Transition In Drosophila Research Articles

Evolution of chemosensory and detoxification gene families across herbivorous Drosophilidae.

Herbivorous insects are exceptionally diverse, accounting for a quarter of all known eukaryotic species, but the genomic basis of adaptations that enabled this dietary transition remains poorly understood. Many studies have suggested that expansions and contractions of chemosensory and detoxification gene families-genes directly mediating interactions with plant chemical defenses-underlie successful plant colonization. However, this hypothesis has been challenging to test because the origins of herbivory in many insect lineages are ancient (>150 million years ago (mya)), obscuring genomic evolutionary patterns. Here, we characterized chemosensory and detoxification gene family evolution across Scaptomyza, a genus nested within Drosophila that includes a recently derived (<15 mya) herbivore lineage of mustard (Brassicales) specialists and carnation (Caryophyllaceae) specialists, and several nonherbivorous species. Comparative genomic analyses revealed that herbivorous Scaptomyza has among the smallest chemosensory and detoxification gene repertoires across 12 drosophilid species surveyed. Rates of gene turnover averaged across the herbivore clade were significantly higher than background rates in over half of the surveyed gene families. However, gene turnover was more limited along the ancestral herbivore branch, with only gustatory receptors and odorant-binding proteins experiencing strong losses. The genes most significantly impacted by gene loss, duplication, or changes in selective constraint were those involved in detecting compounds associated with feeding on living plants (bitter or electrophilic phytotoxins) or their ancestral diet (fermenting plant volatiles). These results provide insight into the molecular and evolutionary mechanisms of plant-feeding adaptations and highlight gene candidates that have also been linked to other dietary transitions in Drosophila.

Ratiometric sensing of Pnt and Yan transcription factor levels confers ultrasensitivity to photoreceptor fate transitions in Drosophila.

Cell state transitions are often triggered by large changes in the concentrations of transcription factors and therefore large differences in their stoichiometric ratios. Whether cells can elicit transitions using modest changes in the ratios of co-expressed factors is unclear. Here, we investigate how cells in the Drosophila eye resolve state transitions by quantifying the expression dynamics of the ETS transcription factors Pnt and Yan. Eye progenitor cells maintain a relatively constant ratio of Pnt/Yan protein, despite expressing both proteins with pulsatile dynamics. A rapid and sustained twofold increase in the Pnt/Yan ratio accompanies transitions to photoreceptor fates. Genetic perturbations that modestly disrupt the Pnt/Yan ratio produce fate transition defects consistent with the hypothesis that transitions are normally driven by a twofold shift in the ratio. A biophysical model based on cooperative Yan-DNA binding coupled with non-cooperative Pnt-DNA binding illustrates how twofold ratio changes could generate ultrasensitive changes in target gene transcription to drive fate transitions. Thus, coupling cell state transitions to the Pnt/Yan ratio sensitizes the system to modest fold-changes, conferring robustness and ultrasensitivity to the developmental program.

Su(var)2-10- and Su(var)205-dependent upregulation of the heterochromatic gene neverland is required for developmental transition in Drosophila.

Animals develop from juveniles to sexually mature adults through the action of steroid hormones. In insect metamorphosis, a surge of the steroid hormone ecdysone prompts the transition from the larval to the adult stage. Ecdysone is synthesized by a series of biosynthetic enzymes that are specifically expressed in an endocrine organ, the prothoracic gland. At the late larval stage, the expression levels of ecdysone biosynthetic enzymes are upregulated through the action of numerous transcription factors, thus initiating metamorphosis. In contrast, the mechanism by which chromatin regulators support the expression of ecdysone biosynthetic genes is largely unknown. Here, we demonstrate that Su(var)2-10 and Su(var)205, suppressor of variegation [Su(var)] genes encoding a chromatin regulator Su(var)2-10 and nonhistone heterochromatic protein 1a, respectively, regulate the transcription of one of the heterochromatic ecdysone biosynthetic genes, neverland, in Drosophila melanogaster. Knockdown of Su(var)2-10 and Su(var)205 in the prothoracic gland caused a decrease in neverland expression, resulting in a defect in larval-to-prepupal transition. Furthermore, overexpression of neverland and administration of 7-dehydrocholesterol, a biosynthetic precursor of ecdysone produced by Neverland, rescued developmental defects in Su(var)2-10 and Su(var)205 knockdown animals. These results indicate that Su(var)2-10- and Su(var)205-mediated proper expression of neverland is required for the initiation of metamorphosis. Given that Su(var)2-10-positive puncta are juxtaposed with the pericentromeric heterochromatic region, we propose that Su(var)2-10- and Su(var)205-dependent regulation of inherent heterochromatin structure at the neverland gene locus is essential for its transcriptional activation.

