Pectoral Fin and Girdle Development in the Basal Actinopterygians Polyodon spathula and Acipenser transmontanus Marcus C. Davis, 1 * Neil H. Shubin, 1 and Allan Force 2 1 Department of Organismal Biology and Anatomy, University of Chicago, Chicago, Illinois 60637 2 Department of Molecular Genetics, Benaroya Research Institute at Virginia Mason, Seattle, Washington 98101 ABSTRACT 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. KEY WORDS: Polyodon; Acipenser; Acipenseriformes; fin development; chondrogenesis; collagen Understanding the mechanisms underlying verte- brate paired appendage development is a key prob- lem in evolutionary developmental biology. While classical embryological studies in the late 19th and early 20th century incorporated a diversity of chon- drichthyans (Mivart, 1879; Balfour, 1881; Goodrich, 1906), lobe-finned fishes (Semon, 1898), and ray- finned fishes (Thacher, 1877; Sewertzoff, 1926; Kryzanovsky, 1927), modern molecular studies of development have been limited to a few derived sarcopterygian (Mus, Gallus) and actinopterygian (Danio) model organisms. To make sense of devel- opmental patterns seen in these derived taxa re- quires an understanding of development in phyloge- netically basal taxa. Acipenseriformes (sturgeons, paddlefish, and their extinct relatives) are particu- larly attractive organisms for research on the evolu- tion of development because of their phylogenetic position near the base of extant actinopterygians. Acipenseriformes are represented by 27 extant spe- cies (Bemis and Grande, 1999) and a fossil record extending to the Lower Jurassic (Grande et al., 2002). The majority of morphological (Nelson, 1969; Patterson, 1982; Gardiner and Schaeffer, 1989; Be- mis et al., 1997; Coates, 1999) and molecular (Le ˆ et al., 1993) phylogenetic hypotheses support Acipens- eriformes as the sister clade to a monophyletic Neopterygii consisting of holosteans (Amia + gar; sensu Gardiner et al., 1996) plus teleosts. However, recent molecular hypotheses based on insertions and deletions in the coding sequences of nuclear genes (Venkatesh et al., 2001) and mitochondrial genomic data (Inoue et al., 2003) support an alter- native topology in which Acipenseriformes and ho- losteans form a monophyletic group as the sister clade to teleosts. In either phylogenetic scenario, Acipenseriformes remain basal to the Teleostei. The pectoral fin skeleton of Acipenseriformes pro- vides a morphological intermediate between the de- Contract grant sponsors: University of Chicago, National Science Foundation (NSF); Contract grant number: 0207721. *Current address and correspondence to: Marcus C. Davis, UCSF, Department of Anatomy, Campus Box 2711, San Francisco, CA 94143-2711. E-mail: mdavis@itsa.ucsf.edu Published online 16 September 2004 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/jmor.10264 JOURNAL OF MORPHOLOGY 262:608 – 628 (2004) © 2004 WILEY-LISS, INC.