OVO transcription factors function antagonistically in the Drosophila female germline

OVO Positively Regulates Essential Maternal Pathways by Binding Near the Transcriptional Start Sites in the Drosophila Female Germline

Differentiation of female germline stem cells into a mature oocyte includes the expression of RNAs and proteins that drive early embryonic development in Drosophila. We have little insight into what activates the expression of these maternal factors. One candidate is the zinc-finger protein OVO. OVO is required for female germline viability and has been shown to positively regulate its own expression, as well as a downstream target, ovarian tumor, by binding to the transcriptional start site (TSS). To find additional OVO targets in the female germline and further elucidate OVO's role in oocyte development, we performed ChIP-seq to determine genome-wide OVO occupancy, as well as RNA-seq comparing hypomorphic and wild type rescue ovo alleles. OVO preferentially binds in close proximity to target TSSs genome-wide, is associated with open chromatin, transcriptionally active histone marks, and OVO-dependent expression. Motif enrichment analysis on OVO ChIP peaks identified a 5'-TAACNGT-3' OVO DNA binding motif spatially enriched near TSSs. However, the OVO DNA binding motif does not exhibit precise motif spacing relative to the TSS characteristic of RNA Polymerase II complex binding core promoter elements. Integrated genomics analysis showed that 525 genes that are bound and increase in expression downstream of OVO are known to be essential maternally expressed genes. These include genes involved in anterior/posterior/germ plasm specification (bcd, exu, swa, osk, nos, aub, pgc, gcl), egg activation (png, plu, gnu, wisp, C(3)g, mtrm), translational regulation (cup, orb, bru1, me31B), and vitelline membrane formation (fs(1)N, fs(1)M3, clos). This suggests that OVO is a master transcriptional regulator of oocyte development and is responsible for the expression of structural components of the egg as well as maternally provided RNAs that are required for early embryonic development.

OVOL2: an epithelial lineage determiner with emerging roles in energy homeostasis

Ovo like zinc finger 2 (OVOL2) is an evolutionarily conserved regulator of epithelial lineage determination and differentiation during embryogenesis. OVOL2 binds to DNA using zinc-finger domains to suppress epithelial-mesenchymal transition (EMT), which is critical for tumor metastasis. However, recent studies have suggested some noncanonical roles of OVOL2 that do not rely on the DNA binding of zinc-finger domains or regulation of EMT. OVOL2 and EMT regulators have emerging roles in adipogenesis, thermogenesis, and lipid metabolism. Here, we review different roles of OVOL2 from embryo development to adult tissue homeostasis, and discuss how OVOL2 and other EMT regulators orchestrate a regulatory network to control energy homeostasis. Last, we propose potential applications of targeting OVOL2 to reduce human obesity.

Female germline expression of OVO transcription factor bridges Drosophila generations

OVO is required for karyotypically female germ cell viability but has no known function in the male germline in Drosophila. ovo is autoregulated by two antagonistic isoforms, OVO-A and OVO-B. All ovo- alleles were created as partial revertants of the antimorphic ovoD1 allele. Creation of new targeted alleles in an ovo+ background indicated that disrupting the germline-specific exon extension of ovo-B leads to an arrested egg chamber phenotype, rather than germ cell death. RNA-seq analysis, including >1K full length cDNAs, indicates that ovo utilizes a number of unannotated splice variations in the extended exon and a minor population of ovo-B transcripts utilizes an alternative splice. This indicates that classical ovo alleles such as ovoD1rv23, are not truly null for ovo, and are likely to be weak antimorphs. To generate bonafide nulls, we deleted the ovo-A and ovo-B promoters showing that only ovo-B is required for female germ cell viability and there is an early and polyphasic developmental requirement for ovo-B in the female germline. To visualize OVO expression and localization, we endogenously tagged ovo and found nuclear OVO in all differentiating female germ cells throughout oogenesis in adults. We also found that OVO is maternally deposited into the embryo, where it showed nuclear localization in newly formed pole cells. Maternal OVO persisted in embryonic germ cells until zygotic OVO expression was detectable, suggesting that there is continuous nuclear OVO expression in the female germline in the transition from one generation to the next.

Hippo pathway and Bonus control developmental cell fate decisions in the Drosophila eye

The canonical function of the Hippo signaling pathway is the regulation of organ growth. How this pathway controls cell-fate determination is less well understood. Here, we identify a function of the Hippo pathway in cell-fate decisions in the developing Drosophila eye, exerted through the interaction of Yorkie (Yki) with the transcriptional regulator Bonus (Bon), an ortholog of mammalian transcriptional intermediary factor 1/tripartite motif (TIF1/TRIM) family proteins. Instead of controlling tissue growth, Yki and Bon promote epidermal and antennal fates at the expense of the eye fate. Proteomic, transcriptomic, and genetic analyses reveal that Yki and Bon control these cell-fate decisions by recruiting transcriptional and post-transcriptional co-regulators and by repressing Notch target genes and activating epidermal differentiation genes. Our work expands the range of functions and regulatory mechanisms under Hippo pathway control.

