Characterization of the brain molecular bases of personalities and assessment of their developmental plasticity in response to environmental stressors using an emerging model fish species, the mangrove rivulus, Kryptolebias marmoratus

Student thesis: Doc typesDocteur en Sciences

Résumé

The widespread existence of personalities across the animal kingdom suggests an evolutionary relevance. The consistent between-individual differences in behavior over time and across contexts, also called personality traits or behavioral individualities, influence various parameters such as food access, social interactions, predator avoidance and, ultimately, the fitness of organisms which corresponds to the ability to survive to reproductive age, find a mate and produce offspring. Although animal behavior constitutes the interactive link between an organism and its environment, behavioral individualities are largely interconnected with ecological dynamics and population evolution. Nowadays, organisms have to face (rapid) changing environmental conditions mostly related to human impacts. Early-life is recognized as a sensitive window during which the environment can have long-lasting effects on the organism phenotype later in life. Discovering out how environmental changes can influence phenotypic variability is crucial to understand individual traits and animal’s ability to acclimate to new environmental conditions during development and adulthood. Hence, the general objective of this thesis was to investigate the developmental plasticity of behavioral traits and the sequence of key molecular events leading to behavioral modifications that permit the organisms to cope with new environmental conditions. A unique vertebrate expressing very low to no genetic variability between lineages was used in this thesis providing an incredible model to identify the genetic and environmental sources of phenotypic variability. Hermaphrodites of the mangrove rivulus fish, Kryptolebias marmoratus, can self-fertilize and naturally produces highly homozygous and isogenic individuals within lineages.
The first chapter of this thesis aimed to characterize boldness and aggressiveness personality traits and the non-genetic mechanisms generating between-individual behavioral variability in hermaphrodites reared in the same controlled conditions. Variability in behaviors expressed by fish was observed despite the absence of genetic and environmental variations. The brain protein expression pattern and DNA methylation landscape in adult fish, globally profiled using respectively label-free quantitation (LFQ) and Reduced Representation Bisulfite Sequencing (RRBS) workflows, provided a list of candidate genes that might be associated to variations in the bold/shy or the aggressive/non-aggressive continuums, some through methylation changes. Proteomic as well as RRBS results have shown no proteins/genes similar to both behavioral continuums indicating the implication of different pathways in the settlement of boldness and aggressiveness. Proteomic results revealed that the bold/shy continuum mostly impacted amino acid metabolic processes as well as structural and cytoskeleton proteins. The aggressive/non-aggressive continuum mostly influenced proteins involved in the central nervous system development, maintenance, plasticity and neurotransmission. The most significant differentially methylated fragment (DMF) between bold and shy fish was situated in the SKI gene body. The difference in methylation level of this gene could indicate potential difference in neurogenesis shaping boldness personality trait. With a methylation change of 40% between aggressive and non-aggressive fish, the toll-interacting protein coding gene controling proinflammatory reaction in response to injury supports, with other significant DMFs, a correlation between the aggressive level and the fish immune response, as it has been discovered in humans. Conserving behavioral individuality across multiple generations even in the absence of considerable environmental variations would maximize survival chances of a population in case of environmental condition change. This concept is known as the bet-hedging strategy, an evolutionary strategy in which a single genotype produces a distribution of phenotypes across offspring with the aim to increase the likelihood that, at least, some individuals are well-adapted to the selection pressure of unpredictable environments.
The second chapter was dedicated to understand the developmental plasticity of personality traits in K. marmoratus submitted to environmentally relevant stimuli during their development (the presence of a conspecific and low salinity environment) by investigating as well the effects on life history traits (growth and reproduction) and brain protein expression profiles. These two stimuli differentially affected fish growth and reproduction as well as the expression of some proteins in their brain, although the exposure stopped a long time before sampling. Social interactions during development induced expression changes of 43 proteins among which, the reticulon 1, the syntaxin-binding protein, the glutaminase, and the sodium-dependent neutral amino acid transporter revealed impacts on vesicle transport and neuronal activity. Low salinity exposure changed the expression of 15 proteins with the prothymosin alpha and the lipoamide acyltransferase indicated that the low salinity increased the lipid metabolism and modulated the immune response. However, no effects of stimuli were detected on fish behaviors. The camouflage of stimuli effects on behavior by the elevated behavioral variability between individuals, the low environmental influence in the determinism of behavior or the choice of stimuli not eliciting strong enough effects were amongst the hypotheses raised and discussed in this chapter.
Those results led to the last experiments carried out during this thesis : exposing fish to an expected more intense stress, a neurotoxin, highly suspected to impact fish behavior, and investigate its immediate and delayed effects. This neurotoxin is the β–N-methylamino-L-alanine (BMAA), produced by cyanobacteria, diatoms and dinoflagellates naturally present in K. marmoratus environment and related to the development of neurodegenerative diseases in humans such as Parkinson’s, amyotrophic lateral sclerosis’ and Alzheimer’s diseases. First, locomotion and prey capture behaviors in the rivulus larvae were investigated after one-week exposure to 2 sublethal BMAA doses (20 µg/L and 15 mg/L) on newly hatched larvae. BMAA significantly increased the maximum velocity as well as the number of failures and trials for capturing preys revealing potential movement and synaptic signaling impairments. Second, the delayed effects of 2-weeks BMAA exposure to both sublethal concentrations were assessed on life history traits, behavioral traits and on the relative expression of 7 genes in fish brain known to be BMAA targets or involved in neurotransmission and/or personality traits. Although no effects on growth, reproduction and behavioral traits were detected, BMAA induced a significant increase of the expression of CaM and MAOA genes at 20 µg/L BMAA compared to the control group. A significant decrease of expression was observed between this lowest BMAA dose and 15 mg/L for DRD4, MAOA and CaM genes. Our results suggest disruption of glutamate turnover, intracellular dopamine depletion and the potential activation of astrocyte protective mechanisms indicating potential long lasting effects of BMAA affecting phenotypic traits with aging.
According to the results gathered along this project, we could conclude that the mangrove rivulus behavioral traits seem to be submitted to low environmental influence even faced to neurotoxic compounds. Personality traits are possibly less plastic to stimuli occurring during rivulus development due to their influence on organism fitness. The hypothetic low environmental determinism of boldness and aggressiveness would avoid trait homogeneity within population to ensure rivulus acclimation and adaptation to new environmental conditions that may occur in adults, hypothesis highly plausible knowing how extreme the environmental conditions in mangrove habitats can be. Our results also suggest a bet-hedging strategy favoring the maintenance of traits diversity within isogenic population of the mangrove rivulus through epigenetic mechanisms such as DNA methylation.
la date de réponse9 sept. 2020
langue originaleAnglais
L'institution diplômante
  • Universite de Namur
SponsorsFSR-FNRS
SuperviseurFREDERIC SILVESTRE (Promoteur), Matthieu Denoël (Copromoteur), Eric Depiereux (Président), Ryan L. Earley (Jury), Serge Aron (Jury), Sophie Prud'homme (Jury) & Anne-Sophie Voisin (Jury)

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