Epigenetic interactions and the structure of phenotypic variation in the cranium

Abstract

SUMMARY Understanding the developmental and genetic basis for evolutionarily significant morphological variation in complex phenotypes such as the mammalian skull is a challenge because of the sheer complexity of the factors involved. We hypothesize that even in this complex system, the expression of phenotypic variation is structured by the interaction of a few key developmental processes. To test this hypothesis, we created a highly variable sample of crania using four mouse mutants and their wild‐type controls from similar genetic backgrounds with developmental perturbations to particular cranial regions. Using geometric morphometric methods we compared patterns of size, shape, and integration in the sample within and between the basicranium, neurocranium, and face. The results highlight regular and predictable patterns of covariation among regions of the skull that presumably reflect the epigenetic influences of the genetic perturbations in the sample. Covariation between relative widths of adjoining regions is the most dominant factor, but there are other significant axes of covariation such as the relationship between neurocranial size and basicranial flexion. Although there are other sources of variation related to developmental perturbations not analyzed in this study, the patterns of covariation created by the epigenetic interactions evident in this sample may underlie larger scale evolutionary patterns in mammalian craniofacial form.

Larger mammals have longer faces because of size-related constraints on skull form

“…Indeed, without integration, fundamental functions could be compromised as size changes during ontogeny and in evolution 9,10 . Integration probably explains why the proportional length and orientation of the cranial base and face account for a large amount of variation in dogs and humans 11 . Consequently, small differences in the genetic regulation of growth factors can, by altering size, indirectly but readily affect shape.…”

A geometric morphometric analysis of heterochrony in the cranium of chimpanzees and bonobos

“…First, total skull length is not a measurement of skull shape. Second, skull length or other measurements of overall skull shape may confound the existence of multiple, dissociated heterochronic shifts, given the partial modular independence of the face and the complex formed by the basicranium and neurocranium (Moss and Young, 1960;Cheverud, 1982Cheverud, , 1995Lieberman et al, 2000a, b;Hallgrimsson et al, 2007). Third, Shea's initial analysis included no data on age, which are necessary to diagnose some forms of paedomorphosis.…”

The Relationship Between Cephalic Scales and Bones in Lizards: A Preliminary Microtomographic Survey on Three Lacertid Species

“…In this framework, functional morphometrics largely deals with the biomechanical environment, trying to identify, describe, and understand the structural relationships between the elements of the anatomical system (Moss and Young, 1960). Under this scenario, applications of geometric modeling to study the morphology and the biomechanical dynamics of skull proved useful to quantify differences between species, providing helpful information on the structural organization of the skull (Hallgrimsson et al, 2007). Lacertids are one of the most diverse groups of lizards in terms of dietary specialization, and although they have been well described in the literature (e.g., Arnold, 1989;Barahona and Barbadillo, 1998;Herrel et al, 2004Herrel et al, , 2008Evans, 2008), the degree to which the mechanics of their skulls differs among species remains unexamined.…”