Larval settlement preference maximizes genetic mixing in an inbreeding population of a simultaneous hermaphrodite (Bugula stolonifera, Bryozoa)

Conspecific aggregations in terrestrial and aquatic organisms can have a significant effect on an individual's survival, growth and reproductive fitness, particularly if these aggregations are composed of closely related individuals. Such aggregations can form passively, as a consequence of dispersal, or actively, as a consequence of kin recognition. In this study, we investigated the genetic composition of individuals in conspecific aggregations in the simultaneous hermaphroditic marine bryozoan Bugula stolonifera. Conspecific larvae routinely metamorphose on adult colonies; the possibility that larvae select or avoid their maternal colony was investigated utilizing 10 newly developed polymorphic microsatellite loci. Adult colonies were collected from Eel Pond, Woods Hole, Massachusetts and inspected for the presence of attached individuals. Adult colonies and their attached individuals were genotyped and compared to assess genetic relatedness within and among these groups relative to the overall genetic variability of the sampling site. Overall, the population of B. stolonifera at this site was found to be outside Hardy-Weinberg equilibrium because of significant levels of inbreeding. No significant genetic differentiation, however, was found between any groups, documenting that a group containing an adult colony and its attached individuals had as much genetic variability as was found for the entire sampling site. Parentage-exclusion analyses showed that the vast majority of attached individuals (>93%) could not have derived from the colony on which they were attached. Kinship analyses showed that the majority of attached individuals (≈63%) shared less than a half-sibling relationship. These results suggest that a colony's nearest neighbours are not composed of siblings, and thus, larval settlement preference can maximize outcrossing in this inbreeding population.     

Genetic diversity and genotypic differentiation between the sexes in swarm aggregations decrease inbreeding in the Formosan subterranean termite

The life of a colony of subterranean termites, such as Coptotermes formosanus Shiraki (Isoptera: Rhinotermitidae), has natural inbreeding and outbreeding cycles. Reproductives of mature colonies can be replaced by their offspring, which increases the degree of inbreeding in each generation. High degrees of inbreeding may lead to inbreeding depression. In this study we focused on mechanisms for inbreeding avoidance during swarming that do not require kin recognition. We investigated genetic differentiation between swarm aggregations (isolation by distance), genetic diversity within swarm aggregations (multiple colony origin) and genetic differentiation between sexes. Alates were collected from five swarm aggregations in New Orleans, La. The genetic make-up of each swarm aggregation was then described by microsatellite genotyping. Alates from the different swarm aggregations were genetically differentiated; however, no isolation by distance up to at least 1000 m was detected. The dispersal distance of alates was sufficient to guarantee mixing of an average of 13 colonies within swarm aggregations. On average, eleven percent of all possible pairs of alates in each swarm aggregation were putative full siblings. Genotypic frequencies differed significantly between males and females. This could not be explained by sex-biased dispersal. We hypothesize sex-biased investment at the colony level to account for this difference. Genetic differentiation between the sexes and dispersal distances sufficient to promote high genetic diversity within swarm aggregations each facilitate inbreeding avoidance. These observations are consistent with the results of previous studies demonstrating that the majority of simple family colonies in Louisiana populations are headed by unrelated and outbred pairs of reproductives. Received 11 August 2005; revised 5 December 2005; accepted 20 December 2005.     

Gene flow on the ice: genetic differentiation among Adélie penguin colonies around Antarctica

Each summer Adélie penguins breed in large disjunct colonies on ice-free areas around the Antarctic continent. Comprising > 10 million birds, this species represents a dominant feature of the Antarctic ecosystem. The patchy distribution within a large geographical range, natal philopatry and a probable history of refugia, suggest that this species is likely to exhibit significant genetic differentiation within and among colonies. We present data from seven microsatellite DNA loci for 442 individuals from 13 locations around the Antarctic continent. With the exception of one locus, there was no significant genic or genotypic heterogeneity across populations. Pairwise FST values were low with no value > 0.02. When all colonies were compared in a single analysis, the overall FST value was 0.0007. Moreover, assignment tests were relatively ineffective at correctly placing individuals into their respective collection sites. These data reveal a lack of genetic differentiation between Adélie penguin colonies around the Antarctic continent, despite substantial levels of genetic variation. We consider this homogeneity in terms of the dispersal of individuals among colonies and the size of breeding groups and discuss our results in terms of the glacial history of Antarctica.     

