Skeletal plasticity in response to embryonic muscular activity underlies the development and evolution of the perching digit of birds
João Francisco Botelho, Daniel Smith-Paredes, Sergio Soto-Acuña, Jorge Mpodozis, Verónica Palma & Alexander O. Vargas
Scientific Reports 5, Article number: 9840 (2015)
Evolutionary developmental biology Palaeontology
Received: 02 November 2014 Accepted: 11 March 2015
Published online: 14 May 2015
Abstract
Most birds have an opposable digit 1 (hallux) allowing the foot to grasp, which evolved from the non-opposable hallux of early theropod dinosaurs. An important morphological difference with early theropods is the twisting of the long axis of its metatarsal. Here, we show how embryonic musculature and the onset of its activity are required for twisting of metatarsal 1 (Mt1) and retroversion of the hallux. Pharmacologically paralyzed embryos do not fully retrovert the hallux and have a straight Mt1 shaft, phenocopying the morphology of early tetanuran dinosaurs. Molecular markers of cartilage maturation and ossification show that differentiation of Mt1 is significantly delayed compared to Mt2-4. We hypothesize on how delayed maturation may have increased plasticity, facilitating muscular twisting. Our experimental results emphasize the importance of embryonic muscular activity in the evolutionary origin of a crucial adaptation.
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