Population genetic structures of the fissiparous seastar Coscinasterias acutispina in the Sea of Japan

The morphological characteristics and the population genetic structures of the fissiparous seastar Coscinasterias acutispina were investigated for eight sites in the Sea of Japan in order to clarify the presence of sexual and asexual reproduction. Morphological observation based on arm length showed that fission was common at all eight sites examined, indicating the likely production of clonal individuals. A random amplified polymorphic DNA (RAPD) marker was used to detect clones arising by fission and to assess gene flow among sites. A simulation approach using RAPD data revealed the presence of clonal individuals at almost all sites, suggesting the existence of asexual reproduction. The result of phylogenetic analysis according to RAPD genotype showed no relationship between genetic and geographic distances. Considering the limited movement ability of seastar species during the adult phase, these observations suggest the existence of marked gene flow among sites, due to dispersal of planktonic larvae produced by sexual reproduction. These observations suggest that multi-locus genotypic compositions depend on the relative amounts of recruitment from sexual and asexual reproduction in each population.     

Genetical structure within populations of the coral Pocillopora damicornis

Genetic variation of allozymes within populations of Pocillopora damicornis from southwestern Australia was consistent with a primary role of local asexual proliferation of clones in population maintenance. Populations were composed typically of two to four multilocus genotypes accounting for 40 to 80% of individuals, with the remainder assigned to genotypes occasionally in twos or threes but more commonly singly. In the three populations where recruitment was examined genetically, 84% of all first-year recruits was assigned to clones represented in the population's resident adults. The majority of these recruits came from the most highly-replicated of the adult clones. The observed genotypic diversity was, on average, about half that calculated to occur for the same allelic frequencies in a sexually-reproducing population with free recombination. Despite the prevalence of asexual reproduction, both through planulae and fragments, the existence of a sexual mode of reproduction was inferred from the high level of variation produced by pooling populations, the existence of novel genotypes and the concordance of clonal gene frequencies at many sites with the predictions of Hardy-Weinberg equilibria.     

Genetic structure of populations of two closely related brittle stars with contrasting sexual and asexual life histories,with observations on the genetic structure of a second asexual species

Ophiocoma pumila Lütken andOphiocomella ophiactoides (H. L. Clark) are morphologically similar brittle stars with contrasting life histories, the former obligately sexual, the latter fissiparous (capable of both sexual reproduction and asexual proliferation by binary fission). Electrophoretic analysis of five polymorphic enzymes was used to assess the genetic consequences of these differing life histories and provide a genetic perspective on the taxonomic relationship between the two species. Genotypic diversity ofOphiocoma pumila collected at Discovery Bay, Jamaica, in 1985 conformed to expectations for a sexually reproducing population. In contrast, genotypic diversity ofOphiocomella ophiactoides at this site was significantly lower than expected for a sexually reproducing population, due largely to the predominance of clonal proliferation over larval recruitment. Large variation in clonal composition over a short (50 m) distance emphasized the very localized scale of clonal mixing in this species. Allozymic data are indicative of a close sibling species relationship betweenOphiocoma pumila andOphiocomella ophiactoides which suggests that the present generic separation of the two species should be re-examined. Electrophoretic analysis was also used to examine the genetic structure of sponge- and alga-dwelling populations of a second fissiparous brittle star,Ophiactis savignyi (Müller & Troschel), which was also collected at Jamaica in 1985. Striking differences in the allelic composition of sponge- and alga-dwellingO. savignyi were observed. Genotypic diversity ofO. savignyi in sponges was very low, each sponge being dominated by a single genotype. Genotypic diversity ofO. savignyi in algae was higher, although still significantly lower than expectations for a sexually reproducing population. In the light of the highly clonal composition of fissiparous brittle-star populations, the adaptive significance of clonal growth may be related to an increase in the overall fitness of dispersed clones (genets), compared to individuals of strictly sexual counterparts, through greater genotypespecific biomass and, hence, fecundity.Contribution No. 476 of the Discovery Bay Marine Laboratory of the University of the West Indies     

Genetic structure of fissiparous populations of Holothuria (Halodeima) atra on the Great Barrier Reef

