THE EVOLUTION OF MSX GENE-FUNCTION - EXPRESSION AND REGULATION OF A SEA-URCHIN MSX CLASS HOMEOBOX GENE

Msx- class homeobox genes, characterized by a distinct and highly cons erved homeodomain, have been identified in a wide variety of metazoans from vertebrates to coelenterates. Although there is evidence that th ey participate in inductive tissue interactions that underlie vertebra te organogenesis, including those that pattern the neural crest, there is little information about their function in simple deuterostomes. B oth to learn more about the ancient function of Msx genes, and to shed Light on the evolution of developmental mechanisms within the lineage that gave rise to vertebrates, we have isolated and characterized Msx genes from ascidians and echinoderms. Here we describe the sequence a nd expression of a sea urchin (Strongylocentrotus purpuratus) Msx gene whose homeodomain is very similar to that of vertebrate Msx2. This ge ne, designated SpMsx, is first expressed in blastula stage embryos, ap parently in a non-localized manner. Subsequently, during the early pha ses of gastrulation, SpMsx transcripts are expressed intensely in the invaginating archenteron and secondary mesenchyme, and at reduced leve ls in the ectoderm. In the latter part of gastrulation, SpMsx transcri pts are concentrated in the oral ectoderm and gut, and continue to be expressed at those sites through the remainder of embryonic developmen t. That vertebrate Msx genes are regulated by inductive tissue interac tions and growth factors suggested to us that the restriction of SpMsx gene expression to the oral ectoderm and derivatives of the vegetal p late might similarly be regulated by the series of signaling events th at pattern these embryonic territories. As a first test of this hypoth esis, we examined the influence of exogastrulation and cell-dissociati on on SpMsx gene expression. In experimentally-induced exogastrulae, S pMsx transcripts were distributed normally in the oral ectoderm, evagi nated gut, and secondary mesenchyme. However, when embryos were dissoc iated into their component cells, SpMsx transcripts failed to accumula te. These data show that the localization of SpMsx transcripts in gast rulae does not depend on interactions between germ layers, yet the act ivation and maintenance of SpMsx expression does require cell-cell or cell-matrix interactions. (C) 1997 Elsevier Science Ireland Ltd.