Viability of a Self-Fertilizing Strain of Philophthalmus megalurus (Trematoda: Digenea)

Abstract

A self-fertilizing strain of Philophthalmus megalurus was carried through 3 successive life cycles with little or no deleterious effects when compared with cross-fertilizing strains. No significant differences in the parameters of growth, eggshell formation, and the production of viable miracidia and cercaria were noted. However, significant reductions in recovery rates of adults from monometacercarial infections in the 2nd and 3rd generations were found. The effects of self-fertilization on different species of digenetic trematodes and cestodes and possible evolutionary adaptations for survival of self-fertilizing strains are discussed. The question of whether the hermaphroditic digenetic trematodes reproduce by selfor cross-fertilization has interested parasitologists for many years. Stunkard (1957) asserted that parasitic flatworms do not occur in interbreeding populations and reproduce by self-fertilization. Smyth (1962) suggested that self-fertilization is the most common type of reproductive process in the digenetic trematodes. On the other hand, Martin (1969) stressed the importance of intraspecific cross-fertilization for interpreting the biological species concept based on data from a study with Philophthalmus megalurus by Nollen (1968) and other studies utilizing different species of digenetic trematodes. The importance of cross-fertilization for development of the life cycle is another question to be answered now that it is known cross-insemination takes place in some species of digenetic trematodes. In the genus Philophthalmus the situation seems to vary among different species. Nollen (1971a) found that isolated adults of P. megalurus grow as well as those in multiple infections, and he was able to carry this eye fluke through one complete life cycle with no evidence of deterioration. On the other hand, Fried (1962) found that isolated adults of P. hegeneri stopped growing after 20 days in a chick's eye and never produced fertile eggs. Working with a similar Received for publication 20 May 1971. * This investigation was supported by the United States-Japan Cooperative Medical Sciences Program administered by the NIAID; Grant AI 09330. fluke, Howell and Bearup (1967) noted that single-worm infections of Philophthalmus burrili were infertile and contained no recognizable seminal receptacle. A similar situation in which species of a genus differ as to the viability of isolated adults also seems to occur in the genus Paragonimus. Sogandares-Bemal (1966) found that monometacercarial infections of cats with Paragonimus kellicotti never developed beyond the wandering preadult stage. However, Fan and Chiang (1970) found single infections of Paragonimus westermani in kittens and puppies developed into mature adults producing eggs capable of hatching. Varied results have been obtained with isolated adults of other digenetic trematodes. Bacha (1966) found that eggs produced by a second generation of isolated adults of Zygocotyle lunata in rats hatched less readily than those from multiple infections. Sogandares-Bernal (1965) found a higher percentage of paired adults of Microphallus opacus, which were excysted and maintained in vitro, produced eggs than those maintained in isolation. Viability of these eggs was not determined. Beaver (1937) and Lie (1965) observed normal miracidial development and hatching in single infections with species of Echinostoma. In this study successive life cycles of P. megalurus were investigated because this species grows and develops normally in isolated situations, and has proved viable for at least one self-fertilizing generation. A major drawback was the difficulty of raising the snail intermediate host, Pleurocera acuta. This was partially