Canalisation of development of scutellar bristles in Drosophila by control of the scute locus.

sex-linked mutant scute (sc), its noma1 allele, and a number of minor Tf:nes which can be shown to exist by selective breeding, form a developmental system which determines the number of bristles on the scutellum of Drosophila mlanogaster. In unselected stocks almost all flies grow four bristles when one or two + genes are present at the scute locus; that is, development is canalised at this level of phenotype. The average number of bristles in sc males in unselected stocks is less than one and in sc/sc females more than one, but there is considerable variation about these means. In stocks segregating for sc and +, selected for high bristle number, + flies may grow a mean of seven or eight bristles. There is considerable variation about these high means also. In high bristle stocks, sc males and sc/sc females average between three and four bristles, most individuals having four bristles but some having three, a few two and a very few having less than two. The scutellar phenotype is controlled at four bristles. If bristle number does not reach as high as four, it varies about some mean lower than four and is not controlled. If bristle number does not fall as low as four it varies about some mean higher than four and is not controlled. But a wide variety of genotypes, despite widely different en~ronmen~al conditi~ns, are con~olled to a level of exactly four bristles. If we write P = (p (M) where P is the phenotype, 111 is Make or that which forms or tends to form bristles and 9 is a function relating P and M , control at four bristles operates over a range of M values equivalent in extent to 60 of M. # in the scutellar system can be represented by a line for which the horizontal axis is M , and the vertical axis is P, with a slope of 45" to a plateau 6aM long followed by a slope of 45". P is 4 at a value of M anywhere under the plateau. Figure 1 shows such a curve relating P to 111. A similar curve based on fewer points was described by an equation containing six terrns in M (RENDEL 1959b). Fuller discussion can be found elsewhere (RENDEL 1962). M receives contributions from the genes at the scute locus and minor genes scattered over the genome. There is evidence suggesting that the major gene at the scute locus is controlled. A comparison of sc/sc, sc/-iand +/+ shows that at all levels of gene background the difference between sc/sc and sc/+ is much greater than the difference between sc/+ and +/+. The second + gene contributes much less than the first and is under some control so that in the presence of +it is less active than in the presence of sc. The suggestion is that the activity of the + gene is under negative feedback control. The more product the less activity until the gene at the scute locus is turned right off.