BIRD, N.C.; MABEE, P.M.; Univ. of South Dakota; Univ. of South Dakota: Homology, individuality, and developmental morphology of the axial skeleton of the zebrafish.

Developmental morphology and analysis of mutant morphology have long been key in evaluating hypotheses of homology. In turn, correct determination of homology is central to understanding the precise sequence and nature of evolutionary changes in development. Our analysis of the development of the zebrafish axial skeleton has provided insights into the homology of long-questioned structures. For example, the os suspensorium, considered a fifth ossicle of the Weberian apparatus, is potentially either a modified pleural rib or an extended parapophysis of the 4th vertebra. Our key and novel observation is that the os suspensorium is a spatially and temporally separate outgrowth of the basiventral of the 4th vertebra, discrete from the second outgrowth. This observation, however, does not resolve the question of homology; neither outgrowth can be definitively identified as a rib. In fact, the individuality of the rib and the parapophysis may be lost. A similar situation is the case for other parts of the Weberian apparatus (e.g. the tripus, claustrum). Although molecular data (e.g. a rib master gene) has the potential to distinguish the identity of the outgrowth as rib or parapophysis, it may not aid in determining the homology of either outgrowth to the same in the basal actinopterygian condition. As specific mutants for the Weberian apparatus are identified, they may aid in homology determination. Our data from vertebral mutants show a variety of developmental defects in this region (i.e. elements are lost, fused, etc.). Our data on the developmental morphology of the axial skeleton provide not only a reference for further work into mechanisms of skeletogenesis in zebrafish, but a starting point for re-consideration of the individuality and homology of highly modified and novel aspects of the zebrafish skeleton.