title: Molecular and cellular changes associated with the evolution of novel jaw muscles in parrots.
abstract: Vertebrates have achieved great evolutionary success due in large part to the anatomical diversification of their jaw complex, which allows them to inhabit almost every ecological niche. While many studies have focused on mechanisms that pattern the jaw skeleton, much remains to be understood about the origins of novelty and diversity in the closely associated musculature. To address this issue, we focused on parrots, which have acquired two anatomically unique jaw muscles: the ethmomandibular and the pseudomasseter. In parrot embryos, we observe distinct and highly derived expression patterns for Scx, Bmp4, Tgfβ2 and Six2 in neural crest-derived mesenchyme destined to form jaw muscle connective tissues. Furthermore, immunohistochemical analysis reveals that cell proliferation is more active in the cells within the jaw muscle than in surrounding connective tissue cells. This biased and differentially regulated mode of cell proliferation in cranial musculoskeletal tissues may allow these unusual jaw muscles to extend towards their new attachment sites. We conclude that the alteration of neural crest-derived connective tissue distribution during development may underlie the spatial changes in jaw musculoskeletal architecture found only in parrots. Thus, parrots provide valuable insights into molecular and cellular mechanisms that may generate evolutionary novelties with functionally adaptive significance.
authors.affiliation: Graduate School of Life and Environmental Sciences, University of Tsukuba, Tenno-dai 1-1-1, Ibaraki, Tsukuba 305-8572, Japan. tokky@biol.tsukuba.ac.jp
authors.forename: Masayoshi
authors.initials: M
authors.lastname: Tokita
authors.forename: Tomoki
authors.initials: T
authors.lastname: Nakayama
authors.forename: Richard A
authors.initials: RA
authors.lastname: Schneider
authors.forename: Kiyokazu
authors.initials: K
authors.lastname: Agata
mesh_terms: D000818:Animals; D005075:Biological Evolution; D055415:Bone Morphogenetic Protein 4 / Q000378:metabolism; D049109:Cell Proliferation; D002642:Chick Embryo / Q000033:anatomy & histology / Q000378:metabolism; D002645:Chickens / Q000033:anatomy & histology / Q000235:genetics / Q000378:metabolism; D004625:Embryo, Nonmammalian / Q000033:anatomy & histology / Q000196:embryology / Q000378:metabolism; D051524:Fibroblast Growth Factor 8 / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental; D007091:Image Processing, Computer-Assisted; D007568:Jaw / Q000033:anatomy & histology / Q000196:embryology; D008410:Masticatory Muscles / Q000033:anatomy & histology / Q000196:embryology* / Q000378:metabolism*; D008445:Maxillofacial Development; D008648:Mesoderm / Q000033:anatomy & histology / Q000166:cytology / Q000196:embryology / Q000378:metabolism; D009432:Neural Crest / Q000166:cytology / Q000196:embryology* / Q000378:metabolism*; D010311:Parrots / Q000033:anatomy & histology / Q000196:embryology* / Q000235:genetics / Q000378:metabolism*; D011784:Quail / Q000033:anatomy & histology / Q000196:embryology / Q000235:genetics / Q000378:metabolism; D012886:Skull / Q000166:cytology / Q000196:embryology; D053781:Transforming Growth Factor beta2 / Q000378:metabolism
chemical_list: D055415:Bone Morphogenetic Protein 4; D053781:Transforming Growth Factor beta2; D051524:Fibroblast Growth Factor 8