Fine mapping of a putative genomic element that mediates the induction of a male-specific muscle in Drosophila

The muscle of Lawrence (MOL) is a pair of male-specific longitudinal muscles located below the tergite of the fifth abdominal segment in adult Drosophila melanogaster. For the MOL to form, the innervating motor neuron (MOL-inducing neuron: Mind neuron) must express male-specific products of the fruitless (fru) gene, collectively called FruM. We localized a cis-regulatory element of the fru gene that promotes its expression in the Mind neuron to a short genomic interval lying between −4.3 and −5.7 kb upstream of the distal promotor P1 of the fru gene. Comparisons of the sequences of this genomic interval between D. melanogaster, which carries a single MOL pair and D. subobscura, a distant relative that carries two MOL pairs, revealed five conserved stretches, Boxes A–E, tandemly aligned in a single copy (D. melanogaster) or in duplicate (D. subobscura). The Gal4 transgenes carrying Boxes A, B, and E together were able to induce the MOL when used to drive UAS-fruM. Deleting Boxes A, B and E from the genome resulted in loss of the MOL, with occasional formation of vestigial MOL-like muscles. Conversely, reintroducing Boxes A, B, and E into the deletant restored the MOL. We conclude that Boxes A, B, and E comprise a major element that promotes fru expression in the Mind neuron for MOL induction, and we propose to call it the cis-element for MOL induction (ceMOi).