The study of interaction between paralogous tandem repeats stellate and suppressor of stellate in the genome of Drosophila melanogaster

Testis-specific expression of tandemly repeated Stellate genes, located in eu- and heterochromatin regions of the X chromosome of Drosophila melanogaster, is suppressed by homologous Suppressor of Stellate repeats located on the Y chromosome. Using transgenic lines, we have demonstrated that three Su(Ste) copies failed to change the expression of the reporter construction carrying the bacterial beta-galactosidase gene under control of the Stellate gene regulatory sequence. Possible mechanisms of the Su(Ste) repeat suppressor activity are discussed.     

Structure of the Y Chromosomal Su(ste) Locus in Drosophila Melanogaster and Evidence for Localized Recombination among Repeats

The structure of the Suppressor of Stellate [Su(Ste)] locus on the Drosophila melanogaster Y chromosome was examined by restriction analysis of both native and cloned genomic DNA. The locus consists of short subarrays of tandem repeats separated by members of other moderately repeated families. Both size variants and restriction variants proved to be common. Most repeats fell into two size classes--2.8 and 2.5 kb--but other size variants were also observed. Restriction variants showed a strong tendency to cluster, both at the gross level where some variants were present in only one of three subintervals of the locus, and at the fine level, where repeats from the same phage clone were significantly more similar than repeats from different clones. Restriction variants were shared freely among repeats of different size classes; however, size variants appeared to be randomly distributed among phage clones. These data indicate that recombination among tandem Su(Ste) repeats occurs at much higher frequencies between close neighbors than distant ones. In addition, they suggest that gene conversion rather than sister chromatid exchange may be the primary recombinational mechanism for spreading variation among repeats at the Su(Ste) locus.     

Molecular evolution of two paralogous tandemly repeated heterochromatic gene clusters linked to the X and Y chromosomes of Drosophila melanogaster

Here we report the peculiarities of molecular evolution and divergence of paralogous heterochromatic clusters of the testis- expressed X-linked Stellate and Y-linked Su(Ste) tandem repeats. It was suggested that Stellate and Su(Ste) clusters affecting male fertility are the amplified derivatives of the unique euchromatic gene betaCK2tes encoding the putative testis-specific beta-subunit of protein kinase CK2. The putative Su(Ste)-like evolutionary intermediate was detected on the Y chromosome as an orphon outside of the Su(Ste) cluster. The orphon shows extensive homology to the Su(Ste) repeat, but contains several Stellate-like diagnostic nucleotide substitutions, as well as a 10-bp insertion and a 3' splice site of the first intron typical of the Stellate unit. The orphon looks like a pseudogene carrying a drastically damaged Su(Ste) open reading frame (ORF). The putative Su(Ste) ORF, as compared with the Stellate one, carries numerous synonymous substitutions leading to the major codon preference. We conclude that Su(Ste) ORFs evolved on the Y chromosome under the pressure of translational selection. Direct sequencing shows that the efficiency of concerted evolution between adjacent repeats is 5-10 times as high in the Stellate heterochromatic cluster on the X chromosome as that in the Y-linked Su(Ste) cluster, judging by the frequencies of nucleotide substitutions and single-nucleotide deletions.     

Heterochromatic DNA repeats in Drosophila and unusual gene silencing in yeast cells

The 1.25-kb heterochromatic Stellate repeats of Drosophila melanogaster are capable of stably persisting in transgenic constructs and silencing the white reporter gene (mosaic position effect variegation). This system reveals an unusual form of silencing, which is insensitive to known modifiers of position effect variegation. The unusual form of silencing was studied with yeast Saccharomyces cerevisiae, a simple eukaryotic model. To be transferred into yeast cells, the D. melanogaster Stellate repeats were cloned in the pYAC4 centromeric vector (CEN4, URA3, TRP1, HIS3). The HIS3 and/or URA3 genes could be inactive in plasmids consisting of pYAC4 and the Stellate insert in yeast cells. Deletion of D. melanogaster DNA from the plasmid was found to activate the URA3 and HIS3 genes. It was assumed that the genes were repressed rather than damaged in the presence of the Stellate repeats and that a new form of gene silencing was revealed in S. cerevisiae.     

