Genetic Diversity in the Endangered Lily Harperocallis flava and a Close Relative, Tofieldia racemosa

We examined genetic diversity in 464 individuals of the monotypic lily Harperocallis flava in its two habitats (seepage bogs and a roadside right-of-way) and five populations of a co-occurring related lily, Tofieldia racemosa. The endangered H. flava, endemic to the Apalachicola lowlands of the Florida panhandle, was monomorphic for the 22 loci scored. In contrast, T. racemosa had a high proportion of polymorphic loci ( P_s = 68.2%; P_p = 47.7%) with moderate genetic diversity (   H_es = 0.134; H_ep = 0.114). Estimated gene flow was moderately high ( Nm = 2.07) for T. racemosa, with most (93%) of the total genetic diversity found within populations. Despite the low level of genetic divergence, some isolation by distance was detected among T. racemosa populations. Harperocallis flava and other species without discernable genetic variation pose special problems for conservation biologists because genetic criteria are not available for the development of ex situ and in situ conservation and management strategies.     

Comparisons of Genetic Diversity in the Endangered Adenophora lobophylla and Its Widespread Congener, A. potaninii

Abstract: Starch-gel electrophoresis was used to examine the levels and distribution of genetic diversity in two Adenophora species: the narrow endangered Adenophora lobophylla and its widespread congener, A. potaninii . Based on allozyme variation at 18 putative loci, we measured high levels of genetic variability both in the endangered and the widespread species, with 83.3% of the loci being polymorphic. The mean expected heterozygosity within populations (   H _ep  ) and within species (   H _es  ) were 0.234 and 0.244 for A. potaninii and were as high as 0.210 and 0.211 for A. lobophylla . There was higher differentiation among populations in A. potaninii (   F _ST = 0.155) than in A. lobophylla (   F _ST = 0.071). The high levels of genetic diversity in the present allozyme survey are consistent with the morphological variation observed in these species and may be attributed to high outcrossing rates in the Adenophora species. In addition, A. lobophylla was identified as a distinct species on the basis of Nei's genetic distances and thus should be given a high priority for protection. It is noteworthy that the endangered A. lobophylla maintains much higher genetic diversity than most endemic or narrowly distributed plant species in spite of its restricted distribution. We hypothesize that A. lobophylla has become endangered for ecological and stochastic reasons, including habitat destruction or environmental changes, mud slides, and human disturbance such as grazing and mowing. Consequently, habitat protection is of particular importance for conserving this endangered species.     

Genetic diversity and genetic structure in the brown alga Halidrys dioica (Fucales: Cystoseiraceae) in Southern California

Genetic diversity and genetic structure in a population of the brown seaweed Halidrys dioica Gardner were evaluated in five sites in southern California, USA, in 1991, using isozyme electrophoresis. H. dioica is relatively long-lived and has an outcrossing mating system and floating reproductive fronds with the potential for longdistance gamete dispersal. Because these characteristics are hypothetically important in determining genetic diversity and structure, we predicted that genetic diversity would be high and genetic structure would be exhibited only at relatively large geographic scales in H. dioica populations. The data were consistent with the prediction: genetic diversity (% polymorphic loci, no. of alleles/locus, average expected heterozygosity) was high compared to that of other seaweed species. Genetic structure (Wright's F statistics, Nei's genetic distance, point-pattern analysis of alleles) was low within and among distinct rocky reefs over 4 km of coast but high in subpopulations separated by 90 km. Life-history characteristics may be useful predictors of genetic diversity and structure in seaweed populations, but information on selection regimes, long-distance dispersal, and the extent of clonal propagation, for example, are critically lacking.     

