Dopamine is a key regulator in the signalling pathway underlying predator-induced defences inDaphnia

Abstract

The waterflea Daphnia is a model to investigate the genetic basis of phenotypic plasticity resulting from one differentially expressed genome. Daphnia develops adaptive phenotypes (e.g. morphological defences) thwarting predators, based on chemical predator cue perception. To understand the genomic basis of phenotypic plasticity, the description of the precedent cellular and neuronal mechanisms is fundamental. However, key regulators remain unknown. All neuronal and endocrine stimulants were able to modulate but not induce defences, indicating a pathway of interlinked steps. A candidate able to link neuronal with endocrine responses is the multi-functional amine dopamine. We here tested its involvement in trait formation in Daphnia pulex and Daphnia longicephala using an induction assay composed of predator cues combined with dopaminergic and cholinergic stimulants. The mere application of both stimulants was sufficient to induce morphological defences. We determined dopamine localization in cells found in close association with the defensive trait. These cells serve as centres controlling divergent morphologies. As a mitogen and sclerotization agent, we anticipate that dopamine is involved in proliferation and structural formation of morphological defences. Furthermore, dopamine pathways appear to be interconnected with endocrine pathways, and control juvenile hormone and ecdysone levels. In conclusion, dopamine is suggested as a key regulator of phenotypic plasticity.

Mechanisms underlying the control of responses to predator odours in aquatic prey

“…Dopamine is stored and released from polyploid cells surrounding the regions where inducible defence structures develop. Exposure to Chaoborus odours results in the upregulation of the genes encoding the dopamine-synthesising enzyme dopamine decarboxylase and the juvenile hormone pathway-related genes JHAMT and Met (Miyakawa et al, 2010;Weiss et al, 2015b). The release of dopamine causes increased sclerotisation that strengthens the carapace (Laforsch et al, 2004).…”

“…Following detection, predator-specific signals are transferred via independent neurosignalling pathways. Responses of D. pulex to fish predators are mediated via the suppression of the inhibitory GABAergic pathways, resulting in the release of ecdysteroids (Dennis et al, 2014;Weiss et al, 2012bWeiss et al, , 2015b. Ecdysteroids promote the reallocation of energy from somatic growth to reproduction, which results in the life-history changes described above.…”

“…The release of dopamine causes increased sclerotisation that strengthens the carapace (Laforsch et al, 2004). Additionally, dopamine also acts as a proliferation agent either alone or in conjunction with juvenile hormones to stimulate the development of neck teeth (Dennis et al, 2014;Miyakawa et al, 2013;Weiss et al, 2015b). Mirroring these morphological changes is a significant up-regulation of genes coding for cuticle-associated proteins required to produce neck teeth and the strengthening of the carapace, along with an up-regulation of the genes involved in chromatin restructuring and cell cyclin -genes likely to be involved in cell proliferation at the site where neck teeth form (Rozenberg et al, 2015).…”

“…The cholinergic system controls the release of dopamine and juvenile hormones, which themselves control the expression of inducible defences in response to invertebrate predators (Dennis et al, 2014;Miyakawa et al, 2013;Weiss et al, 2015b). Dopamine is stored and released from polyploid cells surrounding the regions where inducible defence structures develop.…”

Phenotypic plasticity in threeDaphniagenotypes in response to predator kairomone: evidence for an involvement of chitin deacetylases

“…In Daphnia, juvenile hormones have been demonstrated to be involved in temperaturedependent sex determination (Oda et al, 2005), which suggests that juvenoid-related genes might also be involved in predator-induced polymorphism. Furthermore, Weiss et al (2015) proposed that the neurohormone dopamine is, in concert with juvenoid signalling, not only indirectly involved in the regulation of synthesis and degradation of juvenile hormones (Gruntenko et al, 2005a,b) but also directly by enhancing sclerotization and even the development of morphological defences in Daphnia induced by kairomones of invertebrate predators. Unfortunately, our data do not support the upregulation in the presence of Chaoborus kairomone reported by Miyakawa et al (2010) of the genes TBM, JHAMT, InR, esg and exd in any one of the three clones used in this study, all of which are somehow involved in hormonal signalling.…”