The developmentally timed decay of an essential microRNA family is seed-sequence dependent.

SUMMARYMicroRNA (miRNA) abundance is tightly controlled by regulation of biogenesis and decay. Here, we show that the mir-35 miRNA family undergoes selective decay at the transition from embryonic to larval development in C. elegans. The seed sequence of the miRNA is necessary and largely sufficient for this regulation. Sequences outside the seed (3′ end) regulate mir-35 abundance in the embryo but are not necessary for sharp decay at the transition to larval development. Enzymatic modifications of the miRNA 3′ end are neither prevalent nor correlated with changes in decay, suggesting that miRNA 3′ end display is not a core feature of this mechanism and further supporting a seed-driven decay model. Our findings demonstrate that seed-sequence-specific decay can selectively and coherently regulate all redundant members of a miRNA seed family, a class of mechanism that has great biological and therapeutic potential for dynamic regulation of a miRNA family’s target repertoire.

Cell-type-specific chromatin occupancy by the pioneer factor Zelda drives key developmental transitions in Drosophila

During Drosophila embryogenesis, the essential pioneer factor Zelda defines hundreds of cis-regulatory regions and in doing so reprograms the zygotic transcriptome. While Zelda is essential later in development, it is unclear how the ability of Zelda to define cis-regulatory regions is shaped by cell-type-specific chromatin architecture. Asymmetric division of neural stem cells (neuroblasts) in the fly brain provide an excellent paradigm for investigating the cell-type-specific functions of this pioneer factor. We show that Zelda synergistically functions with Notch to maintain neuroblasts in an undifferentiated state. Zelda misexpression reprograms progenitor cells to neuroblasts, but this capacity is limited by transcriptional repressors critical for progenitor commitment. Zelda genomic occupancy in neuroblasts is reorganized as compared to the embryo, and this reorganization is correlated with differences in chromatin accessibility and cofactor availability. We propose that Zelda regulates essential transitions in the neuroblasts and embryo through a shared gene-regulatory network driven by cell-type-specific enhancers.

RNA degradation is required for the germ-cell to maternal transition in Drosophila

In sexually reproducing animals, the oocyte contributes a large supply of RNAs that are essential to launch development upon fertilization. The mechanisms that regulate the composition of the maternal RNA contribution during oogenesis are unclear. Here, we show that a subset of RNAs expressed during the early stages of oogenesis is subjected to regulated degradation during oocyte specification. Failure to remove these RNAs results in oocyte dysfunction and death. We identify the RNA-degrading Super Killer complex and No-Go Decay factor Pelota as key regulators of oogenesis via targeted degradation of specific RNAs expressed in undifferentiated germ cells. These regulators target RNAs enriched for cytidine sequences that are bound by the polypyrimidine tract binding protein Half pint. Thus, RNA degradation helps orchestrate a germ cell-to-maternal transition that gives rise to the maternal contribution to the zygote.

Maintenance of Cell Fate by the Polycomb Group Gene Sex Combs Extra Enables a Partial Epithelial Mesenchymal Transition in Drosophila.

Epigenetic silencing by Polycomb group (PcG) complexes can promote epithelial-mesenchymal transition (EMT) and stemness and is associated with malignancy of solid cancers. Here we report a role for Drosophila PcG repression in a partial EMT event that occurs during wing disc eversion, an early event during metamorphosis. In a screen for genes required for eversion we identified the PcG genes Sex combs extra (Sce) and Sex combs midleg (Scm). Depletion of Sce or Scm resulted in internalized wings and thoracic clefts, and loss of Sce inhibited the EMT of the peripodial epithelium and basement membrane breakdown, ex vivo. Targeted DamID (TaDa) using Dam-Pol II showed that Sce knockdown caused a genomic transcriptional response consistent with a shift toward a more stable epithelial fate. Surprisingly only 17 genes were significantly upregulated in Sce-depleted cells, including Abd-B, abd-A, caudal, and nubbin. Each of these loci were enriched for Dam-Pc binding. Of the four genes, only Abd-B was robustly upregulated in cells lacking Sce expression. RNAi knockdown of all four genes could partly suppress the Sce RNAi eversion phenotype, though Abd-B had the strongest effect. Our results suggest that in the absence of continued PcG repression peripodial cells express genes such as Abd-B, which promote epithelial state and thereby disrupt eversion. Our results emphasize the important role that PcG suppression can play in maintaining cell states required for morphogenetic events throughout development and suggest that PcG repression of Hox genes may affect epithelial traits that could contribute to metastasis.