Dynamic transcriptome analysis reveals the gene network of gonadal development from the early history life stages in dwarf surfclam Mulinia lateralis

BACKGROUND

Gonadal development is driven by a complex genetic cascade in vertebrates. However, related information remains limited in molluscs owing to the long generation time and the difficulty in maintaining whole life cycle in the lab. The dwarf surfclam Mulinia lateralis is considered an ideal bivalve model due to the short generation time and ease to breed in the lab.

RESULTS

To gain a comprehensive understanding of gonadal development in M. lateralis, we conducted a combined morphological and molecular analysis on the gonads of 30 to 60 dpf. Morphological analysis showed that gonad formation and sex differentiation occur at 35 and 40-45 dpf, respectively; then the gonads go through gametogenic cycle. Gene co-expression network analysis on 40 transcriptomes of 35-60 dpf gonads identifies seven gonadal development-related modules, including two gonad-forming modules (M6, M7), three sex-specific modules (M14, M12, M11), and two sexually shared modules (M15, M13). The modules participate in different biological processes, such as cell communication, glycan biosynthesis, cell cycle, and ribosome biogenesis. Several hub transcription factors including SOX2, FOXZ, HSFY, FOXL2 and HES1 are identified. The expression of top hub genes from sex-specific modules suggests molecular sex differentiation (35 dpf) occurs earlier than morphological sex differentiation (40-45 dpf).

CONCLUSION

This study provides a deep insight into the molecular basis of gonad formation, sex differentiation and gametogenesis in M. lateralis, which will contribute to a comprehensive understanding of the reproductive regulation network in molluscs.

Genomic signatures of recent convergent transitions to social life in spiders

The transition from solitary to social life is a major phenotypic innovation, but its genetic underpinnings are largely unknown. To identify genomic changes associated with this transition, we compare the genomes of 22 spider species representing eight recent and independent origins of sociality. Hundreds of genes tend to experience shifts in selection during the repeated transition to social life. These genes are associated with several key functions, such as neurogenesis, behavior, and metabolism, and include genes that previously have been implicated in animal social behavior and human behavioral disorders. In addition, social species have elevated genome-wide rates of molecular evolution associated with relaxed selection caused by reduced effective population size. Altogether, our study provides unprecedented insights into the genomic signatures of social evolution and the specific genetic changes that repeatedly underpin the evolution of sociality. Our study also highlights the heretofore unappreciated potential of transcriptomics using ethanol-preserved specimens for comparative genomics and phylotranscriptomics.

Dual role of Ovol2 on the germ cell lineage segregation during gastrulation in mouse embryogenesis

In mammals, primordial germ cells (PGCs), the origin of the germ line, are specified from the epiblast at the posterior region where gastrulation simultaneously occurs, yet the functional relationship between PGC specification and gastrulation remains unclear. Here, we show that OVOL2, a transcription factor conserved across the animal kingdom, balances these major developmental processes by repressing the epithelial-to-mesenchymal transition (EMT) that drives gastrulation and the upregulation of genes associated with PGC specification. Ovol2a, a splice variant encoding a repressor domain, directly regulates EMT-related genes and, consequently, induces re-acquisition of potential pluripotency during PGC specification, whereas Ovol2b, another splice variant missing the repressor domain, directly upregulates genes associated with PGC specification. Taken together, these results elucidate the molecular mechanism underlying allocation of the germ line among epiblast cells differentiating into somatic cells through gastrulation.

This article has an associated ‘The people behind the papers’ interview.

Steroid-dependent switch of OvoL/Shavenbaby controls self-renewal versus differentiation of intestinal stem cells

Adult stem cells must continuously fine-tune their behavior to regenerate damaged organs and avoid tumors. While several signaling pathways are well known to regulate somatic stem cells, the underlying mechanisms remain largely unexplored. Here, we demonstrate a cell-intrinsic role for the OvoL family transcription factor, Shavenbaby (Svb), in balancing self-renewal and differentiation of Drosophila intestinal stem cells. We find that svb is a downstream target of Wnt and EGFR pathways, mediating their activity for stem cell survival and proliferation. This requires post-translational processing of Svb into a transcriptional activator, whose upregulation induces tumor-like stem cell hyperproliferation. In contrast, the unprocessed form of Svb acts as a repressor that imposes differentiation into enterocytes, and suppresses tumors induced by altered signaling. We show that the switch between Svb repressor and activator is triggered in response to systemic steroid hormone, which is produced by ovaries. Therefore, the Svb axis allows intrinsic integration of local signaling cues and inter-organ communication to adjust stem cell proliferation versus differentiation, suggesting a broad role of OvoL/Svb in adult and cancer stem cells.

