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EVOLUTION DEVELOPMENT 9 1 76 91 2007 Epigenetic interactions and the structure of phenotypic variation in the cranium B Hallgr mssona 1 D E Liebermanb 1 W Liua A F Ford Hutchinsonc and F R Jirikc a Department of Cell Biology and Anatomy Alberta Bone and Joint Institute Faculty of Medicine University of Calgary 3330 Hospital Dr NW Calgary AB Canada T2N 4N1 b Departments of Anthropology and Organismic and Evolutionary Biology Harvard University 11 Divinity Avenue Cambridge MA 02138 USA c Department of Biochemistry and Molecular Biology Alberta Bone and Joint Institute Faculty of Medicine University of Calgary 3330 Hospital Dr NW Calgary AB Canada T2N 4N1 Author for correspondence email bhallgri ucalgary ca 1 These authors contributed equally to this work 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



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