AGE AND POLYPHYLETIC ORIGINS OF HYBRID AND SPONTANEOUS PARTHENOGENETIC CAMPELOMA (GASTROPODA: VIVIPARIDAE) FROM THE SOUTHEASTERN UNITED STATES

We used mtDNA sequence variation to assess the origin, age, and spatial patterns of sequence divergence of triploid hybrid and diploid spontaneous parthenogens from southeastern United States populations of the freshwater snail Campeloma. There was strong support for multiple origins of both parthenogens using likelihood-ratio tests, and we argue that parthenogens are recently derived from sexuals. Atlantic coastal populations of C. limum or C. floridense were the maternal ancestors of Gulf Coast triploid hybrids in the Florida Panhandle. Sequence divergence within monophyletic groups of both parthenogens is similar to within-population divergence found in sympatric sexuals, and monophyletic clades of hybrid and spontaneous parthenogens are geographically widespread throughout the Atlantic and Gulf Coast rivers. These patterns are consistent with higher dispersal rates and recent range expansion of parthenogens, which should reduce the effects of mutation accumulation or parasitism. Range expansion may have occurred through interdrainage transfer during Pleistocene glacial periods.     

CONTRIBUTIONS OF SEXUAL AND ASEXUAL REPRODUCTION TO POPULATION STRUCTURE IN THE CLONAL SOFT CORAL,ALCYONIUM RUDYI

Numerous studies of population structure in sessile clonal marine invertebrates have demonstrated low genotypic diversity and nonequilibrium genotype frequencies within local populations that are monopolized by relatively few, highly replicated genets. All of the species studied to date produce planktonic sexual propagules capable of dispersing long distances; despite local genotypic disequilibria, populations are often panmictic over large geographic areas. The population structure paradigm these species represent may not be typical of the majority of clonal invertebrate groups, however, which are believed to produce highly philopatric sexual propagules. I used allozyme variation to examine the population structure of the temperate soft coral, Alcyonium rudyi, a typical clonal species whose sexually produced larvae and asexually produced ramets both have very low dispersal capabilities. Like other clonal plants and invertebrates, the local population dynamics of A. rudyi are dominated by asexual reproduction, and recruitment of new sexually produced genets occurs infrequently. As expected from its philopatric larval stage, estimates of genetic differentiation among populations of A. rudyi were highly significant at all spatial scales examined (mean θ = 0.300 among 20 populations spanning a 1100-km range), suggesting that genetic exchange seldom occurs among populations separated by as little as a few hundred meters. Mapping of multilocus allozyme genotypes within a dense aggregation of A. rudyi ramets confirmed that dispersal of asexual propagules is also very limited: members of the same genet usually remain within < 50 cm of one another on the same rock surface. Unlike most previously studied clonal invertebrates, populations of A. rudyi do not appear to be dominated by a few widespread genets: estimates of genotypic diversity (G_o) within 20 geographically distinct populations did not differ from expectations for outcrossing, sexual populations. Despite theoretical suggestions that philopatric dispersal combined with typically small effective population sizes should promote inbreeding in clonal species, inbreeding does not appear to contribute significantly to the population structure of A. rudyi. Genet genotype frequencies conformed to Hardy-Weinberg expectations in all populations, and inbreeding coefficients (f) were close to zero. In general, the population structure of A. rudyi did not differ significantly from that observed among outcrossing sexual species with philopatric larval dispersal. Age estimates suggest, however, that genets of A. rudyi live for many decades. Genet longevity may promote high genotypic diversity within A. rudyi populations and may be the most important evolutionary consequence of clonal reproduction in this species and the many others that share its dispersal characteristics.     