Effects of Population Size, Mating System, and Evolutionary Origin on Genetic Diversity in Spiranthes sinensis and S. hongkongensis

Hong Kong once supported more than 109 species of wild orchids, of which approximately 30% were endemic. Most of the local wild orchids have now become rare or endangered. I conducted a comparative study of genetic diversity in two closely related terrestrial orchids, an allotetraploid, Spiranthes hongkongensis , and its diploid progenitor, S. sinensis , to assess the effects of the population bottleneck associated with the origin of the polyploid and to investigate the relationships between number of breeding individuals, mating system, and level of isozyme variation in their populations. Nearly complete genetic uniformity was observed both within and among populations of S. hongkongensis . In contrast, S. sinensis had high levels of genetic variation for all of the genetic parameters examined. Regression analysis of population size and several components of genetic diversity in S. sinensis revealed that, among various measures of within-population variation, the proportion of polymorphic loci ( P ) and average number of alleles per locus ( A ) or per polymorphic locus ( A _p ) were the most sensitive to population size ( R ² = 0.942, p = 0.001; R ² = 0.932, p = 0.002; and R ² = 0.923, p = 0.002 respectively). The highly negative correlation ( r = −0.999, p < 0.01) between population size and the mean frequency of private alleles in pairwise population comparisons, p (1), indicated that population size may also be used to predict the extent of population differentiation caused by random genetic drift. Conservation of genetic diversity in S. sinensis could be maximized by protecting several of both large and small populations, whereas fewer populations may be needed to achieve this goal for S. hongkongensis.     

Genetic Diversity, Population Size, and Fitness in Central and Peripheral Populations of a Rare Plant Lychnis viscaria

Abstract: Genetic diversity is expected to decrease in small and isolated populations as a consequence of bottlenecks, founder effects, inbreeding, and genetic drift. The genetics and ecology of the rare perennial plant Lychnis viscaria (Caryophyllaceae) were studied in both peripheral and central populations within its distribution area. We aimed to investigate the overall level of genetic diversity, its spatial distribution, and possible differences between peripheral and central populations by examining several populations with electrophoresis. Our results showed that the level of genetic diversity varied substantially among populations (  H _exp = 0.000–0.116) and that the total level of genetic diversity (mean H _exp = 0.056) was low compared to that of other species with similar life-history attributes. The peripheral populations of L. viscaria had less genetic variation (mean H _exp = 0.034) than the central ones (0.114). Analysis of genetic structure suggested limited gene flow (mean F _ST = 0.430) and high differentiation among populations, emphasizing the role of genetic drift (  N _e m = 0.33). Isolation was even higher than expected based on the physical distance among populations. We also focused on the association between population size and genetic diversity and possible effects on fitness of these factors. Population size was positively correlated with genetic diversity. Population size and genetic diversity, however, were not associated with fitness components such as germination rate, seedling mass, or seed yield. There were no differences in the measured fitness components between peripheral and central populations. Even though small and peripheral populations had lower levels of genetic variation, they were as viable as larger populations, which emphasizes their potential value for conservation.     

Developing a Sampling Strategy for Baptisia arachnifera Based on Allozyme Diversity

We analyzed the amount and distribution of genetic variation in Baptisia arachnifera Duncan to develop a sampling strategy for ex situ research. Baptisia arachnifera is an endangered plant species endemic to the coastal plain of Georgia (U.S.) where all populations are within 16 km of each other. A reduction in numbers of individuals has been observed during the last 50 years. Baptisia arachnifera was polymorphic at 24% of the 37 loci examined with an average of 1.32 alleles per locus. The genetic diversity index was relatively low ( H_e = 0.097) as expected for endemic species. Populations were in Hardy-Weinberg equilibrium, suggesting that the species is outcrossing. Consistent with this conclusion is the observation that the majority (approximately 90%) of the genetic variation present in the species is found within individual populations. Indirect evidence of gene flow between populations was detected (   Nm = 2.35). The close proximity of the populations and the recent reduction in population sizes suggest that the populations surveyed may be fragments of a once more continuous gene pool. Based on the observed distribution of genetic diversity among populations (G_ST = 0.096), sampling two populations would capture 99% of the allozyme diversity surveyed. Allozyme data were used to determine which 2 of the 10 populations surveyed should be sampled to maximize the ex situ conservation of genetic diversity. Although the paper-producing companies that own most of the land where Baptisia arachnifera occurs are modifying their harvesting techniques, the species could become extinct without more effective management and preservation efforts.     

