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To demonstrate predator-induced polyphenism, one has to show that the phenotypic modification is caused by the presence of the predator, and that the modification increases the fitness of its bearers when the predator is present (Adler and Harvell 1990; Tollrian and Harvell 1999). Figure 1A shows the typical and predator-induced morphs for several species. Two things characterize each case: (1) the induced morph is more successful at surviving the predator, and (2) soluble filtrate from water surrounding the predator is able to induce the changes. Chemicals that are released by a predator and can induce defenses in its prey are called kairomones.
Several rotifer species will alter their morphology when they develop in pond water in which their predators were cultured (Dodson 1989; Adler and Harvell 1990). The predatory rotifer Asplanchna releases a soluble compound that induces the eggs of a prey rotifer species, Keratella slacki, to develop into individuals with slightly larger bodies and anterior spines 130% longer than they otherwise would be, making the prey more difficult to eat. When exposed to the effluent of the crab species that preys on it, the snail Thais lamellosa develops a thickened shell and a “tooth” in its aperture. In a mixed snail population, crabs will not attack the thicker-shelled snails until more than half of the typical-morph snails are devoured (Palmer 1985).
One of the more interesting mechanisms of predator-induced polyphenism is that of certain echinoderm larvae. When exposed to the mucus of their fish predator, sand dollar plutei larvae clone themselves, budding off small groups of cells that quickly become larvae themselves. The tiny plutei are too small to be seen by the fish, and thereby escape being eaten (Vaughn and Strathmann 2008; Vaughn 2009).
The predator-induced polyphenism of the parthenogenetic water flea Daphnia is beneficial not only to itself but also to its offspring (Harris et al. 2012). When juveniles of D. cucullata encounter the predatory larvae of the fly Chaeoborus, their “helmets” grow to twice the normal size (Figure 1B). This increase lessens the chances that Daphnia will be eaten by the fly larvae. This same helmet induction occurs if the juvenile Daphnia are exposed to extracts of water in which the fly larvae had been swimming. Agrawal and colleagues (1999) have shown that the offspring of such an induced Daphnia are born with this same altered head morphology in the absence of a predator. It is possible that the Chaeoborus kairomone regulates gene expression both in the adult and in the developing embryo. The kairomone consists of a family of long-chain fatty acids conjugated to a glutamine residue (Weiss et al 2018). These are expelled from the animal as it eats. The antennae are critical in perceiving the kairomone, and the signal appears to be mediated through the cholinergic neurons. (Weiss et al. 2012, 2015). The effect does appear to work through the endocrine pathways. The kairomone upregulates the juvenile hormone, insulin, and neuroendocrine signaling pathways, activating the transcription of several transcription factor genes (Figure 1C; Miyakawa et al. 2010). In D. pulex, these transcription factors appear to activate those genes involved in producing the cuticle (An et al 2018). As in the dung beetles, there are trade-offs: the induced Daphnia, having put resources into making protective structures, produce fewer eggs (Tollrian 1995; Imai et al. 2009).
Figure 1 Predator-induced defenses. (A) Typical (upper row) and predator-induced (lower row) morphs of various organisms. The numbers beneath each column represent the percentages of organisms surviving predation when both induced and uninduced individuals were presented with predators (in various assays). (B) Scanning electron micrographs show predator-induced (left) and typical (right) morphs of genetically identical individuals of the water flea Daphnia. In the presence of chemical signals from a predator, Daphnia grows a protective “helmet.” (C) Possible pathway for the development of Daphnia’s defensive phenotype through the endocrine system. DD1 is thought to be involved in kairomone reception and/or fate determination during the embryonic stage. It may play a role in the neural reception of the signal. The other genes are thought to play roles in the morphogenesis of postembryonic juveniles.
References
Am, H. Do, T. D., Karaglzlu, M. Z., and Kim, C-B. 2018. Comparative transcriptome analysis for understanding predator-induced polyphenisms in the water flea Daphnia pulex. Int. J. Mol. Sci. 19: 2110. DOI: 10.3390/ijms19072110
Weiss LC, Albada B, Becker SM, Meckelmann SW, Klein J, Meyer M, Schmitz OJ, Sommer U, Leo M, Zagermann J, Metzler-Nolte N, Tollrian R. 2018. Identification of Chaoborus kairomone chemicals that induce defenses in Daphnia. Nature Chem Biol. 14: 1133-1139. doi: 10.1038/s41589-018-0164-7