How Birds Got Their Beaks

To explore the roles of these genes, Bhullar and Abzhanov treated bird embryos with inhibitors of the WNT and Fgf8 proteins. When the two signals were most curbed, the premaxillary bones became round and never fused, as in birds’ dinosaur relatives, instead of growing long and pointy.

The fused pair of beak-forming bones in a chick embryo (left) remain rounded and paired in treated chicks (middle), resembling those in alligators (right).

And the original article:

The avian beak is a key evolutionary innovation whose flexibility has permitted birds to diversify into a range of disparate ecological niches. We approached the problem of the mechanism behind this innovation using an approach bridging paleontology, comparative anatomy, and experimental developmental biology. First, we used fossil and extant data to show the beak is distinctive in consisting of fused premaxillae that are geometrically distinct from those of ancestral archosaurs. To elucidate underlying developmental mechanisms, we examined candidate gene expression domains in the embryonic face: the earlier frontonasal ectodermal zone (FEZ) and the later midfacial WNT‐responsive region, in birds and several reptiles. This permitted the identification of an autapomorphic median gene expression region in Aves. To test the mechanism, we used inhibitors of both pathways to replicate in chicken the ancestral amniote expression. Altering the FEZ altered later WNT responsiveness to the ancestral pattern. Skeletal phenotypes from both types of experiments had premaxillae that clustered geometrically with ancestral fossil forms instead of beaked birds. The palatal region was also altered to a more ancestral phenotype. This is consistent with the fossil record and with the tight functional association of avian premaxillae and palate in forming a kinetic beak.

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Just remembered this one. It fits with gene study,