Drosophila dosage compensation: males are from Mars, females are from Venus

Natural variation in the maternal and zygotic mRNA complements of the early embryo in Drosophila melanogaster

Background

Maternal gene products supplied to the egg during oogenesis drive the earliest events of development in all metazoans. After the initial stages of embryogenesis, maternal transcripts are degraded as zygotic transcription is activated; this is known as the maternal to zygotic transition (MZT). Recently, it has been shown that the expression of maternal and zygotic transcripts have evolved in the Drosophila genus over the course of 50 million years. However, the extent of natural variation of maternal and zygotic transcripts within a species has yet to be determined. We asked how the maternal and zygotic pools of mRNA vary within and between populations of D. melanogaster. In order to maximize sampling of genetic diversity, African lines of D. melanogaster originating from Zambia as well as DGRP lines originating from North America were chosen for transcriptomic analysis.

Results

Generally, we find that maternal transcripts are more highly conserved, and zygotic transcripts evolve at a higher rate. We find that there is more within-population variation in transcript abundance than between populations and that expression variation is highest post- MZT between African lines.

Conclusions

Determining the natural variation of gene expression surrounding the MZT in natural populations of D. melanogaster gives insight into the extent of how a tightly regulated process may vary within a species, the extent of developmental constraint at both stages and on both the maternal and zygotic genomes, and reveals expression changes allowing this species to adapt as it spread across the world.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08839-4.

A comparative study of the efficiency of Withania somnifera and carbamazepine on lifespan, reproduction and epileptic phenotype - A study in Drosophila paralytic mutant

Background

Seizure disorders are considered a serious health issue because of the vast number of people affected globally and the limited treatment options. Approximately 15 million epileptic patients worldwide do not respond to any of the currently available medications. Carbamazepine (CBZ) is one of the most widely used antiepileptic drugs (AEDs) for the treatment of epilepsy, which is discontinued in less than 5% of epileptic patients due to its side effects. In traditional medicine, to establish the foundation of health care, plant extracts are utilized to a great extent to treat different pathologies. Withania somnifera (W. somnifera) is an herbal component with anticonvulsant properties.

Objectives

To compare the medicinal effects of W. somnifera on lifespan, fecundity, fertility and epileptic phenotype in Drosophila paralytic mutant (para^bss1) model system with CBZ, a commonly used AED.

Material and methods

Flies were exposed to three different doses of W. somnifera or CBZ in standard wheat flour-agar media for six days. Drosophila Oregon-R strain was used as a control.

Results

Results indicate that a high dose of W. somnifera increased the lifespan in Drosophila para^bss1 while remaining safe for fecundity and fertility. CBZ decreased the lifespan of para^bss1 mutant at higher dose (40 μg/ml), as expected, and also reduced the fecundity and fertility of the flies. Our findings indicate that W. somnifera was more effective than CBZ to control epileptic phenotype.

Conclusion

W. somnifera is an effective medication with no side effects for treating epilepsy in Drosophila paralytic mutant.

Cis-acting variation is common across regulatory layers but is often buffered during embryonic development

Precise patterns of gene expression are driven by interactions between transcription factors, regulatory DNA sequences, and chromatin. How DNA mutations affecting any one of these regulatory "layers" are buffered or propagated to gene expression remains unclear. To address this, we quantified allele-specific changes in chromatin accessibility, histone modifications, and gene expression in F1 embryos generated from eight Drosophila crosses at three embryonic stages, yielding a comprehensive data set of 240 samples spanning multiple regulatory layers. Genetic variation (allelic imbalance) impacts gene expression more frequently than chromatin features, with metabolic and environmental response genes being most often affected. Allelic imbalance in cis-regulatory elements (enhancers) is common and highly heritable, yet its functional impact does not generally propagate to gene expression. When it does, genetic variation impacts RNA levels through two alternative mechanisms involving either H3K4me3 or chromatin accessibility and H3K27ac. Changes in RNA are more predictive of variation in H3K4me3 than vice versa, suggesting a role for H3K4me3 downstream from transcription. The impact of a substantial proportion of genetic variation is consistent across embryonic stages, with 50% of allelic imbalanced features at one stage being also imbalanced at subsequent developmental stages. Crucially, buffering, as well as the magnitude and evolutionary impact of genetic variants, is influenced by regulatory complexity (i.e., number of enhancers regulating a gene), with transcription factors being most robust to cis-acting, but most influenced by trans-acting, variation.