Allozyme variation at five polymorphic loci was surveyed in a total of 311 individuals of the sea cucumber Holothuria (Halodeima) atra (Jäger, 1833) collected from two nearshore and two midshelf populations in the Great Barrier Reef in November 1996. Strong deviations in genotype frequencies from those expected under Hardy–Weinberg equilibrium, particularly a large number of heterozygote excesses, confirmed the occurrence of asexual reproduction. Females and males differed significantly in genotype frequencies as a result of differences in either the amount of fission in each sex or in the mortality of asexually produced recruits. The estimated maximum sexual input (number of sexually produced individuals: sample size = N*:N_i) to the two nearshore reefs (38 to 67%) was low relative to that to the two midshelf reefs (74 to 87%). The three ratios and G _o :G _e , N _go :N _i , N*:N _i , (where G _o = observed genotypic diversity, G _e = expected genotypic diversity, N _go = number of genotypes) considered to be indicators of the extent of asexual reproduction, showed a consistent trend in the degree of asexual reproduction similar to that derived from the number of regenerating individuals observed in the populations for which data were available. F-statistic analyses of clonal gene frequencies demonstrated that all populations received sexual recruits from the same gene pool. There was evidence of restricted sexual recruitment to the Fantome population, suggesting that asexual reproduction was dominant only in areas where sexual recruitment was limited by other factors.     

Mode of reproduction,recruitment, and genetic subdivision in the brooding sponge<Emphasis Type="Italic"> Haliclona</Emphasis> sp.

Sponges display a variety of reproductive strategies that have the potential to influence population genetic structure. Histological examination of ten reproductive individuals of the Western Australian sponge Haliclona sp. showed that this species broods embryonic larvae that are potentially limited in dispersal capabilities. Because sponges have the potential to propagate in a number of modes, allozyme electrophoresis was used to assess the relative importance of asexual and sexual reproduction to recruitment, and to quantify genetic subdivision over different spatial scales. Tissue samples from 227 sponges were collected from reefs within two areas 400 km apart: Hamelin Bay and Rottnest Island. Contrary to expectations for highly clonal populations, genotypic diversity within sites was high, no linkage disequilibrium was found, and there was no evidence of genotypic clustering within reefs. There was no genetic evidence that asexual reproduction is important for the maintenance of populations. Genetic comparisons were consistent with mixing of sexually produced recruits within reefs, on a scale up to a few hundred metres, but significant genetic subdivision between reefs (F_ST=0.069 at Hamelin Bay, 0.130 at Rottnest Island) indicated that water gaps of several hundred metres are effective at preventing dispersal. Subdivision between the two areas, separated by 400 km, was moderately greater (F_ST=0.142) than within, but the same alleles were predominant in the two areas. These genetic patterns are consistent with limited dispersal capabilities of brooded larvae.Communicated by G.F. Humphrey, Sydney     

Allozyme electrophoresis demonstrates that the scleractinian coral Montipora digitata is two species

Allozyme electrophoresis at five loci was used to clarify the species status of two morphs of the scleractinian coral Montipora digitata (Dana, 1846), using specimens collected from three locations off the Queensland coast between October 1991 and April 1993. The two morphs, occurring sympatrically, were distinguished by one fixed gene difference at Locus LT-2 and frequency differences at three other loci, indicating reproductive isolation between them. Populations of both morphs were generally in Hardy-Weinberg equilibrium, suggesting that sexual reproduction does occur. Despite sampling designed to reduce the chance of collecting clonemates, the genetic diversity ratio (G _O:G_E) indicated that asexual reproduction also occurs. This is in agreement with the known modes of reproduction for this species. There was significant genetic differentiation between populations of one of the morphs of M. digitata. This could have been influenced by greater degrees of asexual reproduction in this morph, selection within sites, or variation in larval survival and recruitment patterns. Relatively low F _st (differentiation among populations) values found for a broadcast spawning species during this study are in accordance with the hypothesis that brooded planulae are adapted for rapid settlement, whereas planulae produced by broadcast spawners are adapted for widespread dispersal.     

