Developmental and cell cycle progression defects in Drosophila hybrid males

The Integrity of the HMR complex is necessary for centromeric binding and reproductive isolation in Drosophila

Postzygotic isolation by genomic conflict is a major cause for the formation of species. Despite its importance, the molecular mechanisms that result in the lethality of interspecies hybrids are still largely unclear. The genus Drosophila, which contains over 1600 different species, is one of the best characterized model systems to study these questions. We showed in the past that the expression levels of the two hybrid incompatibility factors Hmr and Lhr diverged in the two closely related Drosophila species, D. melanogaster and D. simulans, resulting in an increased level of both proteins in interspecies hybrids. The overexpression of the two proteins also leads to mitotic defects, a misregulation in the expression of transposable elements and decreased fertility in pure species. In this work, we describe a distinct six subunit protein complex containing HMR and LHR and analyse the effect of Hmr mutations on complex integrity and function. Our experiments suggest that HMR needs to bring together components of centromeric and pericentromeric chromatin to fulfil its physiological function and to cause hybrid male lethality.

A major cause of biological speciation is the sterility and/or lethality of hybrids. This hybrid lethality is thought to be the consequence of two incompatible genomes of the two different species. We used the fruit fly Drosophila melanogaster as a model system to isolate a defined protein complex, which mediates this hybrid lethality. Our data suggest that this complex containing six subunits has evolved in one Drosophila species (Drosophila melanogaster) to bring together components of centromeric and pericentromeric chromatin. We show that the integrity of the complex is necessary for its genomic binding patterns and its ability to maintain fertility in female Drosophila melanogaster flies. Hybrid males between Drosophila melanogaster and the very closely related species Drosophila simulans die because they contain elevated levels of this complex. These high levels result in mitotic defects and a misregulation in the expression of transposable elements in those hybrids. Our results show that mutations that interfere with the complex’s function in Drosophila melanogaster also fail to induce lethality in hybrids suggesting that its evolutionary acquired functions in one species induce lethality in interspecies hybrids.

A multi-layered structure of the interphase chromocenter revealed by proximity-based biotinylation

During interphase centromeres often coalesce into a small number of chromocenters, which can be visualized as distinct, DAPI dense nuclear domains. Intact chromocenters play a major role in maintaining genome stability as they stabilize the transcriptionally silent state of repetitive DNA while ensuring centromere function. Despite its biological importance, relatively little is known about the molecular composition of the chromocenter or the processes that mediate chromocenter formation and maintenance. To provide a deeper molecular insight into the composition of the chromocenter and to demonstrate the usefulness of proximity-based biotinylation as a tool to investigate those questions, we performed super resolution microscopy and proximity-based biotinylation experiments of three distinct proteins associated with the chromocenter in Drosophila. Our work revealed an intricate internal architecture of the chromocenter suggesting a complex multilayered structure of this intranuclear domain.

Altered Localization of Hybrid Incompatibility Proteins in Drosophila

Understanding the molecular basis of hybrid incompatibilities is a fundamental pursuit in evolutionary genetics. In crosses between Drosophila melanogaster females and Drosophila simulans males, an interaction between at least three genes is necessary for hybrid male lethality: Hmr ^mel, Lhr ^sim, and gfzf ^sim. Although HMR and LHR physically bind each other and function together in a single complex, the connection between gfzf and either of these proteins remains mysterious. Here, we show that GFZF localizes to many regions of the genome in both D. melanogaster and D. simulans, including at telomeric retrotransposon repeats. We find that GFZF localization at telomeres is significantly different between these two species, reflecting the rapid evolution of telomeric retrotransposon copy number composition between the two species. Next, we show that GFZF and HMR normally do not colocalize in D. melanogaster. In interspecies hybrids, however, HMR shows extensive mis-localization to GFZF sites, thus uncovering a new molecular interaction between these hybrid incompatibility factors. We find that spreading of HMR to GFZF sites requires gfzf ^sim but not Lhr ^sim, suggesting distinct roles for these factors in the hybrid incompatibility. Finally, we find that overexpression of HMR and LHR within species is sufficient to mis-localize HMR to GFZF binding sites, indicating that HMR has a natural low affinity for GFZF sites. Together, these studies provide the first insights into the different properties of gfzf between D. melanogaster and D. simulans, and uncover a molecular interaction between gfzf and Hmr in the form of altered protein localization.

