Xenopus Pax-2/5/8 orthologues: Novel insights into Pax gene evolution and identification of Pax-8 as the earliest marker for otic and pronephric celllineages

Pax genes are a family of transcription factors playing fundamental roles d uring organogenesis. We have recently demonstrated the expression of Pax-2 during Xenopus embryogenesis [Heller N, Brandli AW (1997): Mech Dev 69. 83- 104]. Here we report the cloning and characterization of Xenopus Pax-5 and Pax-8, two orthologues of the Pax-2/5/8 gene family. Molecular phylogenetic analysis indicates thai the amphibian Pax-2/5/8 genes are close relatives of their mammalian counterparts and that all vertebrate Pax-2/5/8 genes are derived from a single ancestral gene. Xenopus Pax-2/5/8 genes are expresse d in spatially and temporally overlapping patterns during development of at least seven distinct tissues. Most strikingly, Xenopus Pax-8 was identifie d as the earliest marker of the prospective otic placode and of the interme diate mesoderm, indicating that Pax-8 may play a central role in auditory a nd excretory system development. Comparison of the expression patterns of F ish, amphibian, and mammalian Pax-2/5/8 genes revealed that the tissue spec ificity of Pax-2/5/8 gene family expression is overall evolutionarily conse rved. The expression domains of individual orthologues can however vary in a species-specific manner. For example, the thyroid glands or mammals expre ss Pax-8, while in Xenopus Pax-2 is expressed instead. Our findings indicat e that differential silencing of Pax-2/5/8 gene expression may have occurre d after the different classes of vertebrates began ia evolve separately. (C ) 1999 Wiley-Liss, Inc.