RESUMEN
The susceptibility of Oncomelania hupensis to Schistosoma japonicum has been shown to be very variable from population to population. This study was undertaken to relate this vari-ation in susceptibility to genetic variability. A total of 33 populations of O. hupensis from sev-eral wide spread localities in mainland China were collected. Snails of all populations were ex-posed in groups to miracidia (20 miracidia per snail) of one laboratory strain of S. japonicum. Infection rate varied from zero to 93. 9%. Population genetic study was carried out by using horizontal starch gel electrophoresis with the 12 specific enzyme systems. A total of 17 genet-ically interpretable loci were observed,7 of these were polymorphic loci. They were Ap,Est -4,Est -5,Got ,Mdh -2,Idh -2 ,Xdh. Allele frequencies were calculated for each population. For logistic regression analysis,susceptibility of the population was scored as 0 when no snail became infected or as 1 when infection rate was more than 0%. Variation in susceptibility was analysed by logistic regression models with genotype frequencies of the 7 polymorphic loci as independent variables. In order to identify the most important factors, both forward and backward stepwise procedures were performed. Results showed that only one variable (genotype frequency of Mdh - 2) entered into the model with a regression coefficient of 13. 706 9 and a constant of -8. 740 9. Results also showed the established model fitted the data well in goodness of fit test,and 93.9% of the populations were correctly classified. It is suggested that a close linkage exists between Mdh-2 gene locus and the susceptibility of O. hupensis.