DAVIS, M.C.; University of Chicago: Chondrogenesis in the developing pectoral fins of the chondrosteans Polyodon and Acipenser

Chondrosteans (sturgeons, paddlefish, and their extinct relatives) have often been included in discussions of basal osteichthyan evolution and relationships, both in the classical embryological literature and more recently in molecular and paleontological studies. Chondrosteans are ideal taxa for testing hypotheses on the evolution of paired appendage development in derived sarcopterygians (Mus, Gallus, Xenopus) and derived actinopterygians (Danio) because of their phylogenetic position near the base of the Actinopterygii. Furthermore, the paired fins of chondrosteans are morphologically intermediate to derived taxa, possessing an elaborate endoskeleton consisting of propterygial, mesopterygial, and metapterygial elements. Developmental series of the paddlefish Polyodon spathula and the sturgeon Acipenser transmontanus have been stained using Type-II collagen immunochemistry and Alcian Blue techniques to reveal the patterns of fin and girdle skeletal development. Earliest expression of Type-II Collagen takes place in the condensation of the scapulocoracoid followed by stripes of expression in the developing radials. The pattern of Type-II Collagen expression does not parallel the pattern of glycosaminoglycan secretion (as visualized by Alcian Blue). The metapterygium is seen early in the ontogeny of Alcian Blue prepared specimens and only at later stages in specimens expressing Type-II collagen. In addition, the expression of Type-II Collagen in the radials is seen first distally and then with increasingly proximal expression in subsequent stages. These distinct differences in the timing of collagen expression and cartilage formation may underlie the different ways in which chondrosteans, teleosts, and tetrapods construct skeletal tissues.