Genetic structure of an Apis dorsata population: the significance of migration and colony aggregation

Eight microsatellite loci were used to investigate the genetic structure of the giant honeybee (Apis dorsata) population in northeast India. This species migrates seasonally between summer and winter nesting sites, and queens appear to return to their previously occupied site. Furthermore, there is a strong tendency for colonies of this species to aggregate at perennially utilized nesting sites that may be shared by more than 150 colonies. These behavioral features suggest that colonies within aggregations should be more related than random colonies, but that the long-distance migration could act to minimize genetic differentiation both between geographical areas and within aggregations. Our genetic study supports these conjectures arising from natural history. A. dorsata aggregations are comprised of colonies that share more alleles than expected by chance. Although queens heading neighboring colonies are not close relatives, fixation indices show significant genetic differentiation among aggregation sites. However, there appears to be sufficient gene flow among aggregations to prevent high degrees of relatedness developing between colonies within aggregations. The results also suggest that there is significant population structuring between geographical regions, although the level of structuring caused by aggregation exceeds the differentiation attributable to geographic region.     

Population genetic evidence for sex‐specific dispersal in an inbred social spider

Dispersal in most group‐living species ensures gene flow among groups, but in cooperative social spiders, juvenile dispersal is suppressed and colonies are highly inbred. It has been suggested that such inbred sociality is advantageous in the short term, but likely to lead to extinction or reduced speciation rates in the long run. In this situation, very low levels of dispersal and gene flow among colonies may have unusually important impacts on fitness and persistence of social spiders. We investigated sex‐specific differences in dispersal and gene flow among colonies, as reflected in the genetic structure within colonies and populations of the African social spider Stegodyphus dumicola Pocock, 1898 (Eresidae). We used DNA fingerprinting and mtDNA sequence data along with spatial mapping of colonies to compare male and female patterns of relatedness within and among colonies at three study sites. Samples were collected during and shortly after the mating season to detect sex‐specific dispersal. Distribution of mtDNA haplotypes was consistent with proliferation of social nests by budding and medium‐ to long‐distance dispersal by ballooning females. Analysis of molecular variance and spatial autocorrelation analyses of AFLPs showed high levels of genetic similarity within colonies, and STRUCTURE analyses revealed that the number of source populations contributing to colonies ranged from one to three. We also showed significant evidence of male dispersal among colonies at one site. These results support the hypothesis that in social spiders, genetic cohesion among populations is maintained by long‐distance dispersal of female colony founders. Genetic diversity within colonies is maintained by colony initiation by multiple dispersing females, and adult male dispersal over short distances. Male dispersal may be particularly important in maintaining gene flow among colonies in local populations.     

Fine-scale spatial genetic structure and dispersal among spotted salamander (Ambystoma maculatum) breeding populations

We examined fine-scale genetic variation among breeding aggregations of the spotted salamander (Ambystoma maculatum) to quantify dispersal, interpopulation connectivity and population genetic structure. Spotted salamanders rely on temporary ponds or wetlands for aggregate breeding. Adequate breeding sites are relatively isolated from one another and field studies suggest considerable adult site fidelity; therefore, we expected to find population structure and differentiation at small spatial scales. We used microsatellites to estimate population structure and dispersal among 29 breeding aggregations in Tompkins County, New York, USA, an area encompassing 1272 km(2). Bayesian and frequency-based analyses revealed fine-scale genetic structure with two genetically defined demes: the North deme included seven breeding ponds, and the South deme included 13 ponds. Nine ponds showed evidence of admixture between these two genetic pools. Bayesian assignment tests for detection of interpopulation dispersal indicate that immigration among ponds is common within demes, and that certain populations serve as sources of immigrants to neighbouring ponds. Likewise, spatial genetic correlation analyses showed that populations < or = 4.8 km distant from each other show significant genetic correlation that is not evident at higher scales. Within-population levels of relatedness are consistently larger than expected if mating were completely random across ponds, and in the case of a few ponds, within-population processes such as inbreeding or reproductive skew contribute significantly to differentiation from neighbouring ponds. Our data underscore the importance of these within-population processes as a source of genetic diversity across the landscape, despite considerable population connectivity. Our data further suggest that spotted salamander breeding groups behave as metapopulations, with population clusters as functional units, but sufficient migration among demes to allow for potential rescue and recolonization. Amphibian habitats are becoming increasingly fragmented and a clear understanding of dispersal and patterns of population connectivity for taxa with different ecologies and life histories is crucial for their conservation.     

