Gene turnover and diversificationof the α-and β-GlobinGene families in sauropsid vertebrates
The genes that encode the α- and β-chain subunits of vertebrate hemoglobin have served as a model system for elucidating general principles of gene family evolution, but little is known about patterns of evolution in amniotes other thanmammals and birds. Here,we report a comparative genomic analysis of the α- and β-globin gene clusters in sauropsids (archosaurs and nonavian reptiles). The objectives were to characterize changes in the size and membership composition of the α- and β globin gene familieswithin and among themajor sauropsid lineages, to reconstruct the evolutionary history of the sauropsid α- and β-globin genes, to resolve orthologous relationships, and to reconstruct evolutionary changes in the developmental regulation of gene expression. Our comparisons revealed contrasting patterns of evolution in the unlinked α- and β-globin gene clusters. In the α-globin gene cluster,which has remained in the ancestral chromosomal location, evolutionary changes in gene content are attributable to the differential retention of paralogous gene copies thatwere present inthe common ancestor of tetrapods. In the β-globin gene cluster,which was translocated to a newchromosomal location, evolutionary changes in gene content are attributable to differential gene gains (via lineage-specific duplication events) and gene losses (via lineage-specific deletions and inactivations).Consequently, allmajor groups ofamniotes possess unique repertoires ofembryonic and postnatally expressed β-typeglobingenes that diversifiedindependently ineachlineage.These independentlyderived β-typeglobinsdescend from a pair of tandemly linked paralogs in the most recent common ancestor of sauropsids.