Evolution of regulation and molecular characterization ofmsp130, a mesenchyme cell lineage-specific protein of the sea urchin

Most sea urchins develop via a pluteus larva, which feeds for several weeks before metamorphosing into a juvenile. Some species, however, have lost the pluteus stage and develop directly into a juvenile in a few days. Direct developing sea urchins provide a system for examining morphological and molecular changes involved in the evolution of development. I examined development of three direct developers, Heliocidaris erythrogramma, Phyllacanthus parvispinus, and Asthenosoma ijimai. These species have, in parallel, lost many larval traits, gained novel features, and accelerated adult rudiment formation. I describe modifications in cell cleavage patterns and gene expression, indicating that the embryos of direct developers are not merely degenerate plutei. The results suggest that the evolution of early development is not as constrained as generally thought.;Analysis of the msp130 promoter region from indirect and direct developers was undertaken to study genomic changes that cause heterochronic shifts. I carried out a major portion of the characterization of the msp130 coding sequence and promoter region from an indirect developer. The putative amino acid sequence exhibits no homology to known proteins. It contains a secretion signal sequence, two glycine-rich domains and a hydrophobic carboxyl terminus similar to those found in proteins bound to the cell surface by phosphatidylinositol lipid anchors. I confirm, by phospholipase analysis, that msp130 is linked to the membrane via a lipid anchor. The promoter sequence is compared with another mesenchyme-specific protein, SM50, and regions possibly involved in temporal and spatial regulation in indirect developers are defined.;In indirect developers, mesenchyme cells function in two phases of skeletogenesis, secreting both larval skeletal rods and adult skeletal elements. Using antibodies to a skeletogenic mesenchyme-specific protein, msp130, I demonstrate that msp130 is associated with mesenchyme in both modes of skeletogenesis. In direct developing embryos, I find no larval skeleton made and adult elements accerelated relative to gastrulation. The expression pattern of msp130 parallels this alteration, with expression delayed until the beginning of adult rudiment formation. This provides direct evidence that heterochronies (relative timing changes) at the molecular level accompany those at the cellular and morphological levels during early development.