Ontogeny of body form and metamorphosis: insights from the crested newts
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Exploring the pattern of size and shape changes during the ontogeny of closely related species is of fundamental importance because it provides an insight into the processes that lead to the evolutionary diversification of body forms and into understanding the adaptations and constraints during growth and development. In this study, we explored the interspecific variation of body size and shape changes during postembryonic development (from the mid-larval period up to the end of metamorphosis) of four crested newt species. We analysed ontogenetic changes in the body size and shape, growth rate and the dynamics of shape variance patterns. We found a consistent pattern of changes in variance across the species studied, with the mid-larval and juvenile stages being highly constrained and canalized and the period of metamorphosis as the most variable stage. The ontogenetic trajectories of larval shape diverge in both the direction and the rate of shape changes along species-specific trajectories. These divergences are concordant with interspecific differences in adult body form and species-specific ecological preferences. However, crested newt species reach the juvenile stage at similar size and shape, indicating that metamorphosis, which is a key point between aquatic and terrestrial morphs, 'resets' the ontogenetic trajectories of larvae. Thus, metamorphosis interrupts the pattern of interspecific divergence, causing species to converge in body form. We speculate that such a pattern of developmental regulation could play crucial roles in the evolution of the body form in amphibians with a biphasic life cycle.