Dosage Compensation of X-Linked Muller Element F Genes but Not X-Linked Transgenes in the Australian Sheep Blowfly

In most animals that have X and Y sex chromosomes, chromosome-wide mechanisms are used to balance X-linked gene expression in males and females. In the fly Drosophila melanogaster, the dosage compensation mechanism also generally extends to X-linked transgenes. Over 70 transgenic lines of the Australian sheep blowfly Lucilia cuprina have been made as part of an effort to develop male-only strains for a genetic control program of this major pest of sheep. All lines carry a constitutively expressed fluorescent protein marker gene. In all 12 X-linked lines, female larvae show brighter fluorescence than male larvae, suggesting the marker gene is not dosage compensated. This has been confirmed by quantitative RT-PCR for selected lines. To determine if endogenous X-linked genes are dosage compensated, we isolated 8 genes that are orthologs of genes that are on the fourth chromosome in D. melanogaster. Recent evidence suggests that the D. melanogaster fourth chromosome, or Muller element F, is the ancestral X chromosome in Diptera that has reverted to an autosome in Drosophila species. We show by quantitative PCR of male and female DNA that 6 of the 8 linkage group F genes reside on the X chromosome in L. cuprina. The other two Muller element F genes were found to be autosomal in L. cuprina, whereas two Muller element B genes were found on the same region of the X chromosome as the L. cuprina orthologs of the D. melanogaster Ephrin and gawky genes. We find that the L. cuprina X chromosome genes are equally expressed in males and females (i.e., fully dosage compensated). Thus, unlike in Drosophila, it appears that the Lucilia dosage compensation system is specific for genes endogenous to the X chromosome and cannot be co-opted by recently arrived transgenes.     

A male-specific lethal mutation in Drosophila melanogaster that transforms sex.

Sex-lethal, male-specific allele #1 (Sxl^M#1, 1–19.2) is a dominant, X-linked mutation that is lethal to males. It has no effect in females other than to rescue them from the otherwise lethal maternal effect of the autosomal mutation, daughterless. From a study of the effects of Sxl^M#1 on the development of sexually mosaic flies (gynandromorphs), it was observed that this lethal mutation can cause genetically male (haplo-X) tissue to differentiate as if it were female. With respect to its effect on sexual differentiation, the mutation transformer (tra, 3–45) is epistatic to Sxl^M#1, though the lethal effects of Sxl^M#1 are not modified by tra. In addition to its effect on sexual differentiation, Sxl^M#1 reduces the size of haplo-X imaginal disc and histoblast derivatives in general in a cell autonomous fashion. The viability of gynandromorphs with Sxl^M#1 tissue is very low, and the surviving mosaics have relatively little haplo-X tissue, suggesting that there is no single localized “lethal focus” for this mutation. The relationship between Sxl and daughterless is discussed, as well as the possible involvement of the Sxl locus in X-chromosome dosage compensation.     

Sex determination and dosage compensation in Drosophila melanogaster: production of male clones in XX females

Sex determination and dosage compensation in Drosophila are implemented by the ratio of X-chromosomes to sets of autosomes (X:A ratio). Our aim was to change this X:A ratio during development, and to assess the response of the affected cells in sexually dimorphic structures. For this purpose, clones of XO constitution were produced in female embryos and larvae of two genotypes in which almost the entire euchromatic arm of one X-chromosome was translocated to the third chromosome. Genotype I was heterozygous for the X-linked recessive mutations Sxl^fLS, genotype II was homozygous for Sxl⁺. The Sxl⁺ gene (sex-lethal) is involved in mediating sex determination and dosage compensation. In genotype I (Sxl^fLS), male clones could be generated up to 48 h in genitalia and analia, up to 72 h in the sex comb region and up to 96 h in 5th and 6th tergites. In genotype II (Sxl⁺), male clones only appeared in the tergites, and only up to 24 h. The difference in these results is ascribed to the presence of Sxl^fLS in genotype I: when homozygous, this mutation causes XX clones to differentiate male structures; most of the male clones produced in genotype I must therefore be XX. In contrast, male clones produced in genotype II must be XO. Since these were only found when generated in embryos we conclude that the X:A ratio expresses itself autonomously in clones by setting the state of activity of the Sxl gene around blastoderm stage. Once this is achieved, the X:A signal is no longer needed, and the state of activity of the Sxl⁺ gene determines sex and dosage compensation.     

