Pectoral Fin and Girdle Development in the Basal Actinopterygians Polyodon spathula and Acipenser transmontanus

The pectoral fins of Acipenseriformes pos- sess endoskeletons with elements homologous to both the fin radials of teleosts and the limb bones of tetrapods. Here we present a study of pectoral fin development in the North American paddlefish, Polyodon spathula, and the white sturgeon, Acipenser transmontanus, which reveals that aspects of both teleost and tetrapod endoskeletal patterning mechanisms are present in Acipenseriformes. Those elements considered homologous to teleost radials, the propterygium and the mesopterygial radials, form via subdivision of an initially chondrogenic plate of mesenchy- mal cells called the endoskeletal disc. In Acipenseri- formes, elements homologous to the sarcopterygian metapterygium develop separately from the endoskeletal disc as an outgrowth of the endoskeletal shoulder girdle that extends into the posterior margin of the finbud. As in tetrapods, the elongating metapterygium and the metapterygial radials form in a proximal to distal order as discrete condensations from initially nonchondrogenic mesenchyme. Patterns of variation seen in the Acipens- eriform fin also correlate with putative homology: all vari- ants from the "normal" fin bauplan involved the metapterygium and the metapterygial radials alone. The primary factor distinguishing Polyodon and Acipenser fin development from each other is the composition of the endoskeletal extracellular matrix. Proteoglycans (visual- ized with Alcian Blue) and Type II collagen (visualized by immunohistochemistry) are secreted in different places within the mesenchymal anlage of the fin elements and girdle and at different developmental times. Acipenseri- form pectoral fins differ from the fins of teleosts in the relative contribution of the endoskeleton and dermal rays. The fins of Polyodon and Acipenser possess elaborate en- doskeletons overlapped along their distal margins by der- mal lepidotrichia. In contrast, teleost fins generally pos- sess relatively small endoskeletal radials that articulate with the dermal fin skeleton terminally, with little or no proximodistal overlap. J. Morphol. 262:608 - 628, 2004. © 2004 Wiley-Liss, Inc.