Reduced DNA polytenization of a minichromosome region undergoing position-effect variegation in Drosophila.
Molecular analysis of a Drosophila minichromosome, Dp(1;f)1187, revealed a relationship between position-effect variegation and the copy number reductions of heterochromatic sequences that occur in polytene cells. Heterochromatin adjacent to a defined junction with euchromatin underpolytenized at least 60-fold. Lesser reductions were observed in euchromatic sequences up to 103 kb from the breakpoint. The copy number changes behaved in all respects like the expression of yellow, a gene located within the affected region. Both copy number and yellow expression displayed a cell-by-cell mosaic pattern of reduction, and adding a Y chromosome, a known suppressor of variegation, increased both substantially. We discuss the possibility that changes in replication alter copy number locally and also propose an alternative model of position-effect variegation based on the somatic elimination of heterochromatic sequences.
Howard Hughes Medical Institute Research Laboratories, Carnegie Institution of Washington, Department of Embryology, Baltimore, Maryland 21210.
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D000818:Animals; D002872:Chromosome Deletion; D002874:Chromosome Mapping; D004247:DNA / Q000235:genetics*; D004330:Drosophila / Q000033:anatomy & histology / Q000235:genetics*; D005260:Female; D006570:Heterochromatin / Q000502:physiology / Q000648:ultrastructure; D008297:Male; D009154:Mutation; D009693:Nucleic Acid Hybridization; D015183:Restriction Mapping; D012469:Salivary Glands / Q000502:physiology
D006570:Heterochromatin; D004247:DNA