Dying Neurons Utilize Innate Immune Signaling to Prime Glia for Phagocytosis during Development

Glia continuously survey neuronal health during development, providing trophic support to healthy neurons while rapidly engulfing dying ones. These diametrically opposed functions necessitate a foolproof mechanism enabling glia to unambiguously identify those neurons to support versus those to engulf. To ensure specificity, glia are proposed to interact with dying neurons via a series of carefully choreographed steps. However, these crucial interactions are largely obscure. Here we show that dying neurons and glia communicate via Toll-receptor-regulated innate immune signaling. Neuronal apoptosis drives processing and activation of the Toll-6 ligand, Spätzle5. This cue activates a dSARM-mediated Toll-6 transcriptional pathway in glia, which controls the expression of the Draper engulfment receptor. Pathway loss drives early-onset neurodegeneration, underscoring its functional importance. Our results identify an upstream priming signal that prepares glia for phagocytosis. Thus, a core innate immune pathway plays an unprecedented role setting the valence of neuron-glia interactions during development.

The Hybrid Incompatibility Genes Lhr and Hmr Are Required for Sister Chromatid Detachment During Anaphase but Not for Centromere Function

Crosses between Drosophila melanogaster females and Drosophila simulans males produce hybrid sons that die at the larval stage. This hybrid lethality is suppressed by loss-of-function mutations in the D. melanogaster Hybrid male rescue (Hmr) or in the D. simulans Lethal hybrid rescue (Lhr) genes. Previous studies have shown that Hmr and Lhr interact with heterochromatin proteins and suppress expression of transposable elements within D. melanogaster It also has been proposed that Hmr and Lhr function at the centromere. We examined mitotic divisions in larval brains from Hmr and Lhr single mutants and Hmr; Lhr double mutants in D. melanogaster In none of the mutants did we observe defects in metaphase chromosome alignment or hyperploid cells, which are hallmarks of centromere or kinetochore dysfunction. In addition, we found that Hmr-HA and Lhr-HA do not colocalize with centromeres either during interphase or mitotic division. However, all mutants displayed anaphase bridges and chromosome aberrations resulting from the breakage of these bridges, predominantly at the euchromatin-heterochromatin junction. The few dividing cells present in hybrid males showed fuzzy and irregularly condensed chromosomes with unresolved sister chromatids. Despite this defect in condensation, chromosomes in hybrids managed to align on the metaphase plate and undergo anaphase. We conclude that there is no evidence for a centromeric function of Hmr and Lhr within D. melanogaster nor for a centromere defect causing hybrid lethality. Instead, we find that Hmr and Lhr are required in D. melanogaster for detachment of sister chromatids during anaphase.

The clock is ticking. Ageing of the circadian system: From physiology to cell cycle

The circadian system is the responsible to organise the internal temporal order in relation to the environment of every process of the organisms producing the circadian rhythms. These rhythms have a fixed phase relationship among them and with the environment in order to optimise the available energy and resources. From a cellular level, circadian rhythms are controlled by genetic positive and negative auto-regulated transcriptional and translational feedback loops, which generate 24h rhythms in mRNA and protein levels of the clock components. It has been described about 10% of the genome is controlled by clock genes, with special relevance, due to its implications, to the cell cycle. Ageing is a deleterious process which affects all the organisms' structures including circadian system. The circadian system's ageing may produce a disorganisation among the circadian rhythms, arrhythmicity and, even, disconnection from the environment, resulting in a detrimental situation to the organism. In addition, some environmental conditions can produce circadian disruption, also called chronodisruption, which may produce many pathologies including accelerated ageing. Finally, some strategies to prevent, palliate or counteract chronodisruption effects have been proposed to enhance the circadian system, also called chronoenhancement. This review tries to gather recent advances in the chronobiology of the ageing process, including cell cycle, neurogenesis process and physiology.

The Drosophila speciation factor HMR localizes to genomic insulator sites

Hybrid incompatibility between Drosophila melanogaster and D. simulans is caused by a lethal interaction of the proteins encoded by the Hmr and Lhr genes. In D. melanogaster the loss of HMR results in mitotic defects, an increase in transcription of transposable elements and a deregulation of heterochromatic genes. To better understand the molecular mechanisms that mediate HMR’s function, we measured genome-wide localization of HMR in D. melanogaster tissue culture cells by chromatin immunoprecipitation. Interestingly, we find HMR localizing to genomic insulator sites that can be classified into two groups. One group belongs to gypsy insulators and another one borders HP1a bound regions at active genes. The transcription of the latter group genes is strongly affected in larvae and ovaries of Hmr mutant flies. Our data suggest a novel link between HMR and insulator proteins, a finding that implicates a potential role for genome organization in the formation of species.

