Microsatellite analysis of 6-hour-old embryos reveals no preferential intraspecific fertilization between cupped oysters Crassostrea gigas and Crassostrea angulata

Experimental examination of reproductive isolation is the first step in und erstanding li,hybridization processes. Here, we studied preferential fertil ization between 2 cupped oyster taxa, Crassostrea angulata and Crassostrea gigas, as a potential prezygotic reproductive isolation. Early examination of sperm competition is now possible by molecular analysis of oyster embryo s. This avoids the confounding effect of differential mortality during the larval stage. Six hundred embryos were sampled from 2 crosses. Three micros atellite loci were enough to determine without ambiguity the taxa of contri buting sires of embryos. No evidence of preferential fertilization between gametes from the same taxa was shown. A significantly higher contribution o f the C. gigas males was revealed with the C. angulata females, but not wit h the C. gigas females, which might suggest early heterosis or interaction differences between gametes. In the light of these results, natural hybridi zation between both taxa call be expected in cases of their geographical co existence, as in the Southern European populations in which both taxa. are in contact as a result of aquaculture development.