科研之友
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摘要
Gene duplication and neofunctionalization are known to be important processes in the evolution of phenotypic complexity. They account for important evolutionary novelties that confer ecological adaptation, such as the major histocompatibility complex (MHC), a multigene family crucial to the vertebrate immune system. In birds, two MHC class II beta (MHCII beta) exon 3 lineages have been recently characterized, and two hypotheses for the evolutionary history of MHCII beta lineages were proposed. These lineages could have arisen either by 1) an ancient duplication and subsequent divergence of one paralog or by 2) recent parallel duplications followed by functional convergence. Here, we compiled a data set consisting of 63 MHCII beta exon 3 sequences from six avian orders to distinguish between these hypotheses and to understand the role of selection in the divergent evolution of the two avian MHCII beta lineages. Based on phylogenetic reconstructions and simulations, we show that a unique duplication event preceding the major avian radiations gave rise to two ancestral MHCII beta lineages that were each likely lost once later during avian evolution. Maximum likelihood estimation shows that following the ancestral duplication, positive selection drove a radical shift from basic to acidic amino acid composition of a protein domain facing the alpha-chain in the MHCII alpha beta-heterodimer. Structural analyses of the MHCII alpha beta-heterodimer highlight that three of these residues are potentially involved in direct interactions with the alpha-chain, suggesting that the shift following duplication may have been accompanied by coevolution of the interacting alpha- and beta-chains. These results provide new insights into the long-term evolutionary relationships among avian MHC genes and open interesting perspectives for comparative and population genomic studies of avian MHC evolution.
- 出版日期2010-10
