Taxonomie et génétique des populations de Biomphalaria spp. dans quelques régions du grand Sud-Cameroun

Abstract

Basommatophore gastropods of the genus Biomphalaria are intermediate hosts of Schistosoma mansoni, one of parasites responsible of intestinal schistosomiasis in the intertropical zone. The disease is endemic in all administrative regions of Cameroon. Despite the reduction in morbidity due to mass drug administration of praziquantel, its transmission remains very dynamic and heterogeneous. Vector control seem to be the most effective way to achieve elimination. For this, it is necessary to improve the knowledge on the species identification, biology and genetic diversity of these vectors. We therefore undertook to conduct a study on the taxonomy and genetic structure of few Biomphalaria spp. populations mainly sampled in the Southern part of Cameroon. Snails collected during two sampling campaigns were divided into 3 groups destinated to the 3 main study experiments. Biomphalaria species were identified based on their morphology, shell measurement, and molecular (PCR-RFLP) method. The morphometric parameters of 490 specimens were analyzed and univariate and multivariate statistics were performed. The parasites present in living mollusc specimens (225 individuals) were identified by performing a natural cercarial shedding test during 30 days. Microsatellite alleles were revealed in 698 snails, followed by the study of the genetic diversity and the analysis of the demographic parameters within well identified Biomphalaria spp. populations. A total of 1231 and 435 snails were collected respectively at the first and at the second sampling campaigns. Cercarial emission revealed the presence of Echinostoma (Trematoda) larva in 13.77% of snails from only few B. camerunensis populations. Although principal component analysis failed to make a clear distinction between B. pfeifferi and B. camerunensis, univariate statistics revealed three key features that could be used to differentiate them. PCR-RFLP method could easily distinguish B. camerunensis from B. pfeifferi. Phylogenetic analyzes revealed the presence of six and three haplotypes respectively in B. camerunensis and in B. pfeifferi samples. Analysis of ITS molecular diversity revealed a low gene flow among Biomphalaria populations. These snail samples were not demographically expanding according to neutrality tests (D from Tajima and F from Fu). The study of 8 microsatellite markers showed a very poor level of polymorphism with only 28 alleles revealed. The average allelic richness being 2.053 and 1.965 respectively in B. camerunensis and in B. pfeifferi samples. The level of genetic diversity Hs was also very low, the average values being 0.289 and 0.192 respectively in B. camerunensis and B. pfeifferi populations. Observed heterozygosity Ho were also found to be very low (0.075 and 0.053) respectively in B. camerunensis and in B. pfeifferi) samples. In addition, we recorded significant positive values of the FIS inbreeding coefficient, with an

average of 0.742 for B. camerunensis and 0.726 for B. pfeifferi populations, suggesting a high level of apparent heterozygote deficits in the various studied populations. The major cause was the short allele dominance of the loci used. Populations of Biomphalaria spp. studied were then considered to be in Hardy Weinberg equilibrium. The mean fixation index (𝐹ST) obtained across all loci was 0.456 and 0.117 in B. camerunensis and B. pfeifferi populations. We noticed by the hierarchical analysis of the 𝐹, a significant differentiation between samples of B. camerunensis from different sites (𝐹sites/city =0.3284, P = 0.001), different cities (𝐹city/ Region = 0.2336, P = 0.001) and different Regions (𝐹Regions/Total = -0.0322, P = 0.016). Patterning was also significant between the two samples of B. pfeifferi from different sites in the same city (𝐹sites/city = 0.2611, P = 0.001). Pairwise FST estimates between all samples were significantly different (P < 0.00055 and P = 0.001 respectively for B. camerunensis and B. pfeifferi). However, the level of divergence between samples of B. camerunensis were not correlated with the geographical distance existing between the collection sites. Demographic parameters have shown that Biomphalaria populations collected at different dates have a low power of dispersal suggesting a hight effect of genetic drift, especially since the values of the effective population have been very low overall, with the migration rate almost null. This Biomphalaria spp population structure is favourable for a rapid decrease in the population density if vector control is implemented.