Direct Proof of a Variegated-Type Position Effect at the White Locus in Drosophila Melanogaster.

ARIEGATED-type position effect in Drosophila melunoguster has received a V considerable amount of attention from many investigators (See review by LEWIS 1950). Direct proof of this phenomenon has been obtained for only two cases however. DUBININ and SIDOROV (1935) showed that the action of the gene for hairy (h) was affected when this locus was placed near heterochromatin of the fourth chromosome by a translocation between the third and fourth chromosomes. PANSHIN (1935) studying another 3-4 translocation showed that the curled gene (cu) also became unstable when placed near heterochromatin. By taking advantage of a small amount of crossing over between the variegating gene and the break point of the translocation, these workers were able to show in both cases that the gene in question behaved as the standard + allele when placed in a chromosome of normal structure. Proof of the position effect came from a comparison of translocation heterozygotes that differed in the position of the + and mutant genes. Variegation occurred in heterozygotes having the + allele in the rearranged chromosome and the mutant allele in the normal homolog, while heterozygotes in which the position of these two alleles had been exchanged gave a wild type phenotype. It was shown that the variegation is not the result of structural heterozygosis but depends only on the position of the + allele. The present investigation provides direct proof of variegated-type position effect for the white locus. White-mottled types have been widely studied but only indirect evidence for position effect has been provided (SCHULTZ 1936; SACHAROV 1936; DEMEREC 1941; and others). The white gene provides more satisfactory material for working out this direct proof than does either curled or hairy. DUBININ and SIDOROV and particularly PANSHIN were dependent on the phenotype of a population of flies in order to determine the presence or absence of variegation. In the case of white, variegation, or the absence of it, can be detected easily in a single individual since there are some 800 facets in each eye and a change of color in only a few facets can easily be seen.