Apair-rule gene is a type of gene involved in the development of the segmented embryosofinsects. Pair-rule genes are expressed as a result of differing concentrations of gap gene proteins, which encode transcription factors controlling pair-rule gene expression.[1][2] Pair-rule genes are defined by the effect of a mutation in that gene, which causes the loss of the normal developmental pattern in alternating segments.
Pair-rule genes were first described by Christiane Nüsslein-Volhard and Eric Wieschaus in 1980.[3] They used a genetic screen to identify genes required for embryonic development in the fruit fly Drosophila melanogaster. In normal unmutated Drosophila, each segment produces bristles called denticles in a band arranged on the side of the segment closer to the head (the anterior). They found five genes – even-skipped, hairy, odd-skipped, paired and runt – where mutations caused the deletion of a particular region of every alternate segment. For example, in even-skipped, the denticle bands of alternate segments are missing, which results in an embryo having half the number of denticle bands. Later work identified more pair-rule genes in the Drosophila early embryo – fushi tarazu, odd-paired and sloppy paired.[4]
Once the pair-rule genes had been identified at the molecular level it was found that each gene is expressed in alternate parasegments – regions in the embryo that are closely related to segments, but are slightly out of register.[5][6][7][8] Each parasegment includes the posterior part of one (future) segment, and an anterior part of the next (more posterior) segment. The bands of expression of the pair-rule genes correspond to the regions missing in the mutant. The expression of the pair-rule genes in bands is dependent both upon direct regulation by the gap genes [9] and on regulatory interactions between the pair-rule genes themselves.[10]