Pax1/Pax9-related genes in an agnathan vertebrate, Lampetra japonica: Expression pattern of LjPax9 implies sequential evolutionary events toward the gnathostome body plan

Among the transcription factor gene families, Pax genes play important and unique roles in morphological patterning of animal body plans. Of these, Gr oup I Pax genes (Pax1 and Pax9) are expressed in the endodermal pharyngeal pouches in many groups of deuterostomes, and vertebrates seem to have acqui red more extensive expression domains in embryos. To understand the evoluti on of Pax1/Pax9-related genes in basal groups of vertebrates, their cognate s were isolated from the Japanese marine lamprey, Lampetra japonica. RT-PCR of larval lamprey cDNA yielded two different fragments containing vertebra te Pax1- and Pax-9-like paired domains. The Pax9 orthologue was isolated an d named LiPax9. Whole-mount in situ hybridization revealed that this gene w as expressed in endodermal pharyngeal pouches, mesenchyme of the velum (the oral pumping apparatus) and the hyoid arch, and the nasohypophysial plate, but not in the semitic mesoderm of the lamprey embryo. These expression pa tterns could be regarded as a link between the basal chordates and the gnat hostomes and are consistent with the phylogenetic position of the lamprey. Especially, the appearance of neural crest seemed to be the basis of velar expression. Homology of the velum and the jaw is also discussed based on th e LjPax9 expression in the first pharyngeal pouch and in the velar mesenchy me. We conclude that Pax9 genes have sequentially expanded into new express ion domains through evolution as more complicated body plans emerged. (C) 2 000 Academic Press.