IMPLICATIONS OF SIZE-SELECTIVE PREDATION AND MATE AVAILABILITY FOR MATING-SYSTEM EXPRESSION AND EVOLUTION IN A HERMAPHRODITIC SNAIL (PHYSA ACUTA)

摘要

The evolution of environment-specific trait expression (i.e., phenotypic plasticity) represents a seemingly unbeatable evolutionary strategy because a plastic organism may be able to maximize fitness in multiple environments. Traditionally, studies of adaptive plasticity have examined a single type of environment, but organisms in nature may simultaneously adjust their phenotypes to multiple environments. In a series of experiments, I examined whether predation risk and mate availability interact to affect morphology and life history in a hermaphroditic snail (Physa acuta). Predation risk was expected to induce an investment in defense at the expense of reproduction. Mate availability was expected to affect the age at first reproduction where isolated snails are expected to delay selfing because this snail is a preferential outcrosser with the potential for self-fertilization at the cost of inbreeding depression. To establish the adaptive benefit of the predator-induced changes, I induced snails by rearing them in the presence and absence of chemical cues from predatory crayfish and exposed both phenotypes to selection by lethal crayfish. Crayfish induced an increase in mass and shell thickness, and snails with these traits experienced higher survival when exposed to a lethal predator. Therefore, predator-induced plasticity was favored by selection. To establish the adaptive benefit of delayed selfing, I quantified inbreeding depression by comparing the fitness of selfed and outcrossed snails reared in predator and no-predator environments. Inbreeding depression occurred in both environments and therefore, delayed selfing is favored by selection. I went on to demonstrate that inbreeding depression exists for two types of adaptive plasticity (i.e., delayed selfing and an inducible defense). Both types of inbreeding depression in plasticity may act as important constraints on the evolution of self-fertilization. In general, my results highlight the role of enemies in mating-system evolution and the role of mate availability in the evolution of inducible defenses as well as novel forms of constraint on the evolution of plasticity, including the existence of inbreeding depression in adaptive plasticity.