Population structure, parasitism, and survivorship of sexual and autodiploid parthenogenetic Campeloma limum

Two theories for the maintenance of sexual reproduction, the Red Queen hypothesis and mutation accumulation, suggest that the dispersal rates of sexuals and asexuals may determine the elimination or persistence of asexuals. Under higher dispersal rates of asexuals, asexuals may temporarily escape virulent parasites and reduce the effects of deleterious mutations. In the present study, I examine the population structure, parasite loads, and juvenile survivorship of Campeloma limum sexuals and autodiploid parthenogens from the southeastern U.S. Atlantic coastal plain. Using mtDNA sequence variation, it is shown that parthenogenetic haplotypes with limited sequence divergence are geographically widespread throughout this region and there is no significant population differentiation over a broad geographical scale. Sexual C. limum populations show significant mtDNA differentiation among and within river drainages and there is significant isolation by distance. These patterns are consistent with a recent origin and range expansion of parthenogens. Prevalence of infection by digenetic trematodes is significantly higher in autodiploid parthenogens, and the variance of prevalence is also higher in autodiploid parthenogens. I argue that the latter pattern indicates that unparasitized parthenogens have temporarily escaped these virulent parasites, but recolonization of these populations by trematodes results in high infection levels (> 40%), possibly due to reduced variation in resistance genes. I also examined whether the survivorship of juvenile sexuals and parthenogens varied under different stress levels. Sexual juveniles had twofold higher survivorship in all environments. Compared to polyploid parthenogens, autodiploid parthenogens may be less buffered against the effects of deleterious recessive alleles. I propose that the combined effects of higher parasitism and reduced juvenile survivorship of these autodiploid parthenogens accounts for the spatial distribution of sexual and parthenogenetic C. limum in the Atlantic coastal plain. Parthenogens may persist by higher dispersal rates into marginal habitats where there is a temporary escape from digenetic trematodes and competition with sexuals.     

Parthenogenetic populations of the freshwater snail Campeloma limum occupy habitats with fewer environmental stressors than their sexual counterparts

1. Sexual organisms should have half the reproductive rate of their parthenogenetic counterparts (i.e. twofold cost of sex), so the plethora of sexual species relative to parthenogenetic species remains an evolutionary paradox. The rarity of parthenogenesis may in part be due to the accumulation of deleterious mutations. Indeed, parthenogenetic populations of the freshwater snail Campeloma limum have a greater mutation load relative to sexual populations of C. limum, although this does not directly affect their reproductive fitness. We hypothesise that although parthenogenesis has no direct effect on fitness in C. limum, mutation accumulation and environmental stress act synergistically to limit the distribution of parthenogenetic populations. 2. We evaluated this hypothesis, predicting that parthenogenetic populations of C. limum would inhabit sites with fewer environmental stressors than their sexual counterparts. We collected water quality, population density and individual size data at multiple time points from eight parthenogenetic and five sexual populations in the south‐eastern United States (Georgia and South Carolina). 3. Consistent with our hypothesis, sexual populations of C. limum inhabited poorer‐quality areas (sites with significantly lower dissolved oxygen and significantly more faecal coliform bacteria) than parthenogenetic populations. Despite these stressors, sexual populations still exhibited significantly higher population density than parthenogenetic populations. 4. Our findings support the hypothesis that mutation‐laden parthenogenetic C. limum populations occupy habitats with fewer environmental stressors relative to their sexual counterparts. Moreover, sexual C. limum populations inhabit lower‐quality habitats where they can presumably evade the twofold cost of sex in the absence of competition from their parthenogenetic counterparts.     

Host‐associated differentiation and evidence for sexual reproduction in Iranian populations of the cotton aphid,Aphis gossypii

Phytophagous insects with wide host ranges often exhibit host‐associated genetic structure. We used microsatellite analysis to assess the population structure of the cotton aphid, Aphis gossypii Glover (Hemiptera: Aphididae), a serious pest on many economically important crops worldwide. We sampled aphids from five host plant species in Iran and detected strong population subdivision, with an overall multilocus F_ST of 0.191. The matrix of pairwise F_ST values indicated that differentiation between populations collected from different hosts was significantly stronger than between populations from the same hosts. Host‐associated differentiation was further supported by Bayesian clustering analyses, which grouped all samples from cotton together with aubergine, and all samples from cucumber together with pumpkin and hibiscus. This adds to the growing body of evidence that many seemingly generalist aphids are in fact an assemblage of host‐specialized lineages. Although we detected a clear genetic signature of clonal reproduction, the genotypic diversity of A. gossypii in Iran is much higher than in other parts of the world. Particularly samples from cotton exhibited a surprisingly high genotypic diversity, suggesting that many lineages on this host are cyclical parthenogens that engage in regular bouts of sexual reproduction.     

