Genetic studies of mutations at two loci of Drosophila melanogaster which cause a wide variety of homeotic transformations

Summary

The ash-1 locus is in the proximal region of the left arm of the third chromosome of Drosophila melanogaster and the ash-2 locus is in the distal region of the right arm of the third chromosome. Mutations at either locus can cause homeotic transformations of the antenna to leg, proboscis to leg and/or antenna, dorsal prothorax to wing, first and third leg to second leg, haltere to wing, and genitalia to leg and/or antenna. Mutations at the ash-1 locus cause, in addition, transformations of the posterior wing and second leg to anterior wing and second leg, respectively. A similar spectrum of transformations is caused by mutations at yet another third chromosome locus, trithorax. One extraordinary aspect of mutations at all three of these loci is that they cause such a wide variety of transformations. For mutations at both of the loci that we have studied the expression of the homeotic phenotype is both disc-autonomous (as shown by injecting mutant discs into metamorphosing larvae) and cell autonomous (as shown by somatic recombination analysis). The original mutations which identified these two loci, although lethal, manifest variable expressivity and incomplete penetrance of the homeotic phenotype suggesting that they are hypomorphic. The phenotype of double mutants which were synthesized by combining different pairs of those original mutations manifest for two of the four pairs a greater degree of expressivity and slightly more penetrance of the homeotic transformations. This mutual enhancement suggests that the products of both loci interact in the same process. A third double mutant expresses a discless phenotype.

Additional alleles have been recovered at both the ash-1 and the ash-2 loci. Some of these alleles as homozygotes or transheterozygotes express the wide range of transformations revealed first by double mutants. One of the alleles at the ash-1 locus when homozygous and several transheterozygous pairs can cause either the homeotic transformation of discs or the absence of those discs. The fact that these two defects, absence of specific discs and homeotic transformations of those same discs can be caused by mutations within a single gene suggests that the activity of the product of this gene is essential for normal imaginal disc cell proliferation. Loss of that activity leads to the absence of discs, whereas, reduction of that activity leads to homeotic transformations.