The Gcn5 complexes in Drosophila as a model for metazoa

The histone acetyltransferase Gcn5 is conserved throughout eukaryotes where it functions as part of large multi-subunit transcriptional coactivator complexes that stimulate gene expression. Here, we describe how studies in the model insect Drosophila melanogaster have provided insight into the essential roles played by Gcn5 in the development of multicellular organisms. We outline the composition and activity of the four different Gcn5 complexes in Drosophila: the Spt-Ada-Gcn5 Acetyltransferase (SAGA), Ada2a-containing (ATAC), Ada2/Gcn5/Ada3 transcription activator (ADA), and Chiffon Histone Acetyltransferase (CHAT) complexes. Whereas the SAGA and ADA complexes are also present in the yeast Saccharomyces cerevisiae, ATAC has only been identified in other metazoa such as humans, and the CHAT complex appears to be unique to insects. Each of these Gcn5 complexes is nucleated by unique Ada2 homologs or splice isoforms that share conserved N-terminal domains, and differ only in their C-terminal domains. We describe the common and specialized developmental functions of each Gcn5 complex based on phenotypic analysis of mutant flies. In addition, we outline how gene expression studies in mutant flies have shed light on the different biological roles of each complex. Together, these studies highlight the key role that Drosophila has played in understanding the expanded biological function of Gcn5 in multicellular eukaryotes.

Dosage Compensation throughout the Schistosoma mansoni Lifecycle: Specific Chromatin Landscape of the Z Chromosome

Differentiated sex chromosomes are accompanied by a difference in gene dose between X/Z-specific and autosomal genes. At the transcriptomic level, these sex-linked genes can lead to expression imbalance, or gene dosage can be compensated by epigenetic mechanisms and results into expression level equalization. Schistosoma mansoni has been previously described as a ZW species (i.e., female heterogamety, in opposition to XY male heterogametic species) with a partial dosage compensation, but underlying mechanisms are still unexplored. Here, we combine transcriptomic (RNA-Seq) and epigenetic data (ChIP-Seq against H3K4me3, H3K27me3, and H4K20me1 histone marks) in free larval cercariae and intravertebrate parasitic stages. For the first time, we describe differences in dosage compensation status in ZW females, depending on the parasitic status: free cercariae display global dosage compensation, whereas intravertebrate stages show a partial dosage compensation. We also highlight regional differences of gene expression along the Z chromosome in cercariae, but not in the intravertebrate stages. Finally, we feature a consistent permissive chromatin landscape of the Z chromosome in both sexes and stages. We argue that dosage compensation in schistosomes is characterized by chromatin remodeling mechanisms in the Z-specific region.

Transcriptome profiling of the honeybee parasite Varroa destructor provides new biological insights into the mite adult life cycle

Background

The parasite Varroa destructor represents a significant threat to honeybee colonies. Indeed, development of Varroa infestation within colonies, if left untreated, often leads to the death of the colony. Although its impact on bees has been extensively studied, less is known about its biology and the functional processes governing its adult life cycle and adaptation to its host. We therefore developed a full life cycle transcriptomic catalogue in adult Varroa females and included pairwise comparisons with males, artificially-reared and non-reproducing females (10 life cycle stages and conditions in total).

Results

Extensive remodeling of the Varroa transcriptome was observed, with an upregulation of energetic and chitin metabolic processes during the initial and final phases of the life cycle (e.g. phoretic and post-oviposition stages), whereas during reproductive stages in brood cells genes showing functions related to transcriptional regulation were overexpressed. Several neurotransmitter and neuropeptide receptors involved in behavioural regulation, as well as active compounds of salivary glands, were also expressed at a higher level outside the reproductive stages. No difference was detected between artificially-reared phoretic females and their counterparts in colonies, or between females who failed to reproduce and females who successfully reproduced, indicating that phoretic individuals can be reared outside host colonies without impacting their physiology and that mechanisms underlying reproductive failure occur before oogenesis.

Conclusions

We discuss how these new findings reveal the remarkable adaptation of Varroa to its host biology and notably to the switch from living on adults to reproducing in sealed brood cells. By spanning the entire adult life cycle, our work captures the dynamic changes in the parasite gene expression and serves as a unique resource for deciphering Varroa biology and identifying new targets for mite control.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4668-z) contains supplementary material, which is available to authorized users.

