Genetic contribution of sexual and asexual reproduction to the recruitment of a sexually unbalanced population of Coscinasterias tenuispina (Echinodermata: Asteroidea) in Rio De Janeiro, Brazil

摘要

Most asteroid species reproduce sexually, developing planktotrophic or lecithotrophic larvae or brooding offspring. Some fissiparous species, like Coscinasterias tenuispina, have a mixed life history with both sexual and asexual reproduction. A population of C. tenuispina at Itaipu (22° 59' S, 43° 04' W) is dense and consists only of males larger than 5 mm in radius. This suggests recruitment is based on clonal proliferation. To test this hypothesis, six polymorphic loci were surveyed by allozyme electrophoresis. Levels of sexual and asexual contributions were estimated by three different parameters: (1) number of genotypes to sample size (Ngo:N), which indicates the minimum input of sexual reproduction; (2) ratio of observed multi-locus genotypic diversity (Go) to that expected (Ge) under conditions of sexual reproduction and free recombination, which provides a measure of single-locus and multi-locus equilibrium; (3) maximal input of sexual reproduction, given by the ratio of maximal number of individuals sexually produced (N~*) to sample size (N~*:N). 46 of 66 individuals showed distinct multi-locus genotypes. Only 9 of these multilocus genotypes (19.5%) showed a number of individuals unexpected from random mixing. Furthermore, calculations of N:Ngo (0.697), Go:Ge (0.789) and N~*:N (0.994) are similar to those found for other echinoderm species which reproduce mainly sexually. Contrary to expectations for a population consisting only of large individuals of one sex, clonal proliferation is not the major source of recruits maintaining the population of C. tenuispina in Itapu. A strongly biased sex ratio in a population of fissiparous species cannot be taken as a priori evidence of primarily asexual recruitment.