Clonal diversity impacts coral cover in Acropora cervicornis thickets: Potential relationships between density,growth, and polymorphisms

As coral reefs decline, cryptic sources of resistance and resilience to stress may be increasingly important for the persistence of these communities. Among these sources, inter‐ and intraspecific diversity remain understudied on coral reefs but extensively impact a variety of traits in other ecosystems. We use a combination of field and sequencing data at two sites in Florida and two in the Dominican Republic to examine clonal diversity and genetic differentiation of high‐ and low‐density aggregations of the threatened coral Acropora cervicornis in the Caribbean. We find that high‐density aggregations called thickets are composed of up to 30 genotypes at a single site, but 47% of genotypes are also found as isolated, discrete colonies outside these aggregations. Genet–ramet ratios are comparable for thickets (0.636) and isolated colonies after rarefaction (0.569), suggesting the composition of each aggregation is not substantially different and highlighting interactions between colonies as a potential influence on structure. There are no differences in growth rate, but a significant positive correlation between genotypic diversity and coral cover, which may be due to the influence of interactions between colonies on survivorship or fragment retention during asexual reproduction. Many polymorphisms distinguish isolated colonies from thickets despite the shared genotypes found here, including putative nonsynonymous mutations that change amino acid sequence in 25 loci. These results highlight intraspecific diversity as a density‐dependent factor that may impact traits important for the structure and function of coral reefs.     

Widespread gene flow and high genetic variability in populations of water voles Arvicola terrestris in patchy habitats

Theory predicts that the impact of gene flow on the genetic structure of populations in patchy habitats depends on its scale and the demographic attributes of demes (e.g. local colony sizes and timing of reproduction), but empirical evidence is scarce. We inferred the impact of gene flow on genetic structure among populations of water voles Arvicola terrestris that differed in average colony sizes, population turnover and degree of patchiness. Colonies typically consisted of few reproducing adults and several juveniles. Twelve polymorphic microsatellite DNA loci were examined. Levels of individual genetic variability in all areas were high ( H _O= 0.69–0.78). Assignments of juveniles to parents revealed frequent dispersal over long distances. The populations showed negative F _IS values among juveniles, F _IS values around zero among adults, high F _ST values among colonies for juveniles, and moderate, often insignificant, F _ST values for parents. We inferred that excess heterozygosity within colonies reflected the few individuals dispersing from a large area to form discrete breeding colonies. Thus pre-breeding dispersal followed by rapid reproduction results in a seasonal increase in differentiation due to local family groups. Genetic variation was as high in low-density populations in patchy habitats as in populations in continuous habitats used for comparison. In contrast to most theoretical predictions, we found that populations living in patchy habitats can maintain high levels of genetic variability when only a few adults contribute to breeding in each colony, when the variance of reproductive success among colonies is likely to be low, and when dispersal between colonies exceeds nearest-neighbour distances.     

Metapopulation structure of the specialized herbivore Macrosiphoniella tanacetaria (Homoptera,Aphididae)

