Implications of life history for genetic structure and migration rates of southern African coastal invertebrates: planktonic,abbreviated and direct development

The amount of genetic structure in marine invertebrates is often thought to be negatively correlated with larval duration. However, larval retention may increase genetic structure in species with long-lived planktonic larvae, and rafting provides a means of dispersal for species that lack a larval dispersal phase. We compared genetic structure, demographic histories and levels of gene flow of regional lineages (in most cases defined by biogeographic region) of five southern African coastal invertebrates with three main types of larval development: (1) dispersal by long-lived planktonic larvae (mudprawn Upogebia africana and brown mussel Perna perna), (2) abbreviated larval development (crown crab Hymenosoma orbiculare) and (3) direct development (estuarine isopod Exosphaeroma hylecoetes and estuarine cumacean Iphinoe truncata). We hypothesized that H. orbiculare, having abbreviated larval development, would employ a strategy of larval retention, resulting in genetic structure comparable to that of the direct developers rather than the planktonic dispersers. However, regional population structure was significantly lower in all species with planktonic larvae, including H. orbiculare, than in the direct developers. Moreover, nested clade analysis identified demographic histories resulting from low levels of gene flow (isolation by distance and allopatric fragmentation) in the direct developers only, and migration rates were significantly higher in all three species having planktonic larvae than in the direct developers. We conclude that the amount of genetic structure within marine biogeographic regions strongly depends on the presence or absence of free-swimming larvae. Whether such larvae are primarily exported or retained, whether they have long or short larval duration, and whether or not they are capable of active dispersal seems to have little effect on connectivity among populations.     

Extensive gene flow among mytilid (Bathymodiolus thermophilus) populations from hydrothermal vents of the eastern Pacific

Prior studies of the hydrothermal vent mussel Bathymodiolus thermophilus (Bivalvia: Mytilidae), provided conflicting predictions about the dispersal ability and population structure of this highly specialized species. Analyses of morphological features associated with its larval shells revealed a feeding larval stage that might facilitate dispersal between ephemeral vent habitats. In contrast, an allozyme study revealed substantial genetic differentiation between samples taken from populations 2370 km apart on Galápagos Rift (Latitude 0°N) and the East Pacific Rise (13°N). To resolve the discrepancy between these studies, we examined allozyme and mitochondrial (mt) DNA variation in new samples from the same localities plus more recently discovered sites (9° and 11°N) along the East Pacific Rise. Although analysis of 26 enzyme-determining loci revealed relatively low levels of genetic variation within the five populations, no evidence existed for significant barriers to dispersal among populations. We estimated an average effective rate of gege flow (Nm) of 8 migrants per population per generation. Two common mtDNA variants predominated at relatively even frequencies in each population, and similarly provided no evidence for barriers to gene flow or isolation-by-distance across this species' known range. Larvae of this species appear to be capable of dispersing hundreds of kilometers along a continuous ridge system and across gaps separating non-contiguous spreading centers.     

Genetic evidence for contrasting patterns of dispersal in solitary and colonial ascidians

In situ and in vitro observations indicate that brooding colonial ascidians commonly display limited larval dispersal, whilst the larvae of most solitary species are assumed to be widely dispersed. We used allozyme data to determine the population genetic consequences of reproduction and dispersal in a broadcast-spawning solitary ascidian and two brooding colonial species along the central and southern coast of New South Wales, Australia. We surveyed genetic variation at 2 to 9 variable loci for samples collected from 6 to 8 local populations of each of the stalked solitary species Pyura gibbosa gibbosa Heller, 1878; the social Stolonica australis Michaelsen, 1927 and the compound Botrylloides magnicoecum Hartmeyer, 1912. Samples from each local population displayed levels and patterns of genotypic diversity that were consistent with expectations for sexually-derived recruitment of both solitary zooids and separate colonies. However, we found clear differences in the structure of the populations of solitary and colonial species. Genotype frequencies within all nine samples of P. gibbosa gibbosa conformed to expectations for random mating (i.e. Hardy–Weinberg equilibria). Moreover, allele frequencies showed little variation among samples [mean standardised genetic variance (F _{S T} ) =0.002], which implies that local populations are strongly connected by larval dispersal. We estimate (via Wright's “island model”) that gene flow (N _e m) within this set of local populations is 125 effective migrants per generation, which is very similar to estimates obtained for other broadcast-spawning taxa in this region. In contrast, genotype frequencies within samples of both colonial species were characterised by large and statistically significant deficits of heterozygotes, consistent with expectations for highly limited dispersal of larvae or sperm. Moreover, local populations were highly differentiated (F _{S T} =0.201 and 0.202 for S. australis and B. magnicoecum, respectively) and N _e m was estimated to be ∼1.0 in each case. These values of F _{S T} and subsequent estimates of N _e m lie within the range of values reported for other New South Wales taxa with direct larval development, and imply that local populations are effectively closed to immigration. Received: 13 February 1997 / Accepted 18 July 1997     

