In cold blood: Compositional bias and positive selection drive the high evolutionary rate of vampire bats mitochondrial genomes

Abstract

Mitochondrial genomes of animals have long been consideredto evolve under the action ofpurifying selection.Nevertheless, there is increasing evidence that they can also undergo episodes of positive selection in response to shifts in physiological or environmental demands. Vampire bats experienced such a shift, as they are the only mammals feeding exclusively on blood and possessing anatomical adaptations to deal with the associated physiological requirements (e.g., ingestion of high amounts of liquid water and iron).We sequenced eight new chiropteran mitogenomes including two species of vampire bats, five representatives of other lineages of phyllostomids and one close outgroup. Conducting detailed comparative mitogenomic analyses,we found evidence for accelerated evolutionary rates at the nucleotide and amino acid levels in vampires.Moreover, themitogenomes of vampire bats are characterized by an increased cytosine (C) contentmirrored by a decrease in thymine (T) comparedwith other chiropterans. Proteins encoded by the vampire bat mitogenomes also exhibit a significant increase in threonine (Thr) and slight reductions in frequency of the hydrophobic residues isoleucine (Ile), valine (Val), methionine (Met), and phenylalanine (Phe). We show that these peculiar substitution patterns can be explained by the co-occurrence of both neutral (mutational bias) and adaptive (positive selection) processes.We propose that vampire batmitogenomesmay have been impacted by selection on mitochondrial proteins to accommodate the metabolism and nutritional qualities of blood meals.