Analysis of Y chromosome nucleolar organizer mutants in Drosophila melanogaster

Five Y chromosome nucleolar organizer (Y-NO) mutants were analyzed with respect to their rRNA gene numbers, phenotypes and additivity tests with other NO mutants. Four of these are indicative of a class of mutants in which most of the rRNA genes are transcribing functional rRNA. The other mutant has 80 genes, however, lethality and additivity tests suggests that many if not all of these rRNA genes are non-functional. The basis for the observed suppression of rRNA genes of the Y-NO region is discussed.     

Cytological and genetic analysis of the Y chromosome of Drosophila melanogaster

By applying quinacrine-, Hoechst- and N-banding techniques to neuroblast prometaphase chromosomes the Y chromosome of Drosophila melanogaster can be differentiated into 25 regions defined by the degree of fluorescence, the stainability after N-banding and the presence of constrictions. Thus these banding techniques provide an array of cytological landmarks along the Y chromosome that makes it comparable to a polytene chromosome for cytogenetic analysis. — 206 Y-autosome translocations (half of them carrying Y-linked sterile mutations) and 24 sterile y ⁺ Y chromosomes were carefully characterized by these banding techniques and used in extensive complementation analyses. The results of these experiments showed that: (1) there are four linearly ordered fertility factors in Y ^L and two fertility factors in Y ^S . (2) These fertility factors map to characteristic regions of the Y chromosome, specifically stained with the N-banding procedure. (3) The most extensively analyzed fertility factors are defined by a series of cytologically non-overlapping and genetically noncomplementing breaks and deficiencies distributed over large chromosome regions. For example, the breakpoints which inactivate the kl-5 and ks-1 loci are scattered along regions that contain about 3,000 kilobases (kb) DNA. Since these enormous regions formally define single genetic functions, the fertility genes of the Y chromosome have an as yet unappreciated physical dimension, being larger than euchromatic genes by two orders of magnitude.     

Organization of DNA sequences near the centromere of the Drosophila melanogaster Y chromosome

The structural analysis of a yeast artificial chromosome clone from Drosophila melanogaster enriched in dodecasatellite sequences has led us to find a new retrotransposon that we have called Circe. Moreover, this retrotransposon has allowed the isolation of a contig encompassing ∼200 kb near the centromere of the Y chromosome, providing an entry point into a region from which very little sequence information has been obtained to date. The molecular characterization of the contig has shown the presence of HeT-A telomeric retrotransposons close to the centromere of the Y chromosome, suggesting a telocentric origin for this submetacentric chromosome. Received: 19 November 1996 / Accepted: 30 May 1997     

Genomic imprinting absent in Drosophila melanogaster adult females

Genomic imprinting occurs when expression of an allele differs based on the sex of the parent that transmitted the allele. In D. melanogaster, imprinting can occur, but its impact on allelic expression genome-wide is unclear. Here, we search for imprinted genes in D. melanogaster using RNA-seq to compare allele-specific expression between pools of 7- to 10-day-old adult female progeny from reciprocal crosses. We identified 119 genes with allelic expression consistent with imprinting, and these genes showed significant clustering within the genome. Surprisingly, additional analysis of several of these genes showed that either genomic heterogeneity or high levels of intrinsic noise caused imprinting-like allelic expression. Consequently, our data provide no convincing evidence of imprinting for D. melanogaster genes in their native genomic context. Elucidating sources of false-positive signals for imprinting in allele-specific RNA-seq data, as done here, is critical given the growing popularity of this method for identifying imprinted genes.     

Genomic imprinting and position-effect variegation in Drosophila melanogaster

Genomic imprinting is a phenomenon in which the expression of a gene or chromosomal region depends on the sex of the individual transmitting it. The term imprinting was first coined to describe parent-specific chromosome behavior in the dipteran insect Sciara and has since been described in many organisms, including other insects, plants, fish, and mammals. In this article we describe a mini-X chromosome in Drosophila melanogaster that shows genomic imprinting of at least three closely linked genes. The imprinting of these genes is observed as mosaic silencing when the genes are transmitted by the male parent, in contrast to essentially wild-type expression when the same genes are maternally transmitted. We show that the imprint is due to the sex of the parent rather than to a conventional maternal effect, differential mitotic instability of the mini-X chromosome, or an allele-specific effect. Finally, we have examined the effects of classical modifiers of position-effect variegation on the maintenance and the establishment of the imprint. Factors that modify position-effect variegation alter the somatic expression but not the establishment of the imprint. This suggests that chromatin structure is important in maintenance of the imprint, but a separate mechanism may be responsible for its initiation.     

