Drosophila embryonic pattern repair: how embryos respond to bicoid dosage alteration.

The product of the maternal effect gene, bicoid (bcd), is a transcription factor that acts in a concentration-dependent fashion to direct the establishment of anterior fates in the Drosophila melanogaster embryo. Embryos laid by mothers with fewer or greater than the normal two copies of bcd show initial alterations in the expression of the gap, segmentation and segment polarity genes, as well as changes in early morphological markers. In the absence of a fate map repair system, one would predict that these initial changes would result in drastic changes in the shape and size of larval and adult structures. However, these embryos develop into relatively normal larvae and adults. This indicates that there is plasticity in Drosophila embryonic development along the anterior-posterior axis. Embryos laid by mothers with six copies of bcd have reduced viability, indicating a threshold for repairing anterior-posterior mispatterning. We show that cell death plays a major role in correcting expanded regions of the fate map. There is a concomitant decrease of cell death in compressed regions of the fate map. We also show that compression of the fate map does not appear to be repaired by the induction of new cell divisions. In addition, some tissues are more sensitive to fate map compression than others.