Dosage Compensation In Males Research Articles

A novel factor modulating X chromosome dosage compensation in Anopheles

Dosage compensation (DC), a process countering chromosomal imbalance in individuals with heteromorphic sex chromosomes, has been molecularly characterized only in mammals, Caenorhabditis elegans, and fruit flies.1 In Drosophila melanogaster males, it is achieved by an approximately 2-fold hypertranscription of the monosomic X chromosome mediated by the MSL complex.2,3 The complex is not assembled on female X chromosomes because production of its key protein MSL-2 is prevented due to intron retention and inhibition of translation by Sex-lethal, a female-specific protein operating at the top of the sex determination pathway.4 It remains unclear how DC is mechanistically regulated in other insects. In the malaria mosquito Anopheles gambiae, an approximately 2-fold hypertranscription of the male X also occurs5 by a yet-unknown molecular mechanism distinct from that in D. melanogaster.6 Here we show that a male-specifically spliced gene we call 007, which arose by a tandem duplication in the Anopheles ancestral lineage, is involved in the control of DC in males. Homozygous 007 knockouts lead to a global downregulation of the male X, phenotypically manifested by a slower development compared to wild-type mosquitoes or mutant females-however, without loss of viability or fertility. In females, a 007 intron retention promoted by the sex determination protein Femaleless, known to prevent hypertranscription from both X chromosomes,7 introduces a premature termination codon apparently rendering the female transcripts non-productive. In addition to providing a unique perspective on DC evolution, the 007, with its conserved properties, may represent an important addition to a genetic toolbox for malaria vector control.

Cis- and trans-regulation by histone H4 basic patch R17/R19 in metazoan development.

The histone H4 basic patch is critical for chromatin structure and regulation of the chromatin machinery. However, the biological roles of these positively charged residues and the mechanisms by which they regulate gene expression remain unclear. In this study, we used histone mutagenesis to investigate the physiological function and downstream regulatory genes of H4 residues R17 and R19 in Drosophila. We found all histone mutations including R17A/E/H and R19A/E/H (R17 and R19 of H4 are substituted by A, E and H respectively) result in a range of growth defects and abnormalities in chromosomal high-order structures, whereas R17E mutation is embryonic lethal. RNA-seq demonstrates that downregulated genes in both R17A and R19A show significant overlap and are enriched in development-related pathways. In addition, Western and cytological analyses showed that the R17A mutation resulted in a significant reduction in H4K16 acetylation and male offspring, implying that the R17 may be involved in male dosage compensation mechanisms. R19 mutation on the other hand strongly affect Gpp (Dot1 homologue in flies)-mediated H3K79 methylation, possibly through histone crosstalk. Together these results provide insights into the differential impacts of positive charges of H4 basic patch R17/R19 on regulation of gene transcription during developmental processes.

Painting of Fourth and the X-Linked 1.688 Satellite in D. melanogaster is Involved in Chromosome-Wide Gene Regulation.

Chromosome-specific regulatory mechanisms provide a model to understand the coordinated regulation of genes on entire chromosomes or on larger genomic regions. In fruit flies, two chromosome-wide systems have been characterized: The male-specific lethal (MSL) complex, which mediates dosage compensation and primarily acts on the male X-chromosome, and Painting of fourth (POF), which governs chromosome-specific regulation of genes located on the 4th chromosome. How targeting of one specific chromosome evolves is still not understood; but repeated sequences, in forms of satellites and transposable elements, are thought to facilitate the evolution of chromosome-specific targeting. The highly repetitive 1.688 satellite has been functionally connected to both these systems. Considering the rapid evolution and the necessarily constant adaptation of regulatory mechanisms, such as dosage compensation, we hypothesised that POF and/or 1.688 may still show traces of dosage-compensation functions. Here, we test this hypothesis by transcriptome analysis. We show that loss of Pof decreases not only chromosome 4 expression but also reduces the X-chromosome expression in males. The 1.688 repeat deletion, Zhr1 (Zygotic hybrid rescue), does not affect male dosage compensation detectably; however, Zhr1 in females causes a stimulatory effect on X-linked genes with a strong binding affinity to the MSL complex (genes close to high-affinity sites). Lack of pericentromeric 1.688 also affected 1.688 expression in trans and was linked to the differential expression of genes involved in eggshell formation. We discuss our results with reference to the connections between POF, the 1.688 satellite and dosage compensation, and the role of the 1.688 satellite in hybrid lethality.

Identification and characterization of a Masculinizer homologue in the diamondback moth, Plutella xylostella.

