Nuclear intron sequence analysis and its implications for molecular systematics of Asian zoonotic blood flukes (Schistosoma spp.) (Trematoda: Schistosomatidae).

This study explored genetic variation among and within Schistosoma japonicum, S. mekongi, and S. malayensis populations using two intronic regions of the taurocyamine kinase (TK) gene, the bridge intron (TkBridInt) and intron 6 of domain 1 (TkD1Int6). Both introns were successfully amplified across all species and confirmed as true intronic sequences. Sequence analysis of S. japonicum revealed high nucleotide polymorphism in both introns, with the population from Sorsogon (Philippines) exhibiting the highest genetic diversity. Haplotype network analysis indicated several region-specific haplotypes. Pairwise F ST estimates demonstrated significant genetic differentiation among geographical populations, reflecting limited gene flow. The observed low intraspecific but clear interspecific genetic distances further support distinct population structure. Notably, S. malayensis was genetically closer to S. mekongi than to S. japonicum. These findings highlight the potential of intron sequences in detecting intra- and interspecific variation. The findings underscore the importance of applying these markers to more recently collected and geographically comprehensive samples to better elucidate host-associated genetic structuring and transmission dynamics of these zoonotic schistosomes.