The relationship between population genetic structure and pelagic larval duration in coral reef fishes on the Great Barrier Reef

Pelagic larval duration (PLD) is a commonly used proxy for dispersal potential in coral reef fishes. Here we examine the relationship between PLD, genetic structure and genetic variability in geographically widespread and ecological generalist species from one coral reef fish family (Pomacentridae) that differs in mean larval duration by more than a month. The genetic structure was estimated in eight species using a mitochondrial molecular marker (D-loop) and in a sub-set of five species using nuclear molecular markers (ISSRs). Estimates of genetic differentiation were similar among species with pelagic larvae, but differed between molecular markers. The mtDNA indicated no structure in all species except one, while the ISSR indicated some structure between the sampling locations in all species. We detected a relationship between PLD and genetic structure using both markers. These relationships, however, were caused by a single species, Acanthochromis polyacanthus, which differs from all the other species examined here in lacking a larval phase. With this species excluded, there was no relationship between PLD and genetic structure using either marker despite a range of PLDs of more than 20 days. Genetic diversities were generally high in all species and did not differ significantly among species and locations. Nucleotide diversity and total heterozygosity were negatively related to maximum PLD but again these relationships were caused by A. polyacanthus and disappeared when this species was excluded. These genetic patterns are consistent with moderate gene flow among well-connected locations and indicate that at this phylogenetic level (i.e., within family) the duration of the pelagic larval phase is unrelated to the patterns of genetic differentiation.     

Gene flow among vestimentiferan tube worm (Riftia pachyptila) populations from hydrothermal vents of the eastern Pacific

Allozyme data are presented for six discrete populations of the giant hydrothermal vent tube worm Riftia pachyptila Jones, 1981 collected throughout the species' known range along mid-ocean spreading ridges of the eastern Pacific Ocean. Contrary to an earlier report, levels of genetic variation are relatively high in this species. Estimates of gene flow based on F-statistics revealed that dispersal throughout the surveyed region is sufficiently high to counter random processes that would lead to losses of genetic diversity and significant population differentiation. R. pachyptila, like other species of tube worms, displays considerable morphologic variation among populations, but this diversity is not reflected in allozyme variation. Vestimentifera, in general, appear to show extensive phenotypic plasticity. In the light of the available genetic data, caution is warranted when making inferences about the taxonomic status of collections based on morphological variation alone. A general decrease in estimated rates of gene flow between geographically more distant populations supports the hypothesis that dispersal in this species follows a stepping-stone model, with exchange between neighboring populations in great excess of long-distance dispersal. High levels of gene flow have been recorded in a variety of vent fauna and may be a prerequisite for success of species found in the ephemeral habitats associated with regions of sea-floor hydrothermal activity.     

Latitudinal variability in spatial genetic structure in the invasive ascidian, Styela plicata

Increases in temperature can shorten planktonic larval durations, so that higher temperatures may reduce dispersal distances for many marine animals. To test this prediction, we first quantified how minimum time to settlement is shortened at higher temperatures for the ascidian Styela plicata. Second, using latitude as a correlate for ocean temperature and spatial genetic structure as a proxy for dispersal, we tested for a negative correlation between latitude and spatial genetic structure within populations, as measured by anonymous DNA markers. Spatial genetic structure was variable among latitudes, with significant structure at low and intermediate latitudes (high and medium temperatures) and there was no genetic structure within high-latitude (low temperature) populations. In addition, we found consistently high genetic diversity across all Australian populations, showing no evidence for recent local bottlenecks associated S. plicata’s history as an invasive species. There was, however, significant genetic differentiation between all populations indicating limited ongoing gene flow.     

