| The Pacific oyster Crassostrea gigas is a major global aquaculture species, ranking first in the production among all other aquaculture fish and shellfish species. China produced over 3.5 million tons of cultured oysters in 2007, and C. gigas is one of the most important species. China has become the country with the largest C. gigas production worldwide. However, nearly all the broodstock used today remains unselected, with many hatcheries using wild or cultured brood oyster. In order to improve the productivity traits of C. gigas, a selective breeding program for faster growth was initiated in China in 2006. From 2007, three successive generations of mass selection lines for fast growth in shell height have been established by using three cultured stocks from China, Japan and Korea. This paper evaluates the successive generations of selection including estimates of realized heritability, selection response. Genetic parameters of important traits have been studied by using different families (including dark shell, light shell, dark mantle and light mantle). Meanwhile, growth, survival and genetics for various shell and mantle color have also been studied. The major results and conclusions are as follows:1. Response to selection and realized heritability in successive generationsThe mean increase in shell height for the selection lines was over the controls range 8-14% after three generations of selection. In the first generation, for Stock China, Japan and Korea, response to selection were 0.567±0.044, 0.684±0.041 and 0.269±0.049, respectively; realized heritability were 0.334±0.028, 0.402±0.024 and 0.149±0.027, respectively. In the second generation, for Stock China, Japan and Korea, response to selection were 0.834±0.262, 0.584±0.133 and 0.573±0.015, respectively; realized heritability were 0.457±0.143, 0.312±0.071 and 0.332±0.009, respectively. In the third generation, for Stock China, Japan and Korea, response to selection were 0.812±0.411, 0.868±0.547 and 0.676±0.307, respectively; realized heritability were 0.447±0.226, 0.471±0.297 and 0.367±0.167, respectively. 2. Comparison of quantitative traits among the breeding lines of different geographic populationsThere was significant difference in quantitative traits among different lines. The Japanese line (JS) had significantly higher shell height and total weight than the Chinese line (CS) and Korean line (KS), showing better growth performance. The shell shape index showed no significant difference between CS and KS, but index C of JS was significantly higher than that of CS and KS. There was significant difference in pigmentation of left shell, mantle and adductor muscle scar among the different lines. The level of pigmentation of left shell was the highest in KS, followed by JS and CS. The pigmentation intensity of mantle and adductor muscle scar was in the following order: JS > CS > KS.3. Genetic variability and effective population size assessed by microsatellites in mass selection lines of the Pacific oysterTen polymorphic microsatellite loci were used to examine the genetic diversity, effective population size, genetic relatedness and family structure of two wild populations and four successive mass selection lines. High levels of genetic diversity of Pacific oyster in all populations were detected at ten microsatellite loci, as evidenced by large numbers of alleles per locus (N = 24.0-29.4) and high levels of heterozygosity (He = 0.925-0.956, Ho = 0.718-0.809) within populations, respectively. The different Ne values estimated by different methods revealed relatively uniform trends across all populations, and showed no significant Ne reductions in mass selection lines. The low Rxy values (?0.015-0.054) and no dominant family including large number of individuals existing in any population demonstrated that the genetic variability have not been affected by the selective pressures. This study shows retentions of genetic diversity and effective population sizes in mass selection lines of the Pacific oyster. This information should be useful for future genetic improvement by selective breeding and facilitate the design of suitable management guidelines for these genetic materials. 