Meeting Abstract

P2.103  Sunday, Jan. 5 15:30  Polyembryony and larval development in the stenolaemate bryozoan, Crisia pugeti DIMARCO-TEMKIN, M.P.*; TEMKIN, M.H.; State Univ. of New York, Canton; St. Lawrence University dimarcotemkinm@canton.edu

Polyembryony is a specialized mode of asexual reproduction that occurs between fertilization and the juvenile stage in the life cycles of some plants and animals. Among the Bryozoa, polyembryony occurs only in the Stenolaemata. In stenolaemates, all of the events of polyembryony occur within specialized individuals within stenolaemate colonies, called gonozooids. Here, we report on gonozooid ontogeny, primary embryogenesis, secondary embryogenesis, and larvagenesis in the stenolaemate bryozoan Crisia pugeti. In C. pugeti, the egg cleaves within the ovary to form the primary embryo. While it is still inside of the ovary, the primary embryo grows to become hundreds of microns in length and develops a lobate structure. During this growth period the ovary degenerates, and cytolysosomes and shrunken nuclei are numerous in ovarian tissue. Some of the lobes of the primary embryo, elongate to form bi-layered, finger-like processes or budding loci, which produce secondary embryos. Newly budded secondary embryos consist of 8 inner cells surrounded by an epithelium of about 32 outer cells. As the secondary embryos develop, the outer cell layer proliferates more rapidly than the inner cell layer. Larvae retain the two-layered structure of secondary embryos. During larvagenesis, the outer cell layer invaginates at the anterior end to form an apical sinus, which is lined by a cuticle. At the posterior end of the larva, the outer layer invaginates to form the adhesive sac. These observations indicate that the basic organization of the larva, and therefore, of the ancestrula (the first member of a new colony) is established very early in the primary embryo. The formation of secondary embryos from a pluripotent, bi-layered primary embryo may represent a heterochronic change in the onset of asexual reproduction in the life cycle of this group of bryozoans.