Population genetic structure of Biomphalaria snails found in East Africa’s river systems

There is a need for current and more detailed information on the population genetic structure of Biomphalaria snails in East Africa’s river systems. Here, we examine the population genetic structure of Biomphalaria populations in East Africa using the mitochondrial cytochrome oxidase 1 (CO1) gene and the internal transcribed spacer 2 (ITS-2) region of nuclear ribosomal DNA. Biomphalaria snails were found at 31 of the 172 sites surveyed, with B. pfeifferi snails found at 23 sites and snails of the Biomphalaria ‘Nilotic Species Complex’ found at 9 sites. Biomphalaria pfeifferi formed a monophyletic group in both CO1 and ITS-2 phylogenetic trees. Similarly, the ‘Nilotic Species Complex’ also formed a monophyletic group in both CO1 and ITS-2 trees but while some individual species within the 'Nilotic Species Complex’ were monophyletic, others were not monophyletic and intermingled in the trees.

A total of 17 CO1 haplotypes (3 shared haplotypes and 14 private haplotypes) were identified for B. pfeifferi populations with a haplotype diversity of 0.798 and nucleotide diversity of 0.004. For ITS-2, a total of 27 B. pfeifferi haplotypes (3 shared haplotypes and 24 private haplotypes) were identified, with a haplotype diversity of 0.471 and nucleotide diversity of 0.002. For the ‘Nilotic Species Complex’, a total of 21 CO1 haplotypes (all private) were identified, with a haplotype diversity of 0.897 and nucleotide diversity of 0.018. For ITS-2, a total of 23 ‘Nilotic Species Complex’ ITS-2 haplotypes (3 shared haplotypes and 20 private haplotypes) were identified with a haplotype diversity of 0.951 and nucleotide diversity of 0.008.

Analysis of Molecular Variance (AMOVA) revealed low genetic variation within B. pfeifferi populations (between 4.83 % and 5.86 %) and high genetic differentiation among populations (between 94.14 % and 95.17 %). For the ‘Nilotic Species Complex’, the genetic diversity was moderate within populations (between 27.83 % and 31.96 %) and among populations (between 68.04 % and 72.17 %). Geographical distance plays a role in bringing about genetic differentiation among Biomphalaria populations in East Africa’s river systems, with sequences from Biomphalaria populations that were found at sites close to each other geographically, taking positions close to each other in the phylogenetic trees.

B. pfeifferi populations studied herein were characterised by low intra-population genetic diversity and high inter-population genetic diversity suggestive of low levels of gene flow and high levels of inbreeding. In the case of the ‘Nilotic Species Complex’, there was moderate to high levels of intra-population genetic diversity and moderate to high levels of inter-population genetic differentiation suggestive of high levels of gene flow. It was established that the population genetic structure of Biomphalaria snails is a possible determinant of the success of S. mansoni infections in the snails.