Role of abd-A and Abd-B in development of abdominal epithelia breaks posterior prevalence rule.

Hox genes that determine anteroposterior body axis formation in all bilaterians are often found to have partially overlapping expression pattern. Since posterior genes dominate over anterior Hox genes in the region of co-expression, the anterior Hox genes are thought to have no function in such regions. In this study we show that two Hox genes have distinct and essential functions in the same cell. In Drosophila, the three Hox genes of the bithorax complex, Ubx, abd-A and Abd-B, show coexpression during embryonic development. Here, we show that in early pupal abdominal epithelia, Ubx does not coexpress with abd-A and Abd-B, while abd-A and Abd-B continue to coexpress in the same nuclei. The abd-A and Abd-B are expressed in both histoblast nest cells and larval epithelial cells of early pupal abdominal epithelia. Further functional studies demonstrate that abd-A is required in histoblast nest cells for their proliferation and suppression of Ubx to prevent first abdominal segment like features in posterior segments while in larval epithelial cells it is required for their elimination. We also observed that these functions of abd-A are required in its exclusive as well as the coexpression domain with that of Abd-B. The expression of Abd-B is required in histoblast nest cells for their identity while it is dispensable in the larval epithelial cells. The higher level of Abd-B in the seventh abdominal segment, that down-regulates abd-A expression, leads this segment to be absent in males or of smaller size in females. We also show that abd-A in histoblast nest cells positively regulates expression of wingless for the formation of the abdominal epithelia. Our study reveals an exception to the rule of posterior prevalence and shows that two different Hox genes have distinct functions in the same cell, which is essential for the development of abdominal epithelia.