The Pseudoallelism of White and Apricot in Drosophila Melanogaster

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Abstract
Evidence is presented to show that the genes for white eyes (w) and for apricot eyes (designated apr instead of wa) occupy separate loci. Attached-X diploid females were used, with the phenotype yellow-2 and dilute apricot, and having the following (Type-A) constitution: y2 apr ec/y sc w spl. These females were made heterozygous for chromosomal rearrangements in the 2d and/or 3d chromosomes, using 2 Curly (Cy) inversions for the 2d chromosome and a complex termed Ultrabithorax -130 (Ubx130) for the 3d, since such rearrangements were known to increase crossing-over in the X-chromosome. In a preliminary mating using a yellow-2 dilute-apricot female heterozygous for only the Ubx130 rearrangement, a single yellow-2 red echinus female appeared which, by progeny tests, must have had one X-chromosome derived from a crossover between the w locus and an apr locus to the left of w. A 2d mating was made using yellow-2 dilute-apricot females heterozygous for Cy and Ubxl30. From 794 adequately constituted cultures there were 12 red-eyes females among 40,100 attached -X offspring. Analysis of these flies provided a total of 18 cross-overs between apr and w, with an observed crossover value of .03% under conditions giving 2-6 times the normal amt. of crossing over. The standard map distance between apr and w was calculated to be 0.005 to 0.02 units. Indirect proof was obtained for the occurrence of 6 double mutant (apr w) crossovers from mating 2, but in none of the cases could the apr w homozygote be distinguished from w. The heterozygote apr w/++ had red eyes, whereas apr +/+w and apr w/apr + females had pinkish yellow eyes. The apr w/+w females had white eyes, as did 2 apr w males resulting from detachment of the X chromosomes. Direct proof for the presence of apr in the double mutant combn. was obtained by using apr w/++ females heterozygous for Ubxl30. From 1 culture a single dilute-apricot female arose which must have represented simultaneous recovery of the w+ and apr + crossover chromatids after crossing over between w and apr in the apr w/++ female. The results obtained in these expts. are discussed in the light of phenotypic differences between the "apricot" and the "eosin" mutants, which may reflect a functional difference between the apr+ and w+ genes in relation to position pseudo-allelism. It is suggested that apr+ may be located in the same doublet of the salivary gland chromosomes as w+, namely 3C2-3, in which case both genes may represent the duplication of a single ancestral gene, with divergence in function due to mutation.

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