Predator identity and the role of chemical communication in Daphnia

在水生环境中，信息传递很大程度上依赖于化学线索，即所谓的信息化学物质。特别是，通过信息化学物质远程识别捕食者的能力能够诱导潜在猎物在行为、生活史或形态上的适应性变化。虽然这种由捕食者的信息化学物质（如kairomones）诱导的防御在水生系统中很普遍，但对所涉及的线索的化学性质知之甚少。水蚤属的小甲壳类动物在湖泊和池塘中起着关键作用，因为它们是浮游生物初级生产者的主要消费者，也是更高营养水平的重要猎物。水蚤已经成为捕食者释放的激素诱导防御的教科书范例。作为水蚤主要捕食者的鱼类释放的激素，水蚤在形态上和生活史上都表现为垂直迁移行为（DVM）。直到最近，我们才发现鲤科鱼释放的可引起DVM和形态变化的氯酮是胆汁盐5α-硫酸氯醇（5α-CPS）。然而，在其他淡水鱼中不存在5α-CPS，尽管它们在化学上诱导水蚤具有相同的防御作用。这就提出了一个问题，即水蚤对非鲤科鱼类的防御诱导的化学基础是什么，以及如何实现不同的捕食者身份，因为不同的kairomones汇聚到水蚤中诱导相同的防御。鲈鱼是一种分布广泛的浮游动物，是重要的捕食者。鲈鱼不合成5α-CPS，我们的初步工作表明，鲈鱼孵育水提取物可诱导DVM，改变水蚤的生活史和形态，而鲈鱼胆汁则不会。因此，与鲤科的情况不同，在鲈鱼胆汁中不存在未知的鲈鱼激素。我建议对鲈鱼孵育水的提取物进行反复的生物测定指导分离，通过HPLC结合高分辨率MS和随后的非靶向代谢组学来鉴定诱导DVM和形态学和生活史变化的鲈鱼kairomones。如果完成了对凯罗酮的鉴定，那么从鲈鱼中释放的凯罗酮的速率和诱导DVM的阈值浓度将允许确定水蚤是否对不同的鱼类有不同的敏感性，即对鲤科鱼和鲈鱼。我们将通过测定不同体型水蚤中5-α-CPS和鲈鱼kairomone的阈值浓度，进一步验证体型较小的水蚤诱导DVM的kairomone阈值较高的假设。

In aquatic environments information transfer largely depends on chemical cues, so called infochemicals. In particular, the capability to remotely recognize predators by infochemicals enables the induction of adaptive changes in behavior, life-history or morphology in potential prey. Although this induction of defenses by infochemicals (i.e. kairomones) from predators is widespread in aquatic systems, little is known about the chemical nature of the cues involved. Microcrustaceans of the genus Daphnia play a key role in lakes and ponds, as they are major consumers of planktonic primary producers and are important prey for higher trophic levels. Daphnia have become text book examples for the induction of defenses by kairomones released by predators. In response to kairomones released by fish, the major predator of daphnids, Daphnia deploy anti-predator defenses as diel vertical migration behavior (DVM), changes in morphology and in life-history. Only recently we have identified the kairomone the kairomone released from cyprinid fish that induces DVM and morphological changes as the bile salt 5α-cyprinol sulfate (5α-CPS). However, 5α-CPS is absent in other freshwater fish, although they chemically induce the same defenses in Daphnia. This raises the question what the chemical basis of induction of defenses in Daphnia in response to non-cyprinid fish species is and how it is achieved that different predator identities in fish coming as different kairomones converge into the induction of identical defenses in Daphnia. Here I propose to elucidate the respective kairomones of perch, which is widespread and an important predator of zooplankton. Percidae do not synthesize 5α-CPS, and our preliminary work demonstrates that extract of perch incubation water induces DVM and changes in life-history and morphology of Daphnia, but perch bile does not. Hence, different from the situation in Cyprindae, the unknown perch kairomones is not present in perch bile. I propose to use repeated rounds of bioassay-guided fractionation of extract of perch incubation water by HPLC coupled to high-resolution MS and subsequent non-targeted metabolomics to identify the perch kairomones that induce DVM and changes in morphology and life-history. If identification of the kairomones is accomplished, kairomone release rates from perch and threshold concentrations for the induction of DVM will allow to determine if Daphnia have different sensitivities to different fish species, i.e. to cyprinid fish and to perch. We will further test the hypothesis that Daphnia species with smaller body size have higher kairomone thresholds for the induction of DVM by determining threshold concentration of 5-α-CPS and of the perch kairomone across Daphnia species with different body sizes.