Convergent evolution of the genomes of marine mammals

Abstract

Marine mammals from different mammalian orders share several phenotypic traits adapted to the aquatic environment and therefore represent a classic example of convergent evolution. To investigate convergent evolution at the genomic level, we sequenced and performed de novo assembly of the genomes of three species of marine mammals (the killer whale, walrus and manatee) from three mammalian orders that share independently evolved phenotypic adaptations to a marine existence. Our comparative genomic analyses found that convergent amino acid substitutions were widespread throughout the genome and that a subset of these substitutions were in genes evolving under positive selection and putatively associated with a marine phenotype. However, we found higher levels of convergent amino acid substitutions in a control set of terrestrial sister taxa to the marine mammals. Our results suggest that, whereas convergent molecular evolution is relatively common, adaptive molecular convergence linked to phenotypic convergence is comparatively rare.

Figure 1

Phylogeny of 20 eutherian mammalian genome sequences, rooted with a marsupial outgroup. Branches representing the independent evolution of marine mammal lineages, for which tests for positive selection and parallel non-synonymous amino acid substitutions were performed, are coloured red. Branches of the control set of terrestrial taxa, for which tests for positive selection and parallel non-synonymous amino acid substitutions were also performed, are coloured black. Marine mammal illustrations by Uko Gorter.

Figure 2

Genome scans for convergence. Marine mammals genome revealed a large number of parallel substitutions (blue shaded bars) that occurred along the branches of at least two marine mammal lineages since they evolved from a terrestrial ancestor. Parallel substitutions that occurred in positively selected genes are shaded red.