Divide and broadcast: sexual reproduction in the West Indian brittle star Ophiocomella ophiactoides and its relationship to fissiparity

It is known that asexual reproduction by fission is the dominant mode of propagation in the small (disc diameter <5.0 mm) ophiocomid brittle star Ophiocomella ophiactoides (H. L. Clark). This species is, however, able to reproduce sexually as well. Sexual reproduction occurs throughout much or all of the year in a population of O. ophiactoides at Jamaica. Gonads are often present in recently split individuals, indicating that sexual and asexual reproduction could occur simultaneously. Regeneration of new gonads in the new disc half is underway by the time an individual is competent to split again. Unsexable individuals are significantly smaller than males, which in turn are significantly smaller than females. The unsexable individuals appear to be below a threshold size for gonad production. Fecundity is low, with the largest females producing a maximum of about 7 400 eggs of small (mean diameter 80 m) size. Fertilized eggs develop into planktotrophic ophioplutei which are typical of the larvae of other species of ophiocomid brittle stars in morphology, rate of development and duration of planktonic period. Egg size and larval type for O. ophiactoides are characteristic of brittle stars with the planktotrophic mode of development, but egg number is uncharacteristically low. Fissiparity in O. ophiactoides may have evolved in association with small body size, concomitant low fecundity and a high-risk planktotrophic larva as a means of ensuring reliable recruitment. Available data, although sparse, suggest that this hypothesis may be applicable to fissiparous brittle stars in general. Fission in brittle stars has a broad systematic incidence at the familial level which suggests that it has evolved recurrently. The ability both to divide and broadcast appears to be a useful combination for O. ophiactoides: it can recruit reliably at large size to occupy favourable habitats quickly, but, it can still maintain low level dispersal of larvae which could provide escape from deteriorating habitats and the potential to colonize new habitats.Contribution No. 328 of the Discovery Bay Marine Laboratory, University of the West Indies     

Fissiparity and population genetics of Coscinasterias calamaria

The effect of asexual reproduction on the population genetics of the fissiparous seastar Coscinasterias calamaria was examined at Rottnest Island and the adjacent mainland, Western Australia, in 1985. Field samples and laboratory observations on growth rate and regeneration showed that fission is common at all 14 sites examined. Electrophoretic analysis of six polymorphic enzymes revealed low genotypic diversity and strong genetic disequilibrium at each site, confirming the highly clonal structure of local populations. Striking variation in clonal composition over distances as short as 50 m emphasizes the very localized subdivision of these populations. In the combined sample from all sites, however, the expected range of multilocus genotypes was found in the proportions expected from random mixing of sexually produced larvae, confirming that clonal diversity results from sexual reproduction. Local genotypic diversity was not correlated with fission, emphasizing the difficulty of determining the short-term roles of asexual and larval recruitment in the maintenance of populations. Subtidal populations, however, appear to have both lower incidence of fission and lower larval recruitment than do intertidal populations.     

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.     

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.     

Variation among Desert Topminnows in Their Susceptibility to Attack by Exotic Parasites

Exposure of native species to novel disease organisms is a major threat posed by introduced exotic species. The genetic background of a native population should affect its response to an exotic disease. We evaluate the susceptibility of sexually and asexually reproducing lineages of fish (genus Poeciliopsis ) to infection by a novel trematode parasite ( Gyrodactylus turnbulli ). The parasite commonly infects guppies ( Poecilia reticulata ), which have obtained a worldwide distribution as a consequence of their value in the pet trade. We examined two sexual species and two related clonal (nonrecombinant) lineages of Poeciliopsis that coexist naturally in Sonora, Mexico. One sexual species ( P. monacha ) and one clone ( ML/VIII ) were completely refractory to infection by G. turnbulli . The other sexual species ( P. lucida ) was relatively resistant. The second clone ( ML/VII ) was susceptible, however, and infections led to increased mortality. Chance fixations of alleles that occurred during the hybrid origins of these clonal lineages probably were responsible for the differences in susceptibility between lineages. Lack of heritable genetic diversity within a clonal lineage would hamper its ability to quickly evolve a suitable response to an exotic parasite and would make it particularly vulnerable to extinction.     

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.     

