Impact of asymmetric gene repertoire between cyclostomes and gnathostomes

Extant vertebrates are divided into the two major groups, cyclostomes and gnathostomes (jawed vertebrates). The former includes jawless fishes, hagfishes and lampreys, and the latter includes all extant jawed vertebrates. In many research fields, the phenotypic traits of the cyclostomes have been considered crucial in understanding the evolutionary process from invertebrates to vertebrates. Recent studies have suggested that the common ancestor of the extant vertebrates including hagfishes and lampreys underwent two-round of whole genome duplications, and thus the genome expansion solely does not account for phenotypic differences between cyclostomes and gnathostomes. Emerging evidence from molecular phylogeny of individual gene families indicates that the gene repertoire expanded at the common ancestor of vertebrates were later reshaped asymmetrically between the two lineages, resulting in the retention of differential gene sets. This also confuses interpretation of conserved synteny which often serves as indicator of orthology and the ploidy level. In this review, current controversy and future perspectives of cyclostome genomics are discussed with reference to evolutionary developmental biology.