Ectopic recombination between interspersed repeat sequences generates chromosomal rearrangements that have a major impact on genome structure. A survey of ectopic recombination in the region flanking the white locus of Drosophila melanogaster identified 25 transposon-mediated rearrangements from four parallel experiments. Eighteen of the 25 were generated from females carrying X chromosomes heterozygous for interspersed repeat sequences. The cytogenetic and molecular analyses of the rearrangements and the parental chromosomes show: (1) interchromosomal and intrachromosomal recombinants are generated in about equal numbers; (2) ectopic recombination appears to be a meiotic process that is stimulated by the interchromosomal effect to about the same degree as regular crossing over; (3) copies of the retrotransposon roo were involved in all of the interchromosomal exchanges; some copies were involved much more frequently than others in the target region; (4) homozygosis for interspersed repeat sequences and other sequence variations significantly reduced ectopic recombination.
Data are presented that indicate that the z locus of Drosophila melanogaster represses w locus activity, but the repression is effective only on paired or physically adjacent w loci. Various mutant alleles of z and w were combined in a series of different doses to determine the effect of dosage and physical position in the nucleus on gene expression. In z/z individuals, paired white alleles fail to be expressed, while unpaired alleles are expressed normally. The results are discussed in terms of a model postulating that the z gene product represses the transcription of wr by complexing with an RNA produced by part of the white locus itself. In order to be effective in repression, there must be two w+ genes producing the RNA in a limited volume in the nucleus. Such a model necessarily imposes a specific architecture on the chromatin of interphase nuclei.Examples of regulatory interactions between loci in eukaryotes are rare; communication between two alleles at a single locus is even less well documented. We present evidence that a particular locus of Drosophila melanogaster functions as a repressor of another locus. Repression is generally apparent, however, only if two alleles of the repressed locus are physically adjacent or paired.Gans (1) discovered the mutation zeste (z; 1-1.0) that produces yellow eye color in females but is wild type in males. She showed that the expression of the zeste phenotype in the females is due to the presence of two wild-type alleles of the white locus (w; 1-1.5). Certain duplications of w+ allow zeste phenotype expression in males. On the other hand, females heterozygous for a white locus deletion (z w + /z w -) are wild type in eye color. Some mutant alleles of w act in the same fashion as a deletion of the locus (1). Such white locus alleles were determined by Green (2) to map in the proximal part of the white locus (here designated as wPrX). Those alleles of white located in the distal portion of the locus (wdst) allow expression of zeste just as w + does. In fact, all the mutations that appear to upset regulatory function of the white locus map in WPrx. Pattern mutants that produce mosaics of pigmented and nonpigmented facets are located in WPrX, and all mutants in the region fail to show dosage compensation, which is characteristic of w+. A tandem duplication of only the wPorx portion of the locus behaves with respect to z as two doses of w+ (3). On the basis of these and some other distinguishing properties, Judd (4) has suggested that WPrX is involved in the regulation of the white locus while wdst contains the structural sequence.Historically the zeste-white interaction has been stated in terms of white locus deletions and mutants in wPrx acting as dominant suppressors of zeste. We believe this is not an accurate description of the system, and we will here describe the interaction from the point of view of repression of w+ by z. The reduction in pigmentation in z w + /z w + females results from the continued repression of w+ even when the gene should be active. Th...
An explanation for the origins of four classes of regularly occurring duplication and deficiency chromosomes is provided through examination of their molecular structures. The duplications and deficiencies occur as the reciprocal products of crossing-over, following two different patterns of asymmetrical synapsis between transposons positioned in and proximal to the white locus of Drosophila melanogaster. Three copies of the retrovirus-like transposon roo are involved in the exchanges. Evidence suggests that transposon-mediated asymmetrical exchange is a general phenomenon in eukaryotes, which adds significantly to the effects of transposons in the restructuring of eukaryotic genomes. We have investigated a series of asymmetrical exchange chromosomes that are duplications or deficiencies for regions in and around the white locus of Drosophila melanogaster. Previous genetic analyses of these products (3, 4) indicated that females heterozygous for white-apricot (Wa) and whitebuff (wbf) generated reciprocal asymmetrical recombinant chromosomes offour types. This highly unusual situation has now been examined at the molecular level. MATERIALS AND METHODSStrains. The duplication and deficiency strains were obtained from two sets of crosses explained in detail by Judd (3, 4). The first involved y2 Wa spl ec/wbf fS; SMI /+; Ubx130/+ females crossed to y w spl sn3 males. Genetic symbols are explained in Lindsley and Grell (5). The offspring from this cross were examined for eye colors unlike those of the parents of the heterozygous females-i.e., white-apricot (Wa) or white-buff (wbf. No wild-type recombinants were found among -85,000 offspring, but 18 recombinants composing three classes were recognized, all of which had white eyecolor: 7 y2 "w" that complement with the WSPJ allele, indicating that this class is not deficient for the proximal portion of the white locus (6); 2 y2 "W" that fail to complement WSPJ, indicating a deficiency for the proximal part of the white locus; and 9 "w" spl ec individuals, all of which complement w1P'. The appearance of three recombinant classes, none ofwhich on subsequent genetic analysis proved to be those expected from straightforward reciprocal exchange between mutant white alleles, prompted the recombination experiment to be repeated using attached-X chromosomes (4), which allows simultaneous recovery of reciprocal recombination products. The genotype of the attached-X-chromosome females was y sc Z wa ec/y2 wbf spl; SMJ/+; Ubx130/+. The zeste (z) mutation was added to the combination to detect a white locus duplication because it was recognized from the work of Gans (7) that zeste expression depends on the number of doses of white locus. Among =31,000 female offspring, 2 y "w" daughters were recovered. Both proved to be of the reciprocal crossover class. Genetic analysis of their X chromosomes showed that one chromosome, when made homozygous, produced a whiteeyed phenotype and had the markers y sc z spl, while the homologous chromosome carried y2 ec markers and produced ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.