Digitor/dASCIZ Has Multiple Roles in Drosophila Development

In this study we provide evidence that the spindle matrix protein Skeletor in Drosophila interacts with the human ASCIZ (also known as ATMIN and ZNF822) ortholog, Digitor/dASCIZ. This interaction was first detected in a yeast two-hybrid screen and subsequently confirmed by pull-down assays. We also confirm a previously documented function of Digitor/dASCIZ as a regulator of Dynein light chain/Cut up expression. Using transgenic expression of a mCitrine-labeled Digitor construct, we show that Digitor/dASCIZ is a nuclear protein that is localized to interband and developmental puff chromosomal regions during interphase but redistributes to the spindle region during mitosis. Its mitotic localization and physical interaction with Skeletor suggest the possibility that Digitor/dASCIZ plays a direct role in mitotic progression as a member of the spindle matrix complex. Furthermore, we have characterized a P-element insertion that is likely to be a true null Digitor/dASCIZ allele resulting in complete pupal lethality when homozygous, indicating that Digitor/dASCIZ is an essential gene. Phenotypic analysis of the mutant provided evidence that Digitor/dASCIZ plays critical roles in regulation of metamorphosis and organogenesis as well as in the DNA damage response. In the Digitor/dASCIZ null mutant larvae there was greatly elevated levels of γH2Av, indicating accumulation of DNA double-strand breaks. Furthermore, reduced levels of Digitor/dASCIZ decreased the resistance to paraquat-induced oxidative stress resulting in increased mortality in a stress test paradigm. We show that an early developmental consequence of the absence of Digitor/dASCIZ is reduced third instar larval brain size although overall larval development appeared otherwise normal at this stage. While Digitor/dASCIZ mutant larvae initiate pupation, all mutant pupae failed to eclose and exhibited various defects in metamorphosis such as impaired differentiation, incomplete disc eversion, and faulty apoptosis. Altogether we provide evidence that Digitor/dASCIZ is a nuclear protein that performs multiple roles in Drosophila larval and pupal development.

A genomic approach to identify hybrid incompatibility genes

Uncovering the genetic and molecular basis of barriers to gene flow between populations is key to understanding how new species are born. Intrinsic postzygotic reproductive barriers such as hybrid sterility and hybrid inviability are caused by deleterious genetic interactions known as hybrid incompatibilities. The difficulty in identifying these hybrid incompatibility genes remains a rate-limiting step in our understanding of the molecular basis of speciation. We recently described how whole genome sequencing can be applied to identify hybrid incompatibility genes, even from genetically terminal hybrids. Using this approach, we discovered a new hybrid incompatibility gene, gfzf, between Drosophila melanogaster and Drosophila simulans, and found that it plays an essential role in cell cycle regulation. Here, we discuss the history of the hunt for incompatibility genes between these species, discuss the molecular roles of gfzf in cell cycle regulation, and explore how intragenomic conflict drives the evolution of fundamental cellular mechanisms that lead to the developmental arrest of hybrids.

An essential cell cycle regulation gene causes hybrid inviability in Drosophila

Speciation, the process by which new biological species arise, involves the evolution of reproductive barriers, such as hybrid sterility or inviability between populations. However, identifying hybrid incompatibility genes remains a key obstacle in understanding the molecular basis of reproductive isolation. We devised a genomic screen, which identified a cell cycle-regulation gene as the cause of male inviability in hybrids resulting from a cross between Drosophila melanogaster and D. simulans. Ablation of the D. simulans allele of this gene is sufficient to rescue the adult viability of hybrid males. This dominantly acting cell cycle regulator causes mitotic arrest and, thereby, inviability of male hybrid larvae. Our genomic method provides a facile means to accelerate the identification of hybrid incompatibility genes in other model and nonmodel systems.

