COMPARISONS OF LIFE-HISTORY TRAITS BETWEEN CLONAL AND SEXUAL FISH (POECILIOPSIS, POECILIIDAE) RAISED IN MONOCULTURE AND MIXED TREATMENTS

Clonal and sexual co-existence is common in a number of vertebrate tax a, even though the 'cost of sex' makes such co-existence theoretically unlikely. The frozen niche-variation (FNV) model explains this coexis tence on the basis of differences in overall niche breadth and competi tion between clones and sexuals. In the present study I examined two p redictions of the FNV model. First, I examined the prediction that gen etically variable populations have higher relative fitness when compar ed with monoclonal populations by comparing the performances of clonal and outcrossed sexual strains of Poeciliopsis in monocultures at two densities. The prediction of increased overall productivity for the se xuals was verified, with net reproductive rates for the sexuals being between two and four times as high as the clones. Second, I tested the prediction that derived clones will successfully compete with their s exual progenitor(s) in the narrow range to which the clones are adapte d, while the sexuals should co-exist because of their ability to use a wider range of resources than any single clone. I examined this predi ction by comparing performance variables (e.g. growth, fecundity and s urvival) of each strain in pure culture with their partitioned perform ance from the mixed treatments. Clonal performance increased in mixtur es compared to monocultures, as expected. However, the expectation tha t the sexual's performance would be less affected by mixtures than the clones' performance, was not met. The sexuals had reduced growth and fecundity on a par with the increase in both variables in the clones. Therefore, support for the FNV model was mixed. Although the performan ce in monocultures suggests that the sexuals have a wider niche breadt h than the clones, performances in mixtures do not indicate such a rel ationship. Switching of behaviours or resource-use patterns between mi xed and pure cultures may have caused the equivocal results.