Differential patterns of molecular evolution among Haemosporidian parasite groups

Malaria parasites have had profound effects on human populations for millennia, but other terrestrial vertebrates are impacted by malaria as well. Entire species of birds have been driven to extinction, and many others are threatened by population declines. Recent studies have shown that host-switching is quite common among malaria parasite lineages, and these switches often involve a significant shift in the environment in which the parasites find themselves, including nucleated vs non-nucleated red blood cells and red vs white blood cells. Therefore, it is important to understand how parasites adapt to these different host environments. The mitochondrial cytochrome b (cyt b) gene shows evidence of adaptive molecular evolution among malaria parasite groups, putatively because of its critical role in the electron transport chain (ETC) in cellular metabolism. Two hypotheses were addressed here: (1) mitochondrial components of the ETC (cyt b and cytochrome oxidase 1 [COI]) should show evidence of adaptive evolution (i.e. selection) and (2) selection should be evident in host switches. Overall we found a signature of constraint (e.g. purifying selection) across the four genes included here, but we also found evidence of positive selection associated with host switches in cyt b and, surprisingly, in (apicoplast) caseinolytic protease C. These results suggest that evidence of selection should be widespread across these parasite genomes.