The species delimitation problem in the Simulium damnosum complex, blackfly vectors of onchocerciasis

The Simulium damnosum Theobald complex (Diptera: Simuliidae) comprises 57 cytoforms grouped into six subcomplexes. Previous phylogenetic studies using gene sequences have not completely resolved the evolutionary relationships of the cytoforms. The present study investigated the systematics of the complex using a phylogeographic approach. The differentiation between eastern and western forms observed in the phylogenetic studies is confirmed in the estimated haplotype networks. However, haplotypes tend to group in geographical clades and not according to cytoforms. Spatial analyses of the molecular variance also resulted in optimal groupings of sequences that did not correspond to cytoform boundaries. Moreover, Mantel tests showed significant correlations, although not strong, between genetic and geographical distances. This suggests an isolation-by-distance model of differentiation. Furthermore, there are instances in which genetic differentiation between cytoforms is low and not significant. These results indicate a lack of clear genetic differentiation between the cytoforms, which may be explained either by a separation of the taxa recent enough to allow the accumulation of few genetic differences or by recombination between the genomes of the cytoforms, which may be the result of hybridization with introgression or of non-independent evolutionary lineages. The results also emphasize the need for further sampling and for the use of more variable markers in order to clarify the evolutionary history of the group.