Diversification of Body Plans and Body Parts

[We suggest that] evolutionary changes in anatomy and way of life are more often based on changes in the mechanisms controling the expression of genes than on sequence changes in proteins.

—Mary-Claire King and Allan Wilson (1975)

Chapters 2 and 3 described the general principles of the underlying unity in developmental regulatory mechanisms, and Chapter 4 detailed the widespread conservation of the genetic toolkit for development among bilaterians. These chapters set the stage for the consideration of longstanding questions about body plan evolution. Given the extensive genetic similarities of living animals, how did new and vastly different forms evolve from a common bilaterian ancestor? What are the genetic differences that underlie the diversity of animal body patterns? This chapter focuses on ways in which evolutionary changes in the regulation of toolkit genes during development contributed to morphological change. Here we examine the relationship between body plan evolution and regulatory evolution, concentrating primarily on the diversification of repeated structures along the primary body axis and of homologous parts between lineages.

The identification of genetic mechanisms underlying body pattern diversity relies on a comparative approach encompassing both model organisms and their relatives. Our understanding of the genetic basis of the radiation of body plans within a phylum is mostly limited to arthropods (including the insect Drosophila melanogaster) and chordates (including vertebrate model systems such as chicks, mice, frogs, and zebrafish). Fortunately, these phyla are also exemplary regarding the degree of body plan diversification they display within the framework of a shared body organization. The incredible diversity of extant arthropods, particularly of crustaceans and insects, in combination with the many arthropod and onychophoran forms present in the Cambrian period (530 Ma), provides a rich experimental and historical foundation for a case study of body plan evolution. Similarly, the evolution and subsequent diversification of the axial morphologies of modern chordates provide dramatic examples of large-scale morphological diversification.

Much of our understanding of the role played by regulatory evolution in shaping animal body patterns comes from the Hox genes. The Hox genes are the best-characterized developmental regulatory genes within and between Metazoan phyla. The genes themselves predate the radiation of bilaterian body plans (see Chapter 4). In this chapter, we examine how Hox genes are used during development in related organisms to understand how evolutionary changes in these and other selector genes contribute to body plan evolution. Comparative analyses of Hox gene expression domains reveal that major transitions in body organization in both arthropods and tetrapods correlate with shifting spatial boundaries of Hox gene expression. In particular, differences in the regulation of Hox genes correlate with the diversity of the number and identity of repeated units, such as arthropod and annelid segments and appendages and vertebrate axial elements.

More closely related animal groups with a more conserved body organization, such as insects, exhibit fewer large-scale differences in Hox gene expression. The diversification of homologous structures in the context of a stable body plan is largely characterized by regulatory changes downstream of Hox (or other selector) gene function. For example, the morphological diversity of insect hindwings and of vertebrate forelimbs is a consequence of evolutionary changes in the assortment of target genes regulated by the Hox (or other selector) genes that pattern these appendages. A second mechanism of diversification acting in organisms that share a particular body plan is the modification of Hox expression patterns within developmental fields. In this chapter, we discuss the best-understood case studies that illustrate the relationship between morphological diversity and evolutionary changes in the regulation of the Hox genes and of their downstream targets.

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