| The Pacific oyster Crassostrea gigas. one of the important economic bivalves, is cultured in the world. The oyster is known as "milk of the sea" for its rich nutrition and delicious taste. In China, the Pacific oyster is widely cultured and has high market demand. Excellent varieties with good resistance, fast growth and higher fecundity will promote the rapid development of China’s oyster industry. In this study, intra-specific hybridization was carried out using two strains of C. gigas, which were successively mass selected for five generation and six generation, respectively. We analyzed the heterosis and genetic diversity of hybrids between the two selected strains, and the major results are as follows:1. Heterosis and combining ability of hybrids between Chinese and Japanese populations of the Pacific oyster C. gigasA complete diallel crosses between two Chinese and Japanese populations of the Pacific oyster C. gigas was carried out. Chinese population (C) was mass selected for five generation, while Japanese population (J) was mass selected for six generation. Growth and survival performances were compared among hybrid and pure populations in different growth stages. Heterosis and combining ability of growth traits were analyzed. The results indicated that the growth and survival of the two reciprocal crosses showed weak heterosis in larval stage and Japanese population got high general combining ability (GCA). The purebred group (JJ) and hybrid cross JC showed significant growth advantage, but purebred group (CC) and the cross (CJ) live well, indicating that there were significant maternal effects. In larval stage, the CJ cross had highest survival rate, with the heterosis ranged from 4.67% to 40.30%. In spat and adult stages, growth of the two reciprocal crosses showed heterosis compared with the purebred groups. Among the four traits (shell height, shell length, shell width and body weight), body weight had the highest heterosis, which varied from 13.61% to 21.71%. Besides, growth of the cross CJ showed positive heterosis in the whole spat and adult stages. The female GCA of Chinese population and male GCA of Japanese population were positive during the later stage. And the special combining ability (SCA) of the reciprocal crosses presented positive in the whole growth stages based on the GCA, suggesting that the CJ cross was the best hybrid cross with meaningful productive potential.2. Development and characterization of polymorphic microsatellite markers in the Pacific oyster C. gigasA total of 480 novel simple sequence repeat (SSR) markers derived from expressed sequence tags (ESTs) and whole genome sequences were developed in the Pacific oyster C. gigas. The number of alleles per locus ranged from 2 to 33, and the observed and expected heterozygosity varied from 0 to 1.0000 and from 0.0655 to 0.9690, respectively.196 loci showed significant departure from hardy-weinberg equilibrium and 23 pairs of loci was detected significant linkage disequilibrium after Bonferroni correction. These microsatellite loci will be useful for studies on QTL mapping and genetic diversity in this species.3. Genetic diversity of 2×2 full diallel crosses of Chinese and Japanese populations of the Pacific oyster C. gigas based on microsatellites and its association with growth-related traitsThirty microsatellite markers were selected to analyze the genetic diversity of 2×2 full diallel crosses of Chinese and Japanese populations of C. gigas and the association between microsatellite markers and growth traits. The results indicated that the average numbers of alleles for the four Pacific oyster stocks ranged from 7.4 to 10.8. The mean levels of observed heterozygosity, expected heterozygosity and PIC varied from 0.5247 to 0.5742,0.7042 to 0.7520 and 0.6572 to 0.7139, respectively. It showed that there was no significant difference between reciprocal cross populations (CJ and JC) and pure populations (CC and JJ). An UPGMA tree based on genetic distance demonstrated that JC was genetically near to JJ and CJ was similar to both pure populations. The association between these microsatellite loci and growth traits was analyzed by least square linear model. The results showed that significant differences were detected between different genotypes of six loci in the four Pacific oyster populations, respectively. The association analysis showed that the genotype BD at locus CgGJ152 was positively associated with the shell length and shell width and the allele C at this locus may have an advantage over D. The genotype AC at locus CgEJ493, DD at locus CgGJ222, allele E at locus CgGJ246 may have positive association with growth traits of the Pacific oyster while the CC at locus CgEJ041, AB at locus CgEJ538 and BE at locus CgGJ222 were favorable genotypes. The results would be valuable for marker-assisted breeding in the oyster.4. Heterozygosity-fitness correlations of microsatellite markers in the Pacific oyster C. gigasHeterozygosity-fitness correlations (HFCs) are frequently used to examine the relationship between genetic diversity and fitness. We investigated HFCs in the full diallel cross breeding populations of C. gigas. Relationships between multilocus heterozygosity, single-locus heterozygosity at microsatellite loci and growth traits were analyzed in 1-year-old cohorts of the Pacific oyster (four populations,30 loci), respectively. When pooled the four populations to analyze HFC, there were positive correlations between multilocus heterozygosity and shell width, body weight while heterozygosity based on single-locus were associated with shell height and shell length. In consideration of spurious HFC caused by population stratification and merging data from several populations, we repeated the analyses using within-population size deviates. The results indicated that no association between heterozygosity and growth traits was detected. This reminded us to be cautious when study HFC in a merging data. It may be better to study HFC in an appropriate age and large number of samples using abundant makers. |
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