Evolutionary History and Not Heterochronic Modifications Associated with Viviparity Drive Head Shape Differentiation in a Reproductive Polymorphic Species, Salamandra salamandra

Identifying the evolutionary processes that underlie morphological variation at the intraspecific level is cornerstone for understanding the drivers of phenotypic diversity at higher macro-evolutionary scales. The fire salamander, Salamandra salamandra, exhibits exceptional intraspecific variation in multiple phenotypic traits (i.e. body size, head shape, coloration, reproductive modes). Pueriparity (females laying fully metamorphosed juveniles) in S. salamandra entails modifications during embryonic development in comparison to larviparity (females laying aquatic larvae), which is the ancestral reproductive mode. These heterochronic modifications involve a general acceleration of development and mainly focus on cephalic structures to facilitate intrauterine active feeding, which might impact head shape in the adult stage. In the present study, we (i) describe the main features of head shape variation in adults of the two distinct subspecies of Salamandra salamandra that independently evolved to pueriparity, and (ii) explore the morphological consequences of developmental and functional changes related to this major evolutionary shift. Our results show that evolutionary history, and not reproductive mode, is the main driver of head shape variation in S. salamandra. These results suggest different evolutionary processes acting differentially on each subspecies, at least at the adult stage. The present study highlights the importance of comparative studies integrating evolutionary histories and ontogenetic trajectories to explore the different sources of observed morphological diversification.