Abstract
Biologists have long been captivated by novel traits because they provide insights into both the origin of and constraints on morphological variation. The iconic adaptive radiations of cichlid fishes have led to incredible diversity of form, including some species with an exaggerated snout. This novelty is mechanically integrated with the upper jaw, appears to be under directional selection, and is found in one of the most ecologically successful cichlid lineages. We used protein manipulation, gene expression, and genetic mapping to implicate the Tgfβ pathway in the development of this unusual trait. Given the functions of Tgfβ signaling in tissue proliferation, migration, invasion, and organ fibrosis, this represents an example of the cooption of existing pathways in the evolution of novelty.
Phenotypic novelties are an important but poorly understood category of morphological diversity. They can provide insights into the origins of phenotypic variation, but we know relatively little about their genetic origins. Cichlid fishes display remarkable diversity in craniofacial anatomy, including several novelties. One aspect of this variation is a conspicuous, exaggerated snout that has evolved in a single Malawi cichlid lineage and is associated with foraging specialization and increased ecological success. We examined the developmental and genetic origins for this phenotype and found that the snout is composed of two hypertrophied tissues: the intermaxillary ligament (IML), which connects the right and left sides of the upper jaw, and the overlying loose connective tissue. The IML is present in all cichlids, but in its exaggerated form it interdigitates with the more superficial connective tissue and anchors to the epithelium, forming a unique ligament–epithelial complex. We examined the Transforming growth factor β (Tgfβ) → Scleraxis (Scx) candidate pathway and confirmed a role for these factors in snout development. We demonstrate further that experimental up-regulation of Tgfβ is sufficient to produce an expansion of scx expression and concomitant changes in snout morphology. Genetic and genomic mapping show that core members of canonical Tgfβ signaling segregate with quantitative trait loci (QTL) for snout variation. These data also implicate a candidate for ligament development, adam12, which we confirm using the zebrafish model. Collectively, these data provide insights into ligament morphogenesis, as well as how an ecologically relevant novelty can arise at the molecular level.
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