The steroid-induced microRNA let-7 regulates developmental growth by targeting cdc7 in the Drosophila fat body.

In insects, 20-hydroxyecdysone (20E) limits systemic growth by triggering developmental transitions. Previous studies have shown that 20E-induced let-7 exhibits crosstalk with the cell cycle. Here, we examined the underlying molecular mechanisms and physiological functions of 20E-induced let-7 in the fat body, an organ for energy storage and nutrient mobilization which plays a critical role in the larval growth. First, the overexpression of let-7 decreased the body size and led to the reduction of both nucleolus and cell sizes in the larval fat body. In contrast, the overexpression of let-7-Sponge increased the nucleolus and cell sizes. Moreover, we found that cdc7, encoding a conserved protein kinase that controls the endocycle, is a target of let-7. Notably, the mutation of cdc7 in the fat body resulted in growth defects. Overall, our findings revealed a novel role of let-7 in the control of endoreduplication-related growth during larval-prepupal transition in Drosophila.

All-Atomic Molecular Dynamic Studies of Human and Drosophila CDK8: Insights into Their Kinase Domains, the LXXLL Motifs, and Drug Binding Site.

Cyclin-dependent kinase 8 (CDK8) and its regulatory partner Cyclin C (CycC) play conserved roles in modulating RNA polymerase II (Pol II)-dependent gene expression. To understand the structure and function relations of CDK8, we analyzed the structures of human and Drosophila CDK8 proteins using molecular dynamics simulations, combined with functional analyses in Drosophila. Specifically, we evaluated the structural differences between hCDK8 and dCDK8 to predict the effects of the LXXLL motif mutation (AQKAA), the P154L mutations, and drug binding on local structures of the CDK8 proteins. First, we have observed that both the LXXLL motif and the kinase activity of CDK8 are required for the normal larval-to-pupal transition in Drosophila. Second, our molecular dynamic analyses have revealed that hCDK8 has higher hydrogen bond occupation of His149-Asp151 and Asp151-Asn156 than dCDK8. Third, the substructure of Asp282, Phe283, Arg285, Thr287 and Cys291 can distinguish human and Drosophila CDK8 structures. In addition, there are two hydrogen bonds in the LXXLL motif: a lower occupation between L312 and L315, and a relatively higher occupation between L312 and L316. Human CDK8 has higher hydrogen bond occupation between L312 and L316 than dCDK8. Moreover, L312, L315 and L316 in the LXXLL motif of CDK8 have the specific pattern of hydrogen bonds and geometries, which could be crucial for the binding to nuclear receptors. Furthermore, the P154L mutation dramatically decreases the hydrogen bond between L312 and L315 in hCDK8, but not in dCDK8. The mutations of P154L and AQKAA modestly alter the local structures around residues 154. Finally, we identified the inhibitor-induced conformational changes of hCDK8, and our results suggest a structural difference in the drug-binding site between hCDK8 and dCDK8. Taken together, these results provide the structural insights into the roles of the LXXLL motif and the kinase activity of CDK8 in vivo.

Feedforward Regulation of Glucose Metabolism by Steroid Hormones Drives a Developmental Transition in Drosophila

