Abstract
Many bones in the vertebrate skeleton, including the limb bones, axial skeleton, and bones of the floor of the cranium, grow through the process of endochondral ossification, under the control of growth plates. The cellular and molecular mechanisms of endochondral ossification are conserved across these cartilaginous growth plates, increasing the tendency of skeletal elements to covary in size and shape. Covariation at the phenotypic, developmental, and genetic levels has been hypothesized to lead to correlated changes in parts of the skeleton not under direct selection. We tested this hypothesis using the selectively bred Longshanks mouse, in which the sole target of selection was relative tibia length. We use x-ray micro-computed tomography (µCT) and geometric morphometrics in a large, multi-generation sample of Longshanks and random-bred wildtype mice to characterize shape changes in the Longshanks cranium. We show that Longshanks skulls became longer, flatter, and narrower in a stepwise intergenerational process. Moreover, we show that these morphological changes likely resulted from underlying developmental changes in the growth plates of the cranial base, that mirror changes in the process of endochondral ossification observed in Longshanks’ tibia growth plate. Taken together, these results show that indirect, and potentially non-adaptive, skeletal changes can occur due to developmental overlap among distant anatomical elements, with important implications for interpreting the evolutionary history of vertebrate skeletal form.
Competing Interest Statement
The authors have declared no competing interest.