Limited gene misregulation is exacerbated by allele-specific upregulation in lethal hybrids between Drosophila melanogaster and Drosophila simulans

Misregulation of gene expression is often observed in interspecific hybrids and is generally attributed to regulatory incompatibilities caused by divergence between the two genomes. However, it has been challenging to distinguish effects of regulatory divergence from secondary effects including developmental and physiological defects common to hybrids. Here, we use RNA-Seq to profile gene expression in F1 hybrid male larvae from crosses of Drosophila melanogaster to its sibling species D. simulans. We analyze lethal and viable hybrid males, the latter produced using a mutation in the X-linked D. melanogaster Hybrid male rescue (Hmr) gene and compare them with their parental species and to public data sets of gene expression across development. We find that Hmr has drastically different effects on the parental and hybrid genomes, demonstrating that hybrid incompatibility genes can exhibit novel properties in the hybrid genetic background. Additionally, we find that D. melanogaster alleles are preferentially affected between lethal and viable hybrids. We further determine that many of the differences between the hybrids result from developmental delay in the Hmr(+) hybrids. Finally, we find surprisingly modest expression differences in hybrids when compared with the parents, with only 9% and 4% of genes deviating from additivity or expressed outside of the parental range, respectively. Most of these differences can be attributed to developmental delay and differences in tissue types. Overall, our study suggests that hybrid gene misexpression is prone to overestimation and that even between species separated by approximately 2.5 Ma, regulatory incompatibilities are not widespread in hybrids.

Fine mapping of dominant X-linked incompatibility alleles in Drosophila hybrids

Sex chromosomes have a large effect on reproductive isolation and play an important role in hybrid inviability. In Drosophila hybrids, X-linked genes have pronounced deleterious effects on fitness in male hybrids, which have only one X chromosome. Several studies have succeeded at locating and identifying recessive X-linked alleles involved in hybrid inviability. Nonetheless, the density of dominant X-linked alleles involved in interspecific hybrid viability remains largely unknown. In this report, we study the effects of a panel of small fragments of the D. melanogaster X-chromosome carried on the D. melanogaster Y-chromosome in three kinds of hybrid males: D. melanogaster/D. santomea, D. melanogaster/D. simulans and D. melanogaster/D. mauritiana. D. santomea and D. melanogaster diverged over 10 million years ago, while D. simulans (and D. mauritiana) diverged from D. melanogaster over 3 million years ago. We find that the X-chromosome from D. melanogaster carries dominant alleles that are lethal in mel/san, mel/sim, and mel/mau hybrids, and more of these alleles are revealed in the most divergent cross. We then compare these effects on hybrid viability with two D. melanogaster intraspecific crosses. Unlike the interspecific crosses, we found no X-linked alleles that cause lethality in intraspecific crosses. Our results reveal the existence of dominant alleles on the X-chromosome of D. melanogaster which cause lethality in three different interspecific hybrids. These alleles only cause inviability in hybrid males, yet have little effect in hybrid females. This suggests that X-linked elements that cause hybrid inviability in males might not do so in hybrid females due to differing sex chromosome interactions.

Allelic asymmetry of the Lethal hybrid rescue (Lhr) gene expression in the hybrid between Drosophila melanogaster and D. simulans: confirmation by using genetic variations of D. melanogaster

In the cross between Drosophila melanogaster females and D. simulans males, hybrid males die at the late larval stage, and the sibling females also die at later stages at high temperatures. Removing the D. simulans allele of the Lethal hybrid rescue gene (Lhr (sim) ) improves the hybrid incompatibility phenotypes. However, the loss-of-function mutation of Lhr (sim) (Lhr (sim0) ) does not rescue the hybrid males in crosses with several D. melanogaster strains. We first describe the genetic factor possessed by the D. melanogaster strains. It has been suggested that removing the D. melanogaster allele of Lhr (Lhr (mel) ), that is Lhr (mel0) , does not have the hybrid male rescue effect, contrasting to Lhr (sim0) . Because the expression level of the Lhr gene is known to be Lhr (sim) > Lhr (mel) in the hybrid, Lhr (mel0) may not lead to enough of a reduction in total Lhr expression. Then, there is a possibility that the D. melanogaster factor changes the expression level to Lhr (sim) < Lhr (mel) . But in fact, the expression level was Lhr (sim) > Lhr (mel) in the hybrid irrespectively of the presence of the factor. At last, we showed that Lhr (mel0) slightly improves the viability of hybrid females, which was not realized previously. All of the present results are consistent with the allelic asymmetry model of the Lhr gene expression in the hybrid.