Studying X chromosome inactivation in the single-cell genomic era

Single-cell genomics is set to revolutionise our understanding of how epigenetic silencing works; by studying specific epigenetic marks or chromatin conformations in single cells, it is possible to ask whether they cause transcriptional silencing or are instead a consequence of the silent state. Here, we review what single-cell genomics has revealed about X chromosome inactivation, perhaps the best characterised mammalian epigenetic process, highlighting the novel findings and important differences between mouse and human X inactivation uncovered through these studies. We consider what fundamental questions these techniques are set to answer in coming years and propose that X chromosome inactivation is an ideal model to study gene silencing by single-cell genomics as technical limitations are minimised through the co-analysis of hundreds of genes.

Faster-X evolution: Theory and evidence from Drosophila

A faster rate of adaptive evolution of X-linked genes compared with autosomal genes can be caused by the fixation of recessive or partially recessive advantageous mutations, due to the full expression of X-linked mutations in hemizygous males. Other processes, including recombination rate and mutation rate differences between X chromosomes and autosomes, may also cause faster evolution of X-linked genes. We review population genetics theory concerning the expected relative values of variability and rates of evolution of X-linked and autosomal DNA sequences. The theoretical predictions are compared with data from population genomic studies of several species of Drosophila. We conclude that there is evidence for adaptive faster-X evolution of several classes of functionally significant nucleotides. We also find evidence for potential differences in mutation rates between X-linked and autosomal genes, due to differences in mutational bias towards GC to AT mutations. Many aspects of the data are consistent with the male hemizygosity model, although not all possible confounding factors can be excluded.

Stoichiometric balance of protein copy numbers is measurable and functionally significant in a protein-protein interaction network for yeast endocytosis

Stoichiometric balance, or dosage balance, implies that proteins that are subunits of obligate complexes (e.g. the ribosome) should have copy numbers expressed to match their stoichiometry in that complex. Establishing balance (or imbalance) is an important tool for inferring subunit function and assembly bottlenecks. We show here that these correlations in protein copy numbers can extend beyond complex subunits to larger protein-protein interactions networks (PPIN) involving a range of reversible binding interactions. We develop a simple method for quantifying balance in any interface-resolved PPINs based on network structure and experimentally observed protein copy numbers. By analyzing such a network for the clathrin-mediated endocytosis (CME) system in yeast, we found that the real protein copy numbers were significantly more balanced in relation to their binding partners compared to randomly sampled sets of yeast copy numbers. The observed balance is not perfect, highlighting both under and overexpressed proteins. We evaluate the potential cost and benefits of imbalance using two criteria. First, a potential cost to imbalance is that ‘leftover’ proteins without remaining functional partners are free to misinteract. We systematically quantify how this misinteraction cost is most dangerous for strong-binding protein interactions and for network topologies observed in biological PPINs. Second, a more direct consequence of imbalance is that the formation of specific functional complexes depends on relative copy numbers. We therefore construct simple kinetic models of two sub-networks in the CME network to assess multi-protein assembly of the ARP2/3 complex and a minimal, nine-protein clathrin-coated vesicle forming module. We find that the observed, imperfectly balanced copy numbers are less effective than balanced copy numbers in producing fast and complete multi-protein assemblies. However, we speculate that strategic imbalance in the vesicle forming module allows cells to tune where endocytosis occurs, providing sensitive control over cargo uptake via clathrin-coated vesicles.

Protein copy numbers are often found to be stoichiometrically balanced for subunits of multi-protein complexes. Imbalance is believed to be deleterious because it lowers complex yield (the dosage balance hypothesis) and increases the risk of misinteractions, but imbalance may also provide unexplored functional benefits. We show here that the benefits of stoichiometric balance can extend to larger networks of interacting proteins. We develop a method to quantify to what degree protein networks are balanced, and apply it to two networks. We find that the clathrin-mediated endocytosis system in yeast is statistically balanced, but not perfectly so, and explore the consequences of imbalance in the form of misinteractions and endocytic function. We also show that biological networks are more robust to misinteractions than random networks when balanced, but are more sensitive to misinteractions under imbalance. This suggests evolutionary pressure for proteins to be balanced and that any conserved imbalance should occur for functional reasons. We explore one such reason in the form of bottlenecking the endocytosis process. Our method can be generalized to other networks and used to identify out-of-balance proteins. Our results provide insight into how network design, expression level regulation, and cell fitness are intertwined.