Metabolism must be coupled with developmental transition to fulfill the energy requirements during an organism's life cycle. In most animals, steroid hormones are crucial regulators of life-stage transitions until adulthood. In the fruit fly Drosophila, ecdysteroid titers drive developmental transitions, such as molting and metamorphosis [1-4]. Although the timings of molting and larval-pupal transition are dependent on environmental and internal conditions, metamorphosis is a temporally controlled life transition event that solely relies on energetic macromolecules accumulated during the larval period. The ecdysteroid signaling cascade and the energetics of metamorphosis have been determined [5-8]. However, the molecular mechanisms that regulate metabolic homeostasis during metamorphosis remain largely unknown. Here, I show that the programmed regulation of carbohydrate metabolism by steroid hormones directs the prepupal-pupal transition in Drosophila. I found that pupation is associated with a transient increase in glucose oxidation. Mechanistically, after pupariation, ecdysteroid signaling and the competence factor Ftz-F1 regulates the systematic degradation of circulating trehalose via the transient induction of trehalose transporters and trehalase (Treh) in a timely manner. Trehalose metabolism is crucial for energy homeostasis at the prepupal-pupal transition. Moreover, trehalose catabolism acts upstream of ecdysteroid biosynthesis and signaling. My findings lead to the hypothesis that trehalose breakdown not only defines energy costs by providing a carbon source but also facilitates pupation by inducing water loss in the puparium. My work sheds light on the ways in which a life-stage transition is driven by the preprogrammed coordination between steroid hormones and catabolism of stored nutrients.

The Drosophila nuclear receptors EcR and ERR jointly regulate the expression of genes involved in carbohydrate metabolism

The rate of carbohydrate metabolism is tightly coordinated with developmental transitions in Drosophila, and fluctuates depending on the requirements of a particular developmental stage. These successive metabolic switches result from changes in the expression levels of genes encoding glycolytic, tricarboxylic acid cycle (TCA), and oxidative phosphorylation enzymes. In this report, we describe a repressive action of ecdysone signaling on the expression of glycolytic genes and enzymes of glycogen metabolism in Drosophila development. The basis of this effect is an interaction between the ecdysone receptor (EcR) and the estrogen-related receptor (ERR), a specific regulator of the Drosophila glycolysis. We found an overlapping DNA-binding pattern for the EcR and ERR in the Drosophila S2 cells. EcR was detected at a subset of the ERR target genes responsible for carbohydrate metabolism. The 20-hydroxyecdysone treatment of both the Drosophila larvae and the S2 cells decreased transcriptional levels of ERR targets. We propose a joint action mode for both the EcR and ERR, for at least a subset of the glycolytic genes. We find that both receptors bind to the same regulatory regions and may form or be part of a joint transcriptional regulatory complex in the Drosophila S2 cells.

CDK8 mediates the dietary effects on developmental transition in Drosophila

The complex interplay between genetic and environmental factors, such as diet and lifestyle, defines the initiation and progression of multifactorial diseases, including cancer, cardiovascular and metabolic diseases, and neurological disorders. Given that most of the studies have been performed in controlled experimental settings to ensure the consistency and reproducibility, the impacts of environmental factors, such as dietary perturbation, on the development of animals with different genotypes and the pathogenesis of these diseases remain poorly understood. By analyzing the cdk8 and cyclin C (cycC) mutant larvae in Drosophila, we have previously reported that the CDK8-CycC complex coordinately regulates lipogenesis by repressing dSREBP (sterol regulatory element-binding protein)-activated transcription and developmental timing by activating EcR (ecdysone receptor)-dependent gene expression. Here we report that dietary nutrients, particularly proteins and carbohydrates, modulate the developmental timing through the CDK8/CycC/EcR pathway. We observed that cdk8 and cycC mutants are sensitive to the levels of dietary proteins and seven amino acids (arginine, glutamine, isoleucine, leucine, methionine, threonine, and valine). Those mutants are also sensitive to dietary carbohydrates, and they are more sensitive to monosaccharides than disaccharides. These results suggest that CDK8-CycC mediates the dietary effects on lipid metabolism and developmental timing in Drosophila larvae.

Drosophila Caliban mediates G1-S transition and ionizing radiation induced S phase checkpoint