A pair of centromeric proteins mediates reproductive isolation in Drosophila species

Speciation involves the reproductive isolation of natural populations due to the sterility or lethality of their hybrids. However, the molecular basis of hybrid lethality and the evolutionary driving forces that provoke it remain largely elusive. The hybrid male rescue (Hmr) and the lethal hybrid rescue (Lhr) genes serve as a model to study speciation in Drosophilids because their interaction causes lethality in male hybrid offspring. Here, we show that HMR and LHR form a centromeric complex necessary for proper chromosome segregation. We find that the Hmr expression level is substantially higher in Drosophila melanogaster, whereas Lhr expression levels are increased in Drosophila simulans. The resulting elevated amount of HMR/LHR complex in hybrids results in an extensive mislocalization of the complex, an interference with the regulation of transposable elements, and an impairment of cell proliferation. Our findings provide evidence for a major role of centromere divergence in the generation of biodiversity.

Embryonic lethality leads to hybrid male inviability in hybrids between Drosophila melanogaster and D. santomea

The study of the morphological defects unique to interspecific hybrids can reveal which developmental pathways have diverged between species. Drosophila melanogaster and D. santomea diverged more than 10 million years ago, and when crossed produce sterile adult females. Adult hybrid males are absent from all interspecific crosses. We aimed to determine the fate of these hybrid males. To do so, we tracked the development of hybrid females and males using classic genetic markers and techniques. We found that hybrid males die predominantly as embryos with severe segment-specification defects while a large proportion of hybrid females embryos hatch and survive to adulthood. In particular, we show that most male embryos show a characteristic abdominal ablation phenotype, not observed in either parental species. This suggests that sex-specific embryonic developmental defects eliminate hybrid males in this interspecific cross. The study of the developmental abnormalities that occur in hybrids can lead to the understanding of cryptic molecular divergence between species sharing a conserved body plan.

Genetic dissection of Nucleoporin 160 (Nup160), a gene involved in multiple phenotypes of reproductive isolation in Drosophila

Previous reports have suggested that the Nucleoporin 160 (Nup160) gene of Drosophila simulans (Nup160(sim)) causes the hybrid inviability, female sterility, and morphological anomalies that are observed in crosses with D. melanogaster. Here we have confirmed this observation by transposon excision from the P{EP}Nup160(EP372) insertion mutation of D. melanogaster. Null mutations of the Nup160 gene resulted in the three phenotypes caused by Nup160(sim), but revertants of the gene did not. Interestingly, several mutations produced by excision partially complemented hybrid inviability, female sterility, or morphological anomalies. In the future, these mutations will be useful to further our understanding of the developmental mechanisms of reproductive isolation. Based on our analyses with the Nup160(sim) introgression line, the lethal phase of hybrid inviability was determined to be during the early pupal stage. Our analysis also suggested that homozygous Nup160(sim) in D. melanogaster leads to slow development. Thus, Nup160(sim) is involved in multiple aspects of reproductive isolation between these two species.

Cis-by-Trans regulatory divergence causes the asymmetric lethal effects of an ancestral hybrid incompatibility gene

The Dobzhansky and Muller (D-M) model explains the evolution of hybrid incompatibility (HI) through the interaction between lineage-specific derived alleles at two or more loci. In agreement with the expectation that HI results from functional divergence, many protein-coding genes that contribute to incompatibilities between species show signatures of adaptive evolution, including Lhr, which encodes a heterochromatin protein whose amino acid sequence has diverged extensively between Drosophila melanogaster and D. simulans by natural selection. The lethality of D. melanogaster/D. simulans F1 hybrid sons is rescued by removing D. simulans Lhr, but not D. melanogaster Lhr, suggesting that the lethal effect results from adaptive evolution in the D. simulans lineage. It has been proposed that adaptive protein divergence in Lhr reflects antagonistic coevolution with species-specific heterochromatin sequences and that defects in LHR protein localization cause hybrid lethality. Here we present surprising results that are inconsistent with this coding-sequence-based model. Using Lhr transgenes expressed under native conditions, we find no evidence that LHR localization differs between D. melanogaster and D. simulans, nor do we find evidence that it mislocalizes in their interspecific hybrids. Rather, we demonstrate that Lhr orthologs are differentially expressed in the hybrid background, with the levels of D. simulans Lhr double that of D. melanogaster Lhr. We further show that this asymmetric expression is caused by cis-by-trans regulatory divergence of Lhr. Therefore, the non-equivalent hybrid lethal effects of Lhr orthologs can be explained by asymmetric expression of a molecular function that is shared by both orthologs and thus was presumably inherited from the ancestral allele of Lhr. We present a model whereby hybrid lethality occurs by the interaction between evolutionarily ancestral and derived alleles.

