Re-emergence of the Painted Turtle (Chrysemys picta) as a reference species for evo-devo

Disentangling the correlated evolution of body size, life history, and ontogeny in miniaturized chelydroid turtles

Organismal miniaturization is defined by a reduction in body size relative to a large ancestor. In vertebrate animals, miniaturization is achieved by suppressing the energetics of growth. However, this might interfere with reproductive strategies in egg-laying species with limited energy budgets for embryo growth and differentiation. In general, the extent to which miniaturization coincides with alterations in animal development remains obscure. To address the interplay among body size, life history, and ontogeny, miniaturization in chelydroid turtles was examined. The analyses corroborated that miniaturization in the Chelydroidea clade is underlain by a dampening of the ancestral growth trajectory. There were no associated shifts in the early sequence of developmental transformations, though the relative duration of organogenesis was shortened in miniaturized embryos. The size of eggs, hatchlings, and adults was positively correlated within Chelydroidea. A phylogenetically broader exploration revealed an alternative miniaturization mode wherein exceptionally large hatchlings grow minimally and thus attain diminutive adult sizes. Lastly, it is shown that miniaturized Chelydroidea turtles undergo accelerated ossification coupled with a ~10% reduction in shell bones. As in other vertebrates, the effects of miniaturization were not systemic, possibly owing to opposing functional demands and tissue geometric constraints. This underscores the integrated and hierarchical nature of developmental systems.

Transcriptomic similarities and differences between the limb bud AER and unique carapacial ridge of turtle embryos

Evolutionary innovation may arise via major departures from an ancestral condition. Turtle shell morphogenesis depends on a unique structure known as the carapacial ridge (CR). This lateral tissue protrusion in turtle embryos exhibits similar properties as the apical ectodermal ridge (AER)-a well-known molecular signaling center involved in limb development. Still, how the CR influences shell morphogenesis is not entirely clear. The present study aimed to describe the CR transcriptome shortly before ribs were halted within its mesenchyme, as required for shell development. Analyses exposed that the mesenchymal marker VIM was one of the most highly co-expressed genes and numerous appendage formation genes were situated within the core of CR and AER co-expression networks. However, there were tissue-specific differences in the activity of these genes. For instance, WNT5A was most frequently assigned to appendage-related annotations of the CR network core, but not in the AER. Several homeobox transcription factors known to regulate limb bud patterning exhibited their highest expression levels in the AER, but were underexpressed in the CR. The results of this study corroborate that novel body plans often originate via alterations of pre-existing genetic networks. Altogether, this exploratory study enhances the groundwork for future experiments on the molecular underpinnings of turtle shell development and evolution.

Delayed trait development and the convergent evolution of shell kinesis in turtles

Understanding developmental processes is foundational to clarifying the mechanisms by which convergent evolution occurs. Here, we show how a key convergently evolving trait is slowly 'acquired' in growing turtles. Many functionally relevant traits emerge late in turtle ontogeny, owing to design constraints imposed by the shell. We investigated this trend by examining derived patterns of shell formation associated with the multiple (at least 8) origins of shell kinesis in small-bodied turtles. Using box turtles as a model, we demonstrate that the flexible hinge joint required for shell kinesis differentiates gradually and via extensive repatterning of shell tissue. Disproportionate changes in shell shape and size substantiate that this transformation is a delayed ontogenetic response (3-5 years post-hatching) to structural alterations that arise in embryogenesis. These findings exemplify that the translation of genotype to phenotype may reach far beyond embryonic life stages. Thus, the temporal scope for developmental origins of adaptive morphological change might be broader than generally understood. We propose that delayed trait differentiation via tissue repatterning might facilitate phenotypic diversification and innovation that otherwise would not arise due to developmental constraints.