Academic journal article Genetics

Effects of Chromosomal Rearrangements on Transvection at the Yellow Gene of Drosophila Melanogaster

Academic journal article Genetics

Effects of Chromosomal Rearrangements on Transvection at the Yellow Gene of Drosophila Melanogaster

Article excerpt

ABSTRACT

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.

CYTOLOGICAL studies of a wide variety of systems are revealingthe strategies by whicha largeamount 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 byNettie 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 2004; for example, Kopczynski and Muskavitch 1992; Hiraoka et al. 1993; Csink and Henikoff 1998; Fung et al. 1998;Gemkow et al. 1998; Sass andHenikoff 1999; Bantignies et al. 2003; Ronshaugen and Levine 2004; Williams et al. 2007; Hartl et al. 2008), assessment of the frequency of site-specific FLP-mediated recombination (Golic and Golic 1996a), and methods that mark chromosomes with protein tags (Vazquez et al. 2001, 2002, 2006). Here, we present our studies using transvectionas our phenotypic assay for chromosomal pairing in Drosophila.

Transvection is a process by which the pairing of homologous genes results in a change in expression, in some situations causing gene activation and in other situations causing gene repression (reviewed by Pirrotta 1999; Wu and Morris 1999; Duncan 2002; Kennison and Southworth 2002). Because it depends on pairing, transvection can be used as a powerful assay for the paired state of genes. In this study, we used the protocol designed by Ed Lewis during his defining studies of transvection at the Ultrabithorax (Ubx) gene (Lewis 1954). Lewis began his analyses with the observation that certain pairs of Ubx alleles support intragenic complementation. He predicted that this complementation depends on the physical pairing of participating alleles and then confirmed this prediction through the generation and analysis of chromosomal rearrangements that disrupted complementation. Interestingly, the vast majority of the rearrangements had at least one breakpoint in a large chromosomal region between the centromere and Ubx on the order of 12 Mbp in size and covering about one-half the chromosome arm. This region was named the "critical region" and was interpreted as the segment of the chromosome whose integrity is important for homolog pairing, and hence transvection, at Ubx. On the basis of the location of this large critical region between the centromere and Ubx, Lewis (1954) suggested that somatic pairing might initiate at the centromere and proceed distally toward the telomere. …

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