Lampreys, The Jawless Vertebrates, Contain Only Two Parahox Gene Clusters

ParaHox genes (Gsx, Pdx, and Cdx) are an ancient family of developmental genes closely related to the Hox genes. They play critical roles in the patterning of brain and gut. The basal chordate, amphioxus, contains a single ParaHox cluster comprising one member of each family, whereas nonteleost jawed vertebrates contain four ParaHox genomic loci with six or seven ParaHox genes. Teleosts, which have experienced an additional whole-genome duplication, contain six ParaHox genomic loci with six ParaHox genes. Jawless vertebrates, represented by lampreys and hagfish, are the most ancient group of vertebrates and are crucial for understanding the origin and evolution of vertebrate gene families. We have previously shown that lampreys contain six Hox gene loci. Here we report that lampreys contain only two ParaHox gene clusters (designated as alpha- and beta-clusters) bearing five ParaHox genes (Gsx alpha, Pdx alpha, Cdx alpha, Gsx beta, and Cdx beta). The order and orientation of the three genes in the alpha-cluster are identical to that of the single cluster in amphioxus. However, the orientation of Gsx beta in the beta-cluster is inverted. Interestingly, Gsx beta is expressed in the eye, unlike its homologs in jawed vertebrates, which are expressed mainly in the brain. The lamprey Pdxa is expressed in the pancreas similar to jawed vertebrate Pdx genes, indicating that the pancreatic expression of Pdx was acquired before the divergence of jawless and jawed vertebrate lineages. It is likely that the lamprey Pdxa plays a crucial role in pancreas specification and insulin production similar to the Pdx of jawed vertebrates.