A novel conserved evx1 enhancer links spinal interneuron morphology and cis-regulation from fish to mammals

Spinal interneurons are key components of locomotor circuits, driving such diverse behaviors as swimming in fish and walking in mammals. Recent work has linked the expression of evolutionarily conserved transcription factors to key features of interneurons in diverse species, raising the possibility that these interneurons are functionally related. Consequently, the determinants of interneuron subtypes are predicted to share conserved cis-regulation in vertebrates with very different spinal cords. Here, we establish a link between cis-regulation and morphology of spinal interneurons that express the Evx1 homeodomain transcription factor from fish to mammals. Using comparative genomics, and complementary transgenic approaches, we have identified a novel enhancer of evx1, that includes two non-coding elements conserved in vertebrates. We show that pufferfish evx1 transgenes containing this enhancer direct reporter expression to a subset of spinal commissural interneurons in zebrafish embryos. Pufferfish, zebrafish and mouse evx1 downstream genomic enhancers label selectively Evx1(+) V0 commissural interneurons in chick and rat embryos. By dissecting the zebrafish evx1 enhancer, we identify a role for a 25 bp conserved cis-element in V0-specific gene expression. Our findings support the notion that spinal interneurons shared between distantly related vertebrates, have been maintained in part via the preservation of highly conserved cis-regulatory modules.