Vestiges, rudiments and fusion events: the zebrafish caudal fin endoskeleton in an evo-devo perspective

The vertebral column results from a controlled segmentation process associated with two main structures, the notochord and the somites. Pathological fusion of vertebral bodies can result from impaired segmentation during embryonic development or occur postnatally. Here, we explore the process of formation and subsequent fusion of the caudalmost vertebral bodies in zebrafish, where fusion is a normal process, mechanically required to support the caudal fin. To reveal whether the product of fusion is on an evolutionary or a developmental scale, we analyze the mode of formation of vertebral bodies, identify transitory rudiments, and characterize vestiges that indicate previous fusion events. Based on a series of closely spaced ontogenetic stages of cleared and stained zebrafish, parasagittal sections, and detection methods for elastin and mineral, we conclude that the formation of the urostyle involves four fusion events. Although fusion of preural 1 (PU1(+)) with ural 1 (U1) and fusion within ural 2 (U2(+)) are no longer traceable during centrum formation (phylogenetic fusion), fusion between the compound centrum [PU1(+)+U1] and U2(+) (ontogenetic fusion) occurs after individualization of the centra. This slow process is the last fusion and perhaps the latest fusion during the evolution of the zebrafish caudal fin endoskeleton. Newly described characters, such as a mineralized subdivision within U2(+), together with the reinterpretation of known features in an evolutionary-developmental context, strongly suggest that the zebrafish caudal fin endoskeleton is made from more fused vertebral bodies than previously assumed. In addition, these fusion events occur at different developmental levels depending on their evolutionary status, allowing the dissection of fusion processes that have taken place over different evolutionary times.