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キーワード:
ANTIPREDATOR; BODY SIZE; CHAOBORUS; CHEMICAL ECOLOGY; CHEMICAL INDUCTION; DAPHNIA PULEX; DEFENSE; KAIROMONES; LIFE HISTORY; PHENOTYPIC PLASTICITY; PREDATION; RESOURCE ALLOCATION
要旨:
This study was designed to measure and separate the physiological costs of inducible defenses from life history trade-offs and maternal effects in the waterflea Daphnia pulex. Juveniles of D. pulex produce morphological changes (''neckteeth'') and undergo life history shifts as defenses against predatory Chaoborus (phantom midge) larvae. These traits are induced by a chemical cue (kairomone) released by the predator. I performed life history experiments with and without Chaoborus kairomones at different food levels to quantify the induced changes and their potential physiological costs. The Daphnia clone used in this study also increased its body depth in response to the predator substance. Life history shifted toward a larger body size (both length and depth) and higher fecundity, which was balanced by an increased time to reach maturity and by increased adult instar durations, Reproductive effort was higher in the typical morph in the first adult instar, indicating resource allocation shifts towards growth in the protected morph. However, even in the absence of predation the chemically induced protected morph tended to show an increased intrinsic rate of population growth (r).
The longer time to reach maturity was not a direct physiological cost of neckteeth production, but a trade-off for larger body size. The life history shifts are independent of neckteeth formation. Developmental mechanisms leading to life history changes occurred after neckteeth were induced and could thus be uncoupled from neckteeth formation and its direct costs. In this study no direct costs were found. Carbon incorporation rates for the two morphs, at high and low food, were not different.
As a maternal effect, the larger females of the induced morph produced larger neonates which, in turn, matured at a larger size. Morphological changes, life history shifts, and maternal effects acted in concert to form defenses against Chaoborus.
This study shows that the often assumed high physiological costs resulting from the formation or maintenance of the defenses are not necessary to explain the evolution of inducible defenses. As morphological changes increase the visibility of Daphnia pulex, a fitness disadvantage can be caused by a changing predator regime (e.g., fish). The results of this study suggest that environments with changing predator selectivities favor the evolution of inducible defenses
