Dopamine mediates life-history responses to food abundance in Daphnia

Semona Issa,Sigurd Einum,Veerle L B Jaspers,Alexandros G Asimakopoulos,Marlène Gamelon,Kristine Vike-Jonas,Tomasz Maciej Ciesielski

doi:10.1098/rspb.2020.1069

Abstract

Expression of adaptive reaction norms of life-history traits to spatio-temporal variation in food availability is crucial for individual fitness. Yet little is known about the neural signalling mechanisms underlying these reaction norms. Previous studies suggest a role for the dopamine system in regulating behavioural and morphological responses to food across a wide range of taxa. We tested whether this neural signalling system also regulates life-history reaction norms by exposing the zooplankton Daphnia magna to both dopamine and the dopamine reuptake inhibitor bupropion, an antidepressant that enters aquatic environments via various pathways. We recorded a range of life-history traits across two food levels. Both treatments induced changes to the life-history reaction norm slopes. These were due to the effects of the treatments being more pronounced at restricted food ration, where controls had lower somatic growth rates, higher age and larger size at maturation. This translated into a higher population growth rate (r) of dopamine and bupropion treatments when food was restricted. Our findings show that the dopamine system is an important regulatory mechanism underlying life-history trait responses to food abundance and that bupropion can strongly influence the life history of aquatic species such as D. magna. We discuss why D. magna do not evolve towards higher endogenous dopamine levels despite the apparent fitness benefits.

Highlights

Phenotypic plasticity is the propensity of a genotype to produce different phenotypes across environments [1,2]
Our findings show that the dopamine system is an important regulatory mechanism underlying life-history trait responses to food abundance and that bupropion can strongly influence the life history of aquatic species such as D. magna
We examined how dopamine mediates the responses of life-history traits to food abundance in D. magna, through aqueous exposure to dopamine and the antidepressant bupropion, a dopamine reuptake inhibitor

Summary

Introduction

Phenotypic plasticity is the propensity of a genotype to produce different phenotypes across environments [1,2]. We experimentally tested for the effects of dopamine and bupropion exposure on the reaction norms of life-history traits of Daphnia magna in response to high versus restricted food ration. We first tested whether the slopes of the reaction norms of the measured life-history traits in response to food abundance differed among treatments To assess this for DM at maturation and SGR, we used generalized least-squares regression (GLS) models including the effects of the two categorical predictors, treatment and food (high versus restricted) and their interaction. We tested the effects of treatment, food and their interaction on clutch size, age at maturation, age at second reproduction and longevity, using Poisson generalized linear models (Poisson GLMs). The paths models were implemented using the piecewiseSEM package [46]

Results

Findings

Discussion