Identification and expression of the lamprey Pax6 gene: evolutionary origin of the segmented brain of vertebrates

The Pax6 gene plays a developmental role in various metazoans as the master regulatory gene for eye patterning. Pax6 is also spatially regulated in pa rticular regions of the neural tube. Because the amphioxus has no neuromere s, an understanding of Pax6 expression in the agnathans is crucial for an i nsight into the origin of neuromerism in the vertebrates. We have isolated a single cognate cDNA of the Pax6 gene, LjPax6, from a Lampetra japonica cD NA library and observed the pattern of its expression using in situ hybridi zation. Phylogenetic analysis revealed that LjPax6 occurs as an sister grou p of gnathostome Pax6. In lamprey embryos, LjPax6 is expressed in the eye, the nasohypophysial plate, the oral ectoderm and the brain. In the central nervous system, LjPax6 is expressed in clearly delineated domains in the hi ndbrain, midbrain and forebrain. We compared the pattern of LjPax6 expressi on with that of other brain-specific regulatory genes, including LjOtxA, Lj Pax2/5/8, LjDLr1/6, LjEmx and LjTTF1 Most of the gene expression domains sh owed conserved pattern, which reflects the situation in the gnathostomes, c onforming partly to the neuromeric patterns proposed for the gnathostomes. We conclude that most of the segmented domains of the vertebrate brain were already established in the ancestor common to all vertebrates. Major evolu tionary changes in the vertebrate brain may have involved local restriction of cell lineages, leading to the establishment of neuromeres.