A Hierarchical View of Genetic Structure in the Rare Annual Plant Clarkia springvillensis

Genetic structure at several spatial scales was examined in the rare California annual, Clarkia springvillensis . Using seven isozyme-encoding loci as genetic markers, we assessed the amount and distribution of genetic variation among three populations and eight subpopulations. Total genetic variation was lower than in species with similar life history traits but equivalent to that of other endemic plants. Spatial autocorrelation showed some evidence for very limited differentiation within subpopulations at a scale of 1–2 m. The subpopulations, separated by tens of meters, were found to be more differentiated from each other ( F _sp = 0.084) on average than were populations ( F,pt = 0.017). This local genetic differentiation was not correlated with physical distance between subpopulations. The low F_pt estimates suggest that substantial gene flow is occurring among populations. However, the lack of correlation between genetic and geographic distances and the significant differentiation of subpopulations suggest that genetic drift is occurring within populations. Therefore, we believe the apparent homogeneity of populations is due to each population's gene frequencies' being an average of several divergent subpopulations. If drift is causing differentiation within populations, it may eventually cause differentiation between populations. The importance of using a hierarchical approach to evaluating genetic structure is clear. Patterns occurring at one spatial scale may not be evident at others. One should not necessarily conclude that gene flow is substantial and that the risk of genetic erosion via drift is negligible just because differentiation between populations is small; the system may not be at equilibrium. This lesson is particularly important when recent changes in climate or land use are apparent.     

Effective Population Size and Maintenance of Genetic Diversity in Captive-Bred Populations of a Lake Victoria Cichlid

Abstract: We used microsatellite DNA markers to investigate the maintenance of genetic diversity within and between samples of subpopulations (spanning five captive-bred generations) of the haplochromine cichlid Prognathochromis perrieri . The subpopulations are maintained as part of the Lake Victoria Cichlid species survival plan. Changes in the frequencies of 24 alleles, over four polymorphic loci, were used to estimate effective population size (   N _e   ). Point estimates of N _e ranged from 2.5 to 7.7 individuals and were significantly smaller than the actual census size (   N _obs  ) for all subpopulations (32–243 individuals per generation), with the corresponding conservative N _e   /  N _obs ratios ranging from 0.01 to 0.12. Approximately 19% of the initial alleles were lost within the first four generations of captive breeding. Between-generation comparisons of expected heterozygosity showed significant losses ranging from 6% to 12% per generation. Seven private alleles were observed in the last sampled generation of four subpopulations, and analysis of population structure by F _ST indicated that approximately 33% of the total genetic diversity is maintained between the subpopulations from different institutions. To reduce the loss of genetic variation, we recommend that offspring production be equalized by periodically removing dominant males, which will encourage reproduction by additional males. Consideration should also be given to encouraging more institutions to maintain populations, because a significant fraction of the genetic variation exists as among-population differences resulting from random differentiation among subpopulations.     

Genetic Diversity of Lepilemur mustelinus ruficaudatus, a Nocturnal Lemur of Madagascar

The genetic polymorphism of natural populations of Lepilemur mustelinus ruficaudatus was studied by protein electrophoresis. We sampled blood from 72 individuals from four populations separated by geographic or anthropogenic barriers from southwestern Madagascar. Six out of 22 enzyme loci showed genetic variation with a degree of polymorphism of 0.273. The expected and observed degree of genetic heterozygosity over all loci is similar to that of other primates (H_e = 0.058, H_o = 0.036). The F-statistics revealed that the four subpopulations were similar with respect to gene structure (F_ST = 0.065, p = 0.016), but the genotypic structures within subpopulations were inconsistent with random mating. For the total of the four subpopulations the proportion of heterozygous individuals was significantly smaller than expected under random mating (F_IS = 0.373, F_IT = 0.414, p < 0.01). These results correspond closely to what is expected considering the low migration ability of individuals of L. m ruficaudatus leading to small and rather isolated inbred populations.     

