Lamprey Krox Reveals Ancient Origin of Vertebrate Hindbrain Segmentation
The taxon Vertebrata is made up of non-jawed vertebrates and jawed vertebrates, or gnathostomes. Molecular and morphological characteristics distinguish vertebrates from other chordates and also help define phylogenetic relationships among lampreys, the only extant non-jawed vertebrates, and gnathostomes. Studying genes that dictate vertebrate head structure and organization is of particular interest because the specialization of head structures early in development gives rise to many characteristic vertebrate adult morphologies. Additionally, to gain insight into early vertebrate evolution, it is important to understand when duplications of genes that control head structure occur, at the vertebrate origin or at the lamprey-gnathostome divergence. Krox-20, a gene that helps form and maintain hindbrain segmentation in gnathostomes, is expressed in the third and fifth hindbrain segments during embryonic development and contributes to segmental specialization in later development. A paralog, Krox-24, also exists in gnathostomes. In this study, we have cloned, phylogenetically analyzed, and shown the developmental expression of a lamprey Krox homolog in Petromyzon marinus. Our results suggest that hindbrain segmentation is present in lampreys and therefore is characteristic of all vertebrates, and also that lamprey Krox is an ortholog of gnathostome Krox-20. This orthology gives preliminary evidence that the Krox gene duplication event occurred prior to the vertebrate origin.