Changes in the fluorescence patterns of translocated Y chromosome segments in Drosophila melanogaster

Fluorescence analysis after quinacrine staining in squashes of Varese wild stock male larval ganglia confirmed that the Y chromosome has four characteristic sections of bright fluorescence. In one Y/X and in one Y/III translocation the section of bright fluorescence on the short arm of the Y is no longer bright when translocated onto the terminal portion of the X and on the right arm of the III chromosome, respectively. Fluorescence analysis has also permitted the identification of a structurally abnormal Y chromosome in a cell line of Drosophila melanogaster established in vitro. The findings in the two translocations call for caution in the interpretation of structural rearrangements by fluorescence analysis.     

Timing of DNA replication of translocated Y chromosome sections in somatic cells of Drosophila melanogaster

The chronology of DNA replication was studied in cultured somatic cells of three stocks of Drosophila melanogaster marked by the presence of translocations between the Y chromosome and the X, 2nd and 3rd autosome, respectively. In all translocations the Y chromosome is split into two portions differently located. The different Y chromosome segments are always replicating later than euchromatin, but their timing of replication varies independently of the eu- or heterochromatic nature of the adjoining chromosome sections. This variation could be formally described as a position effect without spreading effect. It is concluded that there is evidence for the existence of factors controlling the timing of replication of the Y which are located on the chromosome itself.This is contribution No. 497 of the Euratom Biology Division. In Milano and Pavia this work was supported in part by grants of the Consiglio Nazionale delle Ricerche, Roma.     

Population genetics of the Y chromosome of Drosophila melanogaster: rDNA variation and phenotypic correlates

One means of examining the evolutionary significance of molecular variation on the Y chromosome is to identify phenotypes specifically affected by Y-linked genes, and to quantify the phenotypic variation and its correlation to the molecular variation. The functional importance of the Y-linked array of rRNA genes is demonstrated by the ability of Y chromosome to rescue X-linked bobbed lethal alleles, whose lethality is seen in homozygous females. Because low numbers of X-linked rDNA gene copies result in increased developmental time and shortened bristles, and because there is considerable natural variation in Y-linked copy number, a careful examination of Y-linked variation in these two traits may uncover a mode of selection acting on the multigene family. In this study, 36 Y-chromosome replacement lines were tested to detect subtle variation in bristle phenotypes and developmental rates. Correlations among these traits, rDNA gene copy number, and intergenic sequence length were quantified. The absence of significant correlations between phenotypic characters and rDNA copy number of intergenic sequence length suggests that the extant molecular variation in Y-linked rDNA can have at most very small selective effects.     

Allelic complementation between mutants in the fertility factyors of the Y chromosome in Drosophila melanogaster

Summary Complementation maps of the seven male fertility factors in the Y chromosome of D. melanogaster have been constructed and are linearly consistent in all cases. These observations are further evidence that genetic loci in heterochromatic chromosomes share many characteristics with loci in euchromatic regions of chromosomes. These functional maps are consistent with the hypothesis that the genetic material of the male fertility factors in the Y chromosome is made up of single-copy sequences which become amplified in the primary spermatocyte.     