We investigated population dynamics, genetic diversity and spatial structure in the aphid species Macrosiphoniella tanacetaria, a specialist herbivore feeding on tansy, Tanacetum vulgare. Tansy plants (genets) consist of many shoots (ramets), and genets are grouped in sites. Thus, aphids feeding on tansy can cluster at the level of ramets, genets and sites. We studied aphid population dynamics in 1997 and 2001 and found that within sites: (i). at any time, aphids used only a fraction of the available ramets and genets; (ii). at the level of ramets, most aphid colonies survived only one week; (iii). at the level of genets, mean survival time was less than 4 weeks; and (iv). colonization and extinction events occurred throughout the season. We sampled aphids in seven sites in the Alsace region, France (4-45 km apart) and two sites in Germany in 1999 to study genetic structure within and between populations. Genetic analyses using nine microsatellite loci showed that: (i). genotypic variability was high, (ii). none of the populations was in Hardy-Weinberg equilibrium, (iii). heterozygote deficits and linkage disequilibria were frequent, and (iv). all populations were genetically differentiated, even at a small geographical scale. Renewed sampling of the Alsace sites in 2001 showed that three populations had become extinct and significant genetic changes had occurred in the remaining four populations. The frequencies of extinction and colonization events at several spatial scales suggest a hierarchical metapopulation structure for M. tanacetaria. Frequent population turnover and drift are likely causes for the genetic differentiation of M. tanacetaria populations.     

Temporal recruitment patterns and gene flow in kelp rockfish (Sebastes atrovirens)

Pelagic dispersal of marine organisms provides abundant opportunity for gene flow and presumably inhibits population genetic divergence. However, ephemeral, fine-scale, temporal and spatial genetic heterogeneity is frequently observed in settled propagules of marine species that otherwise exhibit broad-scale genetic homogeneity. A large variance in reproductive success is one explanation for this phenomenon. Here, genetic analyses of 16 microsatellite loci are used to examine temporal patterns of variation in young-of-year kelp rockfish (Sebastes atrovirens) recruiting to nearshore habitat in Monterey Bay, California, USA. Population structure of adults from central California is also evaluated to determine if spatial structure exists and might potentially contribute to recruitment patterns. Genetic homogeneity was found among 414 young-of-year sampled throughout the entire 1998 recruitment season. No substantial adult population structure was found among seven populations spanning 800 km of coastline that includes the Point Conception marine biogeographic boundary. Comparison of young-of-year and adult samples revealed no genetic differentiation and no measurable reduction in genetic variation of offspring, indicating little variance in reproductive success and no reduction in effective population size for this year class. Simulation analyses determined that the data set was sufficiently powerful to detect both slight population structure among adults and a small reduction in effective number of breeders contributing to this year class. The findings of high gene flow and low genetic drift have important implications for fisheries management and conservation efforts.     

Spatio-temporal population genetic structure of the parasitic mite Spinturnix bechsteini is shaped by its own demography and the social system of its bat host

Information about the population genetic structures of parasites is important for an understanding of parasite transmission pathways and ultimately the co-evolution with their hosts. If parasites cannot disperse independently of their hosts, a parasite's population structure will depend upon the host's spatial distribution. Geographical barriers affecting host dispersal can therefore lead to structured parasite populations. However, how the host's social system affects the genetic structure of parasite populations is largely unknown. We used mitochondrial DNA (mtDNA) to describe the spatio-temporal population structure of a contact-transmitted parasitic wing mite ( Spinturnix bechsteini ) and compared it to that of its social host, the Bechstein's bat ( Myotis bechsteinii ). We observed no genetic differentiation between mites living on different bats within a colony. This suggests that mites can move freely among bats of the same colony. As expected in case of restricted inter-colony dispersal, we observed a strong genetic differentiation of mites among demographically isolated bat colonies. In contrast, we found a strong genetic turnover between years when we investigated the temporal variation of mite haplotypes within colonies. This can be explained with mite dispersal occuring between colonies and bottlenecks of mite populations within colonies. The observed absence of isolation by distance could be the result from genetic drift and/or from mites dispersing even between remote bat colonies, whose members may meet at mating sites in autumn or in hibernacula in winter. Our data show that the population structure of this parasitic wing mite is influenced by its own demography and the peculiar social system of its bat host.     

Microspatial genetic heterogeneity and gene flow in stray cats (Felis catus L.): a comparison of coat colour and microsatellite loci

We analysed levels of genetic differentiation between nine local urban colonies of stray cats using eight coat colour and nine microsatellite loci. Both types of markers revealed a strong differentiation between colonies (FST = 0.15 and 0.09 for coat colour and microsatellite loci, respectively). Three coat colour loci showed extreme levels of genetic differentiation comparatively to other loci and are strongly suspected to be under divergent selective pressures. Microsatellite loci showed significant heterozygote deficiency within colonies (FIS = 0.14), suggesting that coat colour loci are not appropriate to investigate genetic structure at a fine scale because coat colour allele frequencies are based on Hardy-Weinberg equilibrium. The reported pattern conformed to that predicted from the social structuring of cat colonies: aggressive exclusion of immigrants, inbreeding and very low dispersal rate.     

