Genetic diversity and structure in a selected strain of hybrid oysters between Crassostrea gigas and C. angulata evaluated from microsatellites and mitochondrial COI sequences

Summer mass mortality is a main problem affecting the production of Pacific oyster (Crassostrea gigas). Selective breeding represents a promising method to improve the summer survival rate and growth of the oyster. However, the pitfall is that selection may significantly decrease genetic diversity, leading to inbreeding depression. The current study presented an analysis of genetic diversity in one hybrid oyster (GA -C. gigas 9 x C. angulata male), parental populations (C. gigas 9 x C. gigas male and C. angulata 9 x C. angulata male) and three generations of mass-strain GA strain (GAF1, GAF2 and GAF3) as well as four wild populations of C. gigas (RC, QD, LY and ZS) using 18 microsatellite loci and mitochondrial COI sequences. Heterozygosity levels of GA (He = 0.69) demostrated an increased genetic diversity in hybrids compared to the parental populations. Heterozygosity (He: 0.64-0.67) and COI haplotypes (Hd: 0.195-0.279) revealed no significant (P > 0.05) loss of genetic diversity in selected strains over three generations. However, the average number of alleles (Na ranging from 8.06 to 7.61), polymorphic information content (I ranging from 1.53 to 1.43), heterozygosity (Ho ranging from 0.72 to 0.55) and effective population size (Ne -lin ranging from 144.1 to 51.2) exhibited a tendency to decrease with selection. Moderate genetic structuring (FST: 0.059-0.124; Nei's D: 0.140-0.326) was found among the selected strains and wild populations (QD, LY and ZS). These results indicate that breeders need to be cautious about artificial selection beyond the third generation and work to improve effective population sizes to maintain genetic gains. The results obtained in this study is that it provides important inference and guidance concerning future genetic improve-ment project.

Automated summary

Summer mass mortality is a major problem for Pacific oyster production. Selective breeding can improve summer survival rate and growth, but may decrease genetic diversity and cause inbreeding depression.