The Analysis Of Effect Of Crossbreeding Among Different Selected Strains And Genotype × Environment Interactions In The Pacific Oyster Crassostrea Gigas

As one of the aquatic animals, the Pacific oyster, Crassostrea gigas has the highest worldwide production. With its fast growth and strong adaptability, it has been widely adopted for farming and rapidly developed into one of the most important commercial species in China. Recently, with large-scale cultivation and rapid increase in Pacific oyster production, loss of genetic variability and seedling quality degeneration have emerged, and become major obstacles to the development of oyster industry. Therefore, genetic improvements are needed to overcome the obstacles and to cultivate exellent lines. In this study, we combined the mass selection with hybridization experiments. A 3×3 complete diallel cross was respectively conducted among three populations of Pacific oyster C. gigas from three selected stains, which were successively mass selected for two generation and three generation, and the effect of crossbreeding was evaluated. All experimental crosses were cultured in two different environments and the genetic effects and genotype×environment interactions on the growth characteristics were analyzed. The main results are as follow:1. Hybridization among the different selected strains of the second generation.The results indicated that the nine experimental groups had higher fertilization and hatching rates. Both maternal origin and mating strategy had significant effects on growth and survival throughout the whole larval stage. The larval growth performances of three parental groups had no significant differences (P < 0.05). The growth performances of the larvae of six hybrid crosses were superior to the three parental groups, showing heterosis in the characteristics of shell height. Especially, the cross CJ had the better heterosis in shell height. The other crosses showed relatively lower heterosis. The larval survival rate of the crosses CJ and CK were higher than that of other hybrid crosses and parental crosses (P < 0.05) and the heterosis of these two hybrid crosses ranged from 13.11% to 26.52% and from 40.77% to 69.18%, respectively, but other crosses had no survival rate heterosis. During the spat and adult stages, the heterosis of cross CJ for all performance indicators was positive, whereas that of the reciprocal cross JC was negative. Both hybrid crosses of CK and KC had positive heterosis value in growth and survival. For the reciprocal crosses JK and KJ, the cross JK had larger heterosis value in growth traits than those of the cross KJ at spat and adult stages, of were better combination for seed production.2. Hybridization among the different selected of the third generation. The results showed that the growth of shell height of parental group KK was slower than the parental group JJ and CC, but the difference was not significant at the larval stage. From day 5 to the later larval stage, the shell height growth of the larvae of six hybrid crosses were faster than the three parental groups, showing heterosis in different extent. Especially, the cross CJ, CK and KC had the better heterosis in larval shell height. The maternal origin had significant effects on the growth of shell height at the later larval stage (P < 0.05). For the larval survival rate, the parental group KK was the lowest, and after day 9 the larval survival rate of hybrid crosses CJ and CK was significant larger than the three parental groups (P < 0.05), showing significant heterosis. The maternal origin had significant effects on survival throughout the whole larval stage (P < 0.05). The larvae produced from eggs of C population had the greatest viability, while those from K population were the lowest. At the spat stage, the growth of shell height and shell length of the six hybrid crosses was faster than the three parental groups, showing heterosis in different extent. The cross CJ, CK and KC had faster growth rate, but no significant difference occurred among the other hybrid crosses (P > 0.05).3. The genetic effects and genotype×environment interactions for growth-related traits in Pacific oyster.Genetic effects of growth traits were analyzed using genotype-environment interaction genetic model. The traits of shell length, shell width and body weight were mainly controlled by dominant effects, and the dominant variance ratio to phenotypic variance were 0.31, 0.23 and 0.36, respectively. The trait of shell height was mainly controlled by dominance-environment interaction effects, and the dominance-environment interaction variance ratio to phenotypic variance was 0.37. Comparing other three traits, the shell height had the biggest additive effect, and the total heritability (hG2 + hGE2) reached 60% or so. As a target trait, shell height was comparatively easy selected, but the effect of selection depended on the environmental conditions. In addition, the prediction of genetic merit in cross combination of C and K was the largest, suggesting that the hybrid progeny would have the largest heterosis value.