Meeting Abstract
The evolution of an exaggerated trait can lead to a novel morphology that allows organisms to exploit new niches. The molecular bases of such phenotypes can reveal insights into the evolution of unique traits, for example, whether a developmental or genetic constraint has limited the evolution of the trait. In the expansive adaptive radiation of cichlid fishes in East Africa’s rift valley lakes, a rare craniofacial morphology has evolved in at least two distantly related lineages, once in Lake Malawi and once in Lake Tanganyika. Fish with this trait have a dramatically overhanging snout that folds in on itself to form a flexible flap that rests on the premaxilla, the tooth-bearing upper jaw bone. Here, we show through gross anatomy and histological staining that this flap appears to arise via hypertrophy of underlying fibrous tissue, mainly the intermaxillary ligament, which extends into and interdigitates with the surrounding connective tissue, forming a novel ligament-epithelial boundary. Through a combination of genetic mapping and comparative gene/protein expression profiles, we demonstrate that this trait is mediated, at least in part, by expression of ADAM12 and TGF-β, which have both been implicated in organ fibrosis in humans and mice. Specifically, ADAM12 has been shown to regulate tissue invasion and extracellular matrix deposition, while TGF-β is involved in important cellular functions such as proliferation, differentiation, and apoptosis and it is also known to induce tendon and ligament progenitor cells. Given these data, we begin to piece together the developmental origins of this unique craniofacial morphology.
