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FIGURE 6.3 A glimpse of the actual wing trans-regulatory landscape. The expression of GFP reports the expression patterns of various wing transcription factors at the time pigmentation genes are being expressed.

a pigmentation gene, then the expression profile of this gene may change. Because the formation of pigmentation patterns requires the coincident deployment of multiple pigmentation genes, it can be anticipated that these genes will fall under the control of a common suite of trans-acting factors.

In this view, diverse pigmentation patterns can arise from the evolution of regulatory connections among pigmentation gene CREs and different combinations of transcription factors (Fig. 6.2). In particular, more elaborate patterns can evolve through the progressive accumulation of regulatory links between components of the trans-landscape and pigmentation genes CREs. This gradual elaboration of complex patterns is reflected in the graded series of pigmentation patterns found in several fly lineages, where new elements are added in more derived species (e.g., the top three wings in the right column in Fig. 6.1). Hence, the complexity of the wing trans-regulatory landscape and the combinatorial nature of gene expression regulation are sufficient to account for the spectacular diversity of wing pigmentation patterns.

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. 6.1 may be accounted for by regulatory changes and would not require, in principle, any coding sequence changes among species.

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