User:Celefin/Work in progress

A pair-rule gene is a type of gene involved in the development of the segmented embryos of insects. 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 Nusslein-Volhard and Eric Wieschaus in 1980. 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 Drosophile early embryo – fushi-tarazu, odd-paired, sloppy paired, and Tenascin major

gap genes in the Drosophila early embryo – giant, huckebein and tailless. Further gap genes are required for the development of the Drosophila head.

Once the gap genes had been identified at the molecular level it was found that each gap gene is expressed in a band in the early embyro generally correlated with the region that is absent in the mutant. In Drosophila the gap genes encode transcription factors, and they directly control the expression of another set of genes involved in segmentation, the pair-rule genes. The gap genes themselves are expressed under the control of maternal effect genes such as bicoid and nanos, and regulate each other to achieve their precise expression patterns.