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Pages 109-128

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From page 109... ... uses available genetic components in the form of preexisting and active transcription factors and CREs to generate novelty; (ii) minimizes the penalty to overall fitness by introducing discrete changes in gene expression; and (iii) Read the entire page →
From page 110... ... A vast body of comparative studies has revealed that morphological differences among taxa are correlated with differences in developmental gene expression patterns, which has supported the proposal that evolutionary modifications of gene expression (i.e., ‘‘regulatory evolution'') are the basis of morphological diversification (King and Wilson, 1975; Carroll, 1995) Read the entire page →
From page 111... ... The elucidation of the mechanisms of Cre evolution in morphological diversification has required the identification of appropriate experimental systems. Because coding sequences are usually sufficiently conserved to identify orthologous sequences among different phyla, it was naïvely assumed initially that the same would hold true for Cres, and that functional comparison of divergent Cres from distantly related taxa would be possible. Read the entire page →
From page 112... ... These principles explain both how and why regulatory sequence evolution is a pervasive, although not the exclusive, mechanism underlying morphological diversification. PIGMENTATION PATTERNS AND GENE EXPRESSION AS MODELS OF REGULATORY EVOLUTION Because morphological evolution is the product of the modification of the expression patterns of underlying genes, to understand how morpho logical changes arise, we must understand how changes in gene expres sion pattern arise. Read the entire page →
From page 113... ... . Therefore, understanding how the expression patterns of the genes encoding these enzymes are established and change among species is key to understanding the formation and diversification of pigmentation patterns. Read the entire page →
From page 114... ... The difference in pigmentation patterns is reflected by differences in pigmentation gene expression. in particular, the product of the yellow (y) Read the entire page →
From page 115... ... in fact, however, the formation of the yellow spot pattern entailed the evolution of binding sites for both activators and repressors involved in the building of the wing. in particular, the transcription factor engrailed, present in cells in the posterior part of the wing, directly represses yellow expression, confining elevated yellow expression and, therefore, the formation of the pigmentation spot to the anterior region (Gompel et al., 2005) Read the entire page →
From page 116... ... A suite of transcription factors control the development of the fly wing, each one being expressed in a particular pattern. Altogether, these expression patterns constitute a wing trans-regulatory landscape, conserved among Drosophila species (Top) Read the entire page →
From page 117... ... More generally, the evolution of wing patterns illustrates a fundamental principle of regulatory evolution: novel patterns arise more readily from the recruitment of available components, Cres, and transcription factors into new regulatory interactions rather than from the de novo creation of genes or Cres. indeed, all of the diversity of wing pigmentation patterns illustrated in Fig. Read the entire page →
From page 118... ... Although we are mainly concerned with the genetic and molecular mechanisms of evolutionary innovations, the existence of selective pressures constantly sifting through the spectrum of emerging variations must be considered, because they constrain the scope of genetic changes permitted under natural selection. The genetic changes contributing to morphological evolution can affect protein function through mutations in gene coding sequences or, instead, gene regulation, mainly through Cre evolution. Read the entire page →
From page 119... ... highly pleiotropic genes (including most developmentally regulated genes) are more likely to contribute to morphological evolution through cis-regulatory changes than through coding sequence alterations. Read the entire page →
From page 120... ... however, studies of pigmentation pattern evolution in flies have revealed that regulatory evolution takes advantage of transcription factors throughout genetic hierarchies in an opportunistic way to generate new regulatory connections. This notion is clearly illustrated by the evolution of abdominal pig mentation in the relatives of D Read the entire page →
From page 121... ... melanogaster A5 A6 D kikkawai FiGUre 6.5 regulatory changes underlying male abdominal pigmentation pattern evolution. Read the entire page →
From page 122... ... The evolution of pigmentation patterns through coopting body-plan regulatory proteins illustrates a third principle of regulatory evolution: association between any transcription factor and a downstream Cre may evolve, irrespective of positions of these components in genetic hierarchies. This opportunistic nature of regulatory interactions contributes to the vast evolutionary potential of Cres. Read the entire page →
From page 123... ... . CONNECTING THE DOTS FROM PIGMENTATION PATTERNS TO BODY PLAN DIVERSIFICATION: THE COMPOUNDING OF REGULATORY CHANGES OVER EONS A long-standing question in evolutionary biology has been whether the genetic and molecular mechanisms underlying morphological changes within populations (so-called ‘‘microevolution'') Read the entire page →
From page 124... ... and not rare genomic rearrangements or duplication events. Therefore, it appears the genetic changes generating morphological differences among species are of the same nature as the ones that arise within populations. Read the entire page →
From page 125... ... . in Diptera, a suite of wing-patterning genes have evolved Ubx-binding sites in their Cres and, as a result, are repressed during hindwing development. Read the entire page →
From page 126... ... Thus, in the same way that abdominal pigmentation pattern evolved by changes in the regulatory connections between Abd-B and downstream pigmentation genes, the two-winged dipteran body plan evolved by changes in the regulatory interactions between Ubx and downstream wing-patterning genes. in this light, we see that the differences between the evolution of modest morphological traits, such as pigmentation pat terns, and changes of larger magnitude, such as body-plan modifications, are not in the nature of the genetic changes but rather in their degree. Read the entire page →
From page 127... ... . Ultimately, such a dynamic picture of Cre evolution will help to reconstruct the mutational paths that lead to the origin of novel gene expression patterns. Read the entire page →

From page 109...

... uses available genetic components in the form of preexisting and active transcription factors and CREs to generate novelty; (ii) minimizes the penalty to overall fitness by introducing discrete changes in gene expression; and (iii)