Population structure and dispersal of the coral‐excavating sponge Cliona delitrix

Some excavating sponges of the genus Cliona compete with live reef corals, often killing and bioeroding entire colonies. Important aspects affecting distribution of these species, such as dispersal capability and population structure, remain largely unknown. Thus, the aim of this study was to determine levels of genetic connectivity and dispersal of Cliona delitrix across the Greater Caribbean (Caribbean Sea, Bahamas and Florida), to understand current patterns and possible future trends in their distribution and effects on coral reefs. Using ten species‐specific microsatellite markers, we found high levels of genetic differentiation between six genetically distinct populations: one in the Atlantic (Florida‐Bahamas), one specific to Florida and four in the South Caribbean Sea. In Florida, two independent breeding populations are likely separated by depth. Gene flow and ecological dispersal occur among other populations in the Florida reef tract, and between some Florida locations and the Bahamas. Similarly, gene flow occurs between populations in the South Caribbean Sea, but appears restricted between the Caribbean Sea and the Atlantic (Florida‐Bahamas). Dispersal of C. delitrix was farther than expected for a marine sponge and favoured in areas where currents are strong enough to transport sponge eggs or larvae over longer distances. Our results support the influence of ocean current patterns on genetic connectivity, and constitute a baseline to monitor future C. delitrix trends under climate change.     

Mode of origin differentially influences the fitness of parthenogenetic freshwater snails

How parthenogenetic lineages arise from sexual ancestors may strongly influence their persistence over evolutionary time. Hybrid parthenogens often have elevated heterozygosity and ploidy, thus making it difficult to disentangle the influence of reproductive mode, hybridity and ploidy on their relative fitness. By comparing the relative fitness of both hybrid and non-hybrid parthenogens to their sexual ancestors, further insight may be gained into how these three factors influence the maintenance of sexual and parthenogenetic reproduction. In the present study, hybrid and non-hybrid parthenogenetic and sexual snails (Campeloma sp.) were compared for the following characteristics: female size-fecundity curves, offspring size, survivorship, and growth. Compared to nearby sexual populations, triploid hybrid parthenogens from the Florida Gulf coast have similar fecundity and offspring size, five-times higher survivorship, and 60% higher growth. Relative to nearby sexual populations, non-hybrid parthenogenetic C. limum from the Atlantic coast have significantly higher fecundity, smaller offspring size, similar survivorship and slightly lower growth. Given the considerable fitness advantages of parthenogens, especially hybrid parthenogens, it is enigmatic as to why these parthenogens occupy marginal natural habitats.     

High genotypic diversity and strong spatial structure in populations of Trifolium alpestre with low seed production

Studies on clonal plants indicate that the proportion between clonal and sexual reproduction can be variable, depending on local habitat conditions and the biological characteristics of the species. In the present study, we assessed this question in Trifolium alpestre by assaying genetic diversity and spatial genotypic structure of natural populations with the use of allozyme markers. Populations revealed high genetic diversity as well as strong spatial structure of multilocus genotypes. The values of genetic diversity were moderately high. Spatially aggregated, identical genotypes spread up to 15 m along linear transects and across 4‐m² plots indicate extensive clonal propagation within populations. However, the existence of numerous unique and small‐sized clones reflects significant contribution from sexual reproduction. Spatially and temporarily stochastic soil disturbances have evidently opened new opportunities for the successful sexual recruitment from the permanent soil seed bank and thus counteracted losses of genetic and genotypic diversity. Seed production in all populations during the three study years was low, in average up to 1.5–2.4 seeds per shoot. The almost total lack of seed set for 57 bagged flower heads on genotypes grown in a common garden indicates that T. alpestre needs pollinators for seed production.     