Spatial genetic structuring in a vagile species, the European wood mouse

We examined the genetic structure of natural populations of the European wood mouse Apodemus sylvaticus at the microgeographic (<3 km) and macrogeographic (>30 km) scales. Ecological and behavioural studies indicate that this species exhibits considerable dispersal relative to its home-range size. Thus, there is potential for high gene flow over larger geographic areas. As levels of population genetic structure are related to gene flow, we hypothesized that population genetic structuring at the microgeographic level should be negligible, increasing only with geographic distance. To test this, four sites were sampled within a microgeographic scale with two additional samples at the macrogeographic level. Individuals ( n =415) were screened and analysed for seven polymorphic microsatellite loci. Contrary to our hypothesis, significant levels of population structuring were detected at both scales. Comparing genetic differentiation with geographic distance suggests increasing genetic isolation with distance. However, this distance effect was non-significant being confounded by surprisingly high levels of differentiation among microgeographic samples. We attribute this pattern of genetic differentiation to the effect of habitat fragmentation, splitting large populations into components with small effective population sizes resulting in enhanced genetic drift. Our results indicate that it is incorrect to assume genetic homogeneity among populations even where there is no evidence of physical barriers and dispersal can occur freely. In the case of A. sylvaticus , it is not clear whether dispersal does not occur across habitat barriers or behavioural dispersal occurs without consequent gene flow.     

RAPD analysis of genetic variation and dispersal of the moss Bryum argenteum in Ross Island and Victoria Land, Antarctica

The Random Amplified Polymorphic DNA technique was used to assess the level of genetic diversity in Bryum argenteum from Ross Island and southern Victoria Land, Antarctica. Samples were collected from three separate transects, and from other geographically distinct populations within 150 km of Ross Island. Moss growth in two transects, sampled down small exposed meltstream channels at Cape Royds and Cape Chocolate, was very sparse with no other moss colonies found within 0.4 or 4 km, respectively. However, samples from these channels showed similar levels of genetic variation to those from a transect at Granite Harbour, where moss colonies were large, luxuriant and turf-like between boulders. In all transects, high levels of genetic diversity were apparent both within and between colonies, and some spatial relationships were observed down the length of the channels, with more extensive variation at the top than the bottom of two transects. Samples from other sites in the region showed varying but high levels of genetic diversity; overall, the majority showed some clustering according to site of collection, with short-distance dispersal of propagules by water and transmission between sites presumably by wind. The extensive genetic diversity observed appears mainly due to somatic mutation within colonies, with some contribution by immigration of propagules from elsewhere into established colonies. Accepted: 10 January 1999     

Low population genetic structuring of two cryptic bat species suggests their migratory behaviour in continental Europe

Although two cryptic pipistrelle bat species, Pipistrellus pipistrellus and Pipistrellus pygmaeus , belong among the most common bat species in Europe, it is still unclear whether they can migrate over long distances between summer and winter roosts. Long-distance migratory species may be expected to show low levels of genetic structuring in large areas due to regular mixing of the gene pool by mating that occurs during migration and/or hibernation. Conversely, the dispersal of gametes in sedentary species is spatially restricted, populations are more genetically structured, and isolation by relatively short distance is visible. By analysing diversity of highly variable microsatellites within and among summer colonies of both studied species in central Europe, we found that differentiation between populations is very weak. Both classical F _ST and Bayesian clustering approach failed to detect genetic structure among colonies and there was no significant isolation-by-distance pattern. The analyses of relatedness, however, revealed that individuals within colonies are more related than random suggesting philopatry of at least one sex. The results were very similar for the two species. The high level of gene flow among central European populations, even on large geographic distances, is discussed in relation with migrations, dispersal, and mating behaviour.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 103–114.     

