How environment and hormones control the life cycle of the clownfish Amphiprion ocellaris. – SENSO

The interplay between life-history strategies, environment and the genome is a central question in biology. We are far from understanding how interactions between the environment and the genome control life history changes and how this may be constrained by internal and external factors. These life history transitions are under the control of hormonal signals that integrate the environmental information and those internally provided by biological timers, leading to the efficient synchronization of vital physiological functions with environmental cues. Analyzing how this integration occurs is crucial to better understand how developmental decisions are taken by the organisms and to better delineate the mechanisms of biological adaptation.
Metamorphosis is one of the most studied life history transition undergone by most animals. In vertebrates, this process is well exemplified by the transformation of a tadpole into a frog. In amphibian models, such as Xenopus, metamorphosis is controlled by thyroid hormones (TH) that orchestrate the intricate changes observed at the cellular and tissue levels. Teleost fishes also undergo very clear metamorphosis, most often coupled to spectacular morphological changes with profound physiological modifications, as well as an ecological transition, as exemplified by flatfishes. However, outside specific laboratory models (zebrafish, medaka) or aquaculture species (flatfishes, salmons), metamorphosis is poorly characterized in teleost fishes.
Teleosts, the largest and most diverse group of vertebrates, provide many examples undergoing morphological, physiological, and behavioral transitions as they progress in their life cycle. Among those transitions, the most impressive one is the larval recruitment of coral reef fishes to the reef. Indeed, these fishes have a pelagic larval phase and a benthic adult phase. The transition between the two phases is done when the larvae migrate and enter reefs where they become juveniles, a step called larval recruitment. This step involves the perception of environmental cues for larvae to localize and settle in the reef, and is accompanied by major morphological changes. This transition of pelagic larvae into reef-associated juveniles is now considered as metamorphosis regulated by TH. Furthermore this step is critical for the survival of these young stages and consequently the maintenance of reef fish populations, but its control remains largely unknown, in particular the modalities of this control within the larvae in the open ocean when the decision to trigger the recruitment process is made.
To tackle this question, SENSO will decipher the connection between hormonal signals and environment in the context of metamorphosis of the clownfish Amphiprion ocellaris. In particular, we will study the mechanisms by which the fish larva integrates information coming from the environment with its metabolic status to take the decision to metamorphose. With this aim, we will scrutinize, using this reef fish model species, how environmental information are integrated by the neuroendocrine system leading to an appropriate physiological answer at a crucial step of its life cycle, the larval recruitment. For this we will: 1/ identify the environmental parameters involved in the early activation of the hypothalamc-pituitary axis, initiating the fish metamorphosis 2/ through transcriptomic analysis, determine the gene networks that are activated during this process and 3/ analyze how these hormonal pathways interact facing environmental information to result in the physiological response in the clownfish life cycle. Bringing information on how this integration occurs is crucial not only to understand the link between genotype and phenotype in animals, but also to detect disturbances in the progress of their life cycle of animals, in the context of global change.