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Conspecific sperm precedence occurs when females are exposed to sperm from males of multiple species, but preferentially use sperm of a conspecific. Conspecific sperm precedence and its mechanisms have been documented widely in terrestrial species, in which complex female behaviors or reproductive tract morphologies can allow many opportunities for female choice and sperm competition, however, the opportunity for conspecific sperm precedence in free spawning marine invertebrates has been largely ignored. Two sea urchin species, Echinometra oblonga and E. sp. C, have high levels of interspecific fertilization in no-choice lab crosses, but no natural hybrids have been found. We performed competitive fertilization assays to test for conspecific sperm precedence and found that eggs of both species showed a marked preference for conspecific sperm when fertilized with heterospecific sperm mixtures. Strong rejection of heterospecific sperm would not have been predicted from no-choice assays and helps explain the lack of natural hybrids. We also found significant variation in hybridization success among crosses. Conspecific sperm precedence in free spawning invertebrates shows that the simple surfaces of eggs and sperm provide ample opportunity for egg choice and sperm competition even in the absence of intricate behavior or complex reproductive morphologies. [ABSTRACT FROM AUTHOR]

Presents a study which focused on the reproductive character displacement and the genetics of gamete recognition in tropical sea urchins. Materials and methods; Results; Discussion.

Comparing geographic variation of noncoding nuclear DNA polymorphisms, which presumably are neutral to natural. selection, with geographic variation of- allozymes is potentially a-good way to detect -the effects of selection on allozyme polymorphisms. A previous study of four anonymous nuclear markers in the American oyster, Crussostrea virginica, found dramatic differences in allele frequency between the Gulf of Mexico and the Atlantic Ocean. In contrast, 14 allozyme polymorphisms were fairly uniform in frequency between the two areas. This led to the conclusion that all of the allozyme polymorphisms were kept uniform in frequency by balancing selection. To test the robustness of this pattern, six additional anonymous nuclear DNA polymorphisms were surveyed in oysters from Panacea, Fla. and Charleston, SC. on the Gulf and Atlantic coasts, respectively. Unlike the previously studied DNA markers, the six DNA polymorphisms examined here show geographic variation that is not significantly greater than that of allozymes. The reason for the discrepancy between the two sets of DNA polymorphisms is unclear. [ABSTRACT FROM AUTHOR]