Recently, a novel sex‐determination system was identified in the silkworm (Bombyx mori) in which a piwi‐interacting RNA (piRNA) encoded on the female‐specific W chromosome silences a Z‐linked gene (Masculinizer) that would otherwise initiate male sex‐determination and dosage compensation. Masculinizer provides various opportunities for developing improved genetic pest management tools. A pest lepidopteran in which a genetic pest management system has been developed, but which would benefit greatly from such improved designs, is the diamondback moth, Plutella xylostella. However, Masculinizer has not yet been identified in this species. Here, focusing on the previously described ‘masculinizing’ domain of B. mori Masculinizer, we identify P. xylostella Masculinizer (PxyMasc). We show that PxyMasc is Z‐linked, regulates sex‐specific alternative splicing of doublesex and is necessary for male survival. Similar results in B. mori suggest this survival effect is possibly through failure to initiate male dosage compensation. The highly conserved function and location of this gene between these two distantly related lepidopterans suggests a deep role for Masculinizer in the sex‐determination systems of the Lepidoptera.

Corytophanids Replaced the Pleurodont XY System with a New Pair of XY Chromosomes.

Almost all lizard families in the pleurodont clade share the same XY system. This system was meticulously studied in Anolis carolinensis, where it shows a highly degenerated Y chromosome and a male-specific X chromosome dosage compensation mechanism. Corytophanids (casque-headed lizards) have been proposed as the only family in the pleurodont clade to lack the XY system. In this study, we worked with extensive genomic and transcriptomic data from Basiliscus vittatus, a member of the Corytophanidae family that inhabits the tropical rainforests of Mexico. We confirmed that B. vittatus underwent a sex chromosome system turnover, which consisted in the loss of the pleurodont XY system and the gain of a new pair of XY chromosomes that are orthologous to chicken chromosome 17. We estimated the origin of the sex chromosome system to have occurred ∼63 Ma in the ancestor of corytophanids. Moreover, we identified 12 XY gametologues with particular attributes, such as functions related to the membrane and intracellular trafficking, very low expression levels, blood specificity, and incomplete dosage compensation in males.

Dosage Compensation via Transposable Element Mediated Rewiring of a Regulatory Network

Transposable elements (TEs) may contribute to evolutionary innovations through the rewiring of networks by supplying ready-to-use cis regulatory elements. Genes on the Drosophila X chromosome are coordinately regulated by the male specific lethal (MSL) complex to achieve dosage compensation in males. We show that the acquisition of dozens of MSL binding sites on evolutionarily new X chromosomes was facilitated by the independent co-option of a mutant helitron TE that attracts the MSL complex (TE domestication). The recently formed neo-X recruits helitrons that provide dozens of functional, but suboptimal, MSL binding sites, whereas the older XR chromosome has ceased acquisition and appears to have fine-tuned the binding affinities of more ancient elements for the MSL complex. Thus, TE-mediated rewiring of regulatory networks through domestication and amplification may be followed by fine-tuning of the cis-regulatory element supplied by the TE and erosion of nonfunctional regions.

The Genomic Distribution of Sex-Biased Genes in Drosophila serrata: X Chromosome Demasculinization, Feminization, and Hyperexpression in Both Sexes

The chromosomal distribution of genes with sex-biased expression is often nonrandom, and in species with XY sex chromosome systems, it is common to observe a deficit of X-linked male-biased genes and an excess of X-linked female-biased genes. One explanation for this pattern is that sex-specific selection has shaped the gene content of the X. Alternatively, the deficit of male-biased and excess of female-biased genes could be an artifact of differences between the sexes in the global expression level of their X chromosome(s), perhaps brought about by a lack of dosage compensation in males and hyperexpression in females. In the montium fruit fly, Drosophila serrata, both these explanations can account for a deficit of male-biased and excess of female-biased X-linked genes. Using genome-wide expression data from multiple male and female tissues (n = 176 hybridizations), we found that testis- and accessory gland-specific genes are underrepresented whereas female ovary-specific genes are overrepresented on the X chromosome, suggesting that X-linkage is disfavored for male function genes but favored for female function genes. However, genes with such sex-specific functions did not fully account for the deficit of male-biased and excess of female-biased X-linked genes. We did, however, observe sex differences in the global expression level of the X chromosome and autosomes. Surprisingly, and in contrast to other species where a lack of dosage compensation in males is responsible, we found that hyperexpression of X-linked genes in both sexes leads to this imbalance in D. serrata. Our results highlight how common genomic distributions of sex-biased genes, even among closely related species, may arise via quite different evolutionary processes.

