EVIDENCE AGAINST SELF-FERTILIZATION IN THE HER- MAPHRODITIC FLATWORM DUGESIA GONOCEPHALA (TRICLADIDA, PALUDICOLA)
by
CARLA VREYS1,3,NICOLAAS K. MICHIELS2 and ERNEST SCHOCKAERT1 (1ResearchGroup Zoology, L.U.C., B-3590 Diepenbeek,Belgium, 2Max-Planck-Institut für Ver- haltensphysiologle,D-82319 Seewiesen,Germany)
ABSTRACT The individual cocoon production of 50 specimens of the simultaneous hermaphroditic flatworm Dugesiagonocephala was followed in the laboratory for nine months. (I) Though cocoons were produced continuously, the number of fertilized cocoons decreased to zero in six months. (II) Twenty-one worms produced no cocoons, 16 produced only unfertilized cocoons and the remaining 13 produced at least one fertilized cocoon. (III) In the latter the proportion of fertilized cocoons relative to the total number produced, decreased with time. This was not compensated for by an increase in the number of hatched young. (IV) In sections of worms that produced either only fertilized cocoons, or only unfertilized cocoons or a combination of first fertilized, than unfertilized and finally no cocoons at all, sperm was only found in the seminal receptacles (near the ovaries) of a worm that produced only fertilized cocoons. All individuals had many sperm in the seminal vesicles of their male reproductive system. These results strongly suggest that self-fertilization does not occur in D. gonocephala. KEY WORDS:hermaphrodite, self-fertilization, Dugesiagonocephala, cocoon production, Platyhelminthes.
INTRODUCTION
Most turbellarians are simultaneous hermaphrodites and can-in prin- ciple-self fertilize. According to SEKERA (1906), self-fertilization is widespread among the Rhabdocoela. In 1909 WILHELMI observed cocoon production in one single isolated individual of Procerodes lobata (Schmidt, 1862). However, he did not report whether the cocoons were fertilized. In Cercyra hastata (Schmidt, 1861 ) young isolated on the day of hatching could produce viable cocoons (GREMINI & NIGRO, 1983). An electrophoretic study demonstrated that Mesostoma lingua (Abildgaard, 1789) does not engage in self-fertilization (HERBERT & PAYNE, 1985), but in Mesostoma ehrenbergii (Focke, 1836) resting eggs are produced by cross-fertilization, while subitaneous eggs are always pro- duced by selfing (HERBERT & BEATON, 1990). 56
On the other hand, based on experiments by VANDEL (1921) on Polycelis cornuta (Kenk, 1930), by YON GELEI (1924) on Dendrocoelum lacteum (Muller, 1774) and by GOETSCH (1925) on Dugesia lugubris (Schmidt, 1861), self-fertilization seems to be rare in freshwater tri- clads. According to ULLYOTT & BEAUCHAMP (1931) this is due to complex modifications of the sexual organs. In D. lugubris, e.g., muscles surrounding the ejaculatory duct should prevent the passage of sperm in periods of sexual quiescence. HYMAN ( 1951 ) stated that in planarians cross-fertilization is the rule and that sperm only leaves the seminal vesicles during copulation. Only in Cura foremanii (Girard, 1852) self fertilization has been demonstrated unequivocally: individuals isolated on the day of hatching produced numerous viable cocoons when reaching sexual maturity (ANDERSON, 1952a,b). Parthenogenesis was not involved (ANDERSON &JOHANN, 1958). Presently we investigate the reproductive strategies and the pro- cesses of sexual selection in the simultaneous hermaphroditic flatworm Dugesia gonocephala (Dugès, 1830). A crucial question is, whether this species can reproduce by selfing. In this report we present first evi- dence against the occurrence of self-fertilization. We looked for changes in the viability of cocoons produced by individuals that were sampled in a natural population and reared in isolation for more than nine months. If young are produced by selfing, we expected to see a rather constant production of fertilized cocoons. We related our obser- vations to the presence or absence of spermatozoa in the seminal receptacles and seminal vesicles in histological sections.
METHODS On 28 October 1992, 94 specimens of D. gonocephalawere collected in the River Noorbeek in the north-eastern part of Belgium (50 °07' N, 5°09' E). Until July 31 th 1993, they were kept individually in small plastic vials (0.3 1), placed in two large containers (35 I). These were filled with continuously filtered and aerated tap water (without chlorine). In every vial two holes (diameter = 34 mm) covered by a 0.5 mm mesh net provided a constant water flow to each individual. The containers were placed in a semi-dark room at a natural dark/light cycle. Temperature ranged between 15° and 18° C. Worms were fed one punctured fresh Chironomuslarva two or three times a week. The following data were recorded: number of cocoons deposited per individual, number of unhatched cocoons and number of hatchlings per cocoon. Except for weekends, individual cocoon production was monitored daily. Undamaged cocoons were kept in isolation until hatching three to six weeks later. Cocoons that had not hatched after eight weeks were ruptured. When no young were found the cocoon was considered "unfertilized". In all analyses, only individuals that survived in the lab during the whole observation period (= 55.6%) are taken into account. For histological study specimens were fixed in Steinmann's fluid (1 part concentrated nitric acid, I part saturated solution of mercuric chloride in 5% sodium chloride, I part distilled water), stored in 70% ethanol and embedded in paraffin. Sagittal sections were