From Heterochromatin to Long Noncoding RNAs in Drosophila: Expanding the Arena of Gene Function and Regulation

Recent years have witnessed a remarkable interest in exploring the significance of pervasive noncoding transcripts in diverse eukaryotes. Classical cytogenetic studies using the Drosophila model system unraveled the perplexing attributes and "functions" of the "gene"-poor heterochromatin. Recent molecular studies in the fly model are likewise revealing the very diverse and significant roles played by long noncoding RNAs (lncRNAs) in development, gene regulation, chromatin organization, cell and nuclear architecture, etc. There has been a rapid increase in the number of identified lncRNAs, although a much larger number still remains unknown. The diversity of modes of actions and functions of the limited number of Drosophila lncRNAs, which have been examined, already reflects the profound roles of such RNAs in generating and sustaining the biological complexities of eukaryotes. Several of the known Drosophila lncRNAs originate as independent sense or antisense transcripts from promoter or intergenic, intronic, or 5'/3'-UTR regions, while many of them are independent genes that produce only lncRNAs or coding as well as noncoding RNAs. The different lncRNAs affect chromatin organization (local or large-scale pan-chromosomal), transcription, RNA processing/stability, or translation either directly through interaction with their target DNA sequences or indirectly by acting as intermediary molecules for specific regulatory proteins or may act as decoys/sinks, or storage sites for specific proteins or groups of proteins, or may provide a structural framework for the assembly of substructures in nucleus/cytoplasm. It is interesting that many of the "functions" alluded to heterochromatin in earlier cytogenetic studies appear to find correlates with the known subtle as well as far-reaching actions of the different small and long noncoding RNAs. Further studies exploiting the very rich and powerful genetic and molecular resources available for the Drosophila model are expected to unravel the mystery underlying the long reach of ncRNAs.

X Chromosome and Autosome Dosage Responses in Drosophila melanogaster Heads

X chromosome dosage compensation is required for male viability in Drosophila. Dosage compensation relative to autosomes is two-fold, but this is likely to be due to a combination of homeostatic gene-by-gene regulation and chromosome-wide regulation. We have baseline values for gene-by-gene dosage compensation on autosomes, but not for the X chromosome. Given the evolutionary history of sex chromosomes, these baseline values could differ. We used a series of deficiencies on the X and autosomes, along with mutations in the sex-determination gene transformer-2, to carefully measure the sex-independent X-chromosome response to gene dosage in adult heads by RNA sequencing. We observed modest and indistinguishable dosage compensation for both X chromosome and autosome genes, suggesting that the X chromosome is neither inherently more robust nor sensitive to dosage change.

Sex-specific embryonic gene expression in species with newly evolved sex chromosomes

Sex chromosome dosage differences between females and males are a significant form of natural genetic variation in many species. Like many species with chromosomal sex determination, Drosophila females have two X chromosomes, while males have one X and one Y. Fusions of sex chromosomes with autosomes have occurred along the lineage leading to D. pseudoobscura and D. miranda. The resulting neo-sex chromosomes are gradually evolving the properties of sex chromosomes, and neo-X chromosomes are becoming targets for the molecular mechanisms that compensate for differences in X chromosome dose between sexes. We have previously shown that D. melanogaster possess at least two dosage compensation mechanisms: the well- characterized MSL-mediated dosage compensation active in most somatic tissues, and another system active during early embryogenesis prior to the onset of MSL-mediated dosage compensation. To better understand the developmental constraints on sex chromosome gene expression and evolution, we sequenced mRNA from individual male and female embryos of D. pseudoobscura and D. miranda, from ∼0.5 to 8 hours of development. Autosomal expression levels are highly conserved between these species. But, unlike D. melanogaster, we observe a general lack of dosage compensation in D. pseudoobscura and D. miranda prior to the onset of MSL-mediated dosage compensation. Thus, either there has been a lineage-specific gain or loss in early dosage compensation mechanism(s) or increasing X chromosome dose may strain dosage compensation systems and make them less effective. The extent of female bias on the X chromosomes decreases through developmental time with the establishment of MSL-mediated dosage compensation, but may do so more slowly in D. miranda than D. pseudoobscura. These results also prompt a number of questions about whether species with more sex-linked genes have more sex-specific phenotypes, and how much transcript level variance is tolerable during critical stages of development.

