Evolutionary Origins

Abstract

Invertebrates are organized around a ventral nerve cord, chordates around a dorsal nerve cord and notochord. Despite this fundamental difference, the molecules that establish the dorso-ventral axes and pattern the dorsal and ventral nervous systems are conserved between Drosophila (and by inference and extension, in all arthropods) and Xenopus (and by inference and extension, in all vertebrates). The short gastrulation gene sog, which specifies ventral in Drosophila, is equivalent in sequence similarity to the gene chordin, which specifies dorsal in Xenopus. Amazingly, Drosophila sog mRNA injected into Xenopus eggs specifies dorsal, while Xenopus chordin mRNA injected into Drosophila eggs specifies ventral. Furthermore, by antagonizing chordin, BMP establishes dorso-ventral polarity of Xenopus neural ectoderm. Decapentaplegic protein, a homologue of BMP, plays the equivalent role in Drosophila, providing further evidence for conservation of genetic control of invertebrate and vertebrate nervous systems. The most parsimonious explanation for such conservation is that these signaling molecules were present in the common invertebrate/chordate ancestor.¹