Adaptive evolution in mammalian proteins involved in cochlear outer hair cell electromotility

Somatic electromotility in cochlear outer hair cells, as the basis for cochlear amplification, is a mammalian novelty and it is largely dependent upon rapid cell length changes proposed to be mediated by the motor-protein prestin, a member of the solute carrier anion-transport family 26. Thus, one might predict that prestin has specifically evolved in mammals to support this unique mammalian adaptation. Using codon-based likelihood models we found evidences for positive selection in the motor-protein prestin only in the mammalian lineage, supporting the hypothesis that lineage-specific adaptation-driven molecular changes endowed prestin with the ability to mediate somatic electromotility. Moreover, signatures of positive selection were found on the alpha10, but not the alpha9, nicotinic cholinergic receptor subunits. An alpha9alpha10-containing nicotinic cholinergic receptor mediates inhibitory olivocochlear efferent effects on hair cells across vertebrates. Our results suggest that evolution-driven modifications of the alpha10 subunit probably allowed the alpha9alpha10 heteromeric receptor to serve a differential function in the mammalian cochlea. Thus, we describe for the first time at the molecular level signatures of adaptive evolution in two outer hair cell proteins only in the lineage leading to mammals. This finding is most likely related with the roles these proteins play in somatic electromotility and/or its fine tuning.