Seasonal and annual genotypic variation and the effect of climate on population genetic structure of the cereal aphid Sitobion avenae in northern France

Changes in the genetic structure and genotypic variation of the aphid Sitobion avenae collected from cereal crops in northern France were examined by analysing variation at five microsatellite loci across several years and seasons. Little regional and temporal differentiation was detected, as shown by very low FST among populations. Repeated genotypes, significant heterozygote deficits, positive FIS values and frequent linkage disequilibria were found in nearly all samples, suggesting an overall pattern of reproductive mode variation in S. avenae populations. In addition, samples from Brittany (Bretagne) showed greater signs of asexual reproduction than those from the north of France, indicating a trend toward increasing sexuality northward. These patterns of reproductive variation in S. avenae are consistent with theoretical models of selection of aphid reproductive modes by climate. Contrasting with little changes in allelic frequencies, genotypic composition varied substantially in time and, to a lesser extent, in space. An important part of changes in genotypic arrays was due to the variation in frequency distribution of common genotypes, i.e. those that were found at several instances in the samples. Genotypic composition was also shown to vary according to climate, as genotypic diversity in spring was significantly correlated with the severity of the previous winter and autumn. We propose that the genetic homogeneity among S. avenae populations shown here across large temporal and spatial scales is the result of two forces: (i) migration conferred by high dispersal capabilities, and (ii) selection over millions of hectares of cereals (mostly wheat) bred from a narrow genetic base.     

Population structure of African buffalo inferred from mtDNA sequences and microsatellite loci: high variation but low differentiation

The African buffalo ( Syncerus caffer ) is widespread throughout sub-Saharan Africa and is found in most major vegetation types, wherever permanent sources of water are available, making it physically able to disperse through a wide range of habitats. Despite this, the buffalo has been assumed to be strongly philopatric and to form large aggregations that remain within separate home ranges with little interchange between units, but the level of differentiation within the species is unknown. Genetic differences between populations were assessed using mitochondrial DNA (control region) sequence data and analysis of variation at six microsatellite loci among 11 localities in eastern and southern Africa. High levels of genetic variability were found, suggesting that reported severe population bottlenecks due to outbreak of rinderpest during the last century did not strongly reduce the genetic variability within the species. The high level of genetic variation within the species was found to be evenly distributed among populations and only at the continental level were we able to consistently detect significant differentiation, contrasting with the assumed philopatric behaviour of the buffalo. Results of mtDNA and microsatellite data were found to be congruent, disagreeing with the alleged male-biased dispersal. We propose that the observed pattern of the distribution of genetic variation between buffalo populations at the regional level can be caused by fragmentation of a previous panmictic population due to human activity, and at the continental level, reflects an effect of geographical distance between populations.     

Population structure is not a simple function of reproductive mode and larval type: insights from tropical corals

1. For a wide range of organisms, heritable variation in life-history characteristics has been shown to be strongly subject to selection, reflecting the impact that variation in characters such as genotypic diversity, duration of larval development and adaptations for dispersal can have on the fitness of offspring and the make-up of populations. Indeed, variation in life-history characteristics, especially reproduction and larval type, have often been used to predict patterns of dispersal and resultant population structures in marine invertebrates. 2. Scleractinian corals are excellent models with which to test this relationship, as they exhibit almost every possible combination of reproductive mode and larval type. Some general patterns are emerging but, contrary to expectations, genetic data suggest that while populations of broadcast spawning species may be genotypically diverse they may be heavily reliant on localized recruitment rather than widespread dispersal of larvae. 3. Here we use microsatellites to test the importance of localized recruitment by comparing the genetic structure of populations of two broadcast spawning corals with contrasting modes of reproduction and larval development; Goniastrea favulus is self-compatible, has sticky, negatively buoyant eggs and larvae and is expected to have restricted dispersal of gametes and larvae. In contrast, Platygyra daedalea is self-incompatibile, spawns positively buoyant egg-sperm bundles and has planktonic development. 4. Surprisingly, spatial-autocorrelation revealed no fine-scale clustering of similar genotypes within sites for G. favulus, but showed a non-random distribution of genotypes in P. daedalea. Both species showed similar levels of genetic subdivision among sites separated by 50-100 m (F(ST) = 0.03), suggesting that larval dispersal may be equivalent in both species. 5. Interestingly, as fragmentation has been considered rare in massive corals, our sample of 284 P. daedalea colonies included 28 replicated genotypes that were each unlikely (P < 0.05) to have been derived independently from sexual reproduction. 6. We conclude that the extreme life history of G. favulus does not produce unusually fine-scale genetic structure and subsequently, that reproductive mode and larval type may not be not good predictors of population structure or dispersal ability.     

