Female control of paternity in the internally fertilizing compound ascidian Diplosoma listerianum . I. Autoradiographic investigation of sperm movements in the female reproductive tract

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Abstract
The hermaphroditic, sessile protochordate Diplosoma listerianum has water-borne sperm and internal cross-fertilization. Sperm enter the female gonopore and travel up the long, narrow oviduct to associate with immature ovarian oocytes, and may accumulate and be stored for weeks in the ovary. Fertilization occurs around the time of discharge of the fully grown ovum into the colonial tunic, where embryonic development occurs. The incompatibility system is different from that of the well-studied compound ascidian Botryllus schlosseri. Autoradiographic investigations of sperm movements within the female reproductive tract of D. listerianum indicate that a particular section of the oviduct, far from the ovary, selects which sperm will reach the ovary. Sperm may be blocked in this region and destroyed by phagocytosis. Self sperm are generally blocked, but so are all non-self sperm one experimental cross, apparently resulting in failure to produce progeny. Reproductive incompatibility in D. listerianum therefore involves a self-intolerant interaction between sperm and diploid, maternal (oviducal) tissue rather than between gametes. The system has analogies to pollen-style interactions in flowering plants. As has been argued for pollen sorting, theoretical considerations suggest that the death of sperm in the oviduct of D. listerianum represents female control of paternity rather than `sperm suicide'.