Growth dynamics and molecular underpinnings of evolutionary novel jaw extensions in halfbeaks and needlefish (Beloniformes)
Belonoid fishes have evolved elongated jaws and, depending on the species, either only the lower (halfbeak phenotype) or upper and lower jaw are elongated (needlefish phenotype). Unlike in other teleosts with elongated jaws, such as gar or pike, these jaws are not simply stretched-out dentary and (pre)maxillary bones, for the lower and upper jaw, respectively. Instead, they consist of a base portion homolog to aforementioned bones and a typically longer, evolutionary novel extension portion anterior to the base portion. The project describes growth dynamics of the jaw bases and extensions in a halfbeak (Dermogenys pusilla) and a needlefish (Xenentodon cancila) species during their larval development. Furthermore, transcriptomes were produced comparing the upper jaw, lower jaw base and lower jaw extension of D. pusillaindividuals during their jaws allometric growth phase. Transcriptomic data corroborated by in situ hybridization suggests that the extension is a unique structure, whose outgrowth is guided by a terminal growth zone. The bone matrix is likely deposited by osteoblast-like cells, but some chondrocyte markers suggest that deposited matrix may be more similar to perichondral or chondroid bone. As the male salmons kype bone shares similar characteristics, we hypothesize that jaw extensions may share deep homology with the regulatory circuitry that allows salmon and other fish to produce often transient bones on their jaws.