Post-transcriptional gene regulation regulates germline stem cell to oocyte transition during Drosophila oogenesis

During oogenesis, several developmental processes must be traversed to ensure effective completion of gametogenesis including, stem cell maintenance and asymmetric division, differentiation, mitosis and meiosis, and production of maternally contributed mRNAs, making the germline a salient model for understanding how cell fate transitions are mediated. Due to silencing of the genome during meiotic divisions, there is little instructive transcription, barring a few examples, to mediate these critical transitions. In Drosophila, several layers of post-transcriptional regulation ensure that the mRNAs required for these processes are expressed in a timely manner and as needed during germline differentiation. These layers of regulation include alternative splicing, RNA modification, ribosome production, and translational repression. Many of the molecules and pathways involved in these regulatory activities are conserved from Drosophila to humans making the Drosophila germline an elegant model for studying the role of post-transcriptional regulation during stem cell differentiation and meiosis.

CRISPR Disruption of BmOvo Resulted in the Failure of Emergence and Affected the Wing and Gonad Development in the Silkworm Bombyx mori

The domesticated silkworm is an economically important insect that is widely used as a lepidopteran insect model. Although somatic sex determination in the silkworm is well characterized, germline sex determination is not. Here, we used the transgenic-based CRISPR/Cas9 genome editing system to study the function of the Ovo gene in Bombyx mori. BmOvo is the homolog of a factor important in germline sex determination in Drosophila melanogaster. BmOvo mutants had abnormally shaped eggs that were disordered in the ovarioles, and gonad development was abnormal. Interestingly, wing discs and wings did not develop properly, and most of the mutants failed to eclose. Gene expression analyses by qRT-PCR showed that BmOvo gene was highly expressed in the wing disc and epidermis. Genes involved in the WNT signaling pathway and wing development genes BmWCP10 and BmE74 were downregulated in the BmOvo mutants when compared with wild-type animals. These results demonstrate that the BmOvo gene product plays an important role in wing metamorphosis. Thus, this study provides new insights into the multiple functions of BmOvo beyond germline sex determination.

Genome-wide identification and expression profiling of the C2H2-type zinc finger protein genes in the silkworm Bombyx mori

Cys2-His2 zinc finger (C2H2-ZF) proteins comprise the largest class of putative eukaryotic transcription factors. The zinc finger motif array is highly divergent, indicating that most proteins will have distinctive binding sites and perform different functions. However, the binding sites and functions of the majority of C2H2-ZF proteins remain unknown. In this study, we identified 327 C2H2-ZF protein genes in the silkworm, 290 in the monarch butterfly, 243 in the fruit fly, 107 in elegans, 673 in mouse, and 1,082 in human. The C2H2-ZF protein genes of the silkworm were classified into three main grouping clades according to a phylogenetic classification, and 312 of these genes could be mapped onto 27 chromosomes. Most silkworm C2H2-ZF protein genes exhibited specific expression in larval tissues. Furthermore, several C2H2-ZF protein genes had sex-specific expression during metamorphosis. In addition, we found that some C2H2-ZF protein genes are involved in metamorphosis and female reproduction by using expression clustering and gene annotation analysis. Among them, five genes were selected, BGIBMGA002091 (CTCF), BGIBMGA006492 (fru), BGIBMGA006230 (wor), BGIBMGA004640 (lola), and BIGBMGA004569, for quantitative real-time PCR analysis from larvae to adult ovaries. The results showed that the five genes had different expression patterns in ovaries, among which BGIBMGA002091 (CTCF) gene expression level was the highest, and its expression level increased rapidly in late pupae and adult stages. These findings provide a basis for further investigation of the functions of C2H2-ZF protein genes in the silkworm, and the results offer clues for further research into the development of metamorphosis and female reproduction in the silkworm.

Functional characterization of BmOVOs in silkworm, Bombyx mori

Background

In our previous study, we identified four isoforms of the Bmovo gene, Bmovo-1, Bmovo-2, Bmovo-3 and Bmovo-4 from the silkworm ovary and verified that ovarian development was regulated by the BmOVO proteins. Results: To understand the regulatory mechanisms of ovarian development, the regulation of four BmOVO isoforms on the B. mori ovarian tumor (Bmotu) promoter activity was investigated with luciferase reporter assays. The results showed the Bmotu promoter activity was positively regulated by BmOVO-1, BmOVO-2, BmOVO-3 and BmOVO-4 in a dose-dependent manner, of which BmOVO-2 had the highest transcriptional activation. However, the first (A1) and third acidic domains (A3) at the N-terminus of BmOVO-1 are transcriptional repression domains, while the fourth (A4) and fifth acidic domains (A5) are transcriptional activation domains. A recombinant BmOVO zinc-finger domain was found to bind to the GTACCGTTGTA sequence located at the Bmotu promoter. Furthermore, the Bmotu promoter activity was negatively regulated by ‘Tal-like’ peptide, which can trigger BmOVO-1 degradation at the N-terminus.