Coexistence and reproductive isolation of the sympatric echinoids<Emphasis Type="Italic"> Diadema savignyi</Emphasis> Michelin and<Emphasis Type="Italic"> Diadema setosum</Emphasis> (Leske) on Kenyan coral reefs

The sympatric echinoids Diadema savignyi and D. setosum coexist in shallow reef lagoons throughout East Africa. The reproductive strategies of these echinoids were studied to investigate reproductive isolation as a possible mechanism for maintaining the coexistence of these closely related species. The annual reproductive cycle and lunar periodicity were determined by gonad index measurements, histological examination of gametogenesis, and induction of spawning with injections of KCl. The peak reproductive period of D. savignyi coincided with the north-east monsoon period (when light and temperatures are high) as gonad indices were high (>8%) beginning in February and peaked at 9.7% in May. Gonad indices subsequently rapidly decreased (by 26%) in June at the beginning of the cooler south-east monsoon period. However, the presence of sperm and ova in most months of the year indicates continuous gametogenesis with reduced reproductive effort during the cooler months. The annual cycle for D. setosum showed less of a seasonal trend as gonad indices remained above 7% throughout much of the year but tended to be highest when temperatures were lower. This is the first confirmation of continuous reproduction in these two species at the equator. The reproductive patterns of both species remained consistent over 2 years of sampling. Both species exhibited a synchronized lunar spawning periodicity during the 3 months sampled, with D. setosum spawning on lunar days 8–10 and D. savignyi spawning after the full moon (lunar days 17–18). Whereas spawning in D. savignyi was very tightly synchronized, 20% of D. setosum individuals still spawned after the peak spawning period. The coexistence of these closely related species appears to be maintained by temporal reproductive isolation during the lunar spawning period reinforced by seasonal differences in reproductive effort.Communicated by O. Kinne, Oldendorf/Luhe     

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.     

Minimal genetic variation among samples of six species of coral reef fishes collected at La Parguera,Puerto Rico,and Discovery Bay,Jamaica

Intraspecific genetic variation among samples of six species of reef fishes,Chromis cyanea, Stegastes partitus, S. planifrons, S. leucostictus, S. dorsopunicans, andThalassoma bifasciatum collected over a 2 wk period in 1990 at La Parguera, Puerto Rico, USA and Discovery Bay, Jamaica, was evaluated using starch-gel electrophoresis. On average, products of 33 protein-coding loci were resolved in each species. Levels of polymorphism (0.95 criterion) ranged from 3.1% inS. dorsopunicans to 42.4% inC. cyanea. Estimates of genetic divergence among samples and indices of genetic subdivision were small in all six study species: mean genetic distances ranged from 0.000 to 0.002 and mean fixation indices ranged from 0.004 to 0.035. Estimates of numbers of migrants per generation (mN _e) ranged from 5.1 to 11.6, indicating that substantial genetic exchange probably occurs over the relatively large geographic distance (ca. 1000 km) separating coral reef communities of La Parguera and Discovery Bay. The estimates ofmN _e may be biased by a sampling strategy involving only two localities, and should therefore be interpreted with caution. With inferences based solely on allozyme frequency data under a primary assumption of neutrality, genetic substructuring of populations of the six study species on a macrogeographic scale appears virtually nonexistent.     

Sexual and asexual production of planulae in reef corals

Electrophoresis was used to provide genetic evidence of the mode of production of brooded planulae in each of four species of scleractinian coral collected from the central region of the Great Barrier Reef during September, October and November 1984. Comparisons were made of the multi-locus genotypes of planulae and their broodparents for two ahermatypic (non zooxanthellate) species,Tubastraea diaphana andT. coccinea and two hermatypic (zooxanthellate) species,Acropora palifera andSeriatopora hystrix. For both ahermatypic species, all planulae were found to be genetically identical to their broodparents, including 91 planulae which were heterozygous for at least one locus. These results are consistent with asexual (ameiotic) reproduction. In contrast, non-parental genotypes were detected in the majority of hermatypic broods, which is consistent with expectations for sexual reproduction with at least some outcrossing. These data confirmed that brooded planulae may be produced both sexually and asexually and countered the suggestion that electrophoretic studies of hermatypic corals may be weakened by the contaminating effect of the enzymes of their symbiotic xooxanthellae.Contribution No. 306 of the Australian Institute of Marine Science     