ABSTRACTCell cycle progression is precisely regulated by diverse extrinsic and intrinsic cellular factors. Understanding the underlying mechanisms of cell cycle regulation is essential to address how normal development and tissue homeostasis are achieved. Here, we present a novel cell cycle regulator Caliban (Clbn), the Drosophila ortholog of human Serologically defined colon cancer antigen 1 (SDCCAG1) gene. We show that ionizing radiation induces expression of clbn, and over-expression of clbn blocks G1-to-S cell cycle transition in Drosophila, while flies loss of clbn have defective S phase checkpoint in response to irradiation. Mechanistically, induced expression of clbn suppressed E2F1 activity and down-regulates the DNA replication and expression of its downstream target cyclin E, a key regulator of G1-to-S transition. Meanwhile, clbn over-expression leads to upregulation of the CDK inhibitor Dacapo (Dap), and upregulated Dap is decreased when e2f1 is over-expressed. Furthermore, expression of clbn is down-regulated in cells with e2f1 over-expression or rbf1 knockdown, indicating that Clbn and E2F1 act antagonistically in mediating G1-to-S transition. Thus we provide genetic evidence that Clbn works together with E2F1 in regulating cell cycle progression, and Clbn is required for S phase cell cycle checkpoint in response to DNA damage.

Antagonistic actions of juvenile hormone and 20-hydroxyecdysone within the ring gland determine developmental transitions in Drosophila

In both vertebrates and insects, developmental transition from the juvenile stage to adulthood is regulated by steroid hormones. In insects, the steroid hormone, 20-hydroxyecdysone (20E), elicits metamorphosis, thus promoting this transition, while the sesquiterpenoid juvenile hormone (JH) antagonizes 20E signaling to prevent precocious metamorphosis during the larval stages. However, not much is known about the mechanisms involved in cross-talk between these two hormones. In this study, we discovered that in the ring gland (RG) of Drosophila larvae, JH and 20E control each other's biosynthesis. JH induces expression of a Krüppel-like transcription factor gene Kr-h1 in the prothoracic gland (PG), a portion of the RG that produces the 20E precursor ecdysone. By reducing both steroidogenesis autoregulation and PG size, high levels of Kr-h1 in the PG inhibit ecdysteriod biosynthesis, thus maintaining juvenile status. JH biosynthesis is prevented by 20E in the corpus allatum, the other portion of the RG that produces JH, to ensure the occurrence of metamorphosis. Hence, antagonistic actions of JH and 20E within the RG determine developmental transitions in Drosophila Our study proposes a mechanism of cross-talk between the two major hormones in the regulation of insect metamorphosis.

The function of Drosophila larval class IV dendritic arborization sensory neurons in the larval-pupal transition is separable from their function in mechanical nociception responses.

The sensory and physiological inputs which govern the larval-pupal transition in Drosophila, and the neuronal circuity that integrates them, are complex. Previous work from our laboratory identified a dosage-sensitive genetic interaction between the genes encoding the Rho-GEF Trio and the zinc-finger transcription factor Sequoia that interfered with the larval-pupal transition. Specifically, we reported heterozygous mutations in sequoia (seq) dominantly exacerbated the trio mutant phenotype, and this seq-enhanced trio mutant genotype blocked the transition of third instar larvae from foragers to wanderers, a requisite behavioral transition prior to pupation. In this work, we use the GAL4-UAS system to rescue this phenotype by tissue-specific trio expression. We find that expressing trio in the class IV dendritic arborization (da) sensory neurons rescues the larval-pupal transition, demonstrating the reliance of the larval-pupal transition on the integrity of these sensory neurons. As nociceptive responses also rely on the functionality of the class IV da neurons, we test mechanical nociceptive responses in our mutant and rescued larvae and find that mechanical nociception is separable from the ability to undergo the larval-pupal transition. This demonstrates for the first time that the roles of the class IV da neurons in governing two critical larval behaviors, the larval-pupal transition and mechanical nociception, are functionally separable from each other.

Global changes of the RNA-bound proteome during the maternal-to-zygotic transition in Drosophila.

The maternal-to-zygotic transition (MZT) is a process that occurs in animal embryos at the earliest developmental stages, during which maternally deposited mRNAs and other molecules are degraded and replaced by products of the zygotic genome. The zygotic genome is not activated immediately upon fertilization, and in the pre-MZT embryo post-transcriptional control by RNA-binding proteins (RBPs) orchestrates the first steps of development. To identify relevant Drosophila RBPs organism-wide, we refined the RNA interactome capture method for comparative analysis of the pre- and post-MZT embryos. We determine 523 proteins as high-confidence RBPs, half of which were not previously reported to bind RNA. Comparison of the RNA interactomes of pre- and post-MZT embryos reveals high dynamicity of the RNA-bound proteome during early development, and suggests active regulation of RNA binding of some RBPs. This resource provides unprecedented insight into the system of RBPs that govern the earliest steps of Drosophila development.

