Miyakawa Hitoshi, Sato Masanao, Colbourne John K, Iguchi Taisen
National Institute for Basic Biology, 5-1 Higashiyama, Myodaiji, Okazaki, Aichi, Japan; Okazaki Institute for Integrative Bioscience, 5-1 Higashiyama, Myodaiji, Okazaki, Aichi, Japan.
National Institute for Basic Biology, 5-1 Higashiyama, Myodaiji, Okazaki, Aichi, Japan; Okazaki Institute for Integrative Bioscience, 5-1 Higashiyama, Myodaiji, Okazaki, Aichi, Japan; Department of Basic Biology, Faculty of Life Science, SOKENDAI (The Graduate University for Advanced Studies), 5-1 Higashiyama, Myodaiji, Okazaki, Aichi, Japan.
PLoS One. 2015 Mar 23;10(3):e0121324. doi: 10.1371/journal.pone.0121324. eCollection 2015.
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.
表型可塑性是许多生物体所具有的一种能力，即在给定基因组的情况下，响应环境刺激（如温度、营养和各种生物相互作用）产生不同的表型。环境信号似乎会引发多种影响个体发育过程的机制性反应。诱导性防御是表型可塑性最显著的例子之一，在这种防御机制中，被捕食动物会改变其形态、行为和/或其他特征，以帮助抵御直接或潜在的捕食威胁。淡水微型甲壳动物大型溞会根据化学线索或信号（在这种情况下是从其捕食者幻影摇蚊幼虫释放的“信息素”）形成齿状防御结构“颈齿”。为了确定参与信息素接收和/或传递的因素，我们使用微阵列分析来识别在短时间暴露于摇蚊信息素后上调的基因。除了识别功能未知的差异表达基因外，我们还发现编码离子型谷氨酸受体的基因显著上调，已知这些受体在许多动物物种的神经传递中发挥作用。这些受体的特异性拮抗剂强烈抑制大型溞颈齿的形成，尽管激动剂本身不会诱导颈齿形成，这表明离子型谷氨酸受体对于大型溞颈齿形成的早期步骤是必要的，但不是充分的。此外，通过将大型溞同时暴露于拮抗剂和生理上介导颈齿形成的保幼激素（JH），我们发现有证据表明拮抗剂的抑制作用不是由于直接抑制JH的合成/分泌。我们的研究结果不仅为大型溞诱导性防御反应提供了一个候选分子，也将有助于理解构成表型可塑性基础的环境变化识别的复杂机制。