The Organization of Genetic Diversity in the Parthenogenetic Lizard CNEMIDOPHORUS TESSELATUS

The parthenogenetic lizard species Cnemidophorus tesselatus is composed of diploid populations formed by hybridization of the bisexual species C. tigris and C. septemvittatus, and of triploid populations derived from a cross between diploid tesselatus and a third bisexual species, C. sexlineatus. An analysis of allozymic variation in proteins encoded by 21 loci revealed that, primarily because of hybrid origin, individual heterozygosity in tesselatus is much higher (0.560 in diploids and 0.714 in triploids) than in the parental bisexual species (mean, 0.059). All triploid individuals apparently represent a single clone, but 12 diploid clones were identified on the basis of genotypic diversity occurring at six loci. From one to four clones were recorded in each population sampled. Three possible sources of clonal diversity in the diploid parthenogens were identified: mutation at three loci has produced three clones, each confined to a single locality; genotypic diversity at two loci apparently caused by multiple hybridization of the bisexual species accounts for four clones; and the remaining five clones apparently have arisen through recombination at three loci. The relatively limited clonal diversity of tesselatus suggests a recent origin. The evolutionary potential of tesselatus and of parthenogenetic forms in general may be less severely limited than has generally been supposed.     

THE ORIGIN AND EVOLUTION OF PARTHENOGENESIS IN HETERONOTIA BINOEI (GEKKONIDAE): EXTENSIVE GENOTYPIC DIVERSITY AMONG PARTHENOGENS

Variation at 18 allozyme loci was assayed among representatives of the geographically widespread, triploid parthenogenetic form of Heteronotia binoei. A minimum of 52 different genotypes were observed among 143 individuals. Virtually all localities sampled had multiple genotypes among the unisexuals. This represents unusually high genotypic diversity for a unisexual vertebrate. Heterozygosity in the triploids was higher than in diploid bisexual populations of H. binoei. Comparison with the alleles present in the diploid bisexuals confirms that the parthenogens are hybrids and indicates that most of the genotypic diversity stems from repetitive hybrid origins. However, the presence of some alleles unique to the parthenogens suggests that mutation adds to their genetic diversity. The genetic structure of this geographically widespread parthenogen suggests the hypothesis that the persistence and spread of the unisexual lineages is facilitated by genotypic diversity.     

Consequences of prolonged clonal growth on local and regional genetic structure and fruiting success of the forest perennial Maianthemum bifolium

The balance between clonal propagation and sexual reproduction varies among species. Although theory predicts an impact of clonal growth on both‐ within‐ and between population genetic structure, most empirical evidence available to date does not reveal sharp differences between sexually reproducing and clonal species. This has been attributed mainly to the fact that even low levels of sexual recruitment can maintain high levels of genetic diversity. Here we study the effects of prolonged clonal growth and very low rates of sexual recruitment on the genetic structure of the perennial Maianthemum bifolium, an outcrossing understorey species of temperate forests. Average genotypic diversity (0.37) of the populations, as revealed by AFLP, was above the average values reported for species of similar characteristics, but some populations were extremely poor in genotypes. Fruiting success was positively correlated with genotypic diversity, probably as a result of shortage in mating types and compatible pollen in populations poor in genotypes. This was confirmed by a pollination experiment. Fruiting success increased by a factor three when individuals were hand‐pollinated with pollen from a nearby population compared to hand‐pollinations with pollen from the own population. Furthermore, the fruiting success after natural pollination (control individuals) was positively related to number of nearby populations which could act as pollen sources. Given the limited colonization capacity of the species (no seed flow), and the long time since fragmentation of the forest fragments studied, between‐population genetic differentiation was relatively low (Φ_st=0.14). Lack of genetic drift due to long generation times and very limited sexual recruitment is probably responsible for this. Our results show that prolonged clonal growth and lack of sexual recruitment may affect within‐ and between‐ population genetic structure and the capability for sexual reproduction.     

