Removal of the Ilium and its Primordium Does Not Affect the Morphology of the Sacral Rib in the Newt Pleurodeles waltl

The sacroiliac joint, articulating the sacrum with the ilium, evolved in a common ancestor of tetrapods as they transitioned from aquatic to terrestrial environments. This skeletal connection between the hindlimb and the trunk plays an important role in terrestrial limb-driven locomotion. Nevertheless, the mechanisms underlying its formation remain almost unexplored. Urodeles are the best potential model group to study this mechanism because they show better preservation of ancient skeletal structures than other extant tetrapods, however, there have been no detailed descriptions of the developmental process of sacroiliac joint formation. We observed the skeletal development of the sacral rib and hindlimb bud-derived bones during the formation of the sacroiliac joint in the newt Pleurodeles waltl to identify the timing of the emergence of these bones. Based on these observations, we then performed hindlimb bud removal and transplantation experiments to examine the interaction between the developing sacral rib and limb bones. Removing either the left or both hindlimb buds from the larval body did not result in reduction of the size of the sacral ribs in this study, in contrast to previous studies which suggested effects of the hindlimb bud on sacral rib development. Autotransplantation of the excised hindlimb bud, either anteriorly or posteriorly, did not produce ectopic sacral rib-like structures on non-sacral vertebrae. Our results suggest that sacral rib development does not require the hindlimb bud.