Nucleotide sequence differences reveal genetic variation in Neotricula aperta (gastropoda: Pomatiopsidae), the snail host of schistosomiasis in the lower Mekong Basin.

Attwood, S. W. and Johnston, DA (2001) Nucleotide sequence differences reveal genetic variation in Neotricula aperta (gastropoda: Pomatiopsidae), the snail host of schistosomiasis in the lower Mekong Basin. Biological Journal of the Linnean Society, 73 (1), 23-41. (doi:10.1111/j.1095-8312.2001.tb01344.x).

Record type: Article

Abstract

To investigate the phylogenetic relationships among the different strains of the polytypic snail Neotricula aperta, mitochondrial-DNA sequence data were sampled from six populations from central and southern Laos, eastern Cambodia, and northeast Thailand. Part of the cytochrome c oxidase subunit I gene was sequenced for 21 individuals from 7 populations, with a 598 bp sequence used in the analyses. Evolutionary distances were estimated as the Kimura two-parameter distance × 100 (D). The samples were taken from the Mekong, Mul and Xé Bang-Fai rivers of the lower Mekong Basin. The snail Tricula bollingi was used as an outgroup. The least amount of genetic divergence was found where the Thai γ-strain was compared with the other N. aperta samples (D, 1.6–6.4), and next for the Xe Bang-Fai river γ-strain sample when compared with the other N. aperta (D, 2.8–7.5). Large distances were apparent between the β-strain and all the γ-strain populations (D, 4.7–8.3). The γ-strain population of northeast Thailand comprised two cryptic taxa which were relatively well diverged (13 = 2.1). The findings agreed with those of earlier studies based on rRNA gene RFLP variation. The findings were consistent with earlier evidence suggesting dispersal of snails from highland streams in central Laos (e.g. Xé Bang-Fai) into the Mekong river of northeast Thailand; this is proposed as an explanation for the cryptic taxa in the region. The γ-strain of Cambodia and southern Laos has been shown to act as intermediate host for Schistosoma mekongi. Such findings are important in the limitation of Mekong schistosomiasis as they relate to the timing and location of snail control measures. A revised phylogeography is presented for the Triculinae on the basis of the present findings and current palaeo-geographic models for Southeast Asia.

Summary
Genetically distinct strains have been demonstrated within N. aperta and the findings agree well with those of Attwood (1999) based on RFLP data for nuclear gene sequences. Again the findings contrasted with those of Staub et al. (1990) in that significant genetic distances were detected among the original strains in northeast Thailand, and no composite cryptic taxa were found. Evidence has been provided for colonization of the lower Mekong by snails from tributaries in central Laos. The indication of colonization via tributaries in Laos and hybridization are important in the planning of disease limitation programs through snail control, as they relate to the timing and location of control applications. Evidence has also been provided for rapid diversification within the N. aperta complex throughout the Pleistocene, probably a result of the stochastic nature of the evolving Mekong river. A revised phy-logeographic model has been described which agrees well with the current biogeographic deployment of the Triculinae and S. japonicum group taxa. The model explains the presence of a Schistosoma sp. on the Lao border in southern Yunnan (transmitted by the snail Jinhongia), the presence of Schistosoma and Triculinae in the Ping river valley of Thailand, the absence of Neotricula from the upper Mekong river, the absence of S. japonicum and Oncomelania from mainland Southeast Asia, and the observation of Davis et al. (1992) that the most conserved taxa of the Triculinae are found closest to the rivers of northern Burma.

Further work is now necessary using more rapidly evolving sequences of mtDNA, perhaps the control regions, to confirm relationships between ALPH and TBOL and to afford a more quantitative account of population phylogeny. It is now also important to perform a molecular genetic assessment of the congeneric status of Chinese and Lao taxa of Neotricula and Tricula, currently based on anatomical characters. Revised palaeogeography suggests that the closest relative of S. mekongi, aside from S. malayensis, will be Thai S. ‘sinensium’ and this requires confirmation. Careful surveys are required to determine reliably the northern limit of Neotricula in the Mekong river of China. Most importantly, molecular data for more taxa, corresponding to all recognized clades of Pomati-opsidae and Schistosoma, are now required in order to resolve better the phylogeography of Schistosoma in Asia.

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