Thesis (M.Sc.)--Chulalongkorn University, 2006

The genetic variation of the two camaenid tree snails, Amphidromus atricallosus and A. inversus which broadly distributed in Southeast Asia was examined. A total of 319 individuals of A. atricallosus were collected from 12 localities in Thailand and one in Singapore, and 144 of A. inversus from 9 localities in Thailand, one in Malaysia and one in Singapore. Tissues from these specimens were subjected to horizontal starch gel electrophoresis. As a result, 13 allozyme loci (including 11 polymorphic) were screened for A. atricallosus and 18 allozyme loci (including five polymorphic) for A. inversus. The degree of heterozygosity was higher in A. atricallosus (Hexp = 0.018-0.201, mean=0.085) than in A. inversus (Hexp = 0-0.023, mean = 0.002). In contrast, overall genetic heterogeneity among local samples was higher in A. inversus (Fst = 0.965) than in A. atricallosus (Fst = 0.781). Within A. atricallosus, the heterogeneity was distinctly higher among the southern Thailand samples (Fst= 0.551) than among the eastern Thailand samples (Fst = 0.144). The high Fst and low heterozygosity values in A. inversus suggest that this species, chiefly occurring off-shore continental-shelf islands experienced a series of strong bottlenecks and subsequent range extensions. The low Fst and high Hexp values for the eastern Thailand of A. atricallosus suggest the frequent gene flows among populations in this region. The southern Thailand populations indicate relative large values in both Fst and Hexp. This may have been involved in an extensive local fragmentation under various selection forces. The souther and eastern samples of A. atricallosus exhibit fixed allele differences at four loci and great genetic distance[D = 0.485-0.946]. The results strongly suggest the separated biological species of snail samples between the two regions.