Differences in inferred genome-wide signals of positive selection during the evolution of Trypanosoma cruzi and Leishmania spp. lineages: A result of disparities in host and tissue infection ranges?

Trypanosoma cruzi and Leishmania spp. are kinetoplastids responsible for Chagas disease and Leishmaniasis, neglected tropical diseases for which there are no effective methods of control. These two human pathogens differ widely in the range of mammal species they can infect, their cell/tissue tropism and cell invasion mechanisms. Whether such major biological differences have had any impact on genome-wide patterns of genetic diversification in both pathogens has not been explored. The recent genome sequencing projects of medically important species of Leishmania and T. cruzi lineages provide unique resources for performing comparative evolutionary analyses to address that question. We show that inferred genome-wide signals of positive selection are higher in T. cruzi proteins than in Leishmania spp. proteins. We report significant differences in the fraction of protein-coding genes showing evidence of positive selection in the two groups of parasites, and also report that the intensity of positive selection and the proportion of sites under selection are higher in T. cruzi than in Leishmania spp. The pattern is unlikely to be the result of confounding factors like differences in GC content, average gene length or differences in reproductive mode between the two taxa. We propose that the greater versatility of T. cruzi in its host range, cell tropism and cell invasion mechanisms may explain the observed differences between the two groups of parasites. Genes showing evidence of positive selection within each taxonomic group may be under diversifying selection to evade the immune system and thus, depending on their functions, could represent viable candidates for the development of drugs or vaccines for these neglected human diseases.