EXPRESSION OF S9 AND ACTIN CYIIA MESSENGER-RNAS REVEALS DORSOVENTRAL POLARITY AND MESODERMAL SUBLINEAGES IN THE VEGETAL PLATE OF THE SEA-URCHIN EMBRYO

We have used whole mount in situ hybridization to analyze the patterns of expression of two genes, S9 and actin Cylla, during the developmen t of the sea urchin, Strongylocentrotus purpuratus. We demonstrate tha t at the late blastula stage, these two mRNAs are expressed specifical ly by cells of the vegetal plate. Their domains of expression, however , are different. S9 mRNA is broadly distributed within most of the veg etal plate except for the central region, while Cylla expression is re stricted to a population of 10-15 cells in the ventral region of the p late. S9-expressing secondary mesenchyme cells (SMCs) migrate from the vegetal plate into the blastocoel early in gastrulation and later pop ulate the dorsal ectoderm. The numbers, morphology, and migratory beha vior of these cells strongly suggest that they are pigment cells. Thro ughout gastrulation, Cylla mRNA is expressed by a population of presum ptive SMCs at the ventral aspect of the archenteron tip. The pattern o f expression of this mRNA is dynamic, however, and by the early pluteu s stage, Cylla mRNA accumulates in primary mesenchyme cells (PMCs), SM Cs, and endodermal cells of the gut. When embryos are treated with NiC l2, a compound that has been shown to ventralize other embryonic tissu es, Cylla mRNA is expressed by an increased number of cells in the veg etal plate in a radially symmetrical pattern. The spatial pattern of C ylla expression provides the first direct molecular evidence that the vegetal plate is polarized along the dorso-ventral (D-V) axis of the e mbryo. This gene product should be a valuable marker in future studies of D-V axis specification, as it can be detected at earlier developme ntal stages than existing molecular markers of this axis. Our observat ions show that the vegetal plate consists of subterritories of gene ex pression, and provide further support for the view that diversificatio n of the presumptive, non-skeletogenic mesoderm begins prior to the on set of invagination.