Darwin's finches are a classic example of species diversification by natural selection. Their impressive variation in beak morphology is associated with the exploitation of a variety of ecological niches, but its developmental basis is unknown. We performed a comparative analysis of expression patterns of various growth factors in species comprising the genus Geospiza. We found that expression of Bmp4 in the mesenchyme of the upper beaks strongly correlated with deep and broad beak morphology. When misexpressed in chicken embryos, Bmp4 caused morphological transformations paralleling the beak morphology of the large ground finch G. magnirostris.

A classic textbook example of adaptive radiation under natural selection is the evolution of 14 closely related species of Darwin's finches (Fringillidae, Passeriformes), whose primary diversity lies in the size and shape of their beaks. Thus, ground finches have deep and wide beaks, cactus finches have long and pointed beaks (low depth and narrower width), and warbler finches have slender and pointed beaks, reflecting differences in their respective diets. Previous work has shown that even small differences in any of the three major dimensions (depth, width and length) of the beak have major consequences for the overall fitness of the birds. Recently we used a candidate gene approach to explain one pathway involved in Darwin's finch beak morphogenesis. However, this type of analysis is limited to molecules with a known association with craniofacial and/or skeletogenic development. Here we use a less constrained, complementary DNA microarray analysis of the transcripts expressed in the beak primordia to find previously unknown genes and pathways whose expression correlates with specific beak morphologies. We show that calmodulin (CaM), a molecule involved in mediating Ca2+ signalling, is expressed at higher levels in the long and pointed beaks of cactus finches than in more robust beak types of other species. We validated this observation with in situ hybridizations. When this upregulation of the CaM-dependent pathway is artificially replicated in the chick frontonasal prominence, it causes an elongation of the upper beak, recapitulating the beak morphology of the cactus finches. Our results indicate that local upregulation of the CaM-dependent pathway is likely to have been a component of the evolution of Darwin's finch species with elongated beak morphology and provide a mechanistic explanation for the independence of beak evolution along different axes. More generally, our results implicate the CaM-dependent pathway in the developmental regulation of craniofacial skeletal structures.

The interplay of evolution and development has been at the heart of evolutionary theory for more than a century. Heterochrony—change in the timing or rate of developmental events—has been implicated in the evolution of major vertebrate lineages such as mammals, including humans. Birds are the most speciose land vertebrates, with more than 10,000 living species representing a bewildering array of ecologies. Their anatomy is radically different from that of other vertebrates. The unique bird skull houses two highly specialized systems: the sophisticated visual and neuromuscular coordination system allows flight coordination and exploitation of diverse visual landscapes, and the astonishing variations of the beak enable a wide range of avian lifestyles. Here we use a geometric morphometric approach integrating developmental, neontological and palaeontological data to show that the heterochronic process of paedomorphosis, by which descendants resemble the juveniles of their ancestors, is responsible for several major evolutionary transitions in the origin of birds. We analysed the variability of a series of landmarks on all known theropod dinosaur skull ontogenies as well as outgroups and birds. The first dimension of variability captured ontogeny, indicating a conserved ontogenetic trajectory. The second dimension accounted for phylogenetic change towards more bird-like dinosaurs. Basally branching eumaniraptorans and avialans clustered with embryos of other archosaurs, indicating paedomorphosis. Our results reveal at least four paedomorphic episodes in the history of birds combined with localized peramorphosis (development beyond the adult state of ancestors) in the beak. Paedomorphic enlargement of the eyes and associated brain regions parallels the enlargement of the nasal cavity and olfactory brain in mammals. This study can be a model for investigations of heterochrony in evolutionary transitions, illuminating the origin of adaptive features and inspiring studies of developmental mechanisms.

Recent studies have postulated that distinct regulatory cascades control myogenic differentiation in the head and the trunk. However, although the tissues and signaling molecules that induce skeletal myogenesis in the trunk have been identified, the source of the signals that trigger skeletal muscle formation in the head remain obscure. Here we show that although myogenesis in the trunk paraxial mesoderm is induced by Wnt signals from the dorsal neural tube, myogenesis in the cranial paraxial mesoderm is blocked by these same signals. In addition, BMP family members that are expressed in both the dorsal neural tube and surface ectoderm are also potent inhibitors of myogenesis in the cranial paraxial mesoderm. We provide evidence suggesting that skeletal myogenesis in the head is induced by the BMP inhibitors, Noggin and Gremlin, and the Wnt inhibitor, Frzb. These molecules are secreted by both cranial neural crest cells and by other tissues surrounding the cranial muscle anlagen. Our findings demonstrate that head muscle formation is locally repressed by Wnt and BMP signals and induced by antagonists of these signaling pathways secreted by adjacent tissues. Vertebrate locomotion crucially depends on trunk skeletal muscles, which all derive from the segmented paraxial mesoderm termed somites (for review, see Christ and Ordahl 1995). During the past decade, the tissues and signaling molecules that induce the formation of skeletal muscle from somites have been intensively studied. These studies have indicated that somitic myogenesis in the trunk is affected by signals emanating from the axial tissues, the surface ectoderm, and the lateral plate mesoderm. Wnt family members expressed in the dorsal neural tube work together with Sonic hedgehog (Shh) expressed in the notochord to activate myogenic bHLH gene expression (that is, Myf-5 and MyoD) in the epaxial component of the myotome (Munsterberg et al. 1995;Stern et al. 1995;Tajbakhsh et al. 1998;Borycki et al. 2000;Gustafsson et al. 2002). In addition, Wnt signals from the dorsal ectoderm have been demonstrated to up-regulate the expression of MyoD in the hypaxial component of the myotome (Tajbakhsh et al. 1998). Furthermore, BMP signals from the lateral plate have been shown to delay the activation of myogenic bHLH gene expression in the hypaxial muscle progenitors relative to those that form the epaxial musculature (Pourquié et al. 1996). In contrast to our understanding of how skeletal muscle is induced in the trunk, the tissues and signaling pathways that induce the formation of skeletal muscle in the head have not yet been elucidated.In the vertebrate head, ∼40 skeletal muscles are present, which, instead of serving for locomotion, rather move the eye, control the cranial openings, or facilitate food uptake and, in humans, speech (for review, see Wachtler and Jacob 1986). Although the hypobranchial muscles, the tongue muscles, and the muscles of the posterior branchial arches (BAs), develop from the somites, the remainder of the head muscles (that is, the "genui...