Digestion of Spermatophore Contents in the Female Genital System of the Hermaphroditic Flatworm Dugesia gonocephala (Tricladida, Paludicola)

Abstract

In Dugesia gonocephala, unlike other planarians, the cells of the copulatory bursa fulfill both a secretory and phagocytic function. Enzymes are synthesized in the absence of a spermatophore and are secreted into the lumen of the bursa when a spermatophore is received from a partner. During the days following copulation, seminal material is engulfed by the cells through cytoplasmic protrusions that extend into the lumen of the bursa. This material disintegrates and, as a result, the cells are filled with phagosomes of different sizes 3-4 days after copulation. Sperm cells are also digested in phagocytic cells that line the proximal side of the vitellarian follicles. Upon arrival in the lumen of the sperm receptacles, sperms penetrate the surrounding epithelial cells. Additional key words: copulatory bursa, resorptive vesicle, sperm receptacle Ejaculates usually contain many more sperms than the few necessary for the fertilization of the eggs. They also contain secretions from accessory glands known to be of considerable nutritional value (Mann 1984). Many invertebrate taxa have therefore become adapted to use these ejaculates as an additional source of food. In bushcrickets and grasshoppers, females actually feed upon parts of the received spermatophore during and after insemination (Wedell 1994). This may lead to an increase in female fecundity and offspring survival, when the nutrients thus obtained are used by the female for the production of the eggs (Bowen et al. 1984; Gwynne 1984, 1988; Boggs 1990; Simmons 1990; Reinhold & Heller 1993). In other taxa, ejaculates may be digested in special organs of the female genital system, e.g., the copulatory bursa (bursa copulatrix) of pulmonate snails (Tompa 1984). In flatworms, little is known about the physiology and fine structure of the different sperm resorptive organs, despite the possible importance of sperm digestion from an evolutionary point of view. Light microscopical studies have shown that excesses of sperms and prostatic secretions are mainly digested in the copulatory bursa, while further resorption of sperms takes place in resorptive vesicles near the vitellarian follicles and in the sperm receptacles near the ovaries, where sperms are stored for fertilization (Cernosvitov 1931, 1932; Sluys 1989a). Ultrastructural studies on the subject are scarce and a Author for correspondence. E-mail: cvreys@luc.ac.be restricted to the copulatory bursa (Farnesi et al. 1979; Fischlschweiger & Clausnitzer 1984). According to Farnesi et al. (1979) the bursal epithelium of Dugesia lugubris (SCHMIDT 1861) consists of two different types of cells, secretory and phagocytic cells, both of which show a positive reaction for acid phosphatase. The latter is in agreement with the histochemical study of Jennings (1968) on two temnocephalid rhabdocoels. Fischlschweiger & Clausnitzer (1984) also recognized a layer of phagocytic cells in the copulatory bursa of D. tigrina (GIRARD 1850). The second type of cell, however, was considered to be a layer of replacement