Figure 1. Contemporary model for the D. melanogaster tergite pigmentation Gene Regulatory Network.

Melanic tergite pigmentation requires the specific expression of the pigmentation genes yellow and tan, while blocking the expression of the yellow-pigment promoting ebony gene. (A) Tergite pigmentation pattern for a D. melanogaster male and the regulatory interactions experienced in the non-melanic male A2-A4 segments (top) and the melanic A5 and A6 segments (bottom). Abd-B is not expressed in the anterior A2-A4 segments and resultantly yellow and tan lack the direct and indirect activating input. In these anterior segments, Abd-A acts as a direct repressor of tan in combination with (direct or indirect) repressive effects of exd and hth. Abd-B is expressed in the posterior A5 and A6 segments, where it functions as a direct activator of yellow and an indirect activator of tan. In these segments, Abd-A acts as an indirect activator of tan. (B) Tergite pigmentation pattern of a D. melanogaster female and the key regulatory inputs experienced in the A2-A4 segments (top) and the A5 and A6 segments (bottom). In the female abdomen, Bab acts as a dominant repressor of yellow and tan expression, overriding the presence of Abd-B and Abd-A. In the GRN schematics, inactive genes are indicated in gray coloring, solid lines connecting genes indicate established direct interactions between a transcription factor and a target gene’s CRE, and dashed connections indicate indirect regulatory interactions or interactions not yet shown to be direct. Lines terminating with an arrowhead indicate regulation in which the transcription factor functions as an activator, and lines terminating in a nail-head shape indicate repression.