Population genetic structure of the gastropod species Bulinus truncatus

Background: Gastropod snails remain strongly understudied, despite their
important role in transmitting parasitic diseases. Knowledge on their
distribution and population dynamics increases our understanding of
processes driving disease transmission. This is the first study using High
Throughput Sequencing (HTS) to elucidate the population genetic structure
of the hermaphroditic snail Bulinus truncatus (Gastropoda, Heterobranchia)
on a regional (17 to 150 km) and an inter-regional (1,000 – 5,400 km)
scale. This snail species acts as an intermediate host of Schistosoma
haematobium and Schistosoma bovis, which cause human and animal
schistosomiasis respectively. Methods: Bulinus truncatus snails were
collected in Senegal, Cameroon, Egypt and France and identified through
DNA barcoding. A single-end Genotyping by Sequencing (GBS) library,
comprising of 87 snail specimens from the respective countries, was built
and sequenced on an Illumina HiSeq 2000 platform. Reads were mapped
against S. bovis and S. haematobium reference genomes to identify
schistosome infections and Single Nucleotide Polymorphisms (SNPs) were
scored using the Stacks pipeline. These SNPs were used to estimate genetic
diversity, assess population structure and to construct phylogenetic trees
of Bulinus truncatus. Results: A total of 10,750 SNPs were scored and used
in downstream analyses. The phylogenetic analysis identified five clades,
each consisting of snails from a single country but with two distinct
clades within Senegal. Genetic diversity was low in all populations,
reflecting high selfing rates, but varied between locations due to habitat
variability. Significant genetic differentiation and isolation by distance
patterns were observed at both spatial scales, indicating that gene flow
is not strong enough to counteract the effects of population bottlenecks,
high selfing rates and genetic drift. Remarkably, the population genetic
differentiation on a regional scale (i.e. within Senegal) was as large as
between populations on an inter-regional scale. The blind GBS technique
was able to pick up parasite DNA in snail tissue, demonstrating the
potential of HTS techniques to further elucidate the role of snail species
in parasite transmission. Conclusions: HTS techniques offer a valuable
toolbox to further investigate the population genetic patterns of
schistosome intermediate host snails and the role of snail species in
parasite transmission.