Conclusions

These results will help us to further understand the regulatory mechanisms of BmOVO isoforms on Bmotu promoter activity and ovarian development in the silkworm.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5697-y) contains supplementary material, which is available to authorized users.

Shavenbaby and Yorkie mediate Hippo signaling to protect adult stem cells from apoptosis

To compensate for accumulating damages and cell death, adult homeostasis (e.g., body fluids and secretion) requires organ regeneration, operated by long-lived stem cells. How stem cells can survive throughout the animal life remains poorly understood. Here we show that the transcription factor Shavenbaby (Svb, OvoL in vertebrates) is expressed in renal/nephric stem cells (RNSCs) of Drosophila and required for their maintenance during adulthood. As recently shown in embryos, Svb function in adult RNSCs further needs a post-translational processing mediated by the Polished rice (Pri) smORF peptides and impairing Svb function leads to RNSC apoptosis. We show that Svb interacts both genetically and physically with Yorkie (YAP/TAZ in vertebrates), a nuclear effector of the Hippo pathway, to activate the expression of the inhibitor of apoptosis DIAP1. These data therefore identify Svb as a nuclear effector in the Hippo pathway, critical for the survival of adult somatic stem cells.

Crinkled employs wingless pathway for wing development in Tribolium castaneum

Crinkled is associated with embryonic denticle formation and auditory organ development in Drosophila melanogaster. However, the functions of Crinkled have not been fully investigated. Additionally, the genes that participate in the Crinkled pathway are unknown. Phylogenetic analysis indicates that crinkled exhibits a one-to-one orthologous relationship in insects. In Tribolium castaneum, the crinkled gene is 6,498 bp in length and consists of six exons. Crinkled expression peaked during two phases in Tribolium: late embryonic and pupal stages. High levels of crinkled mRNA were detected in the fat body, head, epidermis, ovary, and accessory gland of late adults. Knockdown of crinkled using RNA interference (RNAi) severely affected wing morphogenesis in T. castaneum. We further showed that crinkled silencing reduced forked expression through wingless and shaven-baby, and RNAi of forked phenocopied the effects of crinkled knockdown in T. castaneum. This study investigated the development role of crinkled in postembryonic stages and indicated that forked mediates the functions of crinkled during wing morphogenesis in T. castaneum.

Identification of tarsal-less peptides from the silkworm Bombyx mori

The polycistronic and non-canonical gene tarsal-less (tal, known as pri) was reported to be required for embryonic and imaginal development in Drosophila; however, there are few reports of the tal gene in the silkworm Bombyx mori. Here, we cloned a tal-like (Bmtal) gene, and a sequence analysis showed that the Bmtal cDNA (1661 bp) contains five small open reading frames (smORFs) (A1, A2, A3, A4, and B) that encode short peptides of 11-12 (A1-A4) amino acid residues containing an LDPTG(E)L(Q)(V)Y motif that is conserved in Drosophila Tal, as well as a 32-amino-acid B peptide. Reverse transcription-quantitative polymerase chain reaction showed that the expression of the Bmtal gene was highest in the trachea, followed by the silk gland and Malpighian tubule, in day 3 fifth-instar larvae. Subcellular localization showed that BmTal localized in the nucleus. By regulating the expression of the full-length Bmtal gene and the functional smORFs of Bmtal, we showed that the expression levels of the Bmovo gene and genes related to the Notch, transforming growth factor-β, and Hippo signaling pathways changed with changes in BmTal peptide expression. A co-immunoprecipitation assay showed that BmTal interacts with polyubiquitin, which triggered degradation and/or processing of the 14-3-3 protein zeta. A comparative transcriptome analysis showed that 2843 (2045) genes were up- (down)-regulated after Bmtal gene expression was up-regulated. The up- (down)-regulated differentially expressed genes were enriched in 326 (278) gene ontology terms (P ≤ 0.05) and 54 (59) Kyoto Encyclopedia of Genes and Genomes pathways (P ≤ 0.05), and the results indicated that the BmTal peptides could function as mediators of hormone levels or the activities of multiple pathways, including the peroxisome proliferator-activated receptor, Hedgehog, mitogen-activated protein kinase, adipocytokine, and gonadotropin-releasing hormone signaling pathways, as well as the innate immune response. These results increase our understanding of the function and mechanism of BmTal at the genome-wide level.

