Evidence of response to unintentional selection for faster development and inbreeding depression in Crassostrea gigas larvae

Underlying consequences of domestication and artificial selection still remain largely unexplored in most aquacultured species. For species with a two phase life cycle, including the Pacific oyster Crassostrea gigas, most genetic studies have focused on the post-metamorphosis juvenile and adult stages, but relatively few considered the larval stage. To assess the consequence of hatchery practices on larval characters, especially growth, we performed a phenotypic study on larval progenies derived from crosses between Pacific oysters from natural beds and farmed Pacific oysters selected for desirable production traits such as rapid growth, for over seven generations. A set of three microsatellite loci was used to compare the genetic variability between the two parental broodstocks and to establish the relatedness between pairs of individuals within each broodstock. The mean relatedness of the hatchery broodstock was significantly different from expectations under the hypothesis of random association (i.e. no relatedness). On one hand, our results show a lower survival performance in the hatchery broodstock, which is associated with a multimodal distribution of growth rates. On the other hand, the hatchery broodstock had a higher proportion of success at metamorphosis. The results suggest that these larvae suffered from inbreeding depression, but that this was offset by better metamorphosis success. The combined effects are likely the result of unintentional selection for faster development in the hatchery through the practice of culling slow growing larvae and a concomitant reduction in the effective population size leading to inbreeding depression.