High variance in reproductive success of the Pacific oyster (Crassostrea gigas, Thunberg) revealed by microsatellite-based parentage analysis of multifactorial crosses

Marine bivalves are known to have small N-e/N ratios, both in wild and hatchery-propagated populations. This has been interpreted as the consequence of a high variance in reproductive success. However, how much of this variance could be attributed to pre-zygotic reproductive factors or to post-zygotic selection was not yet estimated. To estimate the relative effect of these factors, we have conducted a parentage analysis on two 5 X 5 outbred crosses of Pacific oyster, Crassostrea gigas, using a single microsatellite marker. This was made possible by the high polymorphism encountered at this locus and careful selection of progenitors according to their genotypes. Results show large variance in parental contributions at various developmental stages, leading to a strong reduction of experiment-wide effective population sizes. Segregation distortions fluctuating with time were also observed. Our results show that the observed variance in reproductive success can be attributed to three main factors: gamete quality, sperm-egg interaction and differential viability among genotypes. The comparison of two kinds of crosses, where sperm competition was allowed or not, allowed us to estimate a 20% decrease of the effective population size under sperm competition. A further decrease was observed from larval to juvenile stages. Taken together, the observed gametic, zygotic and genetic effects warn against using too limited a number of progenitors in breeding programmes. (C) 2002 Elsevier Science B.V. All rights reserved.