Dusky works upstream of Four-jointed and Forked in wing morphogenesis in Tribolium castaneum

Dusky (dy) is required for cytoskeletal reorganization during wing morphogenesis in Drosophila melanogaster, but which genes participate together with dy for wing morphogenesis has remained unclear. In Tribolium castaneum, dy is highly expressed at the late embryonic stage. Tissue-specific expression analysis indicated high expression levels of dy in the epidermis, head and fat body of late-stage larvae. RNA interference (RNAi) targeting dy significantly decreased adult wing size and caused improper folding of the elytra. Meanwhile, dy knockdown reduced the transcription of four-jointed (fj) and forked (f). Our results show that fj RNAi reduces adult wing size and that silencing f results in abnormal wing folding in T. castaneum. Interestingly, knocking down fj and f simultaneously phenocopies dy RNAi, suggesting that dy probably acts upstream of fj and f to regulate wing morphogenesis in T. castaneum.

SoxNeuro and Shavenbaby act cooperatively to shape denticles in the embryonic epidermis of Drosophila

During development, extracellular signals are integrated by cells to induce the transcriptional circuitry that controls morphogenesis. In the fly epidermis, Wingless (Wg)/Wnt signaling directs cells to produce either a distinctly shaped denticle or no denticle, resulting in a segmental pattern of denticle belts separated by smooth, or 'naked', cuticle. Naked cuticle results from Wg repression of shavenbaby (svb), which encodes a transcription factor required for denticle construction. We have discovered that although the svb promoter responds differentially to altered Wg levels, Svb alone cannot produce the morphological diversity of denticles found in wild-type belts. Instead, a second Wg-responsive transcription factor, SoxNeuro (SoxN), cooperates with Svb to shape the denticles. Co-expressing ectopic SoxN with svb rescued diverse denticle morphologies. Conversely, removing SoxN activity eliminated the residual denticles found in svb mutant embryos. Furthermore, several known Svb target genes are also activated by SoxN, and we have discovered two novel target genes of SoxN that are expressed in denticle-producing cells and that are regulated independently of Svb. We conclude that proper denticle morphogenesis requires transcriptional regulation by both SoxN and Svb.

Conserved role of Ovo in germline development in mouse and Drosophila

Ovo, which encodes a transcription factor with Zn-finger domains, is evolutionarily conserved among animals. In Drosophila, in addition to its zygotic function for egg production, maternal ovo activity is required in primordial germ cells (PGCs) for expression of germline genes such as vasa and nanos. In this study, we found that maternal Ovo accumulates in PGC nuclei during embryogenesis. In these cells, ovo serves a dual function: activation of genes expressed predominantly in PGCs, and conversely suppression of somatic genes. Reduction of ovo activity in PGCs makes them unable to develop normally into germ cells of both sexes. In mice, knockout of the ovo ortholog, Ovol2, which is expressed in PGCs, decreases the number of PGCs during early embryogenesis. These data strongly suggest that ovo acts as part of an evolutionarily conserved mechanism that regulates germline development in animals.

Effects of Gene Dose, Chromatin, and Network Topology on Expression in Drosophila melanogaster

Deletions, commonly referred to as deficiencies by Drosophila geneticists, are valuable tools for mapping genes and for genetic pathway discovery via dose-dependent suppressor and enhancer screens. More recently, it has become clear that deviations from normal gene dosage are associated with multiple disorders in a range of species including humans. While we are beginning to understand some of the transcriptional effects brought about by gene dosage changes and the chromosome rearrangement breakpoints associated with them, much of this work relies on isolated examples. We have systematically examined deficiencies of the left arm of chromosome 2 and characterize gene-by-gene dosage responses that vary from collapsed expression through modest partial dosage compensation to full or even over compensation. We found negligible long-range effects of creating novel chromosome domains at deletion breakpoints, suggesting that cases of gene regulation due to altered nuclear architecture are rare. These rare cases include trans de-repression when deficiencies delete chromatin characterized as repressive in other studies. Generally, effects of breakpoints on expression are promoter proximal (~100bp) or in the gene body. Effects of deficiencies genome-wide are in genes with regulatory relationships to genes within the deleted segments, highlighting the subtle expression network defects in these sensitized genetic backgrounds.

Comparative transcriptomic analysis of silkworm Bmovo-1 and wild type silkworm ovary

The detailed molecular mechanism of Bmovo-1 regulation of ovary size is unclear. To uncover the mechanism of Bmovo-1 regulation of ovarian development and oogenesis using RNA-Seq, we compared the transcriptomes of wild type (WT) and Bmovo-1-overexpressing silkworm (silkworm^+Bmovo-1) ovaries. Using a pair-end Illumina Solexa sequencing strategy, 5,296,942 total reads were obtained from silkworm^+Bmovo-1 ovaries and 6,306,078 from WT ovaries. The average read length was about 100 bp. Clean read ratios were 98.79% for silkworm^+Bmovo-1 and 98.87% for WT silkworm ovaries. Comparative transcriptome analysis showed 123 upregulated and 111 downregulated genes in silkworm^+Bmovo-1 ovaries. These differentially expressed genes were enriched in the extracellular and extracellular spaces and involved in metabolism, genetic information processing, environmental information processing, cellular processes and organismal systems. Bmovo-1 overexpression in silkworm ovaries might promote anabolism for ovarian development and oogenesis and oocyte proliferation and transport of nutrients to ovaries by altering nutrient partitioning, which would support ovary development. Excessive consumption of nutrients for ovary development alters nutrient partitioning and deters silk protein synthesis.

