Gene activities and segmental patterning in Drosophila: analysis of odd-skipped and pair-rule double mutants.

The odd-skipped (odd) gene is required to generate anterior regions of the odd-numbered segments in Drosophila; homozygous embryos show pattern deletions that are always less than a segment in width and are associated with mirror-image duplications of adjacent regions. To define further the role of odd and determine how it interacts with other segmentation genes, we have described the effects of combining odd with mutations at other pair-rule loci. We have observed phenotypic suppression in double-mutant combinations with even-skipped (eve), paired, sloppy paired, and engrailed (en). In the most thoroughly characterized combination (odd eve), both naked cuticle and specific denticle rows are restored that would normally have been deleted by one of the two mutants alone. In the odd en double mutant, we observe nearly complete suppression of the odd phenotype, such that the mirror image duplications are eliminated and the odd-numbered denticle bands are restored. We conclude that the requirements of pattern elements for specific gene activities are not absolute, and propose mechanisms by which these genes interact to specify cell fates.