Genetic Diversity among Cultured Oysters ( Crassostrea spp.) throughout Thailand

Abstract

Nine samples of Crassostrea belcheri and seven samples of Crassostrea lugubris (=C. iredalei) were collected from up to seven provinces throughout Thailand and analysed for allozyme electrophoresis at six enzyme loci; Aat-B, Est-F2, Lap, Pep-A, Pgi and Pgm. No significant deviations of heterozygosities from Hardy-Weinberg expectations were observed at any locus. Genetic diversity between populations was quantified using the absolute variance of allele frequencies (σ2) and the fixation index (Wright’s FST) in a hierarchical analysis of genetic variation. In C. belcheri samples from both the Andaman Sea and the Gulf of Thailand, on either side of the Malay peninsula, FST was only 0.030. Levels of gene flow estimated as number of migrants per generation (Nm) were similar whether calculated for just Gulf samples (8.2) or for samples from both the Gulf and Andaman Sea (10.9). In C. lugubris, which was only found within the Gulf, FST was smaller at 0.007, and estimated gene flow was substantially higher with Nm = 33.5. For both species, most of genetic variance was found within a few populations in the main culture areas on the west coast of the Gulf of Thailand; 67 % for C. belcheri within four samples from Surat Thani province, and 83 % for C. lugubris within three samples from Chumphon province. Common practises of transplantation appear responsible for homogenisation of allele frequencies in both C. belcheri and C. lugubris.