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290 17 epigenetic Interactions THE DEVELOPMENTAL ROUTE TO FUNCTIONAL INTEGRATION Miriam leah Zelditch and Donald l. Swiderski Epigenetic interactions are obviously necessary for normal development—without them there would be no primary embryonic induction, no epithelial–mesenchymal interactions, and no interactions between differentiated tissues such as muscles and bones. These interactions are necessary not only for normal development but also for normal function, if only because they produce the structures that carry out function. The ability of jawed vertebrates to eat typically requires having a jaw, and without epithelial– mesenchymal interactions there would be no jaw. However, eating requires more than just having a jaw, and epigenetic interactions do more than just produce it. Epigenetic interactions also integrate developmentally heterogeneous tissues into a coherent functional whole, coordinating the development of bones with that of the skeletal muscles that move the bones and with that of the teeth, not to mention the tongue, nerves, and blood vessels. All of these, taken together, comprise a single integrated whole—the feeding system. In the more general case, bones and muscles can be regarded as a single functional system because bones provide skeletal struts and levers that are moved by conTenTs An Overview of Morphological Integration Organisms as Integrated Systems Integration at the Population Level Developmental “and/or” Functional Integration? Why Blur the Distinction? Why Sharpen the Distinction? The Developmental Origins of Integration The Role of Epigenetic Interactions The Evolution of Functional Integration The Mandible: Paradigm for What? Conclusion Acknowledgments References Epigenetics: Linking Genotype and Phenotype in Development and Evolution, ed. Benedikt Hallgrímsson and Brian K. Hall. Copyright © by The Regents of the University of California. All rights of reproduction in any form reserved. epigenetic interactions and integration 291 the forces supplied by muscles (Herring, 1994). Obviously, the system does not work in the absence of its parts; but even if all the parts are there, the system does not work very well when they are disproportionate relative to each other. Excessively strong muscles coupled to a small, slight mandible could generate forces capable of yanking the mandible out of its joint; conversely , excessively weak muscles inserting on a massive jaw could generate forces too weak to open the jaw at all. Even modest disproportions can imperil function, such as when they lead to a misalignment of the jaws and teeth; in the case of a rodent’s ever-growing incisors, malocclusion means that the incisors can grow through the roof of the mouth, which is usually lethal. That epigenetic interactions benefit individuals by integrating functionally interacting parts seems obvious, but that benefit does not mean that epigenetic interactions evolved to serve that particular biological role. Whether epigenetic interactions, or morphological integration more generally, are adaptations is a topic addressed by Hansen in this volume (see Chapter 20); and as is clear from his chapter, this adaptive scenario can be challenged on several grounds. One alternative hypothesis is that integration is an intrinsic feature of developmentally modular systems, a hypothesis that has an important and novel implication because it means that the developmental basis of integration may be critical for understanding its evolutionary origin. Another alternative theory is that integration is not an adaptation in its own right but, rather, a correlated effect of some other adaptation, another hypothesis that emphasizes the theoretical significance of the developmental basis of integration . As we argue herein, the developmental basis of integration may be just as crucial to the evolution of integration as it is to the development of an individual. We begin by briefly reviewing the concept of morphological integration and then focus more specifically on the distinction between developmental and functional integration to provide the context for analyzing the developmental basis of functional integration. In that section, we highlight the distinctive features of epigenetic pleiotropy and why those features might make them distinctive for the evolution of integration in general and for functional integration in particular. Finally, we turn to one classic model system for studies of integration, the mammalian mandible, one that has been regarded as a paradigm for the theory that integration is an adaptation but one which also may exemplify the two alternative theories and that clearly reveals some of the complications of testing theories about integration. An oveRvIew of moRPHologIcAl InTegRATIon oRgAnIsms As InTegRATeD sysTems The idea of morphological integration is grounded in the perception that organisms are not collections of isolated parts but, rather, coherent systems. Consequently, fitness depends on the relationships among traits rather than on their individual...

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