When Shape, Size, and Time Meet: Phylogenetic Analysis of the Evolution of Embryonic Shape Ontogeny in Physalaemus...

A challenging aspect in studying the evolution of development is disentangling the interplay between changes in shape, size, time, and maturation. Building on previous contributions, in this work, we explore the role of heterochrony in modeling the evolution of shape ontogenies in embryos of foam nest-building frogs Physalaemus (Anura: Leptodactylidae). By using methodological approaches that integrate an explicit phylogenetic framework, developmental time (age) data, and shape changes at the onset and offset of developmental trajectories, we describe heterochronic shifts in embryonic shape patterns along Physalaemus evolutionary history, and interpret the temporal dynamics of ontogenetic evolution through identification of the particular heterochronic processes likely involved. We find body shape changes linked to developmental time changes in embryonic trajectories of a set of taxa were paedomorphic (i.e., Physalaemus fernandezae, P. riograndensis) or peramorphic (e.g., P. cicada) development had been invoked in former studies. Rather than by pure types of heterochronic processes, these morphological patterns are mostly explained by combinations of changes concerning the onset, offset, and rate change along the ontogenetic trajectories. Other cases, however, reveal that major changes occur in embryo size instead of shape and developmental timing (P. signifer Clade, P. carrizorum), highlighting the interspecific diversity of the relationships between descriptors of development. These variations allow for further discussion about methodological approaches to the study (e.g., through longitudinal vs. cross-sectional data) and interpretation (e.g., homologation of heterochronic and allometric processes) of the evolution of ontogeny. Finally, differences regarding previous discussion based on pair-wise comparisons of developmental trajectories of related species underline the importance of inferring heterochronic shifts in proper ancestor-descendant relationships.