Expression and regulation of a sea urchin Msx class homeobox gene: insights into the evolution and function of a gene family that participates in the patterning of the early embryo.

Inductive tissue interactions, in which signals from one tissue alter the fate of another, are an essential feature of the development of all metazoans. Secreted and membrane-bound signaling molecules, their receptors, and a series of intracellular signal transducers are key molecular components of inductive interactions. Equally important are systems of transcription factors that respond to and regulate such intracellular signaling pathways (Davidson, 1990). Among these transcription factors are the Msx proteins. Named for the Drosophila msh, the Msx proteins are homeodomain-containing DNA-binding proteins that can function as both transcriptional activators and repressors (Davidson, 1995). Characterized by a distinct and highly conserved homeodomain, Msx proteins have been identified in a wide variety of metazoans from vertebrates to coelenterates. Although there is evidence that they participate in inductive tissue interactions that underlie vertebrate organogenesis, including those that pattern the neural crest, there is little information about their function in simple deuterostomes. To learn more about the ancient function of Msx genes, to shed light on the evolution of developmental mechanisms within the lineage that gave rise to vertebrates, and finally, to provide a molecular baseline for assessing the effects of microgravity on key regulatory