Acanthochromis polyacanthus,a fish lacking larval dispersal,has genetically differentiated populations at local and regional scales on the Great Barrier Reef

Acanthochromis Gill is a monotypic genus within the damselfish family Pomacentridae, erected for an unusual species [A. polyacanthus (Bleeker)] that uniquely lacks larval dispersal. Instead, offspring are reared in the parental territory, in the manner of cichlids, and fledged into the surrounding habitat. Phenotypic and genotypic variation was surveyed on the basis of body colouration and 7 polymorphic loci in 19 populations from 5 regions of the central and southern Great Barrier Reef (GBR). Variation in both characters was found at regional and local scales. Two colour morphs were recognised: a bicoloured morph from the three northern regions and a uniform dark morph from the two southern regions. Isozyme analysis showed a similar pattern with greatest variation between the different morphs, but also with significant variation at both regional and local scales within morphotypes. Heterozygosity was maximal in the central populations, which, together with other measures of variability, suggests a mixing of separate gene pools in this region and denies species status to the two morphotypes despite numerous fixed differences in allele frequencies between the most distant populations. The presence of fixed differences in multiple alleles between populations separated by 1000 km indicates negligible gene flow over such distances and long isolation of these gene pools. These patterns may reflect recolonisation of the GBR after the last sea-level rise by fish from two stocks. Founder effects and random drift in small populations after colonisation are probably the major sources of the local and regional variations observed at smaller spatial scales. This diversity has been maintained among populations at all scales by the very low levels of gene flow possible without an effective strategy for larval dispersal between coral reefs.     

Genetic structure of<Emphasis Type="Italic"> Linckia laevigata</Emphasis> and<Emphasis Type="Italic"> Tridacna crocea</Emphasis> populations in the Palawan shelf and shoal reefs

Allozyme variation of 10 populations of Linckia laevigata at 8 polymorphic loci and 13 populations of Tridacna crocea at 6 polymorphic loci were analyzed to compare genetic variability and genetic affinities among reefs in Palawan, Philippines. Two to five populations were sampled from each of four regions: the shelf reefs in (1) northern Palawan and (2) southern Palawan and the offshore reefs in (3) the Kalayaan island group (KIG) in the South China Sea and (4) the Tubbataha shoals in the Sulu Sea. Heterozygosity was highest in populations of L. laevigata from the south shelf of Palawan and populations of T. crocea from the Tubbataha shoals of the Sulu Sea. The lowest heterozygosity estimates were from the reefs of the KIG in the South China Sea, for both species. Overall F _ST values for both species were significant, with an estimated average number of effective migrants per generation (N _EM) of 4.85 (~5 individuals) for L. laevigata and 3.54 (~4 individuals) for T. crocea. Within-region comparisons showed N_EM ranging from 6.29 to 92.34 for L. laevigata and from 3.40 to 6.30 for T. crocea. The higher gene flow among L. laevigata populations relative to T. crocea is consistent with the greater dispersal potential of the former species. Finer scale genetic structuring was evident in T. crocea populations. For both species, the Tubbataha reefs in the Sulu Sea have higher genetic affinity with the populations from the southern shelf of Palawan, while the reefs in the KIG had higher affinity with the northern Palawan shelf reefs. The north and south shelf populations have the least genetic affinity. Genetic patchiness among reefs within regions suggests the importance of small-scale physical factors that affect recruitment success in structuring populations in small island and shoal reef systems in Palawan.     

Species boundaries in the starfish genus Linckia

 The genetic basis for species boundaries in the starfish genus Linckia was examined using variation observed in 613 base pairs (bp) of sequence from the cytochrome oxidase I gene of mtDNA and 16 allozyme loci. Five groups within Linckia were clearly genetically differentiated; L. columbiae, L. bouvieri, two clades within L. guildingi, and one clade with two sub-clades consisting of both L. laevigata and L. multifora. Genetic divergence among these groups is consistent with interspecific variation. The two clades within L. guildingi suggest the presence of a cryptic, partially sympatric, species. Genetic divergence between these two clades implies that they have been genetically distinct from each other for at least one million years. In contrast, genetic data suggest that L. laevigata and L. multifora are a single species, despite the fact that live individuals can be distinguished by their colour and colour pattern, number of madreporites and ratio of arm length to breadth. There are probably three closely related groups within the L. laevigata/L. multifora clade –L. multifora, and two groups in L. laevigata defined by biogeographic province. It is difficult to determine boundaries for these three entities, since genetic and morphological differences are complicated by phenotypic differences arising from both environmental variation and population genetic structure. The difficulties encountered in defining species boundaries in Linckia, particularly with respect to variation arising from the overlap of Indian and Pacific biogeographic provinces, may be a general issue for many marine organisms from this region. Received: 24 May 1999 / Accepted: 6 October 1999     

