Anotiier Case of Unequal Crossing-over in Drosophila Melanogaster

Females homozygous for the sex-linked dominant, Bar, occasionally give rise to wild-type reversions and to forms with more extreme eye reduction than Bar. This behavior was shown by Sturtevant' to result from unequal crossing-over. The Bar-reverted type was considered to be a deficiency for the Bar gene; while the extreme form, called Ultra-Bar or Double-Bar, was interpreted as a duplication for that gene. Later, Wright2 suggested that Bar itself had something additional present which when lost by unequal crossing-over would give back a normal chromosome (Bar-reverted). The cytological nature of Bar was cleared up independently by Bridges3 and Muller, et al.,4 who investigated the salivary gland chromosomes. They found Bar to be a tandem duplication, in normal order, for an X-chromosome section composed, according to Bridges' detailed analysis, of six bands. Bridges further demonstrated that Bar-reverted had the identical banding of a normal chromosome, whereas Double-Bar had a serial triplication for the region present twice in Bar and once in Bar-reverted. The second case of a tandem duplication being responsible for a dominant "mutation" is that of the sex-linked Hairy wing, which Demerec5 has shown is a repetition for a single heavy band near the tip of the X-chromosome. However, its location in a region of extremely low crossing-over prevented a study of unequal crossing-over. This paper is a preliminary report on an autosomal tandem repeat which was detected as a suppressor of the dominant mutant, Star (S, 2-1.3). An analysis of the salivary gland chromosomes of this suppressing factor, when homozygous, when closely paired with a normal chromosome and when present as an unpaired haploid strand, consistently showed the presence of a tandem duplication in direct order near the left end of the