Genotypic diversity and gene flow in brooding and spawning corals along the Great Barrier Reef, Australia

Marine organisms exhibit great variation in reproductive modes, larval types, and other life-history traits that may have major evolutionary consequences. We measured local and regional patterns of genetic variation in corals along Australia's Great Barrier Reef to determine the relative contributions of sexual and asexual reproduction to recruitment and to infer levels of gene flow both locally (among adjacent sites, < 5 km apart) and regionally (among reefs separated by 500-1,200 km). We selected five common brooding species (Acropora cuneata, A. palifera, Pocillopora damicornis, Seriatopora hystrix, and Stylophora pistillata) and four broadcast spawners (Acropora hyacinthus, A. cytherea, A. millepora, and A. valida), which encompassed a wide range of larval types and potential dispersal capabilities. We found substantial genotypic diversity at local scales in six of the nine species (four brooders, two spawners). For these six, each local population displayed approximately the levels of multilocus genotypic diversity (Go) expected for outcrossed sexual reproduction (mean values of Go:Ge ranged from 0.85 to 1.02), although consistent single-locus heterozygous deficits indicate that inbreeding occurs at the scale of whole reefs. The remaining three species, the brooder S. hystrix and the spawners A. valida and A. millepora displayed significantly less multilocus genotypic diversity (Go) than was expected for outcrossed sexual reproduction (Ge) within each of several sites. Acropora valida and A. millepora showed evidence of extensive localized asexual replication: (1) a small number of multilocus (clonal) genotypes were numerically dominant within some sites (Go:Ge values were as low as 0.17 and 0.20): (2) single-locus genotype frequencies were characterized by both excesses and deficits of heterozygotes (cf. Hardy-Weinberg expectations), and (3) significant linkage disequilibria occurred. For the brooding S. hystrix Go:Ge values were also low within each of four sites (x = 0.48). However, this result most likely reflects the highly restricted dispersal of gametes or larvae, because levels of genetic variation among sites within reefs were extremely high (FSR = 0.28). For all species, we detected considerable genetic subdivision among sites within each reef (high FSR-values), and we infer that larval dispersal is surprisingly limited (i.e., Nem among sites ranging from 0.6 to 3.3 migrants per generation), even in species that have relatively long planktonic durations. Nevertheless, our estimates of allelic variation among reefs (FRT) also imply that for all four broadcast spawning species and three of the brooders, larval dispersal is sufficient to maintain moderate to high levels of gene flow along the entire Great Barrier Reef (i.e., Nem among reefs ranged from 5 to 31). In contrast, widespread populations of S. hystrix and S. pistilata (the two remaining brooders) are relatively weakly connected (Nem among reefs was 1.4 and 2.5, respectively). We conclude that most recruitment by corals is very local, particularly in brooders, but that enough propagules are widely dispersed to ensure that both broadcast spawning and brooding species form vast effectively panmictic populations on the Great Barrier Reef.     

Migration load in plants: role of pollen and seed dispersal in heterogeneous landscapes

Evolution of local adaptation depends critically on the level of gene flow, which, in plants, can be due to either pollen or seed dispersal. Using analytical predictions and individual-centred simulations, we investigate the specific influence of seed and pollen dispersal on local adaptation in plant populations growing in patchy heterogeneous landscapes. We study the evolution of a polygenic trait subject to stabilizing selection within populations, but divergent selection between populations. Deviations from linkage equilibrium and Hardy-Weinberg equilibrium make different contributions to genotypic variance depending on the dispersal mode. Local genotypic variance, differentiation between populations and genetic load vary with the rate of gene flow but are similar for seed and pollen dispersal, unless the landscape is very heterogeneous. In this case, genetic load is higher in the case of pollen dispersal, which appears to be due to differences in the distribution of genotypic values before selection.