Muscles of a Drosophila embryo in red and Stripe expression in the cell nuclei of apodems in red

Stripe activity

We characterised the apodeme-specific transcription factor Stripe and revealed that stripe is exclusively expressed in the precursors of the apodemes and the finally developed muscle attachment sites. In stripe mutant embryos the apodeme cells do not differentiate normally, as shown by the loss of expression of most marker genes of these cells, and the muscles can not attach to the apodemes. Futhermore, ectopic expression of stripe in ectodermal cells is sufficient to transform these cells into apodemes resulting in the expression of known markers. In addition, ectopic expression of stripe is sufficient to attract directed muscle growth towards the ectopic muscle attachemnt sites.

Expression of stripe within the ventral midline of te embryo (bottom) is sufficient to attract ventral muscles (right)

We identified the regulatory elements of the stripe promotor and found that the stripe expression is under direct control of converging Hedgehog (Hh) and Wingless (Wg) signalling activities.

The essential enhancer is only 140 bp in length but contains both Ci and Pan binding sites, which converge their activity to limit the Hedgehog dependent activation only to the cells posterior to the engrailed expression domain that do not receive Wingless, which represses stripe expression anterior to the engrailed domain. This combination of activation and repression restricts the stripe expression to only a single cell row, which eventually forms the segment boundary. This regulation shown for stripe first is functional also for other genes expressed in the segment border like thrombospondin.