Hybridization and introgression between Indo-Pacific species of Diadema

We present the first case of hybridization between echinoid species evaluated through genetic markers and morphology. We examined the three tropical Indo-Pacific species of the genus Diadema: D. paucispinum A. Agassiz, D. savignyi (Audouin) Michelin and D. setosum (Leske). Specimens morphologically intermediate between two of these species, D. savignyi and D. setosumhave previously been noted. Fertile hybrids have also been produced in the laboratory. To determine extent of hybridization, we first assayed the allozyme products of 22 loci in individuals which, on the basis of morphology and collection locality, could be unambiguously assigned to one of the three species. We found four loci that were either diagnostic or semi-diagnostic between D. setosum and the other two species, and one locus semi-diagnostic between D. savignyi and D. paucispinum.We then assayed individuals of intermediate morphology to find out whether they had hybrid genotypes. In the Ryukyu Islands, where D. setosum and D. savignyi coexist, we found one specimen which on the basis of all four diagnostic loci was an F₁ hybrid, and several individuals that could be either F₂ (or later-generation) hybrids or progeny of backcrosses.We also found one individual that on both genetic and morphological grounds appeared to belong to D. paucispinum (even though this spcies has only been reported from Hawaii and Kiribati) and three other individuals that carried alleles characteristic of D. paucispinum. Thus, previous reports of hybridization between D. setosum and D. savignyi were correct; it is also possible that larvae of D. paucispinum occasionally arrive at localities outside Hawaii, reach sexual maturity, and hybridize with the other two species. Counts of pure and hybrid morphotypes in other populations across the western tropical Pacific revealed a low but widespread incidence of apparent F₁ hybrids and backcrosses of D.savignyi and D. setosum. However, the existence of diagnostic or semi-diagnostic loci, low interspecific gene-flow estimates based on F _st statistics, and the lack of Hardy-Weinberg or linkage disequilibria among individuals of pure morphology all suggest that gene introgression between the three species is limited.     

Submersible and laboratory observations on Asteroschema tenue,a long-armed euryaline brittle star epizoic on gorgonians

In situ and laboratory obervations of the biology and behaviour of the deep-sea, euryaline brittle star Asteroschema tenue Lyman, from Jamaica, are described. The brittle star is found clinging to the gorgonian Ellisella barbadensis at depths of 168 to 230 m off Jamaica, and is here restricted to this gorgonian. The brittle star, which is usually cryptically coloured, is active principally nocturnally, when it extends arm loops and employs the tube feet for microphagous filter-feeding. Locomotory activity is limited to extension and retraction along the gorgonian, achieved by means of unusual arm-loop patterns. Association with a gorgonian appears to be essential for the survival of the brittle star. The study was conducted in 1985.Order of authors' names is arbitrary     

Ecological aspects of ophiuroids from the phytal of S. W. Atlantic Ocean warm waters

Nine species of ophiuroids — Amphipholis januarii, Axiognathus squamatus, Hemipholis clongata, Ophiocnida scabriuscula, Ophiactis lymani, Ophiactis savignyi, Ophionereis reticulata, Ophioderma appressum and Ophiothrix angulata —were collected in various habitats from the mid-littoral and upper infralittoral on the northern coast of the State of São Paulo, Brazil. The abundance of 6 species occurring in 28 species of algae was investigated. A total of 220 samples was collected at monthly intervals from April, 1969 to May, 1970. The distributional pattern of the populations, as determined by Mobisita's index, shows aggregation of the ophiuroids. No ophiuroid species was limited to one algal species. Each species prefers certain algal species and/or a different level on the shore. The most abundant species on algae was Ophiactis lymani, with a maximum density of 1,510 individuals/100 ml of algae. Four species of ophiuroids occurred inside sponges. The most abundant species in sponges was O. savignyi, with a maximum density of 1,892 individuals/100 g of dry sponge. Two species of ophiuroids occurred inside the bryozoan Schizoporella unicornis. Feeding observations have shown Axiognathus squamatus to capture its food by mucous nets. Decapod crustaceans fed on ophiuroids in aquaria. The copepod Thespesiopsyllus sp. was found in the body cavity of the 2 Ophiactis species. Simultaneous sexual and asexual reproduction were abserved in O. savignyi.     

Micromorphology and ultrastructure of Caribbean sclerosponges

Fine structural analysis of living tissue of the sclerosponges Ceratoporella nicholsoni (Hickson) and Stromatospongia norae Hartman, collected near Discovery Bay, Jamaica, between 1984 and 1986, was carried out using transmission and scanning electron microscopy (TEM and SEM). The thick dermal membrane of these sponges is covered by exopinacocytes having a T shape in sections perpendicular to the surface. A dense, complex glycocalyx is produced at the surface of these cells. Choanocyte chambers are diplodal and unusually small. The inhalant and exhalant canals of both species are characterized by the presence of valvules, made by transverse lamellipodial processes of the endopinacocytes lining them. An abundant and diversified bacterial community occupies almost 20% of the mesohyl. A single layer of active basopinacocytes lines the mesohyl at the interface between the living tissue and the aragonitic skeleton. Basopinacocytes are presumed to be precursors of the irregular fibrillar organic matrix found in the aragonitic skeleton. Sclerocytes and spongocytes are abundant in the vicinity of the siliceous spicules. Typical lophocytes releasing smooth collagen fibrils are common in the dermal membrane as well as in the choanosome where they can be grouped in bundles. Uniquely, C. nicholsoni elaborates rough intercellular fibrils characterized by periodically spaced thickenings. The endolithic algae Ostreobium sp. is present in the most apical zones of the aragonitic skeleton, but does not seem to interfere with its development. The striking micromorphological resemblances between both species are discussed and compared to demosponges.Contribution no. 472 from the Discovery Bay Marine Laboratory, University of the West Indies, Discovery Bay, Jamaica, West Indies     

