can result in biased and misleading estimates of genetic heterogeneity but microgeographic variation at enzyme loci may not be due to selection at the loci investigated or may even be due to the breeding structure of the mussel populations, as suggested by Tracey et al. (1975).