Genetic Variation in Wild and Hatchery Stocks of Suminoe Oyster (Crassostrea ariakensis) Assessed by PCR-RFLP and Microsatellite Markers

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Abstract
Genetic variation in wild Asian populations and U.S. hatchery stocks of Crassostrea ariakensis was examined using polymerase chain reactions with restriction fragment length polymorphism (PCR-RFLP) analysis of both the mitochondrial COI gene and the nuclear internal transcribed spacer (ITS) 1 region and using 3 microsatellite markers. Hierarchical analysis of molecular variance and pairwise comparisons revealed significant differentiation (P < 0.05) between samples from the northern region, represented by collections from China and Japan, and 2 of 3 samples from southern China. PCR-RFLP patterns were identified that were diagnostic for the northern (N-type) and southern (S-type) groups. Microsatellite marker profiles were used to assign each oyster to one of the two northern or two southern populations. Results for more than 97% of the oysters were consistent with the PCR-RFLP patterns observed for each individual in that oysters with N-type patterns were assigned to one of the northern populations and those with S-type patterns to one of the southern populations. At one site of the Beihai (B) region in southern China a mix of individuals with either the N-type or S-type PCR-RFLP genotypes was found. No heterozygotes at the nuclear ITS-1 locus were found in the sample, possibly indicating reproductive isolation in sympatry. Microsatellite assignment test results of the B individuals were also consistent with identifications as either the N-type or S-type based on PCR-RFLP patterns. The parental population for one hatchery stock was this B sample, which initially was composed of almost equal numbers of northern and southern genetic types. After hatchery spawns, however, more than 97% of the progeny fell into the northern genetic group by PCR-RFLP and microsatellite assignment test analyses, indicating that the individuals with the southern genotype contributed little to the spawn, owing to gametic incompatibility, differential larval survival, or a difference in timing of sexual maturity. Overall, results suggested that oysters collected as C. ariakensis in this study, and likely in other studies as well, include two different sympatric species with some degree of reproductive isolation.