Autoradiographic investigation of uptake and storage of exogenous sperm by the ovary of the compound ascidian Diplosoma listerianum

The movement of radiolabelled non-self sperm within the female reproductive tract of cultured specimens of the hermaphroditic ascidian Diplosoma listerianum Milne Edwards was investigated by light-microscope autoradiography. Passage of male gametes up the oviduct (fertilization canal) to the ovary was demonstrated for sperm known unequivocally to be of external (non-self) origin. The lumen of the ovary was directly confirmed as a site of long-term storage of sperm, as had previously been suggested from circumstantial evidence. The ovary had a branching structure. Sperm entered blind diverticula leading from the main lumen of the ovary, and persisted there adjacent to the oocyte at the end of each of these side-branches. This arrangement included the association of stored sperm with very small, immature oocytes. An investigation of the time-course of sperm uptake showed that exogenous sperm did not enter the oviducts of recipient ramets for some time (> 3 h) after they were initially available, with first presence of sperm noted in the distal half of the duct at 9 h and greatest recorded uptake at 27 h. Sperm had reached the ovary 6.6 h after the first observation of their occurrence in the distal part of the oviduct. Uptake declined steeply between 27 and 46.8 h of continual exposure to sperm released by a single genetic source, and remained low at 81 h. The significance of these observations for the mating pattern of D. listerianum is discussed.