LIVINGSTON, Brian T.; HARMON, Mary; University of South Florida: Early Development of the Ophiuroid Ophiocoma wendtii

Both the morphogenetic movements and the gene regulatory network that control sea urchin development have been characterized in great detail. In particular, we know how micromeres are formed during the fourth cleavage division, and how entry of beta-catenin into the nuclei of these cells as they are formed triggers a cascade of gene expression that causes the micromeres and their descendents to carry out their developmental program. This program includes signaling to adjacent cells to form secondary mesenchyme and endoderm, and concludes with the micromeres expressing a suite of spicule matrix genes and formation of the larval skeleton. Brittle stars also have pluteus larvae with skeletal spicules that resemble those of the urchin. These are formed by mesenchyme cells that ingress during gastrulation, as in sea urchins. Brittle stars do not form micromeres, however, so the origin of the skeleton-forming mesenchime is unclear, as is the mechanism of their specification. We have begun to develop the brittle star system as a means to study evolution of the genes that underlay skeleton formation in echinoderms. We here describe methods to obtain gametes from and the development of Ophiocoma wendtii. We compare this to sea urchin development, and find interesting variations on morphogenetic processes, including a lack of embryonic pigment cells and differences in formation of the larval arms. Skeleton formation seems largely similar to that of sea urchins. We have also begun to isolate brittle star genes homologous to sea urchin genes involved in micromere specification and differentiation. Our analysis of these genes will be described.