Gap gene

A gap gene is a type of gene involved in the development of the segmented embryos of some arthropods. Gap genes are defined by the effect of a mutation in that gene, which causes the loss of contiguous body segments, resembling a gap in the normal body plan. Each gap gene, therefore, is necessary for the development of a section of the organism.

Gap genes were first described by Christiane Nüsslein-Volhard and Eric Wieschaus in 1980.^[1] They used a genetic screen to identify genes required for embryonic development in the fruit fly Drosophila melanogaster. They found three genes – knirps, Krüppel and hunchback – where mutations caused deletion of particular stretches of segments. Later work identified more gap genes in the Drosophila early embryo – giant, huckebein and tailless.^[2][3] Further gap genes including orthodenticle and buttonhead 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 embryo generally correlated with the region that is absent in the mutant.^[4][5] 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.^[6][7] 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.