Effects of environmental change on genetic diversity and distribution pf phelbotomus ariasi, a vector of visceral leishmaniasis in Southwest Europe

Abstract

Leishmania infantum is the causative agent of zoonotic visceral leishmaniasis (ZVL) in the Mediterranean region, with the domestic dog as the main reservoir host. Phlebotomus (Larroussius) ariasi is the principal vector in cooler, forested ecotopes in southwest Europe, which suggests that it might be subject to environmental and geographical isolation. However, the population genetics of P. ariasi had been little studied before this thesis, which investigated how the population differentiation of this vector might affect its ability to spread northwards, or persist in the Mediterranean region, in response to climate and habitat change. Thirty-six spatio-temporal populations of P. ariasi were molecularly characterized across its range, predominantly from southwest France but including geographical outgroups from Spain, Portugal and North Africa. Phylogenetic and population genetic assessments were made based on five DNA sequences: mitochondrial cytochrome b, nuclear elongation factor-la and apyrase, plus two anonymous nuclear loci, AAm20 and AAm24. The results demonstrated the absence of cryptic sibling species of P. ariasi and the selective neutrality of each locus. Mitochondrial DNA revealed a historical phylogeographic structure, which was consistent with Pleistocene climate change driving multiple haplogroup divergences within glacial refuges and phalanx-like population expansions in interglacial periods. Nuclear loci mostly showed isolation by distance, but some supported restricted gene flow between the Pyrenees and the Massif Central, France, as indicated by cytochrome b. A glacial refuge may have existed north of the Pyrenees. The genetic diversity observed in the northeast Pyrenees, France, permitted an assessment of the effects of broadleaf forest fragmentation on the differentiation of P. ariasi. No conclusive evidence was found to support contemporary genetic substructuring or impoverishment associated with a recent increase in forest fragmentation. The salivary peptide apyrase revealed a geographical pattern of polymorphism consistent with the other selectively neutral loci. A range of selection tests indicated that apyrase was not evolving under positive directional or balancing selection and, therefore, a genetic arms race with the mammalian host and/or Leishmania parasite was not supported. The approach taken provides a proof of principle for helping to assess apyrase and other salivary peptides as vaccine candidates against leishmaniasis.