Landscape genetics: the correlation of spatial and genetic distances of Oncomelania hupensis, the intermediate host snail of Schistosoma japonicum in mainland China

Two internal transcribed spacer (ITS1 and ITS2) sequences of the non-coding region of the rDNA and a 16S RNA gene fragment of the mtDNA of Oncomelania hupensis, the intermediate host snail of Schistosoma japonicum in mainland China, have been investigated with a view to illustrate the influence of the environment on genetic differentiation. Thirteen populations of O. hupensis snails, representing four types of ecological settings, were collected to compare genetic and spatial distances. The length of the ITS1-5.8S-ITS2 gene sequence showed a range from 752 to 796 bp with a guanine-cytosine (GC) content of 49.1-50.3%, while that of the 16S sequence ranged from 505 to 508 bp with a GC content of 33.5-35.1%. The 5.8S fragment was shown to be highly conserved and it was therefore removed in the subsequent analysis. In contrast, the ITS flanking sequences and the 16S fragment were found useful for further study as their degree of polymorphism amounted to 13.2% and 8.6%, respectively. The genetic relationship was investigated using tools based on maximum parsimony, minimum evolution and neighbour-joining algorithms. Four branches of O. hupensis were found to be clearly represented on the 16S phylogenetic tree, namely (i) the mountainous region population (from Sichuan and Yunnan provinces); (ii) the Karst region population (from Guangxi autonomous region); (iii) the population representing the region of swamps and lakes along the Yangtze River basin which stretches through the Anhui, Hunan, Hubei, Jiangxi, Jiangsu and Zhejiang provinces; and (iv) the littoral, hilly region population (from Fujian province). These four population branches were found to be correlated to environmental features based on the concept of landscape ecology attributing genetic differentiation to differences in ecological features. However, only three main branches could be found on the ITS1-ITS2 phylogenetic tree. The swamps and lakes population (from the Yangtze River basin) and the littoral, hilly population (from Fujian province) were clustered on the third branch in spite of these two populations not being spatially related, i.e. no firm genetic demarcation between their snail populations was found. Thus, it seems that the Fujian population does not constitute a separate branch but belongs to the third branch. This fact, together with the strong genetic evidence that the subspecies O. hupensis guangxiensis represents a discrete branch, support the hypothesis that genetic differentiation of O. hupensis in mainland China is ultimately structured by landscape ecology.