CROTWELL, P.L.*; MABEE, P.M.; Univ. of South Dakota, Vermillion: Proximal-distal segmentation and evolution of the vertebrate skeleton

Although anterior-posterior subdivision of the body axis is what is typically meant by �segmentation,� proximal-distal segmentation is a common occurrence in the development of the vertebrate skeleton. Fish fin rays and endoskeletal fin supports (radials) are segmented proximo-distally, as are vertebrate limbs. We have hypothesized that the developmental network underlying this proximal-distal skeletal segmentation in fishes was co-opted over several million years of evolution to give rise to tetrapod synovial joints, which form within the regions of segmentation after proximal-distal segmentation has occurred. In our comparison of fish radial to vertebrate limb segmentation, we have found both similarities (e.g. Bmp2, Gdf5, noggin) and differences (e.g. Wnt14) in expression of genes involved in cartilage development and segmentation. Furthermore, we explore the idea that cartilage �segmentation� may be a developmental continuum. At one extreme, what are articulated, segmented pieces in the adult skeleton can form via separation into two pieces of a formerly continuous cartilage condensation (e.g. development of the proximal and medial phalanges in mammalian digits). At the other end of the continuum, the articulated, segmented pieces can form via separate and distinct condensations (e.g. development of the distal-most phalanx). Finally, skeletal segments may form via a process that falls somewhere in between (e.g. development of the pharyngeal arches in zebrafish). Regardless of the actual segmentation process, these developmental patterns reflect the modular nature of their evolution, and the continua of developmental processes may be particularly well illustrated by the proximal-distal segmentation examples we present.