Fig. 7.

Gene inversion within heart complex leads to novel patterning and insect evolution. a The RNA Pol II ChIP-Seq tracks within the ladybird locus from early Drosophila embryos (Chopra VS, Hendrix DA and Levine M. unpublished). The lbe promoter displays a strong stalled Pol II signal, whereas the lbl and C15 promoters are not stalled. This stalled lbe promoter contains insulator activity in a transgenic assay [16]. b In Drosophila, the cardiac enhancer is located 3′ of the ladybird early and ladybird late genes, and is unable to activate C15 expression owing to insulator activity at the ladybird early promoter. The chromosomal inversion in Tribolium relocates this enhancer so that the ladybird promoter is no longer positioned between the enhancer and C15 gene. As a result, the cardiac enhancer is able to activate C15 expression in Tribolium, but not Drosophila. Thus, a novel pattern of gene expression, C15 expression in Tribolium pericardial and cardial cells, is not due to the modification of gene regulatory networks or the de novo evolution of enhancer sequences, but rather results from the interaction of a conserved enhancer with different target genes: ladybird in Drosophila and the neighboring C15 gene in Tribolium [16]