The transcriptional landscape underlying metamorphosis in the Malabar grouper (Epinephelus malabaricus)

Teleost fishes typically have a bi-phasic life cycle with a transition between larval and juvenile phases called metamorphosis, which is known to be regulated by thyroid hormones (TH). However, other hormonal systems might be involved as it is the case in amphibians in which corticosteroids are interacting with TH pathway to trigger and regulate metamorphosis. Unfortunately, such interplay is poorly understood in teleost fishes. In order to investigate the potential involvement of these two hormonal pathways, we used the Malabar grouper ( Epinephelus malabaricus ) as a model system. We assembled a chromosome-scale genome and conducted a transcriptomic analysis of nine larval developmental stages. We studied the expression patterns of genes involved in TH and corticoid pathways, as well as four biological processes known to be regulated by TH in other teleost species: ossification, pigmentation, visual perception, and metabolism. Interestingly, we observed an interplay between TH and corticosteroid coinciding with metamorphosis and, more surprisingly, an early activation of both pathways was observed suggesting their implication in early larval development. Overall, our data reveal that on a common background (TH controlling metamorphosis) evolution is assembling species-specific peculiarities that allow to precisely align the molecular completion of metamorphosis with the ecological constraints.