Genetic structure and geographical variation of Bithynia siamensis goniomphalos sensu lato (Gastropoda: Bithyniidae), the snail intermediate host of Opisthorchis viverrini sensu lato (Digenea: Opisthorchiidae) in the Lower Mekong Basin revealed by mitochondrial DNA sequences

• Three distinct genetic lineages (I–III) of Bithynia siamensis goniomphalos (Bsg) were classified. • Lineages II and III of B. s. goniomphalos were novel. • Lineage III was basal to a clade that included B. s. siamensis (Bss) and lineages I and II of Bsg. • The genetic distance between Bss and Bsg was similar to the level of intraspecific genetic divergence within Bsg. • The genetic makeup of Bsg is sub-structured with genetic structure related to specific catchment systems. The freshwater snail Bithynia siamensis goniomphalos sensu lato is widely distributed in the Lower Mekong Basin where it acts as the first intermediate host of the liver fluke Opisthorchis viverrini , a group 1 carcinogen causing cholangiocarcinoma. This study explores the genetic structure and geographical variation of B. s. goniomphalos from eight previously studied catchments and eight new catchments. These catchments belong to five previously studied catchment systems and one new catchment system (Tonlesap) in the Lower Mekong Basin. Two new catchment systems, Prachin Buri and Bang Pakong from eastern and central Thailand, respectively, were also examined. We collected 289 specimens of B. s. goniomphalos from 15 previously studied localities and 18 new localities in Thailand, Lao PDR (People's Democratic Republic), and Cambodia. The mitochondrial cytochrome c oxidase subunit 1 and 16S ribosomal DNA sequences were used to determine genetic variation. Classification of haplotypes specified 100 at the cox 1 locus and 15 at the rrnL locus. Comparison between 16 catchment populations found significant genetic differences (Ф ST) between all populations. The phylogenetic tree and haplotype network analyses classified B. s. goniomphalos into three evolutionary lineages (lineage I–III). Lineage I contained B. s. goniomphalos from the Mekong, Chi, Mun, Prachin Buri and Bang Pakong catchments in Thailand, including the Nam Ngum catchment in Lao PDR. Lineage II contained all specimens from the Tonlesap catchment, whereas lineage III contained specimens from the Mekong and Sea Bang Heang catchments in Thailand and Lao PDR, respectively. Interestingly, Bithynia siamensis siamensis was placed between lineages I and II of B. s. goniomphalos. This study supports the hypothesis that B. s. goniomphalos is a species complex containing at least three distinct evolutionary lineages in the Lower Mekong Basin, and that comprehensive molecular genetic analyses need to be conducted to further our understanding of the evolutionary and systematic relationships of these Bithynia snail taxa. [ABSTRACT FROM AUTHOR]