Population structure in the sexually reproducing sea anemone Oulactis muscosa

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

Variation in the genetic composition of coral (Pocillopora damicornis and Acropora palifera) populations from different reef habitats

The genetic structure of populations of the corals Pocillopora damicornis and Acropora palifera was examined in three habitats at One Tree Island during March and April 1993, using electrophoretically detectable variation at six allozyme loci. There were significant genetic differences among populations of P. damicornis within each of the reef crest, lagoon and microatoll habitats. The level of differentiation among populations was similar in each of the habitats. Differences between populations of P. damicornis from lagoon and microatolls were no greater than that within habitats, but genetic differentiation of these from crest populations was much higher. There was no difference in the genetic composition of A. palifera populations within or between the lagoon and microatolls, the only habitats where this species was found. Both coral species had observed:expected (G _O:G_E) genotypic diversity rations >0.80, indicating predominantly sexual reproduction. These data, the high genotype diversity and general conformance of genotype frequencies to those expected under conditions of Hardy-Weinberg, suggested panmixis at each site. The high degree of sexual reproduction in the P. damicornis populations is unusual for a species where asexual reproduction has been the dominant mode of reproduction reported to date. Gene flow in both species was considerable between the lagoon and the closed microatolls. The genetic differences between populations of P. damicornis in these habitats and the reef crest may reflect the relative isolation of all populations within the closed One Tree Lagoon from those outside. However, local currents appear to offer effective means of dispersal between the habitats, suggesting that the genetic differences result from natural selection in the different environments within One Tree Lagoon and the reef crest.     

Differences between native and invasive<Emphasis Type="Italic"> Caulerpa taxifolia</Emphasis>: a link between asexual fragmentation and abundance in invasive populations

The tropical and subtropical marine green alga Caulerpa taxifolia has invaded several temperate regions throughout the world, including southeastern Australia. In this study, I examined how invasive C. taxifolia from temperate southeastern Australia differed from native C. taxifolia from subtropical Moreton Bay, Australia, in the traits proposed as being important to its invasion success: thallus size and density, levels of asexual reproduction (fragmentation) and total biomass. Against the prediction of a large size for invasive C. taxifolia, native populations from Moreton Bay had larger stolons and fronds than invasive populations. However, invasive populations consistently had much higher densities of stolons, fronds and fragmented fronds; and a greater biomass compared to native populations. Average densities at invasive sites exceeded 4,700 stolons and 9,000 fronds/m² and were as high as 27,000 stolons and 95,000 fronds/m², which are the highest reported for C. taxifolia anywhere. Average densities of fragmented fronds at invasive sites were as high as 6,000/m² and up to 45% of all stolons at invasive sites could be directly linked to asexual recruitment via fragmented fronds. Importantly, at invasive locations there was a strong association between asexual reproduction and abundance demonstrated by positive correlations between the density of fragmented fronds and total biomass. These findings are the first to describe quantitative differences between native and invasive C. taxifolia and to demonstrate a link between the high levels of asexual reproduction and high abundance in invasive populations. Although the causes and consequences of high levels of asexual reproduction remain to be explored, this study suggests that changes in demographic and life-history traits during the invasion by C. taxifolia into temperate habitats may contribute to its success there.Communicated by M. S. Johnson, Crawley     

Genetic variation and clonal structure in the scleractinian coral Pocillopora damicornis in the Ryukyu Archipelago, southern Japan

Samples of the scleractinian coral Pocillopora damicornis were collected from six sites located around four islands in the Ryukyu Archipelago, southern Japan, and subjected to allozyme electrophoresis. Seven polymorphic loci were examined for their allelic patterns. The ratio of observed to expected genotypic diversity (0.30 < G _o :G _e  < 0.64), the ratio of the observed number of genotypes to the number of individuals (0.47 < N _g :N _i  < 0.75), and deviations from Hardy–Weinberg equilibrium indicated that asexual reproduction plays a major role in the maintenance of established populations. However, populations were not completely dominated by a single or a few clones, and most clones were represented by only a few individual samples. The high frequency of typhoons in the region suggests that, in P. damicornis, fragmentation caused through occasional exposure to powerful waves is a major mode of asexual reproduction, but asexual production of planulae may also be contributing to the maintenance of populations. A significant genetic differentiation (F ST) was found between the six populations examined (0.027 < F ST < 0.092, average F ST = 0.056). The moderate gene flow is discussed according to characteristics of the larval stage of the species, and to circulation patterns in the region. Received: 7 August 1998 / Accepted: 18 May 1999     

