HOM-C evolution in Drosophila: is there a need for Hox gene clustering?

The conservation of Homeotic (Hox) gene clustering and colinearity in many metazoans indicates that functional constraints operate on this genome organization. However, several studies have questioned its relevance in Drosophila. Here, we analyse the genomic organization of Hox and Hox-derived genes in 13 fruitfly species and the mosquito Anopheles gambiae. We found that at least seven different Homeotic complex (HOM-C) arrangements exist among Drosophila species, produced by three major splits, five microinversions and six gene transpositions. This dynamism contrasts with the stable organization of the complex in many other taxa. Although there is no evidence of an absolute requirement for Hox gene clustering in Drosophila, we found that strong functional constraints act on the individual genes.