dUTPase expression correlates with cell division potential in Drosophila melanogaster

dUTP pyrophosphatase (dUTPase) is a dNTP-sanitizing enzyme that prevents the appearance of potentially harmful uracil bases in DNA by hydrolyzing cellular dUTP. This function of dUTPase is found to be essential in many organisms including Drosophila melanogaster. Previously, we showed that the expression pattern of dUTPase determines the extent of uracil accumulation in the genome of different tissues. We wished to find the regulatory mechanism that eventually leaves a set of tissues with a uracil-free and intact genome. We found that the expression pattern established by the promoter of Drosophila dUTPase overlaps with mRNA and protein expression, excluding the involvement of other post-transcriptional contributions. This promoter was found to be active in primordial tissues, such as in the imaginal discs of larvae, in the larval brain and in reproductive organs. In the case of brain and imaginal tissues, we observed that the promoter activity depends on a DNA replication-related element motif, the docking site of DNA replication-related element binding factor, which is known as a transcriptional activator of genes involved in replication and proliferation. These results suggest that dUTPase expression is fine-tuned to meet the requirements of DNA synthesis in tissues where the maintenance of genome integrity is of high importance.

Bmovo-1 regulates ovary size in the silkworm, Bombyx mori

The regulation of antagonistic OVO isoforms is critical for germline formation and differentiation in Drosophila. However, little is known about genes related to ovary development. In this study, we cloned the Bombyx mori ovo gene and investigated its four alternatively spliced isoforms. BmOVO-1, BmOVO-2 and BmOVO-3 all had four C2H2 type zinc fingers, but differed at the N-terminal ends, while BmOVO-4 had a single zinc finger. Bmovo-1, Bmovo-2 and Bmovo-4 showed the highest levels of mRNA in ovaries, while Bmovo-3 was primarily expressed in testes. The mRNA expression pattern suggested that Bmovo expression was related to ovary development. RNAi and transgenic techniques were used to analyze the biological function of Bmovo. The results showed that when the Bmovo gene was downregulated, oviposition number decreased. Upregulation of Bmovo-1 in the gonads of transgenic silkworms increased oviposition number and elevated the trehalose contents of hemolymph and ovaries. We concluded that Bmovo-1 was involved in protein synthesis, contributing to the development of ovaries and oviposition number in silkworms.

Molecular phylogeny of OVOL genes illustrates a conserved C2H2 zinc finger domain coupled by hypervariable unstructured regions

OVO-like proteins (OVOL) are members of the zinc finger protein family and serve as transcription factors to regulate gene expression in various differentiation processes. Recent studies have shown that OVOL genes are involved in epithelial development and differentiation in a wide variety of organisms; yet there is a lack of comprehensive studies that describe OVOL proteins from an evolutionary perspective. Using comparative genomic analysis, we traced three different OVOL genes (OVOL1-3) in vertebrates. One gene, OVOL3, was duplicated during a whole-genome-duplication event in fish, but only the copy (OVOL3b) was retained. From early-branching metazoa to humans, we found that a core domain, comprising a tetrad of C2H2 zinc fingers, is conserved. By domain comparison of the OVOL proteins, we found that they evolved in different metazoan lineages by attaching intrinsically-disordered (ID) segments of N/C-terminal extensions of 100 to 1000 amino acids to this conserved core. These ID regions originated independently across different animal lineages giving rise to different types of OVOL genes over the course of metazoan evolution. We illustrated the molecular evolution of metazoan OVOL genes over a period of 700 million years (MY). This study both extends our current understanding of the structure/function relationship of metazoan OVOL genes, and assembles a good platform for further characterization of OVOL genes from diverged organisms.

Small peptides switch the transcriptional activity of Shavenbaby during Drosophila embryogenesis

A substantial proportion of eukaryotic transcripts are considered to be noncoding RNAs because they contain only short open reading frames (sORFs). Recent findings suggest, however, that some sORFs encode small bioactive peptides. Here, we show that peptides of 11 to 32 amino acids encoded by the polished rice (pri) sORF gene control epidermal differentiation in Drosophila by modifying the transcription factor Shavenbaby (Svb). Pri peptides trigger the amino-terminal truncation of the Svb protein, which converts Svb from a repressor to an activator. Our results demonstrate that during Drosophila embryogenesis, Pri sORF peptides provide a strict temporal control to the transcriptional program of epidermal morphogenesis.

