Meeting Abstract
Unique developmental novelties often mark evolutionary origins of metazoan structures. The anuran urostyle is one such structure. It forms during metamorphosis, as the tail regresses and locomotion changes from an axial-driven mode in larvae to a limb-driven one in adult frogs. Histologically, the urostyle comprises of a mesoderm-derived coccyx and an endoderm-derived hypochord. The coccyx is formed by the fusion of rudimentary caudal vertebrae. Across vertebrates, coccygeal fusions have repeatedly evolved with the loss of the tail. However, the contribution of an endoderm-derived ossifying hypochord to the coccyx in anurans is unique among vertebrates and remains a developmental enigma. Here, we focus on the developmental changes across ontogeny, leading to the anuran urostyle with an emphasis on the ossifying hypochord. We found that thyroid hormone directly affects hypochord formation but not the coccyx. The coccyx development is initiated before the metamorphic climax and depicts an endochondral ossification pattern, whereas the ossifying hypochord undergoes rapid ossification at metamorphic climax and shows hypertrophy. The embryonic hypochord is known to play a significant role in the positioning of the dorsal aorta (DA), but the reason to form an ossifying hypochord during metamorphosis has remained obscure. Interestingly, our results suggest that the ossifying hypochord also plays a role in re-arrangement of the DA in the newly forming adult body by partially occluding the DA in the tail, which subsequently regresses. We propose that the ossifying hypochord induced loss of tail during metamorphosis enabled the evolution of the unique anuran bauplan.