Reproduction and population dynamics of the fissiparous brittle star Ophiactis savignyi (Echinodermata: Ophiuroidea)

Reproductive cycle, asexual reproduction, and population dynamics of the fissiparous brittle star, Ophiactis savignyi, which inhabits the exhalant passages of the sponge Haliclona sp. were examined monthly from February 1991 to January 1992 at Wanlitung, southern Taiwan (22°N; 120°E). Mature gametes were found from March to December, but release was mainly in May and June. Sexual recruits were found from May to December, with the highest frequency (14.1% of population) in June. Sexual recruits composed 2.4% of the 1-yr sample. Sex ratio of male to female was 24:1. Fission occurred throughout the year, although the frequency of recently split individuals was lower from January to June (6 to 31%), and higher from July to December (42 to 52%). The occurrence of fission was highest after spawning. Regenerating individuals composed 48.2% of the 1-yr sample. Population density fluctuated greatly during summer due to recruitment by fission and mortality or dispersal due to the stressful environmental conditions. Both sexual and asexual reproduction of O. savignyi were successful at this site.     

An AFLP-based test of clonality in widespread, putatively asexual populations of Porphyra umbilicalis (Rhodophyta) in the Northwest Atlantic with an in silico analysis for bacterial contamination

Asexual reproduction often predominates in populations of species experiencing range expansion. Locally selected genotypes can lead to the establishment of clonal populations that accumulate genetic diversity through time. Sexual reproduction has never been observed in extensive field and culture studies of the red alga Porphyra umbilicalis from Maine, USA, even though sexual reproduction predominates in this species in the eastern Atlantic (Europe). This suggests that Maine populations are indeed asexual and might consist of one or only a few genetic clones; we have tested this using AFLPs. Individuals were sampled at two sites in Maine (Cobscook Bay [n?=?25], Schoodic Point [n?=?26]) and compared to sexual individuals from England (Sidmouth [n?=?17]). The AFLP analysis determined that individuals at two sites in Maine containing putative asexuals were not strictly clonal; however, two multilocus lineages were sampled more than once. Two genetic clones, one at each Maine site, were comprised of 6 individuals each; the 39 additional Maine individuals had distinctive AFLP genotypes. However, when the individuals from Maine were compared with a known sexual population from Sidmouth, England, much greater genetic diversity was found within the sexual population in England. Finally, we examine how preparation of field-collected material for AFLP investigations can affect the inclusion of non-target DNA and demonstrate an in silico approach for removing some cryptic contaminants from analysis.     

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

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

Patterns of genetic subdivision in populations of a clonal cnidarian,Zoanthus coppingeri,from the Great Barrier Reef

Samples of an intertidal zoanthid, Zoanthus coppingeri, Haddon and Shackelton, 1891, were collected from three localities in the Great Barrier Reef region during 1992–1993, and subjected to allozyme electrophoretic analysis at seven polymorphic loci. The reduced ratio of observed to expected genotypic diversity indicated that populations were partly clonal, but they were not dominated by a few clones as occurs in some other cnidarians. Regular disturbance by wave action is postulated to prevent the formation of large stands of particular clones by clearing space and mixing genotypes over small scales. The sexual origin of clonal genotypes was confirmed by conformance to Hardy-Weinberg predictions of genotype frequencies at all but one locus. Values of the standardised genetic variance among populations, F _ST , were highly significant between localities and between replicate sites within localities separated by only 50 m. Strong genetic structure has not previously been described in a Great Barrier Reef invertebrate species, and is considered to be the consequence of stochastic changes in gene frequencies as a result of low levels of gene flow. High clonal longevity and low recruitment rates may maintain genetic differences over long periods. Similar effects may be seen in other Great Barrier Reef invertebrate species with comparable reproductive patterns.     