Temperature-sensitive mutations in Drosophila melanogaster. XI. Male sterile mutants of the Y chromosome

Eight temperature-sensitive (ts) male sterile mutations have been induced by ethyl methanesulfonate treatment of Y chromosomes derived from a selected temperature-resistant Amherst wild-type stock of Drosophila melanogaster. Males carrying such mutated Y chromosomes (Y^ts) are sterile when raised at 29°C but fertile when reared at 22°C. Complementation tests of the mutants with Y chromosome fragments, deletions, and inter se localized all eight to the long arm of the chromosome in four different complementation groups.When Y^ts-bearing males, reared to adulthood at 22°C, were subjected to a 48-hr regimen at 29°C and mated to fresh virgin females daily, a significant reduction in fertility resulted 5 days after initiation of 29°C treatments. This period of sterility was transient (48–72-hr duration) and corresponded to a temperature-sensitive period (TSP) of spermatogenesis during the primary spermatocyte stage. A more precise definition of the TSP utilized exposure of subadult males to 29°C at selected developmental periods during which only certain germ cell stages are present. Upon eclosion adult males were subjected to a similar schedule of consecutive matings of 12-hr duration in order to detect any delay in the appearance of fertility. Different ts males could be distinguished by the resultant pattern of sterility, and the TSP of different mutations thus localized to either primary spermatocyte or immediately post-meiotic stage.Associated with Y^ts-mediated sterility, spermiogenesis is defective at restrictive temperature as evidenced by the production of nonmotile sperm and a failure to transfer such sperm to the female during copulation. In addition, electron microscopy detected a variety of ultrastructural abnormalities, including defects of axoneme formation, irregularities of Nebenkern derivative development, and failures of separation from the syncitial state or mature cyst with subsequent degeneration.     

Partner choice in heterologous chromosome segregation of the Y chromosome in competitive situations in the oocyte of Drosophila melanogaster

Heterologous segregation of the Y chromosome and secondary non-disjunction of the X chromosomes in female meiosis of Drosophila melanogaster was investigated in ten different crosses where different constellations of translocation/inversion or translocation/translocation systems of the large autosomes were present in the female parent. It appeared that the Y chromosome always segregates from the shortest of the possible heterologous pairing partners. This may be due to size-dependent mechanism of so-called distributive disjunction or to the possibility that the shorter the chromosome element is, the more easily it moves in the nucleus of the oocyte. Secondary non-disjunction of the X chromosomes appeared to be lower the more possible autosomal pairing partners the Y chromosome had, suggesting that the autosomes effectively compete with the X chromosomes for pairing with the Y chromosome. An alternative explanation is that, due to interchromosomal effect on recombination, crossing over in the X chromosomes was different in different experiments.     

A transmissible dicentric chromosome in Drosophila melanogaster

A transmissible dicentric chromosome was recovered in Drosophila melanogaster. The radiation-induced secondary chromosome rearrangement consists essentially of the entire Y and fourth chromosomes joined by 2R heterochromatin. The Y ^S · Y ^L 2Rh4 · chromosome pairs with the X and the free fourth chromosome to form a trivalent in meiosis that is unusual because it forms few chromosome bridges in primary spermatocytes and is transmitted at high frequency. We suggest that the orientation of the weaker fourth chromosome kinetochore eventually fails when opposing the stronger Y kinetochore so that the Y ^S · Y ^L 2Rh4 · moves to the pole to which the Y kinetochore is oriented. There is however an increased frequency of sex chromosome nondisjunction (14%) and of chromosome laggards (6%) in primary spermatocytes; the frequency of exceptional progeny of males containing the Y ^S · Y ^L 2Rh4 · was 7.44% compared with 0.25% in the controls. Disruption of normal sex chromosome disjunction also occurs in females containing the Y ^S · Y ^L 2Rh4 · and a compound X chromosome; the frequency of exceptional progeny was 2.55% versus 0.91% in the controls. Chromosome nondisjunction appears to occur when orientation of the X and Y kinetochores to the same pole is stabilized through tension by the orientation of one or both fourth chromosome kinetochores to the opposite pole. During anaphase, the orientation of the fourth chromosome kinetochore of the Y ^S · Y ^L 2Rh4 · appears to fail and the X and Y ^S · Y ^L 2Rh4 · chromosomes move to the same pole. Y ^S · Y ^L 2Rh4 · chromosome laggards occur with both the Y and fourth chromosome kinetochores amphitelically oriented. This orientation appears to be stable as a result of equal opposing forces toward opposite poles.