Selective Breeding And Genetic Analysis Of Shell Color Traits In The Pacific Oyster,Crassostrea Gigas

The Pacific oyster (Crassostrea gigas) is the most widely distributed shellfish species, ranking first in production among all aquatic animals in the world, and also an important mariculture shellfish species in China. The industry of C. gigas in China mainly relies on bloodstock without systematic genetic improvement, which has several problems such as low growth rate, high mortality rate, poor appearance and so on, and then affects the sound development of C. gigas industry. From a long run, launching the selective breeding program is an effective way to resolve these problems. Except for the growth traits of chiefly concern in breeding, the shell coloration which can affect consumer’s preference and then the product value, becomes an important trait of genetic breeding. The first-generation of four kinds of shell color families were established in 2010, which laid a solid foundation for the selective breeding and the genetic analysis of shell coloration in the Pacific oyster. In this study, the growth performance and survival rate of four shell color families in second-generation were studied. The heterosis of the hybrid offspring of three different shell color families was studied. The breeding progress was estimated with the third and fourth generation of golden-shell C. gigas strains. The inheritance of shell pigmentation and the golden shell color were studied. Molecularmarkers linked to shell coloration were identified. The major results are as follow:1. Comparison of growth and survival of four shell color families in second-generation of the Pacific oyster C. gigas.In this study, four kinds of shell color families (white, black, golden and purple) in the second generation of selection were established, the growth performance and survival rate of these families were measured. The results showed that at day 20 the shell height of golden and purple shell families was significantly larger than that of white and black shell families, while shell height of black families was larger than white families. The survival rate order was:white> black> golden> purple. At day 350. the shell height of white and black shell families was significantly smaller than that of golden and purple shell families, while the survival rates of four shell color families were not significantly different from each other.2. Comparison of growth and survival among the hybrid offspring of three different shell color families of Pacific oyster C. gigasA 3x3 diallel crosses of three different shell color families of Pacific oyster (White/W, Black/B, Purple/P) were conducted. Three parental groups and six reciprocal hybrid crosses were established. The growth, survival rate and heterosis of hybrid crosses at larval and grow-out stages were compared among all experimental groups. The results showed that at the larval stage, hybrid cross PB presented significant growth superiority than other experimental groups. The larval survival rate of all the hybrid crosses were higher than that of the parental crosses. All the hybrid crosses showed obvious larval survival heterosis. At the grow-out stage, the shell height of the parental group PP was significantly larger than that of WW and BB. The growth of hybrid PB was the fastest among the six hybrid crosses, followed by hybrid BP, then hybrid WP and PW were the slowest. The survival rate were not different significantly among all the hybrid crosses and parental crosses during the grow-out stage. The heterosis of shell height, shell length, total weight and survival rate of hybrid cross BP and its reciprocal cross PB were between 3.71-15.27%,-2.00-13.10%,11.23-41.56% and -2.77-9.83%, respectively. The other four hybrid groups had no heterosis during the grow-out stage.3. The progress in the third and fourth generation of selective breeding of golden-shell Pacific oyster C. gigasIn the present study, a successive two-generation selection for shell height in the upward direction was performed based on two golden-shell families of the Pacific oyster. The selection response, genetic gain and realized heritability were estimated. In the third generation selection, the average selection response was 0.436 ± 0.138; the genetic gain was 8.253 ± 1.014%:the realized heritabilily was 0.270 ± 0.086. In the fourth generation selection, the average selection response was 0.453 ± 0.153:the genetic gain was 8.515 ± 1.739%; the realized heritability was 0.266 ± 0.090.4. Estimates of heritability of shell pigmentation in the Pacific oyster C. gigasTwenty-two families derived from parents collected from a cultured population in Rushan Bay, Shandong, were produced in the hatchery and raised on suspended longlines for 1 year. At harvest, we sampled shells from each family and quantified their total left-shell pigmentation using digital image analysis. The estimated narrow-sense heritability by two methods were 0.60 ± 0.07 and 0.65 ± 0.09, respectively. We further examined the distributions of shell pigmentation among individuals within each family and found that in majority of families was normally and continuously distributed. However, offspring within two families segregated into "lighter" and "darker" groups in a 1:1 and 3:1 ratio, respectively, supporting the hypothesis that a major gene is segregating in these families.5. Identification and mapping of molecular markers linked to shell pigmentation of the Pacific oyster C. gigasAn Fi-segregating population exhibiting a bimodal distribution of shell pigmentation intensity was obtained by crossing two wild oysters with opposite shell pigmentation. Genomic DNA from nine individuals with lightest shell pigmentation and nine individuals with darkest shell pigmentation were equally pooled for amplified fragment length polymorphism (AFLP) screening. In bulked segregant analysis, six out of 225 selective primer pair combinations produced seven polymorphic fragments tightly associated with shell pigmentation across the segregating population. The seven AFLP markers were mapped onto a single linkage group, and 80% of phenotypic variance could be explained by this locus. In conversion of the seven fragments into sequence-characterized amplified region (SCAR) markers, only one was successfully converted into a co-dominant SCAR marker, named SP-170. The SCAR marker was integrated into a previously constructed linkage map.6. The inheritance of golden shell color in the Pacific oyster C. gigasA shell color segregation study with progenies from a full factorial cross generated among Pacific oysters exhibiting distinct shell colors (golden, white and black) was conducted to investigate the inheritance of the golden shell color and its correlation with dark pigmentation. Results revealed that golden coloration was inherited in a different pattern from dark pigmentation, indicating its different genetic basis. Dark pigmentation was identified as a foreground color while golden or white color were background ones. The locus controlling background colors has two alleles with the allele for golden background being dominant to the allele for white background. In addition, the overlying foreground pigmentation of shells with a golden background was significantly lighter than that of shells with a white background, which suggested an epistatic effect of background color on shell foreground pigmentation.7. Identification of genetic markers linked to golden shell color in the Pacific oyster C. gigasTo improve the selective breeding progress of true strains with complete golden oysters, research was conducted to identify genetic markers linked to the shell color locus. An F1-segregating population was obtained by crossing two oysters with golden shell and white shell. Genomic DNA from eight progenies with golden shell and eight progenies with white shell were equally pooled for amplified fragment length polymorphism (AFLP) screening. In bulked segregant analysis, six out of 225 selective primer pair combinations produced seven polymorphic fragments tightly linked to shell color across the segregating population. The seven AFLP markers were all derived from the golden dam and mapped onto a single linkage group flanking the shell color locus. In conversion of the AFLPs into single-locus PCR-based markers, a sequence-characterized amplified region (SCAR) marker, named SCARJ8-2 and a single nucleotide polymorphism (SNP) marker, named SNPL2-4 and a simple sequence repeat (SSR) marker, named SSRO11-2 were obtained.