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Genetic analysis of the proximal heterochromatin of chromosome-2 of Drosophila melanogaster Hilliker, Arthur James


The genetic function of Drosophila heterochromatin has been debated since its earliest description by Heitz (1933). To examine the genetic composition of the proximal region of chromosome 2 of Drosophila melanogaster, the generation of proximal deficiencies by the detachment of compound second autosomes appeared to be a promising method. Compound second autosomes were detached by gamma radiation. A fraction of the detachment products were recessive lethals owing to proximal deficiencies. Analysis of these detachment products by inter se complementation, pseudo-dominance tests with proximal mutations and alleleism tests with known deficiencies, provided evidence for at least two loci between the centromere and the light locus in 2L and one locus in 2R between the rolled locus and the centromere. These data in conjunction with cytological observations further demonstrate that rolled and light are located within the proximal heterochromatin of the second chromosome. To further this analysis, lethal alleles of the largest 2L and 2R proximal deficiencies were generated, employing, as a mutagen, ethyl methane sulphonate (EMS). Analysis of the 118 EMS induced recessive lethals and visible mutations recovered provided evidence for seven loci in the 2L heterochromatin and six loci in the 2R heterochromatin, with multiple alleles being obtained for most sites. Of these loci, one in 2L and two in 2R fall near the heterochromatic-euchromatic junction of 2L and 2R respectively. None of the 113 EMS lethals behaved as a deficiency, thereby confirming that, in Drosophila, the EMS mutagenesis method of Lewis and Bacher (1968) results in true "point" mutations. All of the heterochromatic loci uncovered in this study appear to be non-repetitive cistrons. Thus functional genetic loci are found in heterochromatin, albeit at very low density relative to euchromatin.

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