Promoter‐containing ribosomal DNA fragments function as X‐Y meiotic pairing sites in D. melanogaster males

Abstract

The Drosophila melanogaster ribosomal DNA (rDNA) functions as an X-Y meiotic pairing site. Deletions encompassing the X chromosomal rDNA block (located in the heterochromatin) disrupt X-Y pairing and disjunction. Insertions of single, complete rRNA genes at ectopic locations on the heterochromatically deficient X partially restore X-Y pairing capacity. This study was undertaken to test fragments of an rDNA repeat for the ability to stimulate X-Y pairing and disjunction and to test for relationships between pairing capacity and two other phenotypes associated with rDNA insertions: transcription and the ability to organize a nucleolus. Insertions of three different fragments, all of which retained the rDNA promoter and upstream spacer sequences and which differed among each other in the length of downstream sequences, were obtained by P-element mediated transformation. One of the fragments is truncated only 140bp downstream from the promoter. Insertions of all three fragments proved capable of stimulating X-Y disjunction. Double insertions were substantially more effective than single insertions. RNA/PCR analysis was used to show that transcripts initiated at the inserted rDNA promoters are present in testis RNA from all insertions. Treatment with an antinucleolar antibody revealed that none of the insertions was associated with a mininucleolus. Thus promoter-containing rDNA fragments are autonomously capable of being transcribed and of functioning as X-Y pairing sites, but not of forming a mini-nucleolus.

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

“…The present study addresses this issue by examining the molecular structure of the three IGS/promoter insertions using genomic blot analysis and cloning. It is shown that the inserts differ mainly in copy number of the 240 bp repeats, (0, 6 and 11-12), consistent with the suggestion of Merrill et al (1992). One line contains the rDNA promoter and the upstream region of the spacer but lacks 240 bp repeats.…”

“…& p . 2 : pP(rib,ry)10∆3 was prepared as described (Merrill et al 1992). pP(rib,w hs )12 was prepared as follows.…”

“…The first is the molecular basis for the variations among the IGS/promoter construct insertions in ability to stimulate pairing. Merrill et al (1992) suggested that the differences in pairing strength might reflect differences in copy number of the 240 bp repeat resulting from rearrangements during P element transposition. This would be the most economical explanation since the 240 bp repeat has already been shown to mediate pairing capacity in a dose-dependent manner, but other explanations are possible.…”

A general method for identifying major hybrid male sterility genes in Drosophila

“…It is known in D. melanogaster that deficiency of the X heterochromatin causes male sterility. The sperm of these deficient males fail to become individualized and are degraded (Peacock et a!., 1975;McKee & Lindsley, 1978;Merrill et a!., 1992). The phenotype is very similar to the Type 2 sterile males produced from the introgression.…”

Meiosis: Components and process in nuclear differentiation

“…This exceptional meiotic system is a natural variant that has been exploited for examination and manipulation of the nature of pairing sites and segregation mechanisms. Manipulation by both classical chromosome mechanics and by transformation with engineered DNA constructions has been used to demonstrate that ribosomal DNA can function as a n X-Y pairing site in Drosophila males (Merrill et al, 1992 […”