Phylogeny and adaptive evolution of the brain-development gene microcephalin (MCPH1) in cetaceans

Michael R Mcgowen,John Gatesy,Stephen H Montgomery,Clay Clark

doi:10.1186/1471-2148-11-98

Abstract

BackgroundRepresentatives of Cetacea have the greatest absolute brain size among animals, and the largest relative brain size aside from humans. Despite this, genes implicated in the evolution of large brain size in primates have yet to be surveyed in cetaceans.ResultsWe sequenced ~1240 basepairs of the brain development gene microcephalin (MCPH1) in 38 cetacean species. Alignments of these data and a published complete sequence from Tursiops truncatus with primate MCPH1 were utilized in phylogenetic analyses and to estimate ω (rate of nonsynonymous substitution/rate of synonymous substitution) using site and branch models of molecular evolution. We also tested the hypothesis that selection on MCPH1 was correlated with brain size in cetaceans using a continuous regression analysis that accounted for phylogenetic history. Our analyses revealed widespread signals of adaptive evolution in the MCPH1 of Cetacea and in other subclades of Mammalia, however, there was not a significant positive association between ω and brain size within Cetacea.ConclusionIn conjunction with a recent study of Primates, we find no evidence to support an association between MCPH1 evolution and the evolution of brain size in highly encephalized mammalian species. Our finding of significant positive selection in MCPH1 may be linked to other functions of the gene.

Highlights

Representatives of Cetacea have the greatest absolute brain size among animals, and the largest relative brain size aside from humans
This study did not find evidence for a statistically significant association between selection intensity (ω) in MCPH1, a gene associated with microcephaly, and absolute or relative brain size in cetaceans (Table 3), some intriguing patterns emerged from detailed phylogenetic and molecular analyses of MCPH1
Using complete sequences of MCPH1, we identified ω > 1 in only two cases, along the lineage leading to Tursiops and on the branch leading to Hominidae

Summary

Introduction

Representatives of Cetacea have the greatest absolute brain size among animals, and the largest relative brain size aside from humans. The human lineage has undergone a massive expansion in relative brain and forebrain size, cortical surface area, and overall cognitive ability [1]. Many other vertebrates exhibit increased relative brain and forebrain sizes, as well as complex social and cognitive behaviours. Odontocete cetaceans (toothed whales) have some of the largest brains relative to their body mass among extant mammals [2]. Relative brain size in some odontocete species is greater than non-human primates [3], and in absolute terms, the giant sperm whale (Physeter macrocephalus) has the largest brain of any living organism at a maximum of 10 kg [4].

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