Protein Amino Acid Composition: A Genomic Signature of Encephalization in Mammals

Large brains relative to body size represent an evolutionarily costly adaptation as they are metabolically expensive and demand substantial amounts of time to reach structural and functional maturity thereby exacerbating offspring mortality while delaying reproductive age. In spite of its cost and adaptive impact, no genomic features linked to brain evolution have been found. By conducting a genome-wide analysis in all 37 fully sequenced mammalian genomes, we show that encephalization is significantly correlated with overall protein amino acid composition. This correlation is not a by-product of changes in nucleotide content, lifespan, body size, absolute brain size or genome size; is independent of phylogenetic effects; and is not restricted to brain expressed genes. This is the first report of a relationship between this fundamental and complex trait and changes in protein AA usage, possibly reflecting the high selective demands imposed by the process of encephalization across mammalian lineages.相较于躯体尺寸的较大脑容量，属于演化层面成本高昂的适应性性状：其代谢成本高昂，且需耗费大量时间以实现结构与功能的成熟，进而加重后代死亡风险并推迟生殖起始年龄。尽管该特征存在上述成本与适应性影响，迄今尚未发现与脑演化相关的基因组特征。本研究通过对全部37个已完成全基因组测序的哺乳动物基因组开展全基因组分析，证实脑化（encephalization）与整体蛋白质氨基酸组成存在显著相关性。该相关性并非由核苷酸含量、寿命、躯体尺寸、绝对脑容量或基因组大小变化所带来的副产物，且不受系统发育效应的干扰，同时并非仅局限于脑表达基因。这是首次报道该基础且复杂的性状与蛋白质氨基酸使用模式变化之间的关联，该关联或反映了跨哺乳动物谱系的脑化过程所施加的高强度选择压力。