Abstract
Phenotypic plasticity is the ability held in many organisms to produce different phenotypes with a given genome in response to environmental stimuli, such as temperature, nutrition and various biological interactions. It seems likely that environmental signals induce a variety of mechanistic responses that influence ontogenetic processes. Inducible defenses, in which prey animals alter their morphology, behavior and/or other traits to help protect against direct or latent predation threats, are among the most striking examples of phenotypic plasticity. The freshwater microcrustacean Daphnia pulex forms tooth-like defensive structures, “neckteeth,” in response to chemical cues or signals, referred to as “kairomones,” in this case released from phantom midge larvae, a predator of D. pulex. To identify factors involved in the reception and/or transmission of a kairomone, we used microarray analysis to identify genes up-regulated following a short period of exposure to the midge kairomone. In addition to identifying differentially expressed genes of unknown function, we also found significant up-regulation of genes encoding ionotropic glutamate receptors, which are known to be involved in neurotransmission in many animal species. Specific antagonists of these receptors strongly inhibit the formation of neckteeth in D. pulex, although agonists did not induce neckteeth by themselves, indicating that ionotropic glutamate receptors are necessary but not sufficient for early steps of neckteeth formation in D. pulex. Moreover, using co-exposure of D. pulex to antagonists and juvenile hormone (JH), which physiologically mediates neckteeth formation, we found evidence suggesting that the inhibitory effect of antagonists is not due to direct inhibition of JH synthesis/secretion. Our findings not only provide a candidate molecule required for the inducible defense response in D. pulex, but also will contribute to the understanding of complex mechanisms underlying the recognition of environmental changes, which form the basis of phenotypic plasticity.
Highlights
Natural environments surrounding living organisms are changing every moment
In addition to identifying the gammaamino butyric acid (GABA) receptors whose regulatory role for neckteeth formation has been suggested [16], we found that genes coding ionotropic glutamate receptor were significantly up-regulated in response to the kairomone
As mentioned previously, ionotropic glutamate receptor genes appear to be up-regulated in the presence of the kairomone, suggesting that these receptors positively regulate the inducible defenses of D. pulex
Summary
Natural environments surrounding living organisms are changing every moment. As a result, phenotypes adaptive under the present circumstances might not continue to be “adaptive” even in the immediate future. In addition to identifying the GABA receptors whose regulatory role for neckteeth formation has been suggested [16], we found that genes coding ionotropic glutamate receptor were significantly up-regulated in response to the kairomone. We demonstrated more reliable evidence that ionotropic glutamate receptors positively regulate neckteeth formation in D. pulex by exposure experiments using specific agonists and antagonists.
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