Relaxed constraint and thermal desensitization of the cold-sensing ion channel TRPM8 in mammoths

Abstract

Unlike the living elephants, which live in warm tropical and subtropical habitats, mammoths lived in extreme cold environments where average winter temperatures ranged from -30{degrees} to -50{degrees}C. Like other cold adapted and artic species, mammoths evolved of suite of morphological and molecular adaptations that facilitated life in the cold. Here we reanalyze mammoth genomes and find that genes with mammoth-specific amino acid substitutions are enriched in functions related to temperature sensation. Among the genes with mammoth-specific amino acid substitutions is TRPM8, which mediates sensitivity to nonnoxious cool temperatures from 25- 28{degrees}C and cooling sensations induced by the chemical agonists menthol and icilin. We find that TRPM8 evolved rapidly in the mammoth stem-lineage, likely because of an episode of relaxed purifying selection. Functional characterization of resurrected mammoth and ancestral TRPM8 indicates that the mammoth TRPM8 is desensitized to cold but maintains sensitivity to menthol and icilin. These data suggest that as mammoths evolved into a cold tolerant species they lost the need for a cold-sensitive TRPM8.