A role of the Drosophila homeless gene in repression of Stellate in male meiosis

The homeless gene of Drosophila melanogaster encodes a member of the DE-H family of ATPase and RNA helicase proteins. Loss-of-function homeless mutations were previously found to cause female sterility with numerous defects in oogenesis, including improper formation of both the anterior-posterior and dorsal-ventral axes and failure to transport and localize key RNAs required for axis formation. One homeless mutation was also found to affect male meiosis, causing elevated X-Y nondisjunction. Here we further analyze the role of homeless in male meiosis. We show that homeless mutations cause a variety of defects in male meiosis including nondisjunction of the X-Y and 2-2 pair, Y chromosome marker loss, meiotic drive, chromosome fragmentation, chromatin bridges at anaphase, and tripolar meiosis. In addition, homeless mutations interact with an X chromosomal factor to cause complete male sterility. These phenotypes are similar to those caused by deletion of the Suppressor of Stellate [Su(Ste)] locus. Like Su(Ste) deficiencies, homeless mutants also exhibit crystals in primary spermatocytes and derepression of the X-linked Stellate locus. To determine whether the regulatory role of hls is specific for Stellate or includes other repeated sequences as well, we compared testis RNA levels for nine transposable elements and found that all but one, copia, were expressed at the same levels in hls mutants and wild type. Copia, however, was strongly derepressed in hls mutant males. We conclude that hls functions along with Su(Ste) and other recently described genes to repress the Stellate locus in spermatocytes, and that it may also play a role in repressing certain other repeated sequences.     

Genetic Analysis of Stellate Elements of Drosophila Melanogaster

Repeated elements are remarkably important for male meiosis and spermiogenesis in Drosophila melanogaster. Pairing of the X and Y chromosomes is mediated by the ribosomal RNA genes of the Y chromosome and X chromosome heterochromatin, spermiogenesis depends on the fertility factors of the Y chromosome. Intriguingly, a peculiar genetic system of interaction between the Y-linked crystal locus and the X-linked Stellate elements seem to be also involved in male meiosis and spermiogenesis. Deletion of the crystal element of the Y, via an interaction with the Stellate elements of the X, causes meiotic abnormalities, gamete-genotype dependent failure of sperm development (meiotic drive), and deposition of protein crystals in spermatocytes. The current hypothesis is that the meiotic abnormalities observed in cry(-) males is due to an induced overexpression of the normally repressed Ste elements. An implication of this hypothesis is that the strength of the abnormalities would depend on the amount of the Ste copies. To test this point we have genetically and cytologically examined the relationship of Ste copy number and organization to meiotic behavior in cry(-) males. We found that heterochromatic as well as euchromatic Ste repeats are functional and that the abnormality in chromosome condensation and the frequency of nondisjunction are related to Ste copy number. Moreover, we found that meiosis is disrupted after synapsis and that cry-induced meiotic drive is probably not mediated by Ste.     

Drosophila rasiRNA pathway mutations disrupt embryonic axis specification through activation of an ATR/Chk2 DNA damage response

Small repeat-associated siRNAs (rasiRNAs) mediate silencing of retrotransposons and the Stellate locus. Mutations in the Drosophila rasiRNA pathway genes armitage and aubergine disrupt embryonic axis specification, triggering defects in microtubule polarization as well as asymmetric localization of mRNA and protein determinants in the developing oocyte. Mutations in the ATR/Chk2 DNA damage signal transduction pathway dramatically suppress these axis specification defects, but do not restore retrotransposon or Stellate silencing. Furthermore, rasiRNA pathway mutations lead to germline-specific accumulation of gamma-H2Av foci characteristic of DNA damage. We conclude that rasiRNA-based gene silencing is not required for axis specification, and that the critical developmental function for this pathway is to suppress DNA damage signaling in the germline.