P33. Evolution of a fail-safe regulatory system for kidney development

During the chordate evolution, genome duplications have produced a number of paralogous gene pairs. While genetic studies often discover functional compensation between paralog genes, little is known about regulatory mechanisms underlying their interactions. In this study we addressed this issue by studying regulation of vertebrate Pax2 , Pax5 and Pax8 genes that have evolved from a single ancestral gene of early cephalochordates and exhibit overlapping but different expression during eye, ear, brain and kidney development. Comparison of human, chicken and Xenopus genomic sequences in the Pax2/5/8 loci and high-throughput transgenic analysis in Xenopus identified 16 enhancers for Pax2 , 7 for Pax5 , and 3 for Pax8 . Interestingly, Pax5 and Pax8 are associated with enhancers that are active in the tissues expressing Pax2 , but not Pax5 or Pax8 . One of such Pax5 enhancers, which is active in the pronephros, shares sequence features with a Pax2 pronephric enhancer. These two appear to have evolved from the common ancestral enhancer through the genome duplication. This Pax5 enhancer implied a hidden potential of this gene to be activated in the pronephros, and we actually found that Pax2 -knockdowned embryos express Pax5 in the pronephros to rescue expression of WT1 , a downstream target of Pax2 in normal embryos. These results revealed a dynamic balancing mechanism between the paralogs, which involves ectopic gene activation and may reflect regulatory mechanisms of their ancestral gene.