Abstract
In breeding industries, a challenging problem is how to keep genetic diversity over generations. To investigate genetic variation and identify breeding signatures in mass selected lines of Pacific oyster (Crassostrea gigas), three sixth-generation selected lines and four wild populations were assessed using 103 single nucleotide polymorphism (SNP) markers. The genetic diversity data indicated that the selected lines exhibited a significant reduction in the observed heterozygosity and observed number of alleles per locus compared with the wild populations (P≤0.05), indicating the selected lines tended to lose genetic diversity contrasted with the wild populations. The unweighted pair-group method with arithmetic mean (UPGMA) analysis showed that the wild populations and selected lines were not separated into two groups. Using four outlier tests, a total of 17 loci were found under selection at two levels. The global outlier detection suggested that 4 common outlier loci were subject to selection using both the hierarchical island model and Bayesian likelihood approaches. At regional level, 3 SNPs were detected as outlier using at least two outlier tests and one outlier SNP (CgSNP309) was overlapped in the two wild-selected population comparisons. The candidate outlier SNPs provide valuable resources for future association studies in C. gigas.
Highlights
The Pacific oyster (Crassotrea gigas), naturally distributed around Japan, China and Korea, is an important cultivated oyster species worldwide [1]
There was a significant reduction of the observed number of alleles per locus in the selected lines compared with the wild populations (P < 0.05)
Maintenance of genetic variation is known to be important for long-term survival of populations because the level of variation determines their adaptability to environmental changes [38, 39]
Summary
The Pacific oyster (Crassotrea gigas), naturally distributed around Japan, China and Korea, is an important cultivated oyster species worldwide [1]. Many countries has started introducing C. gigas since 1940s, mainly because of its rapid growth rate, high disease resistance and strong environmental adaptability [2]. The C. gigas is one of the most popular oyster species in China, and its main places of production are Shandong and Liaoning provinces [3, 4]. In 2012, China produced 3.94 million tons of oysters with C. gigas as one of the most dominant species [4, 5]. The broodstock used today remains largely unselected and C. gigas has gained little from heredity improvement by selective breeding [6]. With the aim of improving the productivity traits of C. gigas, a breeding program selected for fast growth has been initiated in China.
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