ASEXUAL REPRODUCTION AND ENVIRONMENTAL PREDICTABILITY IN CESTODES (CYCLOPHYLLIDEA: TAENIIDAE).

Abstract

It is generally recognized that genetic diversity of offspring is a major benefit of sexual reproduction (Williams, 1975; Glesener and Tilman, 1978; but see Maynard Smith, 1978). If natural selection is an important factor in the evolution of life histories, organisms that reproduce more than once during the life cycle should be sexual in the phase preceding the period of lowest environmental predictability, there is minimal heritability and maximum likelihood of new genotypes being of greatest fitness (Williams, 1975, p. 23). Conversely, one should expect asexual reproduction if the next stage lives under conditions similar to those encountered by the parent. In an attempt to explain the adaptive advantage of the alternation of sexual and asexual generations, Williams (1975) expanded the aphid-rotifer model, which he claimed might account for the life cycles of many parasites. In this model, a diploid organism lives in a limited, discontinuous habitat. This single organism can saturate the habitat when reproducing asexually. Periodically, dispersing propagules are produced by sexual reproduction. The genetic diversity of such propagules increases the likelihood that some may succeed in the relatively unpredictable habitats to which they are dispersed. This greater chance of success offsets the cost of meiosis, incurred because the sexuallytransmitted gene . . suffers a 50% hazard per generation, relative to asexual alternatives (Williams, 1975, p. 9). Williams (1975) discussed the reliability of this model in predicting the occurrence of sexual reproduction in parasite life cycles. A long list of animal parasites that conform to the pattern could be given. Reproduction within hosts is asexual; production of propagules to colonize new hosts is sexual. Where there is more than one obligate host, the final host (where sexual reproduction takes place) will be the most mobile and disperse the parasite most widely (p. 23). In this case, as in the model case of aphids and rotifers, Williams associates environmental unpredictability with geographic dispersal. I offer an alternative explanation to Williams' (1975) interpretation of parasite life cycles which is based on the facts that 1) many parasitic worms depend on predator-prey relationships for completion of the life cycle, and 2) some parasites appear to be constrained with respect to the diversity of hosts in which they can successfully develop. I argue that such information may obviate the question of where asexual reproduction should occur in the life cycles of parasitic worms. I attempt to test Williams' hypothesis by examining related tapeworms that vary markedly in the extent to which they reproduce asexually and I offer tentative explanations for this variation that do not invoke environmental predictability.