The elimination of an adult segment by the Hox gene Abdominal-B.

Hox gene activity leads to morphological diversity of organs or structures in different species. One special case of Hox function is the elimination of a particular structure. The Abdominal-B Hox gene of Drosophila melanogaster provides an example of such activity, as this gene suppresses the formation of the seventh abdominal segment in the adult. This elimination occurs only in males, and is characteristic of more advanced Diptera. The elimination requires the differential expression or activity of genes that are downstream Abdominal-B, or that work together with it, and which regulate cell proliferation or cell extrusion. Here, we review the mechanisms responsible for such elimination and provide some new data on processes taking place within this segment.

Differential activity of Drosophila Hox genes induces myosin expression and can maintain compartment boundaries.

Compartments are units of cell lineage that subdivide territories with different developmental potential. In Drosophila, the wing and haltere discs are subdivided into anterior and posterior (A/P) compartments, which require the activity of Hedgehog, and into dorsal and ventral (D/V) compartments, needing Notch signaling. There is enrichment in actomyosin proteins at the compartment boundaries, suggesting a role for these proteins in their maintenance. Compartments also develop in the mouse hindbrain rhombomeres, which are characterized by the expression of different Hox genes, a group of genes specifying different structures along their main axis of bilaterians. We show here that the Drosophila Hox gene Ultrabithorax can maintain the A/P and D/V compartment boundaries when Hedgehog or Notch signaling is compromised, and that the interaction of cells with and without Ultrabithorax expression induces high levels of non-muscle myosin II. In the absence of Ultrabithorax there is occasional mixing of cells from different segments. We also show a similar role in cell segregation for the Abdominal-B Hox gene. Our results suggest that the juxtaposition of cells with different Hox gene expression leads to their sorting out, probably through the accumulation of non-muscle myosin II at the boundary of the different cell territories. The increase in myosin expression seems to be a general mechanism used by Hox genes or signaling pathways to maintain the segregation of different groups of cells.