Genetic study of the extent and consequences of sexual and asexual reproduction in the deep-sea epizoic anemones Amphianthus inornata and Kadosactis commensalis (Cnidaria: Anthozoa)

The anemone Amphianthus inornata is found at bathyal depths living on colonies of the gorgonian Acanella arbuscula. Previous studies of the morphology and reproductive stage of this anemone, during different times of the year, have indicated that it reproduces sexually on a seasonal basis. A small proportion of the study population were also reported to be undergoing asexual reproduction by fission. The anemone Kadosactis commensalis is also bathyal, but lives mainly on the holothurian Paroriza prouhoi. Previous morphological studies have indicated that K. commensalis is a protandrous hermaphrodite that exhibits non-seasonal sexual reproduction only. In the present study, allozyme electrophoresis was used to examine the prevalence and genetic consequences of asexual reproduction in a population of Amphianthus inornata from 2 200 m in the Rockall Trough, North Atlantic Ocean. Genetic evidence, from five randomly selected polymorphic enzyme loci, for asexual reproduction in this species was weak. Exact tests indicated that genotype frequencies did not differ significantly from those expected under Hardy–Weinberg equilibrium. F _IS (correlation of homologous alleles with reference to local population, assuming random mating) values also did not differ significantly from zero, and observed heterozygosity (H _o =0.446) and genotypic diversity (G _o =17.0387) were very similar to Hardy–Weinberg expected frequencies (H _e =0.446; G ^* _e =17.0010). Evidence suggests that the contribution of asexual reproduction to recruitment in the study population of A. inornata is low. For a single population of K. commensalis from 4 850 m on the Porcupine Abyssal Plain, North Atlantic Ocean, the hypothesis that inbreeding due to reduced occurrence of outcrossing between anemones on a single holothurian host was examined by electrophoresis of ten randomly selected enzyme loci. Single-locus genotypic frequencies were significantly different from expected frequencies for one locus P≤ 0.05, hexokinase-1 (Hex-1)]. F _IS values were significantly different from zero for two enzyme loci (Hex-1 and Hex-2, P≤ 0.01 and P≤ 0.05, respectively), and the overall observed heterozygosity was lower than the expected heterozygosity (H _o =0.125, H _e =0.140). The hypothesis of inbreeding could not be rejected by the present study, although sample size was small (N=55), leading to possible bias in tests of significance. Genetic variation in A. inornata was higher than that recorded for most eukaryotes, although interlocus sampling error for only five loci is high. High genetic variability has been found in other sea anemones, and has been related to high longevity and mixed reproductive modes. Genetic variation in K. commensalis was in the higher range of that found in other eukaryotes, and is not unusual for anemones. Received: 5 August 1996 / Accepted: 11 December 1996     

Genetic subdivision within the eastern Australian population of the sea anemoneActinia tenebrosa

An electrophoretic survey of allozyme variation revealed substantial genetic differentiation within the eastern Australian population ofActinia tenebrosa. This differentiation appears to reflect the effects of both asexual reproduction and limited gene flow among local populations separated by up to 1050 km. Variation was assessed within groups of 27 to 55 adults sampled between September 1985 and December 1988 collected from small areas of shore within each of 24 local populations. All individuals were collected from stable rock platforms, with the exception of Boulder Bay, where some sea anemones were removed from small mobile boulders. High levels of variability were detected for each of seven enzyme-encoding loci. The patterns of genotypic variation detected imply that local populations are maintained by predominantly asexually generated recruitment. Levels of multi-locus genotypic diversity within samples were consistently less than 50% of the level expected for sexual reproduction with free recombination. This was reflected by the detection of relatively low numbers of multi-locus genotypes and significant departures from expectations for single-locus Hardy-Weinberg equilibria within 17 of the 24 local populations. Standardised genetic variances (F _ST ), calculated from the genotypes of all individual adults were typically much greater than those expected for marine organisms with widely dispersed larvae. The former values were reduced, but were still extremely large when clonal genotype frequencies were substituted into the calculation. These data imply that although widely dispersed larvae may be an important source of initial colonists, levels of gene flow among established local populations are low. Furthermore, cluster analysis revealed a clear subdivision of the population into northern and southern groups. However, this subdivision was largely explained by strong clinal variation at a GPI-encoding locus. For this locus, allele frequencies ranged from fixation of the A allele in samples from the 12 most northern sites to near fixation of the alternative B allele in southern samples. Subdivision of the eastern Australian population is consistent with the predicted off-shore movement of the Eastern Australian Current close to the border between Victoria and New South Wales. However, the split into northern and southern regions, as evidenced by the variation forGpi, could reflect patterns of gene flow and/or other factors such as natural selection or the recent patterns of colonisation.Contribution No. 78 from the Ecology and Genetics Group of the University of Wollongong     

