Inbreeding depression for fitness-related traits and purging the genetic load in the hermaphroditic bay scallop Argopecten irradians irradians (Mollusca: Bivalvia)

To investigate effects of inbreeding on the fitness related traits in the hermaphroditic bay scallop Argopecten irradians irradians, six groups of offspring with inbreeding coefficients (F) ranging from 0.041 to 0.75 were created by self-fertilization and mass-mating using parents with different inbreeding history. Fitness-related traits of hatching success, larval survival, and size in shell length at days 10, 50, and 160 were studied and compared. All fitness-related traits were significantly affected by inbreeding levels. Compared with their counterpart control of mass-mating line, the line experienced successive two generations of self-fertilization produced significantly lower inbreeding depression for almost all tested traits than those only experienced one generation of self-fertilization. Regression analysis indicated that exponential decrease in both hatching success and larval survival with F (P < 0.001), both quadratic decrease in size at day 10 and day 160 with F (P < 0.001), and only linear decrease in size at day 50 with F (P < 0.001). The degree of phenotypic depression is predicted as that a 10% increase in F will result in a 10.48% decrease in hatching, an 11.11% decrease in survival, a 3.38% decrease in larval size at day 10, a 5.85% decrease in spat size at day 50, and a 9.50% decrease in adult size at day 160. Moreover, the present results indicate that significant purging of the genetic load has occurred after two successive-generations of selfing or severe inbreeding in this organism. These results suggest that it is possible to achieve a characteristic-stable line by successive self-fertilization in the hermaphroditic animals.

► Inbreeding significantly affected fitness-related traits. ► Inbreeding depression has occurred after severe inbreeding. ► Genetic load has been partially purged in the hermaphroditic bay scallop.