Meeting Abstract

72.3  Tuesday, Jan. 6  Kinethmoid-Mediated Premaxillary Protrusion: Development of a Complex Trait Provides Clues to Its Evolution STAAB, KL*; HERNANDEZ, LP; George Washington University kstaab@gwmail.gwu.edu

Studies on the evolution of complex biological systems are difficult since their construction cannot be observed over an evolutionary timescale. Complex traits are defined as consisting of multiple elements, often of differing embryological origins, and with multiple linkages that are integrated to form a single functional unit. Developmental studies can show how these units are assembled within individuals, offering clues as to how the trait might have been constructed on an evolutionary timescale. The zebrafish possesses an upper jaw mechanism characterized by significant upper jaw protrusion during feeding. The zebrafish effects premaxillary protrusion via the kinethmoid, a synapomorphy for the order Cypriniformes. The kinethmoid is a sesamoid ossification that is entirely suspended by ligaments to the premaxillae, maxillae, palatines, and neurocranium. Upon mouth opening, the kinethmoid flips on its end as the premaxillae move anteriorly. Along with the bony and ligamentous elements, there are three divisions of the adductor mandibulae complex that render this system functional. We investigated the development of this complex protrusible upper jaw in the zebrafish to generate hypotheses regarding the evolution of this character. Early in development the adductor mandibulae muscle arises as a single unit. The muscle begins to divide after the ossification of the maxillae, on which the A1 division will ultimately insert. The kinethmoid originates as a cartilage within the ligament uniting the two maxillae; it later ossifies at the points of ligamentous attachments. We combine these structural developmental data with functional kinematic data at similar developmental stages which, along with outgroup comparison, has led to hypotheses about how this complex biological system evolved.