| Summary: | <p/> <p>Background</p> <p>In <it>Drosophila melanogaster </it>the <it>Enhancer of split-Complex [E(spl)-C] </it>consists of seven highly related genes encoding basic helix-loop-helix (bHLH) repressors and intermingled, four genes that belong to the Bearded (Brd) family. Both gene classes are targets of the Notch signalling pathway. The <it>Achaete-Scute-Complex [AS-C] </it>comprises four genes encoding bHLH activators. The question arose how these complexes evolved with regard to gene number in the evolution of insects concentrating on Diptera and the Hymenoptera <it>Apis mellifera</it>.</p> <p>Results</p> <p>In Drosophilids both gene complexes are highly conserved, spanning roughly 40 million years of evolution. However, in species more diverged like <it>Anopheles </it>or <it>Apis </it>we find dramatic differences. Here, the <it>E(spl)-C </it>consists of one bHLH (<it>mβ</it>) and one Brd family member (<it>mα</it>) in a head to head arrangement. Interestingly in <it>Apis </it>but not in <it>Anopheles</it>, there are two more <it>E(spl) </it>bHLH like genes within 250 kb, which may reflect duplication events in the honeybee that occurred independently of that in Diptera. The <it>AS-C </it>may have arisen from a single <it>sc/l'sc </it>like gene which is well conserved in <it>Apis </it>and <it>Anopheles </it>and a second <it>ase </it>like gene that is highly diverged, however, located within 50 kb.</p> <p>Conclusion</p> <p><it>E(spl)-C </it>and <it>AS-C </it>presumably evolved by gene duplication to the nowadays complex composition in Drosophilids in order to govern the accurate expression patterns typical for these highly evolved insects. The ancestral ur-complexes, however, consisted most likely of just two genes: <it>E(spl)-C </it>contains one bHLH member of <it>mβ </it>type and one Brd family member of m<it>α </it>type and <it>AS-C </it>contains one <it>sc/l'sc </it>and a highly diverged <it>ase </it>like gene.</p>
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