Near-reciprocal phenotypes caused by inactivation or indiscriminate expression of the Drosophila segmentation gene ftz.

Early in development, Drosophila embryos express the segmentation gene fushi tarazu (ftz) in a 'zebra' pattern of active and inactive stripes, each about the width of a segment primordium. If the ftz gene is prevented from functioning, alternating portions of the body normally derived from the active stripes fail to develop, resulting in larvae which lack the denticle bands normally formed by the mesothorax and odd-numbered abdominal segments (that is, thoracic segment T2 and abdominal segments A1, A3, A5 and A7). Here, using the Drosophila heat shock protein 70 (hsp70) gene promoter to drive widespread expression of ftz transcripts on heat shock, I find that unrestricted ftz activity can cause a reciprocal 'pair-rule' phenotype--that is, the absence of the denticle bands which are normally derived from segments T1, T3, A2, A4, A6 and A8. These results show that both the 'on' and 'off' states of ftz gene expression have instructive roles in the development of alternating regions of the body, and hence suggest that the ftz gene acts combinatorially with other pair-rule genes (for example, even-skipped, odd-skipped, paired) to establish the metameric pattern of the body.