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キーワード:
Ciliaten; Koevolution; Populationsdynamik; Phänotypische Plastizität; Räuber-Beute-Systeme
要旨:
Phenotypic plasticity is common in predator-prey interactions. Prey use inducible defenses to increase their chances of survival in periods of high predation risk. Predators, in turn, display inducible offenses (trophic polyphenisms) and adjust their phenotypes to the prevailing type of prey. In the past, inducible defenses have received considerably more attention than inducible offenses. Here, I point out three areas where taking a predator perspective can increase our understanding of phenotypic plasticity in predator-prey systems. In Part 1, I describe an inducible offense in the predatory ciliate Lembadion bullinum: Mean cell size in a genetically uniform Lembadion population increases with the size of the dominant prey species. This size polyphenism can be explained as the result of a trade-off: Large Lembadion are superior in feeding on large prey, whereas small Lembadion achieve higher division rates when small prey is available. Consequently, inducible predator offenses may evolve as adaptations to environments where important prey characteristics vary over space or time. In Part 2, I investigate the interplay of Lembadion's inducible offense with an inducible prey defense. Lembadion releases a kairomone (i.e. an infochemical) that induces defenses in several prey species. For example, in the herbivorous ciliate Euplotes octocarinatus, it triggers the production of protective lateral "wings". I show that Lembadion can reduce the effect of this defense by activating its inducible offense. This is one of the first known examples of reciprocal phenotypic plasticity in a predator-prey system. While the counter-reaction of Lembadion decreases the fitness of the prey, it could not be shown to significantly increase the fitness of Lembadion itself. Nevertheless, I discuss the hypothesis that phenotypic plasticity in both species is a result of (diffuse) coevolution. In Part 3, I further pursue the idea of coevolution and develop a mathematical model of a coevolving predator-prey pair which displays reciprocal phenotypic plasticity. In this model, the inducible offense is a truly effective counter-adaptation to the prey's defense. The model yields three main conclusions: First, the inducible prey defense can stabilize predator-prey population dynamics. The effect of the inducible counter-offense is less clear and depends on the relative magnitude of its costs and benefits. Second, the maintenance of phenotypic plasticity requires that both the defense and the offense are sufficiently strong. Third, preliminary results suggest that an inducible offense is favored over a constitutive (permanently expressed) one if and only if the model populations perform predator-prey cycles. This leads to the hypothesis that phenotypic plasticity may evolve as an adaptation to temporal heterogeneity created by the internal dynamics of predator-prey systems
