Regulation of the limb shape during the development of the Chinese softshell turtles

Transcriptome data from the interdigital webs of the Chinese Soft-shell Turtle (Pelodiscus sinensis)

The interdigital webs of aquatic tetrapods are a key structure evolved for adaptation, which are formed by inhibiting interdigital cell death (ICD). Diverse interdigital morphologies have independently evolved among species, and the regulatory mechanisms responsible for their development are still not fully understood. The Chinese soft-shell turtle (Pelodiscus sinensis) serves as a good research model that exhibits transitional traits from webless to fully webbed. In this study, we collected eight samples of interdigital webs from the fore- and hindlimbs of turtles at embryonic stage 19 (TK19) for RNA sequencing (RNA-seq) analysis. We identified 608 differentially expressed genes (DEGs). Whole-mount in situ hybridization (WISH) and real-time quantitative PCR (RT-qPCR) of representative genes confirmed the accuracy of the transcriptomic results. These findings not only provide new perspectives and data to support comparative studies of adaptive convergent evolution in aquatic animals but also enhance our understanding of the mechanisms underlying tetrapod limb morphogenesis. Furthermore, these results provide potential molecular targets for research on the plasticity of programmed cell death or senescence.

Convergent deployment of ancestral functions during the evolution of mammalian flight membranes

Lateral flight membranes, or patagia, have evolved repeatedly in diverse mammalian lineages. While little is known about patagium development, its recurrent evolution may suggest a shared molecular basis. By combining transcriptomics, developmental experiments, and mouse transgenics, we demonstrate that lateral Wnt5a expression in the marsupial sugar glider (Petaurus breviceps) promotes the differentiation of its patagium primordium. We further show that this function of Wnt5a reprises ancestral roles in skin morphogenesis predating mammalian flight and has been convergently used during patagium evolution in eutherian bats. Moreover, we find that many genes involved in limb development have been redeployed during patagium outgrowth in both the sugar glider and bat. Together, our findings reveal that deeply conserved genetic toolkits contribute to the evolutionary transition to flight in mammals.

Air-breathing behavior underlies the cell death in limbs of Rana pirica tadpoles

Amphibians shape their limbs by differential outgrowth of digits and interdigital regions. In contrast, amniotes employ cell death, an additional developmental system, to determine the final shape of limbs. Previous work has shown that high oxygen availability is correlated with the induction of cell death in developing limbs. Given the diversity of life histories of amphibians, it is conceivable that some amphibians are exposed to a high-oxygen environment during the tadpole phase and exhibit cell death in their limbs. Here, we examined whether air-breathing behavior underlies the cell death in limbs of aquatic tadpoles of the frog species Rana pirica. Our experimental approach revealed that R. pirica tadpoles exhibit cell death in their limbs that is likely to be induced by oxidative stress associated with their frequent air-breathing behavior.

Regulation of the limb shape during the development of the Chinese softshell turtles

Interdigital cell death is an important mechanism employed by amniotes to shape their limbs; inhibiting this process leads to the formation of webbed fingers, as seen in bats and ducks. The Chinese softshell turtle Pelodiscus sinensis (Reptilia: Testudines: Trionychidae) has a distinctive limb morphology: the anterior side of the limbs has partially webbed fingers with claw‐like protrusions, while the posterior fingers are completely enclosed in webbings. Here, P. sinensis embryos were investigated to gain insights on the evolution of limb‐shaping mechanisms in amniotes. We found cell death and cell senescence in their interdigital webbings. Spatial or temporal modulation of these processes were correlated with the appearance of indentations in the webbings, but not a complete regression of this tissue. No differences in interdigital cell proliferation were found. In subsequent stages, differential growth of the finger cartilages led to a major difference in limb shape. While no asymmetry in bone morphogenetic protein signaling was evident during interdigital cell death stages, some components of this pathway were expressed exclusively in the clawed digit tips, which also had earlier ossification. In addition, a delay and/or truncation in the chondrogenesis of the posterior digits was found in comparison with the anterior digits of P. sinensis, and also when compared with the previously published pattern of digit skeletogenesis of turtles without posterior webbings. In conclusion, modulation of cell death, as well as a heterochrony in digit chondrogenesis, may contribute to the formation of the unique limbs of the Chinese softshell turtles.

Cell death and senescence shape the interdigital webbings of Pelodiscus sinensis.

Delayed chondrogenesis/ossification and truncated tips are found in posterior digits, as well as differential expression of bone morphogenetic proteins and Msh homeobox 1 transcription factors.