Figure 7. Pigmentation gene network model and the evolution of an ancestral CRE regulatory logic.

(A–C) Schematic of the hierarchical structure of the D. melanogaster pigmentation gene network. Direct regulation is represented as solid connections and dashed connections represent connections where regulation has not been shown to be direct. Activation and repression are respectively indicated by the arrowhead and nail-head shapes. This network includes an (A) upper level of patterning genes, including Abd-B and dsx respectively of the body plan and sex-determination pathways, (B) a mid-level tier that integrates patterning inputs, (C) and a lower level that includes pigmentation genes whose encoded products function in pigment metabolism. Although Abd-B directly regulates the pigmentation gene yellow, sexually dimorphic expression of the yellow and tan genes results from the sexually dimorphic output of the bab locus that acts to repress tan and yellow expression in females. (D) A model for the evolution of diverse dimorphic element regulatory activities. The common ancestor of D. melanogaster populations and related species possessed a dimorphic element with both DSX and ABD-B regulatory linkages and that drove expression in the female A6–A8 segments. This ancestral regulatory logic was recurrently modified to increase the levels and expand the segmental domain of activity, or to decrease and contract activity. These changes occurred amidst the preservation of the core ABD-B and DSX regulatory linkages, perhaps though the loss (TF 3) and/or gain (TF 4) of other transcription factor linkages.