Many animals have sex-specific combinations of chromosomes. In humans, for example, females have two X chromosomes while males have one X and one Y. In most species with XX:XY systems, the Y chromosome is degenerate and gene-poor while the X encodes a large number of functional genes. A variety of systems have evolved to ensure that males with one X chromosome and females with two X chromosomes have the same gene expression level for X-linked genes. The vinegar fly D. melanogaster has at least two dosage compensation systems: one that acts early in development, and another active in later stages. In this paper, we determine expression levels for thousands of genes in male and female embryos at different developmental stages in two species, D. pseudoobscura and D. miranda, that have unusually large fractions of their genomes in X or X-like chromosomes. We show that dosage compensation is established slowly during embryogenesis, and that in these species, dosage compensation appears to be absent in early development. This may be due to a lineage-specific loss or gain of compensation mechanism, or possibly because the machinery of dosage compensation cannot effectively handle the increased demand in these species.

Male X-linked genes in Drosophila melanogaster are compensated independently of the Male-Specific Lethal complex

Background

In organisms where the two sexes have unequal numbers of X-chromosomes, the expression of X-linked genes needs to be balanced not only between the two sexes, but also between X and the autosomes. In Drosophila melanogaster, the Male-Specific Lethal (MSL) complex is believed to produce a 2-fold increase in expression of genes on the male X, thus restoring this balance.

Results

Here we show that almost all the genes on the male X are effectively compensated. However, many genes are compensated without any significant recruitment of the MSL-complex. These genes are very weakly, if at all, affected by mutations or RNAi against MSL-complex components. In addition, even the genes that are strongly bound by MSL rely on mechanisms other than the MSL-complex for proper compensation. We find that long, non-ubiquitously expressed genes tend to rely less on the MSL-complex for their compensation and genes that in addition are far from High Affinity Sites tend to not bind the complex at all or very weakly.

Conclusions

We conclude that most of the compensation of X-linked genes is produced by an MSL-independent mechanism. Similar to the case of the MSL-mediated compensation we do not yet know the mechanism behind the MSL-independent compensation that appears to act preferentially on long genes. Even if we observe similarities, it remains to be seen if the mechanism is related to the buffering that is observed in autosomal aneuploidies.

Msl2 is a novel component of the vertebrate DNA damage response

hMSL2 (male-specific lethal 2, human) is a RING finger protein with ubiquitin ligase activity. Although it has been shown to target histone H2B at lysine 34 and p53 at lysine 351, suggesting roles in transcription regulation and apoptosis, its function in these and other processes remains poorly defined. To further characterize this protein, we have disrupted the Msl2 gene in chicken DT40 cells. Msl2^−/− cells are viable, with minor growth defects. Biochemical analysis of the chromatin in these cells revealed aberrations in the levels of several histone modifications involved in DNA damage response pathways. DNA repair assays show that both Msl2^−/− chicken cells and hMSL2-depleted human cells have defects in non-homologous end joining (NHEJ) repair. DNA damage assays also demonstrate that both Msl2 and hMSL2 proteins are modified and stabilized shortly after induction of DNA damage. Moreover, hMSL2 mediates modification, presumably ubiquitylation, of a key DNA repair mediator 53BP1 at lysine 1690. Similarly, hMSL1 and hMOF (males absent on the first) are modified in the presence of hMSL2 shortly after DNA damage. These data identify a novel role for Msl2/hMSL2 in the cellular response to DNA damage. The kinetics of its stabilization suggests a function early in the NHEJ repair pathway. Moreover, Msl2 plays a role in maintaining normal histone modification profiles, which may also contribute to the DNA damage response.

Sequence-specific targeting of dosage compensation in Drosophila favors an active chromatin context

