Meeting Abstract
21.1 Tuesday, Jan. 4 The Prepattern/Cooption Model for the Evolution of the Vertebrate Jaw MEDEIROS, Daniel M*; CATTELL, Maria V; SAUKA-SPENGLER, Tatjana; BRONNER, Marianne; YU , Brian; CERNY, Robert; Univ. of Colorado, Boulder; Univ. of Colorado, Boulder; Caltech; Caltech; Caltech; Charles Univ. in Prague Daniel.Medeiros@Colorado.edu
The elaborate head skeleton of jawed vertebrates is thought to have evolved from the homogenous unjointed gill bars seen in fossil jawless vertebrates. This ancestral unjointed condition appears maintained in living agnathans, which lack distinct skeletal elements and instead possess a fused skeletal basket. How the unjointed agnathan condition gave rise to the jointed gnathostome head skeleton and jaws is an outstanding question in vertebrate evolution. To address this issue, we are examining the expression and function of Endothelins and their downstream targets in the lamprey head. In jawed vertebrates, Endothelin signaling patterns the pharyngeal skeleton along the dorso-ventral (DV) axis and positions the jaw joint through the action of the target genes Hand, Bapx, Chordin, Gdf5, Msx, Goosecoid, Barx , and Dlx. We find gnathostome-like expression of Endothelins and a Endothelin receptors in the lamprey pharynx, suggesting that Endothelin -mediated pharyngeal patterning predates the evolution of the jaw. Furthermore, we find nested expression of Dlx genes, as well as combinatorial expression of Msx, Hand and Gsc genes along the dorso-ventral (DV) axis of the lamprey pharynx, indicating gnathostome-type pharyngeal patterning evolved before the appearance of the jaw. In addition, we find that Bapx and Gdf5/6/7, key regulators of joint formation in gnathostomes, are not expressed in the lamprey first arch, while Barx, which is absent from the intermediate first arch in gnathostomes, marks this domain in lamprey. Taken together, these data support a new scenario for jaw evolution in which incorporation of Bapx and Gdf5/6/7 into a pre-existing Endothelin-based DV patterning program drove the evolution of the jaw by altering the identity of intermediate first arch chondrocytes.