Clonal erosion and genetic drift in cyclical parthenogens – the interplay between neutral and selective processes

The occurrence of alternating phases of clonal and sexual reproduction may strongly impact the interplay between neutral and selective genetic variation in populations. Using a physiologically structured model of the life history of Daphnia, we investigated to what extent clonal erosion associated with selection during the clonal phase affects the genetic structure as observed by neutral markers. Incorporating conservative levels of quantitative genetic variation at 11 physiological and life history traits induces strong clonal erosion, reducing clonal diversity (CD) near the end of the simulations (1000 days) to a level between 1 and 5, even in habitats with high initial CD (10⁸ clones). This strong clonal erosion caused by selection can result in reduced genetic diversity, significant excess of heterozygotes and significant genetic differentiation between populations as observed by neutral markers. Our results indicate that, especially in relatively small habitats, clonal selection may strongly impact the genetic structure and may contribute to the often observed high level of neutral genetic differentiation among natural populations of cyclical parthenogens.     

Parental and hybrid Daphnia from the D. longispina complex: long‐term dynamics in genetic structure and significance of overwintering modes

In recent decades, hybridization has become a focus of attention because of its role in evolutionary processes. However, little is known about changes in genetic structure within and between parental species and hybrids over time. Here, we studied processes of genetic change in parental species and hybrids from the Daphnia longispina complex (Crustacea, Cladocera) over a period of six years across ten habitats. These cyclical parthenogens respond to fluctuating environments by switching from asexual to sexual reproduction. Importantly, sexually produced diapausing eggs, which resist extreme conditions such as low temperatures and serve as dispersal stages, are produced to a lower extent by hybrids. Long‐term microsatellite data revealed clear differences between hybrids and parental species. In hybrids, clonal diversity values were lower, whereas heterozygosity and linkage disequilibrium values were higher compared to parental species. Clonal diversity of hybrids responded to the strength of the winter, with cold winters resulting in few genotypes in the following spring. In time windows when only asexual hybrid females survive, priority effects will favour the establishment of the hybrid offspring before hatchlings from parental diapause eggs can enter the community. The constant high levels of heterozygosity maintained by clonal reproduction in hybrids might lead to their successful establishment over time, when they are able to escape competition from both parental species. Although we found evidence that hybrids diversity depends on fluctuating environments, a direct link between hybrid abundance and the strength of winter was missing. Because of reduced adaptability in clonally reproducing hybrids, multiple factors must contribute to promoting their long‐term success in fluctuating environments.     

The recent spread of <Emphasis Type="Italic">Artemia parthenogenetica</Emphasis> in Western Australia

In Western Australia, populations of Artemia parthenogenetica in coastal salt lakes at Rottnest Island and Lake Hayward, and in salterns at Port Hedland and Shark Bay, are widely accepted to have been introduced by humans. Further, within the past 10 years, populations of A. parthenogenetica have been found in inland playa salt lakes in the wheatbelt of south-west Western Australia, where none had been recorded in previous salt lake studies. Here we hypothesise that birds act as transport vectors for Artemia cysts both within Australia and between the Asian and Australian continents. Allozyme analysis was used to identify clonal types (multi-locus genotypes), clonal frequencies, genotypic diversities and genotypic identity of six populations (three coastal, three inland). Overall, the inland populations displayed almost identical genotypic structure to the coastal population from Lake Hayward, indicating that Lake Hayward could be the major source for dispersal and colonisation of inland populations. Results support the hypothesis of dispersal inland by nomadic bird species. Furthermore, evidence suggests that the inland and Lake Hayward populations may be an example of a metapopulation. The greater variety of genotypes present in the Rottnest population indicates that this population has received a large number of small immigrations, or that it received one large introduction. The former may indicate a long period of suitable salinities, providing a greater time-span over which migration and succession of clonal types could occur in comparison to other populations. While we cannot rule out the possibility of human introduction of A. parthenogenetica to Rottnest, the hypothesis of cyst dispersal along the Austral-Asian flyway remains possible. Guest Editor: John M. Melack Saline Waters and their Biota     

Reproductive strategy, clonal structure and genetic diversity in populations of the aquatic macrophyte Sparganium emersum in river systems