Low genetic differentiation between isolated populations of the colonial ascidian Symplegma rubra Monniot, C. 1972

Pelagic larvae are highly important for maintaining the gene flow among populations of sessile marine invertebrates. Colonial ascidians consist, exclusively, of brooding species, with lecithotrophic larvae that have a limited dispersal. As a result, there is a marked differentiation among populations. In this work, we used allozyme electrophoresis to access the genetic variation in four populations of Symplegma rubra, a colonial ascidian frequently found in the intertidal zone of Southeastern Brazilian coast. High variability was found at three of the four sites sampled, the exception being Praia Grande in the State of Rio de Janeiro. At this site, there was a great preponderance of clones, which possibly reflected the enclosed nature of the location and its low water circulation that reduce the dispersal capabilities of these animals. S. rubra did not conform to expectations for random mating (Hardy–Weinberg equilibrium): there was a deficit of heterozygotes that was more related to the small population size than to inbreeding processes, since F _is analysis per locus revealed a deficiency of heterozygotes at only one locus—MDH*. The greatest variation in allele frequency was found for GPI-2*. Analyses of genetic variability revealed moderate differentiation among the populations (F _ST=0.051), which was unexpected for a species with a low dispersal capability. Rafting, a frequently underestimated means of dispersal, may be the main mode of gene flow in this species over large areas, since colonies of S. rubra are frequently seen growing on drift material and there is no evidence that the larva survive for a long time in the plankton.     

Genetic structure of Dascyllus aruanus populations in French Polynesia

Dascyllus aruanus were collected from 13 different locations in French Polynesia between 15 November 1990 and 15 February 1991 in order to examine larval dispersal on four spatial scales: within-feef, within-island, within-archipelago, among-archipelagoes. Average polymorphism was analysed by protein electrophoresis at two levels (P₉₅=0.285 and P₉₀=0.107) from 12 and 3 polymorphic loci, respectively. Spatial genetic variation displayed a low level of differentiation between populations among archipelagoes, and homogeneity at lower spatial scales. Two hypotheses are proposed to explain the genetic structure observed. The first suggests substantial gene flow between the islands during the pelagic larval phase, the second that the absence of differentiation is the result of recent colonisation. Genetic variation amongst size classes showed a significant heterozygote deficiencies at two loci (PGM ^* and EST-2 ^*) in the smallest size class. This suggests a cyclic selection which affects larvae and adults differentially. The data revealed little differentiation among populations at the different localities, despite the short larval duration of D. aruanus; this suggests that larval duration is not the main factor presently affecting genetic structure in an insular model. Correspondence to: S. Planes at the Université de Perpignan     

Effects of a larval stage on isolation of populations of the trochid<Emphasis Type="Italic"> Austrocochlea constricta</Emphasis> in tidal ponds

To test the importance of special environments for local genetic subdivision in species with a larval phase, we examined allozyme variation among populations of the intertidal snail Austrocochlea constricta, in 18 tidal ponds in the Houtman Abrolhos Islands, Western Australia. Levels of genetic divergence between pond populations were correlated with those of parallel analyses among adjacent shore populations. However, divergence among the isolated ponds, which lack surface connection to the sea, were generally substantially higher, with an overall F_ST of 0.408, compared with 0.274 among the shore sites. The pond populations had less genetic variation than their shore counterparts, and the reduction of heterozygosity was correlated with the isolation of the pond population, as measured by F_ST. Both the degree of isolation and the reduction of heterozygosity were greater in deeper ponds, where snails can produce a local pool of larvae. In contrast, ponds that dry out frequently are less likely to allow production of local recruits, and these appear to be better connected genetically to adjacent shore populations. These patterns contrast sharply with those previously documented at the same sites for the direct-developing snail Bembicium vittatum, which shows greater isolation in ponds that are often dry. The comparison between the two species shows significant interaction between intrinsic and extrinsic impediments to gene flow, and highlights the importance of characteristics of the life history in determining which circumstances favour isolation of local populations.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00277-005-1553-5.Communicated by G.F. Humphrey, Sydney     

Isolation by distance in the scleractinian coral <Emphasis Type="Italic">Seriatopora hystrix</Emphasis> from the Red Sea