Genome assembly,sex-biased gene expression and dosage compensation in the damselfly Ischnura elegans

The evolution of sex chromosomes, and patterns of sex-biased gene expression and dosage compensation, are poorly known among early winged insects such as odonates. We assembled and annotated the genome of Ischnura elegans (blue-tailed damselfly), which, like other odonates, has a male-hemigametic sex-determining system (X0 males, XX females). By identifying X-linked genes in I. elegans and their orthologs in other insect genomes, we found homologies between the X chromosome in odonates and chromosomes of other orders, including the X chromosome in Coleoptera. Next, we showed balanced expression of X-linked genes between sexes in adult I. elegans, i.e. evidence of dosage compensation. Finally, among the genes in the sex-determining pathway only fruitless was found to be X-linked, while only doublesex showed sex-biased expression. This study reveals partly conserved sex chromosome synteny and independent evolution of dosage compensation among insect orders separated by several hundred million years of evolutionary history.     

Maternal Groucho and bHLH repressors amplify the dose-sensitive X chromosome signal in Drosophila sex determination

In Drosophila, XX embryos are fated to develop as females, and XY embryos as males, because the diplo-X dose of four X-linked signal element genes, XSEs, activates the Sex-lethal establishment promoter, SxlPe, whereas the haplo-X XSE dose leaves SxlPe off. The threshold response of SxlPe to XSE concentrations depends in part on the bHLH repressor, Deadpan, present in equal amounts in XX and XY embryos. We identified canonical and non-canonical DNA-binding sites for Dpn at SxlPe and found that cis-acting mutations in the Dpn-binding sites caused stronger and earlier Sxl expression than did deletion of dpn implicating other bHLH repressors in Sxl regulation. Maternal Hey encodes one such bHLH regulator but the E(spl) locus does not. Elimination of the maternal corepressor Groucho also caused strong ectopic Sxl expression in XY, and premature Sxl activation in XX embryos, but Sxl was still expressed differently in the sexes. Our findings suggest that Groucho and associated maternal and zygotic bHLH repressors define the threshold XSE concentrations needed to activate SxlPe and that they participate directly in sex signal amplification. We present a model in which the XSE signal is amplified by a feedback mechanism that interferes with Gro-mediated repression in XX, but not XY embryos.     

How do heterogametic females survive without gene dosage compensation?

When the male is the heterogametic sex (XX♀-XY♂ or XX♀-XO♂), as inDrosophila, orthopteran insects, mammals andCaenorhabditis elegans, X-linked genes are subject to dosage compensation: the single X in the male is functionally equivalent to the two Xs in the female. However, when the female is heterogametic (ZZ♂-ZW♀), as in birds, butterflies and moths, Z-linked genes are apparently not dosage-compensated. This difference between X-linked and Z-linked genes raises fundamental questions about the role of dosage compensation. It is argued that (i) genes which require dosage compensation are primarily those that control morphogenesis and the prospective body plan; (ii) the products of these genes are required in disomic doses especially during oogenesis and early embryonic development; (iii) heterogametic females synthesize and store during oogenesis itself morphogenetically essential gene products - including those encoded by Z-linked genes — in large quantities; (iv) the abundance of these gene products in the egg and their persistence relatively late into embryogenesis enables heterogametic females to overcome the monosomic state of the Z chromosome in ZW embryos. Female heterogamety is predominant in birds, reptiles and amphibians, all of which have megalecithal eggs containing several thousand times more maternal RNA and other maternal messages than eggs of mammals,Caenorhabditis elegans, orDrosophila. This increase in egg size, yolk content and, concomitantly, the size of the maternal legacy to the embryo, may have facilitated female heterogamety and the absence of dosage compensation.     

Restricting Dosage Compensation Complex Binding to the X Chromosomes by H2A.Z/HTZ-1

Dosage compensation ensures similar levels of X-linked gene products in males (XY or XO) and females (XX), despite their different numbers of X chromosomes. In mammals, flies, and worms, dosage compensation is mediated by a specialized machinery that localizes to one or both of the X chromosomes in one sex resulting in a change in gene expression from the affected X chromosome(s). In mammals and flies, dosage compensation is associated with specific histone posttranslational modifications and replacement with variant histones. Until now, no specific histone modifications or histone variants have been implicated in Caenorhabditis elegans dosage compensation. Taking a candidate approach, we have looked at specific histone modifications and variants on the C. elegans dosage compensated X chromosomes. Using RNAi-based assays, we show that reducing levels of the histone H2A variant, H2A.Z (HTZ-1 in C. elegans), leads to partial disruption of dosage compensation. By immunofluorescence, we have observed that HTZ-1 is under-represented on the dosage compensated X chromosomes, but not on the non-dosage compensated male X chromosome. We find that reduction of HTZ-1 levels by RNA interference (RNAi) and mutation results in only a very modest change in dosage compensation complex protein levels. However, in these animals, the X chromosome–specific localization of the complex is partially disrupted, with some nuclei displaying DCC localization beyond the X chromosome territory. We propose a model in which HTZ-1, directly or indirectly, serves to restrict the dosage compensation complex to the X chromosome by acting as or regulating the activity of an autosomal repellant.     

