PRIMUS, A.E.; University of Texas, Austin: Regional specification in the early embryo of the brittle star Ophiopholis aculeate

A fate map has been constructed for the embryo of O. aculeate. The vegetal pole of the egg is the site of gastrulation and gives rise to larval mesoderm and endoderm. The first cleavage plane bisects the zygote near the animal-vegetal axis and divides the embryo into prospective larval ventral and dorsal (oral and aboral) halves; it typically separates the entire presumptive mesoderm, ciliated ventral ectoderm and anterior endoderm (ventrally) from the remainder of the ectoderm and posterior endoderm (dorsally). The second cleavage plane lies near the animal-vegetal axis and bisects the embryo near the larval plane of bilateral symmetry. The third cleavage plane is perpendicular to the plane of the first two cleavages and roughly separates animal and vegetal halves of the embryo. When sister-blastomeres were isolated at the 2-cell stage, both blastomeres frequently gastrulated and gave rise to endodermal, ectodermal and mesodermal components; occasionally both blastomeres developed into relatively normal larvae. These results indicate that while mesodermal fate has segregated by the 2-cell stage, the potential to form mesoderm is still present in both blastomeres. Vegetal halves isolated at the 8-cell stage gastrulated and formed relatively normal larvae with endoderm, ectoderm and mesoderm. Animal halves isolated at the 8-cell stage, however, typically did not gastrulate and gave rise to only ciliated ectoderm. These findings are discussed in the context of what is already known about the process of regional specification in other non-chordate deuterostomes. Supported by NSF Grant IBN-9982025.