Posttranscriptional control of X‐chromosome dosage compensation

RNA regulation plays a major role in the generation of diversity at the molecular and cellular levels, and furnishes the cell with flexibility potential to adapt to changing environments. Often, the regulation by/of RNA dictates when, where, and how the information encoded in the nucleus is revealed. One example is the regulation of X-chromosome dosage compensation. In Drosophila, differences in X-linked gene dosage between males and females are compensated by the transcriptional upregulation of the single male X chromosome. Mechanisms of alternative splicing and translational control, among others, enforce dosage compensation in males while inhibiting this process in females. In this review, we discuss the posttranscriptional RNA regulatory mechanisms that ensure appropriate dosage compensation in Drosophila, drawing parallels with the mammalian system when appropriate.

Requirement of Male-Specific Dosage Compensation in Drosophila Females—Implications of Early X Chromosome Gene Expression

Dosage compensation equates between the sexes the gene dose of sex chromosomes that carry substantially different gene content. In Drosophila, the single male X chromosome is hypertranscribed by approximately two-fold to effect this correction. The key genes are male lethal and appear not to be required in females, or affect their viability. Here, we show these male lethals do in fact have a role in females, and they participate in the very process which will eventually shut down their function—female determination. We find the male dosage compensation complex is required for upregulating transcription of the sex determination master switch, Sex-lethal, an X-linked gene which is specifically activated in females in response to their two X chromosomes. The levels of some X-linked genes are also affected, and some of these genes are used in the process of counting the number of X chromosomes early in development. Our data suggest that before the female state is set, the ground state is male and female X chromosome expression is elevated. Females thus utilize the male dosage compensation process to amplify the signal which determines their fate.

The Drosophila GAGA factor is required for dosage compensation in males and for the formation of the male-specific-lethal complex chromatin entry site at 12DE.

Drosophila melanogaster males have one X chromosome, while females have two. To compensate for the resulting disparity in X-linked gene expression between the two sexes, most genes from the male X chromosome are hyperactivated by a special dosage compensation system. Dosage compensation is achieved by a complex of at least six proteins and two noncoding RNAs that specifically associate with the male X. A central question is how the X chromosome is recognized. According to a current model, complexes initially assemble at approximately 35 chromatin entry sites on the X and then spread bidirectionally along the chromosome where they occupy hundreds of sites. Here, we report that mutations in Trithorax-like (Trl) lead to the loss of a single chromatin entry site on the X, male lethality, and mislocalization of dosage compensation complexes.

Dosage dependent gene regulation and the compensation of the X chromosome in Drosophila males.

Dosage compensation refers to the phenomenon that despite the difference in copy number of X chromosomes in males and females approximately equal expression results from the sex chromosomes. We describe evidence for a model that dosage compensation is caused by an 'inverse dosage effect' that results from an altered stoichiometry of transcriptional regulators on the X versus the autosomes. This imbalance of regulators would cause a two-fold increase in target gene expression throughout the genome in the absence of any modification. The two-fold hyperactivation compensates the X chromosome. However, the MSL (male specific lethal) complex modifies this effect on the autosomes, which would otherwise double their expression, by becoming sequestered to the X chromosome together with a histone acetylase (MOF) and kinase (JIL1). This situation reduces the level of histone 4 Lys16 acetylation and H3 phosphorylation on the autosomes, thus bringing their expression down to near the female level. The presence of the MSL complex on the X modifies chromatin in such a manner that it counteracts any impact of increased histone acetylation and phosphorylation on gene expression. This situation fosters the proper two-fold increase in gene expression needed for X chromosomal dosage compensation in males and reduces the inverse effect on the autosomes to equalize gene expression throughout the genome for the two sexes.

Esa1p is an essential histone acetyltransferase required for cell cycle progression.

Histones are dynamically modified during chromatin assembly, as specific transcriptional patterns are established, and during mitosis and development. Modifications include acetylation, phosphorylation, ubiquitination, methylation, and ADP-ribosylation, but the biological significance of each of these is not well understood. For example, distinct acetylation patterns correlate with nucleosome formation and with transcriptionally activated or silenced chromatin, yet mutations in genes encoding several yeast histone acetyltransferase (HAT) activities result in either no cellular phenotype or only modest growth defects. Here we report characterization of ESA1, an essential gene that is a member of the MYST family that includes two yeast silencing genes, human genes associated with leukemia and with the human immunodeficiency virus type 1 Tat protein, and Drosophila mof, a gene essential for male dosage compensation. Esa1p acetylates histones in a pattern distinct from those of other yeast enzymes, and temperature-sensitive mutant alleles abolish enzymatic activity in vitro and result in partial loss of an acetylated isoform of histone H4 in vivo. Strains carrying these mutations are also blocked in the cell cycle such that at restrictive temperatures, esa1 mutants succeed in replicating their DNA but fail to proceed normally through mitosis and cytokinesis. Recent studies show that Esa1p enhances transcription in vitro and thus may modulate expression of genes important for cell cycle control. These observations therefore link an essential HAT activity to cell cycle progression, potentially through discrete transcriptional regulatory events.