Tribolium castaneum Transformer-2 regulates sex determination and development in both males and females

Tribolium castaneum Transformer (TcTra) is essential for female sex determination and maintenance through the regulation of sex-specific splicing of doublesex (dsx) pre-mRNA. In females, TcTra also regulates the sex-specific splicing of its own pre-mRNA to ensure continuous production of functional Tra protein. Transformer protein is absent in males and hence dsx pre-mRNA is spliced in a default mode. The mechanisms by which males inhibit the production of functional Tra protein are not known. Here, we report on functional characterization of transformer-2 (tra-2) gene (an ortholog of Drosophila transformer-2) in T. castaneum. RNA interference-mediated knockdown in the expression of gene coding for tra-2 in female pupae or adults resulted in the production of male-specific isoform of dsx and both female and male isoforms of tra suggesting that Tra-2 is essential for the female-specific splicing of tra and dsx pre-mRNAs. Interestingly, knockdown of tra-2 in males did not affect the splicing of dsx but resulted in the production of both female and male isoforms of tra suggesting that Tra-2 suppresses female-specific splicing of tra pre-mRNA in males. This dual regulation of sex-specific splicing of tra pre-mRNA ensures a tight regulation of sex determination and maintenance. These data suggest a critical role for Tra-2 in suppression of female sex determination cascade in males. In addition, RNAi studies showed that Tra-2 is also required for successful embryonic and larval development in both sexes.     

X-chromosome inactivation in monkey embryos and pluripotent stem cells

Inactivation of one X chromosome in female mammals (XX) compensates for the reduced dosage of X-linked gene expression in males (XY). However, the inner cell mass (ICM) of mouse preimplantation blastocysts and their in vitro counterparts, pluripotent embryonic stem cells (ESCs), initially maintain two active X chromosomes (XaXa). Random X chromosome inactivation (XCI) takes place in the ICM lineage after implantation or upon differentiation of ESCs, resulting in mosaic tissues composed of two cell types carrying either maternal or paternal active X chromosomes. While the status of XCI in human embryos and ICMs remains unknown, majority of human female ESCs show non-random XCI. We demonstrate here that rhesus monkey ESCs also display monoallelic expression and methylation of X-linked genes in agreement with non-random XCI. However, XIST and other X-linked genes were expressed from both chromosomes in isolated female monkey ICMs indicating that ex vivo pluripotent cells retain XaXa. Intriguingly, the trophectoderm (TE) in preimplantation monkey blastocysts also expressed X-linked genes from both alleles suggesting that, unlike the mouse, primate TE lineage does not support imprinted paternal XCI. Our results provide insights into the species-specific nature of XCI in the primate system and reveal fundamental epigenetic differences between in vitro and ex vivo primate pluripotent cells.     

Sxl in the germline of Drosophila: A target for somatic late induction

In Drosophila, the sex of germ cells is determined by autonomous and inductive signals. Somatic inductive signals can drive XX germ cells into oogenesis or into spermatogenesis. An autonomous signal makes XY germ cells male and unresponsive to sex determination by induction. The elements forming the X:A ratio in the soma and the genes tra, tra2, dsx, and ix that determine the sex of somatic cells have no similar role in the germline. The gene Sxl, however, is required for female differentiation of somatic and germ cells. Inductive signals that are dependent on somatic tra and dsx expression already affect the sex-specific development of germ cells of first instar larvae. At this early stage, however, germline expression of Sxl does not appear to affect the sexual characteristics of germ cells. Since inductive signals dependent on tra and dsx nevertheless influence the choice of sex-specific splicing of Sxl, it can be concluded that Sxl is a target of the inductive signal, but that its product is required late for oogenesis. Other genes must therefore control the early sexual dimorphism of larval germ cells. © 1994 Wiley-Liss, Inc.