Regulation and Function of Maternal Gene Products During the Maternal-to-Zygotic Transition in Drosophila.

Drosophila late-stage oocytes and early embryos are transcriptionally silent. Thus, control of gene expression during these developmental periods is posttranscriptional and posttranslational. Global changes in the transcriptome and proteome occur during oocyte maturation, after egg activation and fertilization, and upon zygotic genome activation. We review the scale, content, and dynamics of these global changes; the factors that regulate these changes; and the mechanisms by which they are accomplished. We highlight the intimate relationship between the clearance of maternal gene products and the activation of the embryo's own genome, and discuss the fact that each of these complementary components of the maternal-to-zygotic transition can be subdivided into several phases that serve different biological roles and are regulated by distinct factors.

Widespread Changes in the Posttranscriptional Landscape at the Drosophila Oocyte-to-Embryo Transition

The oocyte-to-embryo transition marks the onset ofdevelopment. The initial phase of this profound change from the differentiated oocyte to the totipotent embryo occurs in the absence of both transcription and mRNA degradation. Here we combine global polysome profiling, ribosome-footprint profiling, and quantitative mass spectrometry in a comprehensive approach to delineate the translational and proteomic changes that occur during this important transition in Drosophila. Our results show that PNG kinase is a critical regulator of the extensive changes in the translatome, acting uniquely at this developmental window. Analysis of the proteome in png mutants provided insights into the contributions of translation to changes in protein levels, revealing a compensatory dynamic between translation and protein turnover during proteome remodeling at the return to totipotency. The proteome changes additionally suggested regulators of meiosis and early embryogenesis, including the conserved H3K4 demethylase LID, which we demonstrated is required during this period despite transcriptional inactivity.

H3K79 methylation: a new conserved mark that accompanies H4 hyperacetylation prior to histone-to-protamine transition in Drosophila and rat.

ABSTRACTDuring spermiogenesis, haploid spermatids undergo extensive chromatin remodeling events in which histones are successively replaced by more basic protamines to generate highly compacted chromatin. Here we show for the first time that H3K79 methylation is a conserved feature preceding the histone-to-protamine transition in Drosophila melanogaster and rat. During Drosophila spermatogenesis, the Dot1-like methyltransferase Grappa (Gpp) is primarily expressed in canoe stage nuclei. The corresponding H3K79 methylation is a histone modification that precedes the histone-to-protamine transition and correlates with histone H4 hyperacetylation. When acetylation was inhibited in cultured Drosophila testes, nuclei were smaller and chromatin was compact, Gpp was little synthesized, H3K79 methylation was strongly reduced, and protamines were not synthesized. The Gpp isoform Gpp-D has a unique C-terminus, and Gpp is essential for full fertility. In rat, H3K79 methylation also correlates with H4 hyperacetylation but not with active RNA polymerase II, which might point towards a conserved function in chromatin remodeling during the histone-to-protamine transition in both Drosophila and rat.

INO80-dependent regression of ecdysone-induced transcriptional responses regulates developmental timing in Drosophila

Sequential pulses of the steroid hormone ecdysone regulate the major developmental transitions in Drosophila, and the duration of each developmental stage is determined by the length of time between ecdysone pulses. Ecdysone regulates biological responses by directly initiating target gene transcription. In turn, these transcriptional responses are known to be self-limiting, with mechanisms in place to ensure regression of hormone-dependent transcription. However, the biological significance of these transcriptional repression mechanisms remains unclear. Here we show that the chromatin remodeling protein INO80 facilitates transcriptional repression of ecdysone-regulated genes during prepupal development. In ino80 mutant animals, inefficient repression of transcriptional responses to the late larval ecdysone pulse delays the onset of the subsequent prepupal ecdysone pulse, resulting in a significantly longer prepupal stage. Conversely, increased expression of ino80 is sufficient to shorten the prepupal stage by increasing the rate of transcriptional repression. Furthermore, we demonstrate that enhancing the rate of regression of the mid-prepupal competence factor βFTZ-F1 is sufficient to determine the timing of head eversion and thus the duration of prepupal development. Although ino80 is conserved from yeast to humans, this study represents the first characterization of a bona fide ino80 mutation in any metazoan, raising the possibility that the functions of ino80 in transcriptional repression and developmental timing are evolutionarily conserved.