Pelagic dispersal of larvae in sessile marine invertebrates could in principle lead to a homogeneous gene pool over vast distances, yet there is increasing evidence of surprisingly high levels of genetic differentiation on small spatial scale. To evaluate whether larval dispersal is spatially limited and correlated with distance, we conducted a study on the widely distributed, viviparous reef coral Seriatopora hystrix from the Red Sea where we investigated ten populations separated between ~0.150 km and ~610 km. We addressed these questions with newly developed, highly variable microsatellite markers. We detected moderate genetic differentiation among populations based on both F _ST and R _ST (0.089 vs. 0.136, respectively) as well as considerable heterozygote deficits. Mantel tests revealed isolation by distance effects on a small geographic scale (≤20 km), indicating limited dispersal of larvae. Our data did not reveal any evidence against strictly sexual reproduction among the studied populations.     

Mitochondrial DNA variation and population structure in the spiny lobster Panulirus argus

Adult spiny lobsters (Panulirus argus) were collected from nine locations throughout the tropical and subtropical northwest Atlantic Ocean and examined for mitochondrial DNA (mtDNA) variation. 187 different mtDNA haplotypes were observed among the 259 lobsters sampled. Haplotype diversity was calulated to be 0.986 and mean nucleotide sequence-diversity was estimated to be 1.44%; both of these values are among the highest reported values for a marine species. Analysis of molecular variance (AMOVA) and phenetic clustering both failed to reveal any evidence of genetic structure within and among populations of P. argus. The present data are consistent with high levels of gene flow among populations of P. argus resulting from an extended planktonic larval stage and strong prevailing ocean currents.     

Spatial and temporal genetic homogeneity in the Arctic surfclam (<Emphasis Type="Italic">Mactromeris polynyma</Emphasis>)

Commercially harvested marine bivalve populations show a broad range of population-genetic patterns that may be driven by planktonic larval dispersal (gene flow) or by historical (genetic drift) and ecological processes (selection). We characterized microsatellite genetic variation among populations and year classes of the commercially harvested Arctic surfclam, Mactromeris polynyma, in order to test the relative significance of gene flow and drift on three spatial scales: within commercially harvested populations in the northwest Atlantic; among Atlantic populations; and between the Atlantic and Pacific oceans. We found small nonsignificant genetic subdivision among eight populations from the northwest Atlantic (F _ST = 0.002). All of these Atlantic populations were highly significantly differentiated from a northeast Pacific population (F _ST = 0.087); all populations showed high inbreeding coefficients (F _IS = 0.432). We tested one likely source of heterozygote deficits by aging individual clams and exploring genetic variation among age classes within populations (a temporal Wahlund effect). Populations showed strikingly different patterns of age structure, but we found little differentiation among age classes. In one case, we were able to analyze genetic diversity between age classes older or younger than the advent of intensive commercial harvesting. The results generally suggest spatially broad and temporally persistent genetic homogeneity of these bivalves. We discuss the implications of the results for the biology and management of surfclam populations. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The genetic effects of larval dispersal depend on spatial scale and habitat characteristics

The intertidal gastropods Bembicium vittatum and Austrocochlea constricta, which have direct and planktonic larval development, respectively, occur sympatrically at sites across a number of islands at the Houtman Abrolhos archipelago and two harbours at Albany in Western Australia. Their distribution provide an opportunity to examine the effect of dispersal ability on levels of genetic subdivision at a number of spatial scales. F _ST (standardised variance in allelic frequencies) values in the range 0.361 to 0.396, determined from allozyme frequencies at 12 to 13 polymorphic loci, confirm isolation of Abrolhos and Albany populations, which are separated by 900 km of coastline, in both species. Within the Abrolhos and Albany, levels of subdivision in B. vittatum were high, but similar, as indicated by F _ST values of 0.091 and 0.090, respectively. In A. constricta, a mean value of 0.160 at the Abrolhos suggests severe restrictions to gene flow, while 0.021 at Albany indicates much stronger connections among populations. F _ST values at the Abrolhos support previous suggestions that this archipelago favours genetic subdivision in both direct and planktonic-developing species. The Albany harbours favoured subdivision only in B. vittatum, the low values of F _ST in A. constricta being attributed to strong mixing between the harbours, thus facilitating gene flow via planktonic larvae. The isolation of A. constricta populations at the Abrolhos can be explained in terms of highly localised recruitment, the result of limited water movement in complex intertidal habitats. The study illustrates the value of examining sympatric direct and planktonic developers in assessing the role of larval dispersal in patterns of genetic subdivision, and concludes that planktonic larvae may not promote gene flow over broad or even some fine spatial scales.     

