STEWART-SAVAGE, J*; COLE, BJ; Univ. New Orleans; DMC, Univ. of Maine: Ovarian Development in the Colonial Ascidian Botryllus schlosseri

Botryllus schlosseri has a cyclic life history where all of the adult zooids are simultaneously replaced and all zooids can be reproductive. The ovary in the embryonic zooid is initially formed by the migration of oocytes from the juvenile zooid. This migration is restricted to a narrow time window (24 h). After colonization, a squamous outer follicular layer forms around stage 2 oocytes which later becomes cuboidal. Concomitant with the formation of the outer follicular layer, the antral epithelial forms between the pharyngeal basket and ovary and testes. The ovarian stalk, an epithelial cylinder, forms between the antral epithelium and the animal pole of each stage 4 oocyte. Right before ovulation, the ovarian stalk enlarges and the outer follicular layer becomes squamous. After ovulation, the egg is situated in the ovulatory cup which is composed of two epithelial sheets with the outer sheet terminating in testes associated connective tissue. There is close association of the tips of the ovulatory cup with the follicular layer of the egg. The ovulatory cup covers 30-50% of the oocyte surface, but the surface closest to the pharyngeal basket in not covered. To determine the functional role of the ovulatory cup, self transplant experiments were preformed. We found that a large number of the transplanted eggs were not retained in the zooids but were leaving through the exhalent siphon. There was no effect of healing time (10-25 h) or number of transferred eggs (15-60) on egg retention rate (25-75%). Within a colony, there was no difference in the fertilization levels in intact and sham operated zooids, but there was 40% decrease in transferred eggs. Histological examination of isolated eggs demonstrated the presence of an attached ovulatory cup. These data support the hypothesis that the ovulatory cup holds the ovulated egg in the antral cavity against the feeding current. Funded by NFS.