The dynamic ontogenetic patterns of adaptive divergence and sexual dimorphism in Arctic charr

Arctic charr (Salvelinus alpinus) in lake Thingvallavatn (Iceland) is one of the most iconic examples of post-glacial adaptive divergence, resulting in four ecomorphs that diverge along the ecological benthic-limnetic axis (bottom lake versus open water feeders), and are distinct both phenotypically and genotypically. Here, we used geometric morphometrics tools on a common garden setup to determine the factors responsible for genetically based shape variation during the post-embryonic ontogeny of two morphs that represent the benthic-limnetic axis: the small benthic (SB) and the planktivorous (PL). This experiment uses pure crosses and F1 reciprocal hybrids between the two morphs, and includes the onset of sexual maturation, offering an excellent opportunity to explore the genetic component of adaptive divergence and the role of sexual dimorphism in this scenario. We found that growth is the main driver of shape variation across time and provided evidence of a genetically-controlled ontogenetic shift that gives rise to the limnetic morph. Additionally, our results indicate that the onset of sexual maturation triggers differences both in sex ontogenetic trajectories and in static shape variation at different time points, likely dissipating the canalisation for traits traditionally associated with benthic-limnetic adaptations.