RNA Helicase A Mediates Association of CBP with RNA Polymerase II

The coactivator CBP has been proposed to stimulate the expression of certain signal-dependent genes via its association with RNA polymerase II complexes. Here we show that complex formation between CBP and RNA polymerase II requires RNA helicase A (RHA), a nuclear DNA/RNA helicase that is related to the Drosophila male dosage compensation factor mle. In transient transfection assays, RHA was found to cooperate with CBP in mediating target gene activation via the CAMP responsive factor CREB. As a mutation in RHA that compromised its helicase activity correspondingly reduced CREB-dependent transcription, we propose that RHA may induce local changes in chromatin structure that promote engagement of the transcriptional apparatus on signal responsive promoters.

A female-specific lethal lesion in an X-linked positive regulator of the Drosophila sex determination gene, Sex-lethal.

Characterization of a partial-loss-of-function, female-specific lethal mutation has identified an X-linked genetic element (1-34.3; 10B4) that functions as a positive regulator of Sxl, a central gene controlling sex determination in Drosophila melanogaster. The name, sisterless-a, was chosen both to suggest functional similarities that exist between this gene and another positive regulator of Sxl, the maternally acting gene daughterless (da), and also to highlight an important difference; namely, that in contrast to da, it is the zygotic rather than maternal functioning of sis-a that is involved in its interaction with Sxl. As with da, the female-specific lethal phenotype of sis-a is suppressed both by SxlM#1, a gain-of-function mutant allele of the target gene, and, to a lesser extent, by a duplication of Sxl+. Mutations at sis-a, da and Sxl display female-specific dominant synergism, each enhancing the others' lethal effects. The allele specificity with respect to Sxl of these dominant interactions indicates that sis-a and da affect the same aspect of Sxl regulation. As with previous studies of da and Sxl, the masculinizing effects of loss of sis-a function are generally obscured by lethal effects, presumably related to upsets in dosage compensation. The masculinizing effects can be dissociated from lethal effects by analysis of triploid intersexes (XX AAA) or by analysis of diploid females who are also mutant for autosomal genes known to be required for the transcriptional hyperactivation associated with dosage compensation in males. Analysis of foreleg development shows that intersexuality generated by sis-a is of the mosaic type: At the level of individual cells, only male or female development is observed, never an intermediate sexual phenotype characteristic of true intersexes. Sexual development of diplo-X germline and somatic clones of sis-a tissue generated by mitotic recombination during larval stages is normal, as is the sexual phenotype of homozygous sis-a escapers. Considered in their totality, these results indicate that sis-a functions early in development to help establish the activity state of Sxl and thereby initiate the sexual pathway commitment, rather than functioning later in the processes by which Sxl maintains and expresses the sex determination decision.

A Fundamental Theorem of Natural Selection for Sex Linkage or Arrhenotoky

Equation (11) is a general equation for the change in geometric mean fitness at a male haploid or sex-linked locus when the selection intensity is small. The rate of change in fitness depends on the additive genetic variance in fitness, which is defined as the variance of the additive genotypic values when the additive model of gene action assumes no dominance in females and complete dosage compensation in males. If the relative fitnesses of genotypes are constant, then under slow selection the increment in fitness is equal to the additive variance if either (1) the population is in random mating or (2) there is no dominance in females and complete dosage compensation in males. If both conditions hold, then a male haploid population evolves one-third faster than the corresponding diploid population and one-third slower than the corresponding haploid population. In computer-simulated populations undergoing random mating, the additive variance is a good approximation to actual fitness changes over a wide ra...

Some Aspects of Natural Selection in Arrhenotokous Populations

SYNOPSIS. Obligate arrhenotoky is known to have evolved only eight times, once each in the rotifer order Monogonanta, the arachnid order Acarina, the insect orders Hymenoptera and Thysanoptera, and in two families of Homoptera (Coccidae and Aleyrodidae) and Coleoptera (Scolytidae and Micromalthidae). In this paper it is argued that in male haploids the population genetic parameter which is most nearly analogous to the average fitness in ordinary diploids is the geometric mean of the average fitness in the two sexes. The geometric mean fitness is maximized by selection in a random mating population, provided one takes into account the fact that the system of inheritance imposes constraints on the gene frequencies in males and females. Furthermore, under sufficiently small selection intensities, the rate of change in geometric mean fitness is approximately equal to the additive genetic variance infitness, where the additive variance is defined as the variance due to the regression on a model of gene action which assumes no dominance in females and complete dosage compensation in males. Some problems concerning the sex ratio in male haploids when there is extreme inbreeding are also discussed.