Abstract
Increased brain size is thought to have played an important role in the evolution of mammals and is a highly variable trait across lineages. Variations in brain size are closely linked to corresponding variations in the size of the neocortex, a distinct mammalian evolutionary innovation. The genomic features that explain and/or accompany variations in the relative size of the neocortex remain unknown. By comparing the genomes of 28 mammalian species, we show that neocortical expansion relative to the rest of the brain is associated with variations in gene family size (GFS) of gene families that are significantly enriched in biological functions associated with chemotaxis, cell–cell signalling and immune response. Importantly, we find that previously reported GFS variations associated with increased brain size are largely accounted for by the stronger link between neocortex expansion and variations in the size of gene families. Moreover, genes within these families are more prominently expressed in the human neocortex during early compared with adult development. These results suggest that changes in GFS underlie morphological adaptations during brain evolution in mammalian lineages.
Highlights
Increased brain size in mammals when compared with other vertebrate taxa is thought to have played an important role in the expansion of this clade
In order to assess the association between gene family size, GFS, and neocortex expansion, Nr, values were compiled from the literature for 28 mammalian species with fully sequenced genomes
In order to assess whether the observed bias towards strong correlations between GFS and Nr preferentially involves gene families associated with specific biological functions, we assessed the statistical over-representation of functional annotations
Summary
Increased brain size in mammals when compared with other vertebrate taxa is thought to have played an important role in the expansion of this clade. Increased brain size during evolution has been previously related to increased behavioural complexity and the ability to cope with a changing environment [1,2]. The precise evolutionary drivers of brain size expansion in mammals and its relation to behavioural ability are still unclear and remain a topic of much interest and debate. This is complicated by the fact that different mammalian clades have differences in the degree of size-related changes in brain tissue [3]. Large brains are associated with a high metabolic cost [8,9,10,11] as well as higher demands of parental investment and delayed sexual maturation [12 –16]
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