The Drosophila MSL complex mediates dosage compensation by increasing transcription of the single X chromosome in males approximately two-fold. This is accomplished through recognition of the X chromosome and subsequent acetylation of histone H4K16 on X-linked genes. Initial binding to the X is thought to occur at “entry sites” that contain a consensus sequence motif (“MSL recognition element” or MRE). However, this motif is only ∼2 fold enriched on X, and only a fraction of the motifs on X are initially targeted. Here we ask whether chromatin context could distinguish between utilized and non-utilized copies of the motif, by comparing their relative enrichment for histone modifications and chromosomal proteins mapped in the modENCODE project. Through a comparative analysis of the chromatin features in male S2 cells (which contain MSL complex) and female Kc cells (which lack the complex), we find that the presence of active chromatin modifications, together with an elevated local GC content in the surrounding sequences, has strong predictive value for functional MSL entry sites, independent of MSL binding. We tested these sites for function in Kc cells by RNAi knockdown of Sxl, resulting in induction of MSL complex. We show that ectopic MSL expression in Kc cells leads to H4K16 acetylation around these sites and a relative increase in X chromosome transcription. Collectively, our results support a model in which a pre-existing active chromatin environment, coincident with H3K36me3, contributes to MSL entry site selection. The consequences of MSL targeting of the male X chromosome include increase in nucleosome lability, enrichment for H4K16 acetylation and JIL-1 kinase, and depletion of linker histone H1 on active X-linked genes. Our analysis can serve as a model for identifying chromatin and local sequence features that may contribute to selection of functional protein binding sites in the genome.

The genomes of complex organisms encompass hundreds of millions of base pairs of DNA, and regulatory molecules must distinguish specific targets within this vast landscape. In general, regulatory factors find target genes through sequence-specific interactions with the underlying DNA. However, sequence-specific factors typically bind only a fraction of the candidate genomic regions containing their specific target sequence motif. Here we identify potential roles for chromatin environment and flanking sequence composition in helping regulatory factors find their appropriate binding sites, using targeting of the Drosophila dosage compensation complex as a model. The initial stage of dosage compensation involves binding of the Male Specific Lethal (MSL) complex to a sequence motif called the MSL recognition element [1]. Using data from a large chromatin mapping effort (the modENCODE project), we successfully identify an active chromatin environment as predictive of selective MRE binding by the MSL complex. Our study provides a framework for using genome-wide datasets to analyze and predict functional protein–DNA binding site selection.

Sequence signatures involved in targeting the Male-Specific Lethal complex to X-chromosomal genes in Drosophila melanogaster

Background

In Drosophila melanogaster, the dosage-compensation system that equalizes X-linked gene expression between males and females, thereby assuring that an appropriate balance is maintained between the expression of genes on the X chromosome(s) and the autosomes, is at least partially mediated by the Male-Specific Lethal (MSL) complex. This complex binds to genes with a preference for exons on the male X chromosome with a 3' bias, and it targets most expressed genes on the X chromosome. However, a number of genes are expressed but not targeted by the complex. High affinity sites seem to be responsible for initial recruitment of the complex to the X chromosome, but the targeting to and within individual genes is poorly understood.

Results

We have extensively examined X chromosome sequence variation within five types of gene features (promoters, 5' UTRs, coding sequences, introns, 3' UTRs) and intergenic sequences, and assessed its potential involvement in dosage compensation. Presented results show that: the X chromosome has a distinct sequence composition within its gene features; some of the detected variation correlates with genes targeted by the MSL-complex; the insulator protein BEAF-32 preferentially binds upstream of MSL-bound genes; BEAF-32 and MOF co-localizes in promoters; and that bound genes have a distinct sequence composition that shows a 3' bias within coding sequence.

Conclusions

Although, many strongly bound genes are close to a high affinity site neither our promoter motif nor our coding sequence signatures show any correlation to HAS. Based on the results presented here, we believe that there are sequences in the promoters and coding sequences of targeted genes that have the potential to direct the secondary spreading of the MSL-complex to nearby genes.

Balancing sex chromosome expression and satisfying the sexes

Equalizing sex chromosome expression between the sexes when they have largely differing gene content appears to be necessary, and across species, is accomplished in a variety of ways. Even in birds, where the process is less than complete, a mechanism to reduce the difference in gene dose between the sexes exists. In early development, while the dosage difference is unregulated and still in flux, it is frequently exploited by sex determination mechanisms. The Drosophila female sex determination process is one clear example, determining the sexes based on X chromosome dose. Recent data show that in Drosophila, the female sex not only reads this gene balance difference, but at the same time usurps the moment. Taking advantage of the transient default state of male dosage compensation, the sex determination master-switch Sex-lethal which resides on the X, has its expression levels enhanced before it works to correct the gene imbalance. Intriguingly, key developmental genes which could create developmental havoc if their levels were unbalanced show more exquisite regulation, suggesting nature distinguishes them and ensures their expression is kept in the desirable range.