Homologous chromosomes are paired in somatic cells of Drosophila melanogaster. This pairing can lead to transvection, which is a process by which the proximity of homologous genes can lead to a change in gene expression. At the yellow gene, transvection is the basis for several examples of intragenic complementation involving the enhancers of one allele acting in trans on the promoter of a paired second allele. Using complementation as our assay, we explored the chromosomal requirements for pairing and transvection at yellow. Following a protocol established by Ed Lewis, we generated and characterized chromosomal rearrangements to define a region in cis to yellow that must remain intact for complementation to occur. Our data indicate that homolog pairing at yellow is efficient, as complementation was disrupted only in the presence of chromosomal rearrangements that break #650 kbp from yellow. We also found that three telomerically placed chromosomal duplications, containing $700 or more kbp of the yellow genomic region, are able to alter complementation at yellow, presumably through competitive pairing interactions. These results provide a formal demonstration of the pairing-dependent nature of yellow transvection and suggest that yellow pairing, as measured by transvection, reflects the extent of contiguous homology flanking the locus. C YTOLOGICAL studies of a wide variety of systems are revealing the strategies by which a large amount of DNA can be organized into an extraordinarily small volume yet still be accurately expressed, replicated, and passed through cell divisions. In the somatic cells of Drosophila and other dipteran insects, a striking feature of nuclear organization is the extensive amount of pairing that occurs between homologous chromosomes. This pairing was first noted by Nettie Stevens (Stevens 1908) and Charles Metz (Metz 1916) through the examination of mitotic nuclei. Somatic pairing of homologous chromosomes has now been observed in Drosophila interphase nuclei using DNA as well as RNA in situ hybridization techniques (reviewed by McKee
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