Genetic relationships between marine and marginal-marine populations of Cerithium species from the Mediterranean Sea

Results of isozyme electrophoresis were used to explore the genetic relationships between several Mediterranean morphs of Cerithium (Gastropoda: Prosobranchia), for which taxonomy is currently uncertain because of high intraspecific variability and low interspecific differentiation. The large species, classically known as C. vulgatum Bruguière, 1789 was identified at four sites (two in the French Mediterranean and two in southern Spain). Two different larval types were found in the French sites, but poecilogony could not be demonstrated. Individuals collected from harbours were not genetically distinct from open-sea populations of classic C. vulgatum. However, a population in the Embiez lagoon (French Mediterranean) which morphologically resembles C. vulgatum did display distinct genetic traits, supporting its status as a separate species. Of the small Cerithium species usually known as C. rupestre, two sympatric species (C.“rupestre” Risso, 1826 and C. lividulum Risso, 1826) were distinguished. Genotype frequencies within the analysed populations revealed much heterozygote deficiency. F _ST values (fixation index measuring the effects of population subdivision) suggest a higher genetic differentiation for C. lividulum populations than for C. vulgatum populations. We assume that a high larval dispersal capability (via planktotrophy) allows a high gene flow between populations of C. vulgatum. Received: 24 November 1998 / Accepted: 24 September 1999     

Lack of genetically-subdivided population structure in Bullia digitalis, a southern African marine gastropod with lecithotrophic development

Larval shell morphology in fossil and present-day gastropods is often used to infer modes of larval development and levels of dispersal. Dispersal ability influences not only genetic population structure, but also is thought to influence a species' geographical range and evolutionary duration. We tested these predictions in Bullia digitalis, a sandy-beach whelk, by examining genetic variability at 33 protein-coding loci in nine samples (N=739) taken in 1984 to 1985 at localities extending over about three-quarters of the geographical range of this species in southern Africa. Females of this species deposit eggs into benthic or brooded capsules in which larvae develop through the trochophore and veliger stages to emerge as crawling juveniles. Scanning electron micrographs confirmed a protoconch morphology typical for gastropods with lecithotrophic larval development. Contrary to expectations, subpopulations of B.␣digitalis had high levels of variability (H=0.102) and lacked a genetically-fragmented structure (=0.013). The lack of a genetically-subdivided population structure would not have been correctly inferred, if this species were known only from well-preserved fossil shells. Indirect estimates of migration between populations based on and the island model of migration, which assumes drift-mutation equilibrium, ranged between 19 and 23 individuals per generation. Either an undescribed mechanism of dispersal facilitates gene flow between populations, or the geographical range of this species has recently expanded to produce the appearance of high levels of gene flow. Gene-frequency distributions showed that relative to four other species of Bullia, populations of B. digitalis were in mutation-drift disequilibrium, with a significant excess of low-frequency alleles that is consistent with a recent rapid expansion from a small population. Also contrary to expectations, this species has a large geographical range (2 400 km) and an apparently long evolutionary history extending 5 to 20 million years, as estimated from an allozyme phylogeny with four other species of Bullia. Received: 15 January 1997 / Accepted: 28 January 1997     

Genetic uniformity of widely separated populations of the coral reef starfishLinckia laevigata from the East Indian and West Pacific Oceans,revealed by allozyme electrophoresis

Gene flow among 18 widely separated populations of the starfishLinckia laevigata was investigated using allozyme electrophoresis at seven polymorphic enzyme loci. Little genetic differentiation was observed among East Indian-West Pacific populations separated by thousands of kilometres. Gene flow was estimated to be of the same order of magnitude as that found in the highly connected Great Barrier Reef region. The absence of genetic structure over such a broad geographic range is consistent with any of three conclusions: (1) there is extensive dispersal among widely separated populations across the range examined, (2) there has been a rapid expansion of the East Indian-West Pacific populations in the recent past, or (3) the loci surveyed are under the influence of balancing selection. The first two conclusions are not totally exclusive, since a recent expansion over several thousand generations would also require rapid dispersal at some stage, although present-day levels of dispersal need not be of the same order. With the available data, it is not possible to distinguish which of these mechanisms is the most likely. The most parsimonious conclusion is that extremely low levels of population differentiation are consistent with the existence of a large, homogeneous, panmictic population, with extensive dispersal occurring throughout the East Indian-West Pacific.     

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

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