The production of sexual and asexual larvae within single broods of the scleractinian coral, Pocillopora damicornis

The genotypic compositions of two populations of the brooding coral, Pocillopora damicornis, were studied by exhaustive sampling of adult colonies on a fine scale in southern Taiwan. In addition, 100 larvae were randomly selected among more than 1,000 brooded larvae collected from a single broodparent within each population. Using 7 polymorphic microsatellite markers, both populations were found to be highly clonal, and 7 clonal lineages were characterized. One clonal lineage (C1) dominated both study areas and comprises 54.9% of all colonies sampled, while any of the other 6 clonal lineages represented no more than 5%. Among the 100 larvae randomly selected for genotyping from each broodparent, the extent of clonal reproduction was high, and only 29 and 6 larvae, respectively, were found to be genotypically different from their broodparent. Among the 35 genotypically distinct larvae, 33 were thought to be derived from sexual reproduction, and 2 were assumed to be clonal propagules that had undergone somatic mutations. Two genotypically identical larvae were also found in one of the 2 sexually derived larva arrays, indicating the possible existence of polyembryony. The high proportions of clonality in both adult colonies and brooded larvae suggest that asexually produced larvae might significantly contribute to the recruitment of local populations. The dense clonal population of P. damicornis in the study area favors the quick recolonization view of clonal propagules after disturbances.     

Population structure in the coral Pavona cactus: clonal genotypes show little phenotypic plasticity

Electrophoretic data were used to examine the relationship between genotype and growth form, and to assess the contribution of asexual reproduction to recruitment within six local populations of the agaricid coral Pavona cactus from the central and northern Great Barrier Reef. The data revealed the presence of highly replicated clonal genotypes in the five densest populations. In three cases, samples of 50 to 60 colonies collected from 25 m² areas within the Eclipse Island and Pandora Reef populations each consisted of colonies with only two distinct 4-locus genotypes. More intensive sampling of the population at Eclipse Island showed that colonies with the same 4-locus genotypes were separated by distances of up to 93 m. In contrast, the population at Watson's Bay (Lizard Island) consisted of a few widely scattered and genetically distinct colonies. The samples collected from each population contained a range of growth forms including, in some cases, the entire morphological range described for this species. A strong association of genotype and growth form was detected in samples from all populations, with the exception of Watson's Bay where no such comparison was possible. Nevertheless, some genotypes were represented by more than one growth form and this could reflect the effects of limited phenotypic plasticity. The effects of asexual reproduction reduced the value of these data as a test of the genetic connectedness of the six populations studied. The genetic distance between samples was not simply correlated with geographic distance. This may reflect either the true structure of the breeding population(s) or the effects of asexual reproduction on estimates of allelic frequencies.Contribution No. 47 from the Ecology and Genetics Group of the University of Wollongong     

Asexual viviparity and population genetics of Actinia tenebrosa

The intertidal anemone Actinia tenebrosa is viviparous. An electrophoretic study of 3 polymorphic enzymes in Western Australian populations has confirmed genetic identity of adults and their brood young, indicating asexual reproduction. The population effects of this clonal reproduction are seen as linkage disequilibrium and departures from Hardy-Weinberg equilibrium within populations, and large differences between populations. The data also suggest occasional sexual reproduction, and the likelihood of a mixture of reproductive modes in this species.     

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

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