Microsatellite Analysis of Population Structure in the Endangered Hawaiian Monk Seal

Abstract: The endangered Hawaiian monk seal breeds at six locations in the northwestern Hawaiian Islands. To determine whether significant genetic differentiation exists among these sites, we used microsatellite loci to examine the monk seal population structure at the five largest breeding colonies. Of 27 loci isolated from other seal species, only 3 were polymorphic in an initial screening of one individual from each breeding site. Only two alleles were found at each of these 3 loci in samples of 46–108 individuals. This extremely low variation is consistent with other measures of genetic variability in this species and is probably the result of a recent severe population bottleneck, combined with a long-term history of small population sizes. Although the smallest monk seal subpopulation in this study ( Kure Atoll) showed some evidence of heterozygote deficit, possibly indicative of inbreeding, the next smallest ( Pearl and Hermes Reef) had an apparent excess of heterozygous individuals. Genetic differentiation was detected between the two subpopulations at extreme ends of the range ( Kure and French Frigate Shoals). This trend was significant only at the microsatellite locus for which we had the largest sample size ( Hg6.3: R _ST = 0.206, p = 0.002; allelic goodness of fit G _h = 15.412, p < 0.005). French Frigate Shoals is the source population for translocated animals that have been released primarily at Kure Atoll. Differentiation between these sites consisted of allele frequency differences (with the same allele predominant in each location at all three loci), rather than the preservation of alternative alleles. Although the translocations have had positive demographic effects, we recommend continued genetic monitoring of both the source and recipient populations because translocated individuals are now entering the breeding population.     

山西高原油松种群遗传多样性

用酸性聚丙烯酰胺凝胶电泳（A-PAGE）技术，分析了山西高原9个油松种群在醇溶蛋白水平上的遗传多样性。135份材料共分离出23条带，其中3条为共有带，多态性高达86．95％。全部材料共出现53种带型，9个不同油松种群的带型有差异，同一种群不同个体的带型也有所不同，说明山西高原的油松在遗传上已产生一定程度分化，在醇溶蛋白水平上呈现出遗传多态性。从供试材料的带型计算出油松遗传分化系数为0．1547。即在种群间的变异占总变异的15．47％，种群内变异为84．53％，大部分的遗传变异存在于种群内，但种群间的分化程度在松属树种中也属于较高水平。根据23个多态位点计算遗传相似系数和遗传距离，进行聚类分析，将山西高原9个油松种群聚为3个类群。     

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     

Unprecedented Low Levels of Genetic Variation and Inbreeding Depression in an Island Population of the Black-Footed Rock-Wallaby

Abstract: It has been argued that demographic and environmental factors will cause small, isolated populations to become extinct before genetic factors have a significant negative impact. Islands provide an ideal opportunity to test this hypothesis because they often support small, isolated populations that are highly vulnerable to extinction. To assess the potential negative impact of isolation and small population size, we compared levels of genetic variation and fitness in island and mainland populations of the black-footed rock-wallaby ( Petrogale lateralis [Marsupialia: Macropodidae]). Our results indicate that the Barrow Island population of P. lateralis has unprecedented low levels of genetic variation (  H _e = 0.053, from 10 microsatellite loci) and suffers from inbreeding depression (reduced female fecundity, skewed sex ratio, increased levels of fluctuating asymmetry). Despite a long period of isolation ( ∼ 1600 generations) and small effective population size (  N _e ∼ 15), demographic and environmental factors have not yet driven this population to extinction. Nevertheless, it has been affected significantly by genetic factors. It has lost most of its genetic variation and become highly inbred (  F _e = 0.91), and it exhibits reduced fitness. Because several other island populations of P. lateralis also exhibit exceptionally low levels of genetic variation, this phenomenon may be widespread. Inbreeding in these populations is at a level associated with high rates of extinction in populations of domestic and laboratory species. Genetic factors cannot then be excluded as contributing to the extinction proneness of small, isolated populations.     

