Evolutionary genomics of anthroponosis in Cryptosporidium

Human cryptosporidiosis is the leading protozoan cause of diarrhoeal mortality worldwide, and a preponderance of infections is caused by Cryptosporidium hominis and C. parvum. Both species consist of several subtypes with distinct geographical distributions and host preferences (that is, generalist zoonotic and specialist anthroponotic subtypes). The evolutionary processes that drive the adaptation to the human host and the population structures of Cryptosporidium remain unknown. In this study, we analyse 21 whole-genome sequences to elucidate the evolution of anthroponosis. We show that Cryptosporidium parvum splits into two subclades and that the specialist anthroponotic subtype IIc-a shares a subset of loci with C. hominis that is undergoing rapid convergent evolution driven by positive selection. C. parvum subtype IIc-a also has an elevated level of insertion and deletion mutations in the peri-telomeric genes, which is also a characteristic of other specialist subtypes. Genetic exchange between Cryptosporidium subtypes plays a prominent role throughout the evolution of the genus. Interestingly, recombinant regions are enriched for positively selected genes and potential virulence factors, which indicates adaptive introgression. Analysis of 467 gp60 sequences collected from locations across the world shows that the population genetic structure differs markedly between the main zoonotic subtype (isolation-by-distance) and the anthroponotic subtype (admixed population structure). We also show that introgression between the four anthroponotic Cryptosporidium subtypes and species included in this study has occurred recently, probably within the past millennium.