Single-locus modification of position-effect variegation in Drosophila melanogaster. I. White variegation.

mechanism of position-eff ect variegation has been and remains a fasciT?ating puzzle, tan talizing the student of the genetics of somewhat higher organisms with the hope that here is a clue to the riddle of the sequential shifts of control during development. This view is elaborated in BAKER’S (1963) able review of his own and his colleagues’ work. One of the more refractory aspects of the puzzle, and at the same time one source of its fascination, is the extraordinary susceptibility of the process to modification by a diverse array of genetic and environmental factors Rigorous physiological and biochemical analyses of the phenomenon await the identification and disentanglement of the effects of these modifying factors at the gross level. Most of these factors have been exposed by the pioneering work of SCHULTZ (1941, 1950, 1956). Several single-locus variegation modifiers have been isolated (SCHULTZ, in MORGAN et al. 1942), but none have been fully described in the literature as yet. The present investigation began with the segregation of variegation suppression in crosses between two Drosophila melanogaster stocks coisogenic except for the third chromosome: onre was isogenic for a normal third chromosome; the other had both this and another carrying a short duplication including the white locus from the X chromosomie. Since the X chromosomes carried the mutant w (white). the isogenic stock had white eyes and the other segregated mottledand whiteeyed flies. The second <stock was maintained by mating duplication-bearing ( D p ) females with isogenic inales from the companion stock. Two classes of expression of the variegating w” locus were soon distinguishable. The larger class had sizable patches of red facets in the eyes of most Dp/ f flies. The smaller D p / f class had white eyes though thtiy transmitted Dp to progeny. (In both cases, of course, there were white-eyed sibs homozygous for the normal third chromosome.) In the presence of extra heterochromatin-a Y in a female or both an attached X Y plus a free Y in a male-a small amount of eye pigment appeared in flies of the second class. Sublines of the stock were established, “Dp’,” expressible in free-X genotypes, and ‘‘DpU,’’ expressible only in extra-Y genotypes ( SPOFFORD 1961 ) . Upon intercrossing D p f / f males and females, a third class of expression was found: virtually full vvild-type pigmentation even in flies with normal amounts of heterochromatin. A stock of this latter was preserved, still segregating normal and Dp-bearing third chromosomes. Males homozygous for the duplication are either lethal or sterile.