Effects of Habitat Fragmentation on Effective Population Size in the Endangered Rio Grande Silvery Minnow

Abstract:  We assessed spatial and temporal patterns of genetic diversity to evaluate effects of river fragmentation on remnant populations of the federally endangered Rio Grande silvery minnow ( Hybognathus amarus ). Analysis of microsatellite and mitochondrial DNA detected little spatial genetic structure over the current geographic range, consistent with high gene flow despite fragmentation by dams. Maximum-likelihood analysis of temporal genetic data indicated, however, that present-day effective population size ( N_eV ) of the largest extant population of this species was 78 and the ratio of effective size to adult numbers ( N_eV/N ) was ∼ 0.001 during the study period (1999 to 2001). Coalescent-based analytical methods provided an estimate of historical (river fragmentation was completed in 1975) effective size ( N_eI  ) that ranged between 10⁵ and 10⁶. We propose that disparity between contemporary and historical estimates of N_e and low contemporary N_e/N result from recent changes in demography related to river fragmentation. Rio Grande silvery minnows produce pelagic eggs and larvae subject to downstream transport through diversion dams. This life-history feature results in heavy losses of yearly reproductive effort to emigration and mortality, and extremely large variance in reproductive success among individuals and spawning localities. Interaction of pelagic early life history and river fragmentation has altered demographic and genetic dynamics of remnant populations and reduced N_e to critically low values over ecological time.     

Genetic consequences of long larval life in the starfish Linckia laevigata (Echinodermata: Asteroidea) on the Great Barrier Reef

Gene flow between populations of the asteroid Linckia laevigata (Linnaeus) was investigated by examining over 1000 individuals collected from ten reefs throughout the Great Barrier Reef (GBR), Australia, for genetic variation at seven polymorphic enzyme loci. Despite geographic separations in excess of 1000 km, Nei's unbiased genetic distance (0 to 0.003) and standardised genetic variation between populations (F _ST) values (mean 0.0011) were small and not significant. Genetic homogeneity among L. laevigata populations is consistent with the long-distance dispersal capability of its 28 d planktonic larval phase, and is greater than that observed for other asteroid species, including another high-dispersal species, Acanthaster planci, which has a 14 d larval phase. Variation within populations was also higher than previously recorded for asteroids (mean heterozygosity=0.384; number of alleles per locus ranged from 5.1 to 6.0 in each population). Among asteroids, dispersal ability is positively correlated with gene flow and levels of variation, and negatively correlated with levels of differentiation.     

Genetic variability and differentiation of sporophytes and gametophytes in populations of Gelidium arbuscula (Gelidiaceae: Rhodophyta) determined by isozyme electrophoresis

Genetic differentiation and genetic variability of sporophytic and gametophytic populations of Gelidium arbuscula (Bory) from three localities sampled in 1989 and 1990 in the Canary Islands (Spain) were examined by isozyme electrophoresis. Twenty-three to 29 putative alleles corresponding to 22 gene loci, were compared. High deviations in Hardy-Weinberg equilibrium, and significant differences between allelic frequencies of sporophytic and gametophytic subpopulations at the same locality were found, suggesting a predominant asexual reproduction of G. arbuscula. The genetic variability (percentage of polymorphic loci, mean number of alleles per locus and average gene diversity) of haploid subpopulations was lower than that of diploid subpopulations at all three localities, being the lowest described for seaweeds. No correlation between genetic and geographical distance was found. The high genetic differentiation coefficient between all subpopulations suggests a very reduced genetic flow between subpopulations of the same and of different localities. These results suggest that the genetic structure of the populations of G. arbuscula from the Canary Islands is due to a founder-effect combined with a predominance of asexual reproduction. This is the first report comparing allelic frequencies between sporophytic and gametophytic subpopulations of seaweeds.     

