OtxExpression during Lamprey Embryogenesis Provides Insights into the Evolution of the Vertebrate Head and Jaw

Abstract

Agnathan or jawless vertebrates, such as lampreys, occupy a critical phylogenetic position between the gnathostome or jawed vertebrates and the cephalochordates, represented by amphioxus. In order to gain insight into the evolution of the vertebrate head, we have cloned and characterized a homolog of the head-specific geneOtxfrom the lampreyPetromyzon marinus.This lampreyOtxgene is a clear phylogenetic outgroup to both the gnathostomeOtx1andOtx2genes. Like its gnathostome counterparts, lampreyOtxis expressed throughout the presumptive forebrain and midbrain. Together, these results indicate that the divergence ofOtx1andOtx2took place after the gnathostome/agnathan divergence and does not correlate with the origin of the vertebrate brain. Intriguingly,Otxis also expressed in the cephalic neural crest cells as well as mesenchymal and endodermal components of the first pharyngeal arch in lampreys, providing molecular evidence of homology with the gnathostome mandibular arch and insights into the evolution of the gnathostome jaw.