Ovol2 suppresses cell cycling and terminal differentiation of keratinocytes by directly repressing c-Myc and Notch1

Ovol2 belongs to the Ovo family of evolutionarily conserved zinc finger transcription factors that act downstream of key developmental signaling pathways including Wg/Wnt and BMP/TGF-beta. We previously reported Ovol2 expression in the basal layer of epidermis, where epidermal stem/progenitor cells reside. In this work, we use HaCaT human keratinocytes to investigate the cellular and molecular functions of Ovol2. We show that depletion of Ovol2 leads to transient cell expansion but a loss of cells with long term proliferation potential. Mathematical modeling and experimental findings suggest that both faster cycling and precocious withdrawal from the cell cycle underlie this phenotype. Ovol2 depletion also accelerates extracellular signal-induced terminal differentiation in two- and three-dimensional culture models. By chromatin immunoprecipitation, luciferase reporter, and functional rescue assays, we demonstrate that Ovol2 directly represses two critical downstream targets, c-Myc and Notch1, thereby suppressing keratinocyte transient proliferation and terminal differentiation, respectively. These findings shed light on how an epidermal cell maintains a proliferation-competent and differentiation-resistant state.

Insulator and Ovo proteins determine the frequency and specificity of insertion of the gypsy retrotransposon in Drosophila melanogaster

The gypsy retrovirus of Drosophila is quite unique among retroviruses in that it shows a strong preference for integration into specific sites in the genome. In particular, gypsy integrates with a frequency of > 10% into the regulatory region of the ovo gene. We have used in vivo transgenic assays to dissect the role of Ovo proteins and the gypsy insulator during the process of gypsy site-specific integration. Here we show that DNA containing binding sites for the Ovo protein is required to promote site-specific gypsy integration into the regulatory region of the ovo gene. Using a synthetic sequence, we find that Ovo binding sites alone are also sufficient to promote gypsy site-specific integration into transgenes. These results indicate that Ovo proteins can determine the specificity of gypsy insertion. In addition, we find that interactions between a gypsy provirus and the gypsy preintegration complex may also participate in the process leading to the selection of gypsy integration sites. Finally, the results suggest that the relative orientation of two integrated gypsy sequences has an important role in the enhancer-blocking activity of the gypsy insulator.

Drosophila germline sex determination: integration of germline autonomous cues and somatic signals

The Drosophila testis and ovary are major genetically tractable systems for studying stem cells and their regulation. This has resulted in a deep understanding of germline stem cell regulation by the microenvironment, or niche. The male and female germline niches differ. Since sex is determined through different mechanisms in the soma than in the germline, genetic or physical manipulations can be used to experimentally mismatch somatic and germline sexual identities. The phenotypic consequences of these mismatches have striking similarities to those resulting from manipulations of signals within the niche. A critical role of the germline sex determination pathway may therefore be to ensure the proper receipt and processing of signals from the niche.

The creation of sexual dimorphism in the Drosophila soma

Animals have evolved a fascinating array of mechanisms for conducting sexual reproduction. These include producing the sex-specific gametes, as well as mechanisms for attracting a mate, courting a mate, and getting the gametes together. These processes require that males and females take on dramatically different forms (sexual dimorphism). Here, we will explore the problem of how sex is determined in Drosophila, and pay particular attention to how information about sexual identity is used to instruct males and females to develop differently. Along the way, we will highlight new work that challenges some of the traditional views about sex determination. In Drosophila, it is commonly thought that every cell decides its own sex based on its sex chromosome constitution (XX vs. XY). However, we now know that many cell types undergo nonautonomous sex determination, where they are told what sex to be through signals from surrounding cells, independent of their own chromosomal content. Further, it now appears that not all cells even "know" their sex, since key members of the sex determination pathway are not expressed in all cells. Thus, our understanding of how sex is determined, and how sexual identity is used to create sexual dimorphism, has changed considerably.

Ovol2/Movo, a homologue of Drosophila ovo, is required for angiogenesis, heart formation and placental development in mice

The zinc-finger transcription factor Ovol2 (Movo, Movo2) is a mouse homologue of Drosophila ovo, which is essential for the survival and differentiation of female germ line cells. To elucidate OVOL2 function in mammals, we generated Ovol2-deficient mice by gene targeting. The Ovol2 mutants died at embryonic days 9.5-10.5 (E9.5-E10.5), as a result of defects in extraembryonic and embryonic vascularization, and in heart formation. Although the Ovol2 expression was weak, severe defects were detected in extraembryonic and embryonic vascularization, and in heart formation at E8.5-E9.5. In Ovol2(-/-) placentas, allantoic blood vessel expansion and development of the labyrinthine layer were impaired at E10.5. In an endothelial cell line, siRNAs for Ovol2 reduced the expression of Ovol2 and inhibited the capillary-like network formation on Matrigel in vitro. These results demonstrate that Ovol2 may play a critical role in vascular angiogenesis during early embryogenesis.

