title: Greater Growth of Proximal Metatarsals in Bird Embryos and the Evolution of Hallux Position in the Grasping Foot.
abstract: In early theropod dinosaurs-the ancestors of birds-the hallux (digit 1) had an elevated position within the foot and had lost the proximal portion of its metatarsal. It no longer articulated with the ankle, but was attached at about mid-length of metatarsal 2 (mt2). In adult birds, the hallux is articulated closer to the distal end of mt2 at ground level with the other digits. However, on chick embryonic day 7, its position is as in early theropods at half-length of mt2. The adult distal location is acquired during embryonic days 8-10. To assess how the adult phenotype is acquired, we produced fate maps of the metatarsals of day 6 chicken embryos injecting the lipophilic tracer DiI. The fates of these marks indicate a larger expansion of the metatarsals at their proximal end, which creates the illusory effect that d1 moves distally. This larger proximal expansion occurs concomitantly with growth and early differentiation of cartilage. Histological analysis of metatarsals shows that the domains of flattened and prehypertrophic chondrocytes are larger toward the proximal end. The results suggest that the distal position of the hallux in the avian foot evolved as a consequence of an embryological period of expansion of the metatarsus toward the proximal end. It also brings attention to the developmental mechanisms leading to differential growth between epiphyses and their evolutionary consequences.
authors.affiliation: Departamento de Biología, Laboratorio de Ontogenia y Filogenia, Facultad de Ciencias de la Universidad de Chile, Santiago, RM, Chile.|Department of Anatomy, Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand.|Instituto Nacional de Ciência e Tecnologia em Estudos Interdisciplinares e Transdisciplinares em Ecologia e Evolução (IN-TREE), Salvador, BA, Brazil.|Departamento de Biología, Laboratorio de Células Troncales y Biología del Desarrollo, Facultad de Ciencias de la Universidad de Chile, Santiago, RM, Chile.
authors.forename: João Francisco
authors.initials: JF
authors.lastname: Botelho
authors.affiliation: Departamento de Biología, Laboratorio de Ontogenia y Filogenia, Facultad de Ciencias de la Universidad de Chile, Santiago, RM, Chile.
authors.forename: Daniel
authors.initials: D
authors.lastname: Smith-Paredes
authors.affiliation: Departamento de Biología, Laboratorio de Ontogenia y Filogenia, Facultad de Ciencias de la Universidad de Chile, Santiago, RM, Chile.|Área de Paleontología, Museo Nacional de Historia Natural, Santiago, RM, Chile.
authors.forename: Sergio
authors.initials: S
authors.lastname: Soto-Acuña
authors.affiliation: Departamento de Biología, Laboratorio de Ontogenia y Filogenia, Facultad de Ciencias de la Universidad de Chile, Santiago, RM, Chile.
authors.forename: Daniel
authors.initials: D
authors.lastname: Núñez-León
authors.affiliation: Departamento de Biología, Laboratorio de Células Troncales y Biología del Desarrollo, Facultad de Ciencias de la Universidad de Chile, Santiago, RM, Chile.
authors.forename: Verónica
authors.initials: V
authors.lastname: Palma
authors.affiliation: Departamento de Biología, Laboratorio de Ontogenia y Filogenia, Facultad de Ciencias de la Universidad de Chile, Santiago, RM, Chile.
authors.forename: Alexander O
authors.initials: AO
authors.lastname: Vargas
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000818:Animals; D005075:Biological Evolution; D002642:Chick Embryo; D025061:Dinosaurs / Q000033:anatomy & histology*; D005528:Foot / Q000196:embryology*; D005529:Foot Bones / Q000196:embryology; D008682:Metatarsal Bones / Q000196:embryology*