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Abstract
BackgroundBeak morphology exhibits considerable adaptive plasticity in birds, which results in highly varied or specialized forms in response to variations in ecology and life history. As the only parid species endemic to the Qinghai-Tibet Plateau, the Ground Tit (Parus humilis) has evolved a distinctly long and curved beak from other parids. An integration of morphometrics, phylogenetics, transcriptomics and embryology allows us to address the evolutionary and developmental mechanisms of the adaptive beak structure observed in the Ground Tit.ResultsA morphometric approach quantified that the Ground Tit has a comparatively longer and more decurved upper beaks than other parids. We estimated that the ancestor of the Ground Tit likely had a short straight upper beak similar to most current recognized parid species using an ancestral state reconstruction. This morphological specialization is considered an adaptation to its ground-oriented behavior on the high plateau. To identify genetic mechanisms behind this adaptive change, a comparative transcriptomic analysis was applied between the Ground Tit and its closely related species, the Great Tit (Parus major). We detected that 623 genes were significantly differentially expressed in embryonic upper beaks between the two species, 17 of which were functionally annotated to correlate with bone development and morphogenesis, although genes related to bone development were not found to undergo accelerated evolution in the Ground Tit. RT-qPCR validation confirmed differential expression of five out of eight genes that were selected from the 17 genes. Subsequent functional assays in chicken embryos demonstrated that two of these genes, FGF13 and ITGB3, may affect beak morphology by modulating levels of osteoblasts and osteoclasts.ConclusionsOur results provide preliminary evidence that development of the long decurved beak of the Ground Tit is likely regulated by transcriptional activities of multiple genes coordinating osteoblasts and osteoclasts. The integration of multiple approaches employed here sheds light on ecological and genetic mechanisms in the evolution of avian morphology.
Highlights
Beak morphology exhibits considerable adaptive plasticity in birds, which results in highly varied or specialized forms in response to variations in ecology and life history
A derived long decurved beak in the Ground Tit Length, width and depth of upper beaks from 349 skin specimens were measured for linear analysis (Fig. 1a)
The linear discriminant analysis (LDA) without beak length could not discriminate Ground Tits from other parids (Fig. 1c; see Additional file 1: Tables S2 and S3), which indicated that beak length is a better discriminant variable than beak width and depth
Summary
Beak morphology exhibits considerable adaptive plasticity in birds, which results in highly varied or specialized forms in response to variations in ecology and life history. Studies of the genetic mechanisms underlying variation in beak morphology have identified multiple genes responsible for the development of beak morphology in birds. Two recent genomic analyses on Darwin’s finches revealed that variations of ALX1 and HMGA2 are associated with beak shape [11] and beak size [12], respectively. These results show that the development of beak morphology is regulated by a complex genetic system that involves multiple genes. It is very similar to the craniofacial development across vertebrates, which is controlled by the conserved and complicated multi-gene pathways [13,14,15]
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