| An essential challenge facing evolutionary biologists studying insect-parasite associations is to establish more convincingly cause-and-effect to the relationship between parasitism and variability in host fitness. The central goal of this research has been to integrate field and laboratory experiments to describe and measure selection imposed by an ectoparasitic mite, Macrocheles subbadius (Berlese), on its natural host, Drosophila nigrospiracula (Patterson and Wheeler). Contrary to the widespread belief that macrochelid mites form only phoretic associations with adult flies, it is demonstrated that mites pierce adult fly integument and actually ingests haemolymph while attached to their host. Field and laboratory experiments have revealed that parasitism by mites delays the period prior to oviposition, and reduces host fecundity, lifetime reproduction and survivorship. Field and laboratory experiments demonstrated that mites per se can generate host sexual selection, and that this effect is not merely an incidental outcome of correlated effects such as inbreeding depression. Associations between parasitism, host fluctuating asymmetry and sexual selection were also studied. Fluctuating asymmetry (minor deviations from perfect bilateral symmetry) is manifested by individuals less able to buffer environmental stress during development. Adult male D. nigrospiracula infected as larvae with an allantonematid nematode exhibited greater fluctuating bristle asymmetry than did mite-infested and unparasitized males. There was also a significant positive relation between nematode burden and degree of asymmetry. Moreover, female flies burdened with mites produced sons with significantly higher fluctuating asymmetry than females carrying no mites. Thus, if low-fluctuating asymmetry males enjoy a mating advantage, then given appropriate genetic variability, sexual selection could drive the evolution of host resistance in fly populations. However, this hypothesis was not supported since variability in fluctuating asymmetry did not influence male mating success in nature. Thus, although male fluctuating asymmetry is causally associated with parasitism, asymmetry-based sexual selection is unlikely to influence the evolution of resistance in D. nigrospiracula. |