Many aquatic and riparian plant species are characterized by the ability to reproduce both sexually and asexually. Yet, little is known about how spatial variation in sexual and asexual reproduction affects the genotypic diversity within populations of aquatic and riparian plants. We used six polymorphic microsatellites to examine the genetic diversity within and differentiation among 17 populations (606 individuals) of Sparganium emersum, in two Dutch-German rivers. Our study revealed a striking difference between rivers in the mode of reproduction (sexual vs. asexual) within S. emersum populations. The mode of reproduction was strongly related to locally reigning hydrodynamic conditions. Sexually reproducing populations exhibited a greater number of multilocus genotypes compared to asexual populations. The regional population structure suggested higher levels of gene flow among sexually reproducing populations compared to clonal populations. Gene flow was mainly mediated via hydrochoric dispersal of generative propagules (seeds), impeding genetic differentiation among populations even over river distances up to 50 km. Although evidence for hydrochoric dispersal of vegetative propagules (clonal plant fragments) was found, this mechanism appeared to be relatively less important. Bayesian-based assignment procedures revealed a number of immigrants, originating from outside our study area, suggesting intercatchment plant dispersal, possibly the result of waterfowl-mediated seed dispersal. This study demonstrates how variation in local environmental conditions in river systems, resulting in shifting balances of sexual vs. asexual reproduction within populations, will affect the genotypic diversity within populations. This study furthermore cautions against generalizations about dispersal of riparian plant species in river systems.     

Population genetics of the fissiparous holothurian Stichopus chloronotus (Aspidochirotida) on the Great Barrier Reef, Australia

 Population genetic structure was studied in one nearshore and two offshore populations of Stichopus chloronotus, a common holothurian species on Indo-Pacific coral reefs. Genetic variation at five polymorphic loci was examined using allozyme electrophoresis. The nearshore population consisted almost exclusively of male individuals, and more males than females were found in all populations studied. Deviations of heterozygosity from that predicted under Hardy-Weinberg equilibrium indicated that asexual reproduction occurred in all populations. Estimates of the level of asexual reproduction using the ratios of the number of sexually produced individuals to sample size, observed genotypic diversity to expected genotypic diversity, and number of genotypes to sample size confirmed that this reproductive mode was more important at the nearshore reef compared to the two offshore reefs. There were large differences in genotypic frequencies between males and females. F-statistics on clonal genotypic frequencies were not statistically significant between populations for neither females or males, suggesting high dispersal of larvae between reefs. A higher mortality of females during larval or early post-settlement stages, or reduced dispersal capability of female larvae are the most likely reasons for biased sex ratios. Accepted: 23 November 1998     

The effect of herbivores on genotypic diversity in a clonal aquatic plant

In clonal plants, vegetative parts may outcompete seeds in the absence of disturbance, limiting the build‐up of genotypic diversity through repeated seedling recruitment (RSR). Herbivory may provide disturbance and trigger establishment of strong colonizers (seeds) at the expense of strong competitors (clonal propagules). In the clonal aquatic fennel pondweed Potamogeton pectinatus, two distinct herbivore guilds may modify the dynamics of propagation. In winter, Bewick's swans may deplete patches of tubers, promoting seedling establishment in spring. In summer, seed consumption by waterfowl can reduce the density of viable seeds but grazing may also reduce tuber production and hence facilitate seedling establishment. This study is among the first to experimentally test herbivore impact on plant genotypic diversity. We assess the separate and combined effects of both herbivore guilds on genotypic diversity and structure of fennel pondweed beds. Using microsatellites, we genotyped P. pectinatus from an exclosure experiment and assessed the contribution of herbivory, dispersal and sexual reproduction to the population genetic structure. Despite the predominance of clonal propagation in P. pectinatus, we found considerable genotypic diversity. Within the experimental blocks, kinship among genets decreased with geographic distance, clearly identifying a role for RSR in the maintenance of genotypic diversity within the fennel pondweed beds. However, over a period of five years, none of the herbivory treatments affected genotypic diversity. Hence, sexual reproduction on a local scale is important in this putatively clonal plant and possibly sufficient to ensure a relatively high genotypic diversity even in the absence of herbivores. Although we cannot preclude a role of herbivory in shaping genotypic diversity of a clonal plant, after five years of exclusion of the two investigated herbivore guilds no measurable effect on genotypic diversity was detected.     