When two different species mate, the hybrid progeny are often sterile or lethal. Such hybrid incompatibilities cause reproductive isolation between species and are an important mechanism for maintaining species as separate units. A gene called Lethal hybrid rescue (Lhr) is part of the cause of hybrid lethality between Drosophila species. Like many other hybrid incompatibility genes, Lhr protein sequences in the hybridizing species have diverged from one another by natural selection. This and other findings led to the hypotheses that the function of Lhr has changed between the two species, and this is what makes Lhr a hybrid lethality gene. Using a series of genetic, molecular, and cytological assays, we report evidence contrary to these hypotheses, that hybrid lethal activity is instead a function shared by both species and inherited from their common ancestor. This result is particularly surprising because the Lhr genes from the two species have different effects on hybrid viability. We discovered that these differential effects are caused by differences in expression levels of Lhr in hybrids rather than by changes in its protein-coding sequence. Our results demonstrate that, while natural selection may be important in evolving hybrid incompatibilities, how it does so in this case remains mysterious.

Genetic testing of the hypothesis that hybrid male lethality results from a failure in dosage compensation

Several recent studies have suggested that F(1) hybrid male lethality in crosses between Drosophila melanogaster and D. simulans is due to a failure in dosage compensation, caused by incompatibilities between D. simulans dosage compensation proteins and the D. melanogaster X chromosome. Contrary to the predictions of this hypothesis, mutations in four essential D. melanogaster dosage compensation genes are shown here to moderately increase rather than decrease hybrid male viability.

Reduced fertility of Drosophila melanogaster hybrid male rescue (Hmr) mutant females is partially complemented by Hmr orthologs from sibling species

The gene Hybrid male rescue (Hmr) causes lethality in interspecific hybrids between Drosophila melanogaster and its sibling species. Hmr has functionally diverged for this interspecific phenotype because lethality is caused specifically by D. melanogaster Hmr but not by D. simulans or D. mauritiana Hmr. Hmr was identified by the D. melanogaster partial loss-of-function allele Hmr1, which suppresses hybrid lethality but has no apparent phenotype within pure-species D. melanogaster. Here we have investigated the possible function of Hmr in D. melanogaster females using stronger mutant alleles. Females homozygous for Hmr mutants have reduced viability posteclosion and significantly reduced fertility. We find that reduced fertility of Hmr mutants is caused by a reduction in the number of eggs laid as well as reduced zygotic viability. Cytological analysis reveals that ovarioles from Hmr mutant females express markers that distinguish various stages of wild-type oogenesis, but that developing egg chambers fail to migrate posteriorly. D. simulans and D. mauritiana Hmr+ partially complement the reduced fertility of a D. melanogaster Hmr mutation. This partial complementation contrasts with the complete functional divergence previously observed for the interspecific hybrid lethality phenotype. We also investigate here the molecular basis of hybrid rescue associated with a second D. melanogaster hybrid rescue allele, In(1)AB. We show that In(1)AB is mutant for Hmr function, likely due to a missense mutation in an evolutionarily conserved amino acid. Two independently discovered hybrid rescue mutations are therefore allelic.

Positive selection at the binding sites of the male-specific lethal complex involved in dosage compensation in Drosophila

In many taxa, males and females differ with respect to their sex chromosomes, and dosage compensation mechanisms have evolved to equalize X-linked gene transcription. In Drosophila, the male-specific lethal (MSL) complex binds to hundreds of sites along the male X chromosome and mediates twofold hypertranscription of the single male X. Two recent studies found evidence for lineage-specific adaptive evolution in all five core protein-coding genes of the MSL complex in Drosophila melanogaster. In particular, dramatic positive selection was detected in domains shown to be responsible for their specific targeting to the X chromosome. Here I use population genetics to show that three previously characterized MSL-binding DNA segments on the X themselves underwent adaptive evolution in D. melanogaster, but not in its close relatives D. simulans and D. yakuba. MSL components have been shown to not correctly target the D. melanogaster X chromosome in hybrids between D. melanogaster and D. simulans. My finding supports the idea of selection-driven coevolution among DNA-protein interactions of the dosage compensation machinery and suggests that misregulated dosage compensation could contribute to male hybrid inviability in Drosophila.