Epigenetic interactions and the structure of phenotypic variation in the cranium

B Hallgrímsson,W Liu,D.E Lieberman,A.F Ford‐Hutchinson,F.R Jirik

doi:10.1111/j.1525-142x.2006.00139.x

Abstract

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.

Highlights

In order to test hypotheses about evolutionary changes in phenotype, it is often necessary to understand their underlying genotypic and developmental bases. This is a challenge because complex phenotypes arise from many sequential interactions among genes, cells, tissues, organs and the environment
HN2, is that changes that directly affect the morphogenesis of a particular region will have widespread effects on other regions of the cranium
Sample To control for age and strain, we composed a sample of four mutant strains as well as wild-type strains of adult mice (490 days) from similar genetic backgrounds (C57BL/6J)

Summary

INTRODUCTION

In order to test hypotheses about evolutionary changes in phenotype, it is often necessary to understand their underlying genotypic and developmental bases. (Fig. 1): the basicranium (derived from the chondrocranium), the neurocranium (the dermatocranial bones of the cranial vault), and the face (derived initially from the splanchnocranium with subsequent development of dermatocranial elements) These regions behave as modules by varying somewhat independently (Cheverud 1982b, 1989, 1995; Lieberman et al 2000b; Hallgrımsson et al 2004b). HN2, is that changes that directly affect the morphogenesis of a particular region will have widespread effects on other regions of the cranium Such a result would be expected if craniofacial shape derives from multiple distributed interactions. We test several alternative hypotheses based on the suggestion that some regions, notably the basicranium, have greater effects on craniofacial shape than others via particular, asymmetrical interactions (i.e., more localized effects in contrast with widespread effects above).

MATERIALS AND METHODS

RESULTS

Findings

DISCUSSION