Molecular phylogenetic investigations of Bulinus (Gastropoda: Planorbidae) in Lake Malawi with comments on the topological incongruence between DNA loci

Freshwater snails of the genus Bulinus, subfamily Bulininae, are widespread in Africa and the Middle East. Several species are intermediate hosts for schistosomes, mammalian blood flukes that cause schistosomiasis. The genus is split into four species groups and without recourse to further molecular DNA studies, phylogenetic relationships within and between species-groups remain obscure. For example, the Bulinus truncatus/tropicus species complex, which cannot be adequately defined with morphological characters, appears to be monophyletic but increased taxon sampling is warranted for confirmation when this group appears to contain species with limited sequence divergence in either mitochondrial 16S rRNA or Cytochome Oxidase subunit I (COI) loci. In the present study, taxonomic sampling is increased through addition of several taxa, including B. nyassanus and B. succinoides both endemic to Lake Malawi. From DNA analyses, these two species appear basal within the B. truncatus/tropicus species complex which provides an interesting evolutionary insight into its origins. The relationship of the B. truncatus/tropicus species complex with the three other groups, however, is more problematic as incongruence between loci and computational methodologies exist. This phenomenon is likely due to mutational saturation of the COI as evidenced by the transition : transversion ratio which also results in placement of the discoidal Indoplanorbis exustus, a member of the Bulininae, within the ingroup. To resolve these more basal incongruencies between species groups inspection of more evolutionary conserved molecular loci is needed, for example, the nuclear histone H3 gene, and 18S and 28S rDNA.