Roles of rDNA spacer and transcription unit-sequences in X - Y meiotic chromosome pairing in Drosophila melanogaster males

Abstract

Meiotic pairing of the X and Y chromosomes in Drosophila melanogaster males is mediated by the rDNA repeats, which are present in two tandem clusters, one in the centric X heterochromatin and the other near the base of the short arm of the Y chromosome. Deletion of the X chromosomal rDNA cluster disrupts X-Y pairing and causes high frequences of X-Y nondisjunction. Pairing can be partly restored by insertions of cloned complete rRNA genes or by rDNA fragments that include the intergenic spacer (IGS) region. A 240 bp repeated sequence in the IGS was shown to be effective in promoting pairing when present at copy numbers above five. This study further defines the rDNA sequences involved in mediating pairing. Germline insertions of a P element construct containing most of the rDNA transcription unit but no promoter or IGS region were obtained. Two single-copy insertions and four two-copy insertions proved unable to stimulate X-Y disjunction when located on an rDNA-deficient X chromosome. In addition, three insertions of a P element construct consisting of the IGS and promoter regions of the rDNA were characterized molecularly. These three insertions had previously been shown to range in pairing ability from very weak to quite strong. Molecular analysis revealed that the three insertions also vary in copy number of the 240 bp IGS repeat and that these structural differences correlate with the differences in pairing ability. These data indicate that 240 bp repeats are considerably more effective than other regions of the rDNA in stimulating chromosome pairing.

Evolutionary aspects of the ZZ/ZW sex chromosome system in the Characidae fish, genus Triportheus. A monophyletic state and NOR location on the W chromosome

“…Besides Tripotheus, NORs located on fish sex chromosomes were already found in Fundulus diaphanus (Howell and Black, 1979), in the Arctic char Salvelinus alpinus (Reed and Phillips, 1997), and Hoplias malabaricus (Born and Bertollo, 2000). Among invertebrates, such as Drosophila and many Coleoptera, where the synaptonemal complex does not occur between the X and Y chromosomes, the synapsis is assured by the association between the rDNA present in these chromosomes, allowing a regular segregation during gametogenesis (Irick, 1994;Ren et al, 1997). Stitou et al (1997) also described latent NORs in the sex chromosomes of the rodent Lemniscomys barbarus, suggesting some role for the rDNA during the synapsis process of the X and Y chromosomes.…”

Yeast RNA Polymerase I Enhancer Is Dispensable for Transcription of the Chromosomal rRNA Gene and Cell Growth, and Its Apparent Transcription Enhancement from Ectopic Promoters Requires Fob1 Protein

“…Thus, the possibility that these intergenic elements might have a function(s) only indirectly related to Pol I transcription (rather than direct enhancement of Pol I transcription) cannot be excluded. It is interesting that rDNA enhancer elements in Drosophila melanogaster are known to play an essential role in X-Y meiotic chromosome pairing (17,24). It may be prudent to await more rigorous future experimentation before accepting the conclusion that these enhancer elements really function directly in stimulating Pol I transcription within the native chromosomal rDNA genes in vivo.…”

Natural variation of the Y chromosome suppresses sex ratio distortion and modulates testis-specific gene expression in Drosophila simulans

“…Expression of the few protein-coding loci requires major chromatin remodeling in the testis (Ashburner et al, 2005). The D. melanogaster Y chromosome also harbors a functional rDNA array that comprises hundreds of active rRNA genes and non-transcribed spacer repeats (Bonaccorsi and Lohe, 1991), the latter constituting the paring sites of the X and Y chromosomes during male meiosis (McKee et al, 1992;Ren et al, 1997). In Drosophila simulans, a sibling species of D. melanogaster, the rDNA array on the Y chromosome consists only of the non-transcribed spacer repeats (Brosseau, 1960;Lohe and Roberts, 1990), which are presumed to be the X-Y pairing site during meiosis in this species.…”