Genetic variation in Neotricula aperta, the snail intermediate host of Schistosoma mekongi in the lower Mekong Basin

Restriction fragment length polymorphism (RFLP) was studied for six populat ions of the polytypic snail Neotricula aperta using seven different restric tion enzymes. Samples of all three strains of N. aperta from north-eastern Thailand were examined, as well as topotypes from southern Laos and materia l from Kampuchea and central Laos. Genetic distances were estimated as Nei & Miller's (1990) distance x 100 (D). The samples were taken from the Mekon g, Mul and Xe-Bang-Fai (XBF) rivers of the lower Mekong basin. The gamma-st rain of Kampuchea and southern Laos has been shown to act as intermediate h ost for Schistosoma mekongi. The least amount of genetic divergence was fou nd where the alpha-strain was compared with other taxa (D, 2.1-3.7), and th e largest values for comparisons involving samples of the XBF strain (D, 2. 1-6.4). Large distances were apparent between the b-strain and all the gamm a-strain populations (D, 4.0-5.5). The beta- and XBF-strain may represent a pair of sibling species with respect to N. aperta. The gamma-strain popula tion of north-eastern Thailand comprised two cryptic taxa which were relati vely well diverged (D = 1.6). Only one of these cryptic taxa showed close a ffinity with other gamma-strain taxa; this finding may be important in the control of Mekong schistosomiasis.