Genetic influence on caste determination underlying the asexual queen succession system in a termite

The question of how reproductives and sterile workers differentiate within eusocial groups has long been a core issue in the study of social insects. Recent studies have shown that not only environmental factors but also genetic factors affect caste differentiation. In the termite Reticulitermes speratus, queens produce their replacements (neotenics) asexually but use normal sexual reproduction to produce other colony members. Here, we demonstrate a genetic influence on caste determination underlying the asexual queen succession system in this termite species. Thelytoky in termites is accomplished by automixis with terminal fusion, yielding almost completely homozygous offspring; thus, parthenogenetically and sexually produced offspring profoundly differ in heterozygosity. An analysis of the relationship between the reproductive dominance of female neotenics obtained from experimentally orphaned colonies and their genotypes at five microsatellite loci showed that homozygosity at two loci influenced the developmental priority and/or reproductive quality of neotenics. These results suggest the existence of a multi-locus system affecting the queen fecundity and explain why parthenogens have genetic priority to become neotenics in this termite species.     

Variations in benthic community structure in a sub-tropical estuary

Abundances of macroinfaunal invertebrates in sand dollar (Dendraster excentricus) beds and in adjacent areas free of sand dollars were studied at 10 intertidal sites in the Pacific Northwest region of the USA and Canada. Each site was sampled once in late summer of 1977. There were no significant differences in overall diversity of species between the beds and adjacent areas; nor did cluster analysis indicate a sand dollar bed community. However, a polychaete (Armandia brevis) was significantly more abundant inside the beds than just outside them. Conversely, in comparison to the beds, adjacent regions contained significantly more specimens of the following species: a bivalve (Transennella tantilla), several tubicolous crustaceans (Corophium spp., Ampelisca agassizi, and Leptochelia savignyi) and two polychaetes (Glycinde polygnatha and Malacoceros arenicola). Sediment characteristics at each study site were not correlated with the presence or absence of sand dollars; moreover, a comparison among the 10 sites revealed no sediment characteristics typical of sand dollar beds. At most of the sites, significantly fewer tube-building invertebrates occurred inside sand dollar beds than outside. The discussion considers the possibility that mechanical disturbance by the sand dollars tends to exclude some macroinvertebrates (especially tubicolous ones) from sand dollar beds.     

Spatial autocorrelation analysis of small-scale genetic structure in a clonal soft coral with limited larval dispersal

The philopatric larval dispesal and small effective population sizes characteristic of many clonal species should promote the development of significant small-scale genetic structure within populations as a result of isolation-by-distance. We used spatial autocorrelation statistics to detect genetic structure, arising from both clonal reproduction and philopatric dispersal of sexual propagules, for five allozyme loci within populations of the soft coral Alcyonium sp. In a population on Tatoosh Island, Washington, USA, sampled in 1991/1992, we found significant positive spatial autocorrelation at all loci among individuals separated by <40 cm, reflecting the presence of significant smallscale genetic structure due to associations among clonemates. For 4 of 5 loci, however, we detected no significant spatial autocorrelation among the different clones within this population over distances of 1 to 40 m. Analysis of soft-coral populations from six additional, topographically diverse sites in the north-east Pacific also did not reveal significant spatial autocorrelation among clones at any loci. This general lack of spatial autocorrelation of genotypes among clones suggests that significant small-scale genetic structure has not arisen in populations of Alcyonium sp. as a consequence of isolation-by-distance.     

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     

Genetic similarity between two morphologically similar species of polychaetes

Glycera dibranchiata Ehlers and G. americana Leidy are two closely related and similar species of polychaetes. Electrophoretic studies of 11 enzyme systems encoded by 15 loci reveal that both species are polymorphic at about 50% of their enzyme loci, but that G. dibranchiata is the more variable as judged by heterozygosity per individual, genetic diversity, and effective number of alleles per locus. Both species show marked heterozygote deficiencies at nearly all polymorphic loci, possibly as the result of life-history features. Genetic similarity between the Glycera species suggests divergence at a level between species and sibling species compared to the Drosophila willistoni complex.     

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