Shadow enhancers as a source of evolutionary novelty.

The dorsal-ventral patterning of the Drosophila embryo is controlled by Dorsal, a sequence-specific transcription factor that is related to mammalian NF-κB. Previous ChIP-chip assays predicted that as many as a third or even half of all Dorsal target genes contain multiple enhancers for the same or similar expression pattern. Here we show that some of these secondary enhancers, or “shadow enhancers”, produce gene expression patterns that overlap those produced by the primary enhancers in transgenic embryos. We suggest that shadow enhancers help ensure the precision of embryonic patterning and discuss their significance in the evolution of genetic novelty.