V-Type Position Effects at the Light Locus in Drosophila Melanogaster.

group of 35 exceptional flies exhibiting variegated type position effects at the A light locus (lt:2-55.0) has been produced, and its analysis is the subject of this paper. Variegation at It has been shown by SCHULTZ and DOBZHANSKY (1934) and further studied by MORGAN, BRIDGES and SCHULTZ (1932, 1935), MORGAN, SCHULTZ and CURRY (1941), MORGAN and SCHULTZ (1942) and SCHULTZ (1936, 1941). However, no detailed work is available which reveals certain characteristics this writer has found typical of variegations for light. The light gene, normally located in heterochromatin, may show variegation if placed near euchromatin or if euchromatin is placed near it. Thus It+/lt produces wild type pigmentation, while R (It + ) /It yields within the compound eye a mixture in varying proportions of wild type and light pigmentation. DE ZULUETA (1931) established the position of I t as 55.0 crossover units from the tip of 2L, and SCHULTZ (1936) established that this locus is in proximal heterochromatin of 2L. The location of It on the salivary chromosome is not precisely described in the literature. However, it is possible that it may be to the right of salivary band 40B and to the left of 40D (EILEEN S. GERSH, personal communication). The reader is referred to the review of LEWIS (1950) for an extensive treatment of the subject of position effect and to the review of HANNAH (1951) on the function of heterochromatin. (HANNAH lists the location of It as 40F.) Although position effects of loci situated in euchromatin have been studied extensively, only a few thorough studies have been made on variegation produced at loci normally found in heterochromatin. The cubitus interruptus locus (ci: 4-0.0) has been extensively investigated by DUBININ and SIDIROV (1935), KHVOSTOVA (1939), and STERN and KODANI (1955). SCHULTZ has made longterm studies of heterochromatin including some work on It, and BAKER (1953) has investigated the peach locus (5-203) in Drosophila uirilis. One distinctive feature of the rearrangements causing position effects of genes located in heterochromatin is the non random distribution of breaks. In this study the rearrangements which produced variegation at I t (and which were analyzed cytologically) may be characterized as involving one break in the proximal heterochromatin of 2L, and therefore either proximal or distal to It, and another