4. Estimates of heritabilities and genetic correlations for growth in the Pacific oyster larvaeThe shell height and shell length of larvae were measured on day 5, 10, 15, 20 and 25 post-insemination (30 individuals per family). The estimates of heritabilities in the narrow sense were 0.161-0.771 for shell height, 0.139-0.814 for shell length. Heritabilities in narrow sense of paternal half-sib were 0.387, 0.364, 0.262, 0.378 and 0.161 for shell height, and 0.383, 0.398, 0.302, 0.361 and 0.139 for shell length at different stages, respectively. The medium heritability 0.1-0.4 at five different stages shows that it is possible to increase the shell size by selection during larval stage. The estimation of genetic and phenotypic correlation coefficients between the two growth traits at different stages are positive in 0.091-0.820 and 0.224-0.360, respectively. The medium heritability estimates and positive estimated genetic correlations indicate both shell height and shell length should respond favor ably to both direct and indirect selection for growth.5. Correlations analysis and estimates of genetic parameters for important traits in adult Pacific oysterFor growth traits, there were significant positive correlations between shell height and shell length, shell width, total weight, shell weight and meat weight (P < 0.05); there was no correlation between shell height and meat ratio (P > 0.05). For shell indexes, there were significant negative correlations between shell height and shell index A, B and E (P < 0.05); there was significant positive correlation between shell height and shell index C (P < 0.05); there was no correlation between shell height and shell index D (P > 0.05). Shell indexes showed low heritabilities: 0.2±0.09 for shell index B, less than 0.1 or 0 for others. Heritabilities of shell height, shell length, shell width, total weight, shell weight, meat weight and meat ratio were 0.35±0.15, 0.20±0.11, 0.15±0.10, 0.27±0.13, 0.08±0.16, 0.14±0.11 and 0.19±0.14, respectively. All phenotypic correlations between shell height and other growth traits were positive, with the rank of shell weight > total weight > meat weight > shell width > shell length > meat ratio. All genetic correlations between shell height and other growth traits were also positive, with the rank of meat weight > shell length > shell width >total weight > shell weight > meat ratio. 6. Estimates of genetic parameters for growth traits of the Pacific oyster at different agesFor shell height, heritability ranged from 0.13 to 0.34 at different age, and was the lowest (0.13±0.12) on day 500. Phenotypic correlations ranged from 0.09 to 0.24 at different age. Genetic correlations between any two ages before 360 days were positive (0.49-0.83). Negative genetic correlations between 500 and 100 and 200 days were -0.38±0.88, -0.25±0.91, respectively. For shell length, heritability ranged from 0.12 to 0.44 at different age. Phenotypic correlations were 0.13-0.31 from 100 to 360 days. Negative phenotypic correlations between 500 and 100 and 200 days were -0.02±0.10 and -0.01±0.11, respectively. Genetic correlations were 0.23-0.99 from 100 to 360 days. Negative genetic correlations between 500 and 100 days, 280 and 500 days were -0.74±1.40, -0.38±1.23 and -0.19±0.93, respectively. For shell width, heritability ranged from 0.14 to 0.39 at different age, being the lowest on day 500 (0.14±0.12). Positive phenotypic and genetic correlations were observed at different age, 0.01-0.23 and 0.44-0.98, respectively. For total weight, heritability ranged from 0.15 to 0.42 at different age, and was the lowest on day 500 (0.15±0.14). Positive phenotypic and genetic correlations were 0.12-0.30 and -0.28-0.25, respectively.7. Family selection for shell pigmentation in the Pacific oysterAt 350 and 450 days old, shell height in both dark and light shell groups was larger than the control, but there was no significant difference among groups (P > 0.05). For survival rate, no significant difference was found among dark, light and control groups at different ages (P > 0.05). No light shell oysters were found in any dark shell family, and all dark families were dominated by dark shell oysters. In light families, two families were dominated by light shell oysters, but two families contained oysters with four classes shell pigmentation. Most oysters belonged to second or third glass shell pigmentation in control families. The"light"allele in left shell may be dominant over the"dark"allele.8. Family selection for mantle pigmentation in the Pacific oysterAt 350 and 450 days old, there was no significant difference for shell height among different mantle crosses (P > 0.05). For survival rate, no significant difference was found among different mantle crosses at different age (P > 0.05). No light mantle oysters were found in three dark mantle families, and the percentage of dark mantle oysters in dark families was over 90%. In two light families, the proportion of light mantle oysters in family YY1 was above 90%, but family YY2 contained oysters with four classes mantle pigmentation. In reciprocal cross families, the percentages of four classes oysters were almost equal. The"light"allele in mantle may be dominant over the"dark"allele. |
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