Brain volume increase and neuronal plasticity underly predator-induced morphological defense expression in Daphnia longicephala

A Graeve,J Reinhard,Lc Weiss,I Ioannidou,A Faissner,D M Görl

doi:10.1038/s41598-021-92052-y

Abstract

Predator-induced phenotypic plasticity describes the ability of prey to respond to an increased predation risk by developing adaptive phenotypes. Upon the perception of chemical predator cues, the freshwater crustacean Daphnia longicephala develops defensive crests against its predator Notonecta spec. (Heteroptera). Chemical predator perception initiates a cascade of biological reactions that leads to the development of these morphological features. Neuronal signaling is a central component in this series, however how the nervous system perceives and integrates environmental signals is not well understood. As neuronal activity is often accompanied by functional and structural plasticity of the nervous system, we hypothesized that predator perception is associated with structural and functional changes of nervous tissues. We observe structural plasticity as a volume increase of the central brain, which is independent of the total number of brain cells. In addition, we find functional plasticity in form of an increased number of inhibitory post-synaptic sites during the initial stage of defense development. Our results indicate a structural rewiring of nerve-cell connections upon predator perception and provide important insights into how the nervous system of prey species interprets predator cues and develops cost–benefit optimized defenses.

Highlights

Predator-induced phenotypic plasticity describes the ability of prey to respond to an increased predation risk by developing adaptive phenotypes
The freshwater crustacean Daphnia is a prime example of predator-induced d efenses[2], which range from morphological features such as helmets in D. cucullata[3], neck teeth in D. pulex[4] or crests in D. longicephala[5] to changes in life h istory[6,7] or adaptive behavior[8]
Morphological defenses are expressed in a predator-density specific manner, but rather they are finetuned to the predation risk, so that the conspecific density is decisional for the degree of defense expression[9]

Summary

Introduction

Predator-induced phenotypic plasticity describes the ability of prey to respond to an increased predation risk by developing adaptive phenotypes. Daphnia perceive chemical cues via the antennules, upon which a signaling cascade involving cholinergic, dopaminergic, glutamatergic and GABAergic components is initiated, which leads to the transformation of the undefended into the defended p henotype[10,11,12,13,14,15] It is still unknown how the nervous system detects and integrates environmental information when the Daphnia brain morphologically appears rather simple (Fig. 1A). The protocerebrum comprises the largest part of the brain and is discussed to be centrally involved in the analysis and interpretation of environmental sensory signals[17] Such environmental signal integration is often associated with changes in form and function of nervous systems. We demonstrate that there is structural and functional plasticity independent of developmental age associated with predator perception

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