Neotricula aperta (Gastropoda : Pomatiopsidae), the intermediate host of Schistosoma mekongi: allozyme variation and relationships between Khmer, Lao, and Thai populations

Variation was studied at 13 electrophoretically detected allozyme loci for samples of six populations of the polytypic snail Neotricula aperta; this w as the first time that topotypic material from southern Laos and material f rom Kampuchea had been so examined. In addition, samples of all three strai ns of N. aperta from North East Thailand were examined and the findings com pared with those of Staub et al. (1990). The samples were taken from the Me kong and Mul rivers of the Lower Mekong Basin. A small sample was also incl uded from a gamma-strain population in the Xe-Bang-Fai (XBF) river of centr al Laos, The gamma-strain population of southern Laos and of Kampuchea has been shown to act as an intermediate host for Schistosoma mekongi. In contrast to the work of Staub et al. (1990) significant multilocus genet ic differences (Nei's D, D-N = 0.6-1.2) were detected between the three str ains from Thailand and no cryptic taxon, that included both alpha- and gamm a-strains could be demonstrated. However, there was agreement with the 1990 report in that the beta-strain was divisible into two new taxa, with a red uction in heterozygote deficiency, although the genetic distance was not ma rked (D-N = 0.12). Significant genetic distances were found between gamma-s train snails from the type locality and beta- and gamma-strain samples from North East Thailand (D-N = 2.0 and 1.5, respectively). On the basis of gen etic distance measures, the p-strain is probably a sibling species of N. ap erta and the gamma-strain of North East Thailand may also be a separate spe cies. The population at XBF appears to be N. aperta s.s. The public health implications of the findings are discussed, particularly in the context of Mekong river water resource development.