Effects of clonality on the genetic variability of rare,insular species: the case of Ruta microcarpa from the Canary Islands

Many plant species combine sexual and clonal reproduction. Clonal propagation has ecological costs mainly related to inbreeding depression and pollen discounting; at the same time, species able to reproduce clonally have ecological and evolutionary advantages being able to persist when conditions are not favorable for sexual reproduction. The presence of clonality has profound consequences on the genetic structure of populations, especially when it represents the predominant reproductive strategy in a population. Theoretical studies suggest that high rate of clonal propagation should increase the effective number of alleles and heterozygosity in a population, while an opposite effect is expected on genetic differentiation among populations and on genotypic diversity. In this study, we ask how clonal propagation affects the genetic diversity of rare insular species, which are often characterized by low levels of genetic diversity, hence at risk of extinction. We used eight polymorphic microsatellite markers to study the genetic structure of the critically endangered insular endemic Ruta microcarpa. We found that clonality appears to positively affect the genetic diversity of R. microcarpa by increasing allelic diversity, polymorphism, and heterozygosity. Moreover, clonal propagation seems to be a more successful reproductive strategy in small, isolated population subjected to environmental stress. Our results suggest that clonal propagation may benefit rare species. However, the advantage of clonal growth may be only short‐lived for prolonged clonal growth could ultimately lead to monoclonal populations. Some degree of sexual reproduction may be needed in a predominantly clonal species to ensure long‐term viability.     

Restricted dispersal in a continuously distributed marine species: common bottlenose dolphins Tursiops truncatus in coastal waters of the western North Atlantic

The marine environment provides an opportunity to examine population structure in species with high dispersal capabilities and often no obvious barriers to genetic exchange. In coastal waters of the western North Atlantic, common bottlenose dolphins, Tursiops truncatus, are a highly mobile species with a continuous distribution from New York to Florida. We examine if the highly mobile nature coupled with no obvious geographic barriers to movement in this region result in a large panmictic population. Mitochondrial control region sequences and 18 microsatellite loci indicate dolphins are partitioning the habitat both latitudinally and longitudinally. A minimum of five genetically differentiated populations were identified among 404 samples collected in the range of New Jersey to northern Florida using both genetic marker types, some inhabiting nearshore coastal waters and others utilizing inshore estuarine waters. The genetic results reject the hypothesis of a single stock of coastal bottlenose dolphins put forth after the 1987–1988 epizootic that caused a large‐scale die‐off of dolphins and suggest instead the disease vector was transferred from one population to the next as a result of seasonal migratory movements of some populations. These coastal Atlantic populations also differ significantly from bottlenose dolphin samples collected in coastal waters of the northern Gulf of Mexico, implying a long‐term barrier to movement between the two basins.     

CLONAL-DIVERSITY PATTERNS AND BREEDING-SYSTEM VARIATION IN DAPHNIA PULEX,AN ASEXUAL-SEXUAL COMPLEX

Some individuals of the cladoceran crustacean, Daphnia pulex, reproduce by cyclic parthenogenesis, while others are obligate parthenogens. Cyclic parthenogenesis is the primitive breeding system; the transition to obligate parthenogenesis has been linked to sex-limited meiosis-suppression. Detailed study of patterns of breeding-system distribution and clonal diversity is justified because D. pulex is the first species in which the loss of sex has been related to this mechanism. The present study investigated the genotypic characteristics of 10 D. pulex populations from each of 22 sites in the Great Lakes watershed. This analysis revealed that populations reproducing by cyclic parthenogenesis were uncommon and restricted to southern sites. Most populations reproduced by obligate parthenogenesis, with the electrophoretic survey revealing an average of three clones per pond and 145 unique clones over the watershed. A combinatorial analysis was used to examine the relationships between clone discovery in the asexual populations and both sample size and genetic-sampling intensity. This analysis showed that the few clones found in individual ponds were readily discriminated, while diversity on a regional scale was underestimated. These methods provide a quantitative basis for assessing the level of clonal diversity in asexual populations and in asexually transmitted segments of the genome.