Genetic diversity in a genetically improved line of the Pacific oyster Crassostrea gigas with orange shell based on microsatellites and mtDNA data

The shell color is of interest to the shellfish industry and has been regarded as a target trait for a better commercial value. Based on an inbred variant line of the Pacific oyster Crassostrea gigas with orange shell obtained previously, a genetically improved orange-shell line was formed through hybridization and subsequent mass selection. In the present study, the genetic diversity among the base population (G0) and two successive massselected generations of the improved orange-shell line of C. gigas (G1 and G2) were assessed by both 15 microsatellites and mitochondrial cytochrome oxidase I sequences (mtCOI) compared to four outbred populations and three progenitor selected strains. The microsatellite results showed that the genetic variation over two generations of mass selection were maintained since no significant reduction in average number of alleles (Na: 4.87-5.40), expected heterozygosity (He: 0.59-0.61) and expected heterozygosity (Ho: 0.48-0.54) compared with those of G0. The number of different alleles in the generations of improved orange-shell line ranged from 73 to 81 alleles, which was approximate 55-64% less than the 176-204 alleles found in the oysters from wild populations. The Na in G0-G2 were obviously higher than that of the inbred orange-shell line without improvement. The differences in alleles in the improved orange-shell line compared to four wild populations is likely due to small broodstock size resulting in genetic drift over successive generations. Effective population size of G1 and G2 calculated by linkage disequilibrium methods was 63.2 and 89.8, respectively. On a population level, pairwise FST value (FST: 0.14-0.21) along with AMOVA analysis indicated medium to high genetic differentiation between the improved orange-shell line and outbred populations and progenitor selected strains. Overall, the results of microsatellite suggested that the genetic diversity of the improved orange shell line was not affected greatly by mass selection during the process of improvement. This study provides insight into utilization of shell color variants and future genetic improvement through selective breeding in aquaculture.