Description:
<jats:p>
<jats:italic>Irregular facets</jats:italic>
(
<jats:italic>If</jats:italic>
) is a dominant mutation of
<jats:italic>Drosophila</jats:italic>
that results in small eyes with fused ommatidia. Previous results showed that the gene
<jats:italic>Krüppel</jats:italic>
(
<jats:italic>Kr</jats:italic>
), which is best known for its early segmentation function, is expressed ectopically in
<jats:italic>If</jats:italic>
mutant eye discs. However, it was not known whether ectopic
<jats:italic>Kr</jats:italic>
activity is either the cause or the result of the
<jats:italic>If</jats:italic>
mutation. Here, we show that
<jats:italic>If</jats:italic>
is a gain-of-function allele of
<jats:italic>Kr</jats:italic>
. We then used the
<jats:italic>If</jats:italic>
mutation in a genetic screen to identify dominant enhancers and suppressors of
<jats:italic>Kr</jats:italic>
activity on the third chromosome. Of 30 identified
<jats:italic>Kr</jats:italic>
-interacting loci, two were cloned, and we examined whether they also represent components of a natural
<jats:italic>Kr</jats:italic>
-dependent developmental pathway of the embryo. We show that the two genes,
<jats:italic>eyelid</jats:italic>
(
<jats:italic>eld</jats:italic>
) and
<jats:italic>extramacrochaetae</jats:italic>
(
<jats:italic>emc</jats:italic>
), which encode a Bright family-type DNA binding protein and a helix-loop-helix factor, respectively, are necessary to achieve the singling-out of a unique
<jats:italic>Kr</jats:italic>
-expressing cell during the development of the Malpighian tubules, the excretory organs of the fly. The results indicate that the
<jats:italic>Kr</jats:italic>
gain-of-function mutation
<jats:italic>If</jats:italic>
provides a tool to identify genes that are active during eye development and that a number of them function also in the control of
<jats:italic>Kr</jats:italic>
-dependent developmental processes.
</jats:p>