Genetic Estimates of Variability and Relatedness for Conservation of an Ozark Cave Crayfish Species Complex

Allozyme-based genetic distances were used to determine the distinctness of six species of cave crayfish from the Ozark Plateau in Missouri, Arkansas, and Oklahoma. One of the cave species is in the subgenus Erebicambarus and the others are in Jugicambarus . Four of the six species are very rare and are found in only one to three known sites each. In addition, most populations of all the species are presumed to be small; rarely are more than a few individuals observed. A chela (claw) was collected from sixty individuals representing the six species, including all known populations of the four rare species. Variability and distance estimates were based on 20 presumptive gene loci. Population samples with identical genotypes were pooled. Thirteen loci were polymorphic, but average heterozygosity was low (H= 1%) compared to epigean crayfish species. Pairwise genetic distances within Jugicambarus ranged from D = 0.051 to 0.522, and mean distance between subgenera was D = 0.676. The underground water systems in Ozark caves are defined by discreet recharge zones. Groundwater pollution threatens the stability of cave ecosystems, including the survival of cave crayfish. If restoration of threatened or extirpated populations becomes necessary, a database of genetic variability and relatedness estimates for known populations of all the species will aid decisions about numbers and sources of individuals for propagation or transfer.     

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.     

Using environmental and geographic data to optimize ex situ collections and preserve evolutionary potential

Maintenance of biodiversity through seed banks and botanical gardens, where the wealth of species’ genetic variation may be preserved ex situ, is a major goal of conservation. However, challenges can persist in optimizing ex situ collections if trade-offs exist among cost, effort, and conserving species evolutionary potential, particularly when genetic data are not available. We evaluated the genetic consequences of population preservation informed by geographic (isolation by distance [IBD]) and environmental (isolation by environment [IBE]) distance for ex situ collections for which population provenance is available. We used 19 genetic and genomic data sets from 15 plant species to assess the proportion of population genetic differentiation explained by geographic and environmental factors and to simulate ex situ collections prioritizing source populations based on pairwise geographic distance, environmental distance, or both. Specifically, we tested the impact prioritizing sampling based on these distances may have on the capture of neutral, functional, or putatively adaptive genetic diversity and differentiation. Individually, IBD and IBE explained limited population genetic differences across all 3 genetic marker classes (IBD, 10–16%; IBE, 1–5.5%). Together, they explained a substantial proportion of population genetic differences for functional (45%) and adaptive (71%) variation. Simulated ex situ collections revealed that inclusion of IBD, IBE, or both increased allelic diversity and genetic differentiation captured among populations, particularly for loci that may be important for adaptation. Thus, prioritizing population collections based on environmental and geographic distance data can optimize genetic variation captured ex situ. For the vast majority of plant species for which there is no genetic information, these data are invaluable to conservation because they can guide preservation of genetic variation needed to maintain evolutionary potential within collections.     

Genetic differentiation of populations of the planktonic copepod Labidocera aestiva

Planktonic populations of the calanoid copepod Labidocera aestiva show significant biochemical genetic heterogeneity along the Atlantic coast of the USA. In summer, 1981, copepods were collected by surface tows at Beaufort Inlet, North Carolina; Fort Pierce Inlet, Florida; and Vineyard Sound, Massachusetts. Genetic variation within each population and genetic differentiation among the three populations were studied by micro-acrylamide gel electrophoresis of six loci encoding four enzymes. All six enzyme loci were polymorphic when all populations were considered together, but the North Carolina population was monomorphic at two loci. High genetic variability was indicated by the following: (1) the number of alleles per locus averaged over all loci was 2.57±0.26 SD; (2) the average proportion of loci for which the frequency of the most common allele was not greater than 0.95 was 0.78±0.10; (3) the frequency of heterozygous individuals was 0.25±0.07. Genetic differentiation among population samples in the three regions was demonstrated in several ways: allele frequencies at one aminopeptidase-I locus, Lap-1, differed significantly among samples of the three populations, and there were unique alleles of high frequency at this locus in two population samples. Values of the statistic of genetic distance (D) averaged 0.20±0.08 for pairwise comparisons between all samples. Compared to expected heterozygosity if individuals across the entire range sampled mated at random, there were highly significant heterozygote deficiencies at five of the six loci. Genetic differentiation of populations of L. aestiva may result from (1) differential selection on populations in the three regions, or (2) restricted gene flow between the populations. Gene flow may be limited by geographic separation or differences in life history, such as seasonal presence in the plankton and diapause egg production.Contribution No. 5810 of Woods Hole Oceanographic Institution