Meeting Abstract
Cichlid fishes display remarkable phenotypic diversity, especially in regards to their craniofacial anatomy. One unique corner of this diversity is in the soft tissue surrounding the upper jaw. In a handful of the over 2,000 species of East African cichlids, that soft tissue has undergone severe hypertrophy, resulting in a conspicuous fleshy flap. Just beneath the surface of the flap is the intermaxillary ligament that stretches across the premaxilla to connect the right and left maxillary heads. This ligament is present in all cichlids, but in its hypertrophied form we find upon examination of histological sections that it also interdigitates with the more superficial connective tissue and anchors to the epithelium, forming a novel ligament-epithelial connection. Based on genetic/genomic mapping experiments, we identified ADAM12 as a candidate gene for flap hypertrophy. ADAM12 has been shown to regulate tissue invasion and extracellular matrix deposition and it is an upstream regulator of TGFβ. TGFβ is a known regulator of ligament development via the tendon and ligament-specific transcription factor scleraxis. Taking advantage of this pathway, we implanted beads soaked in the TGFβ protein into juvenile cichlids with and without flaps. We found that this experimental manipulation resulted in an expansion of scleraxis expression, ligament hypertrophy, and concomitant changes in flap morphology. The evolution of phenotypic novelty by trait exaggeration is a common theme in evolutionary biology. Here we provide molecular inroads into the developmental mechanism through which one such trait has evolved.