Core promoter sequences contribute to ovo-B regulation in the Drosophila melanogaster germline

Utilization of tightly linked ovo-A vs. ovo-B germline promoters results in the expression of OVO-A and OVO-B, C(2)H(2) transcription factors with different N -termini, and different effects on target gene transcription and on female germline development. We show that two sex-determination signals, the X chromosome number within the germ cells and a female soma, differentially regulate ovo-B and ovo-A. We have previously shown that OVO regulates ovarian tumor transcription by binding the transcription start site. We have explored the regulation of the ovo-B promoter using an extensive series of transgenic reporter gene constructs to delimit cis-regulatory sequences as assayed in wild-type and sex-transformed flies and flies with altered ovo dose. Minimum regulated expression of ovo-B requires a short region flanking the transcription start site, suggesting that the ovo-B core promoter bears regulatory information in addition to a "basal" activity. In support of this idea, the core promoter region binds distinct factors in ovary and testis extracts, but not in soma extracts, suggesting that regulatory complexes form at the start site. This idea is further supported by the evolutionarily conserved organization of OVO binding sites at or near the start sites of ovo loci in other flies.

Characterization of the isoforms of MOVO zinc finger protein, a mouse homologue of Drosophila Ovo, as transcription factors

We previously described two isoforms (MOVO-A and -B) of a novel zinc finger protein MOVO, a mouse homologue of Drosophila Ovo protein. Here, we isolated cDNA encoding the third isoform MOVO-C, which had a transactivation domain and zinc finger domain, but lacked an N-terminal potential repression domain that was present in MOVO-A. Three isoform mRNAs were expressed highly in mouse testis and also in the ovary at lower levels. The structural analyses of the isolated Movo gene and mRNAs demonstrated that three different Movo transcripts were differentially processed to generate three isoforms. Major mRNA species encoded MOVO-B with a zinc finger domain alone, and minor mRNA species encoded MOVO-A (potential repressor) and MOVO-C (potential activator). To assign MOVO to a transcriptional factor, we characterized DNA-binding and transactivation properties. Random oligonucleotide selection, electrophoretic mobility shift assay and footprinting indicated that MOVO bound to the sequence, 5'-G(G/C/T)GGGGG-3'. These motifs were found in the 5'-flanking regions of Movo and other testis-specific genes. Nuclear proteins binding to this motif were detected in mouse testis, and the expression of MOVO mRNA was restricted in spermatocytes. The luciferase assay demonstrated that MOVO-C activated Movo promoter and MOVO-A repressed it, but MOVO-B had no effects. Mutated MOVO-binding motifs in the Movo promoter reduced the luciferase activity. All the isoforms had no effects on SV40 promoter without MOVO-binding motifs. MOVO-A partially rescued oogenesis of a Drosophila ovo mutant. These results suggest that MOVO isoforms are transcription factors to regulate genes carrying the MOVO-binding motifs in the testis.

The dual function of ovo/shavenbaby in germline and epidermis differentiation is conserved between Drosophila melanogaster and the olive fruit fly Bactrocera oleae

The olive fruit fly Bactrocera oleae (B. oleae) is a major olive damaging pest in the Mediterranean area. As a first molecular analysis of a developmental gene in this insect, we characterised the ovo/shavenbaby (ovo/svb) gene. In Drosophila, ovo/svb encodes a family of transcription regulators with two distinct functions: ovo is required for female germline differentiation and svb controls morphogenesis of epidermal cells. Here, we report the cloning and characterisation of ovo/svb in B. oleae, showing that the ovo genomic organisation and complex pattern of germline transcription have been conserved between distantly related Dipterae. We further show that B. oleae svb embryonic expression precisely prefigures the pattern of larval trichomes, supporting the conclusion that regulatory changes in svb transcription underlie evolutionary diversification of trichome patterns seen among Dipterae.

The Ovo/Shavenbaby transcription factor specifies actin remodelling during epidermal differentiation in Drosophila

In Drosophila, differentiation of the epidermis results in a stereotyped array of cells with F-actin-based extensions at their apical face. We identified Ovo/Shavenbaby (Svb) as a transcription factor that governs changes in epidermal cell shape. Svb is required for the formation of apical extensions and cells deficient in svb differentiate a smooth surface. In both the embryo and the adult, we show that Svb is necessary and sufficient for the cells to grow extensions and that the tight regulation of ovo/svb activity is critical for morphogenesis to occur correctly. We establish that Svb triggers early F-actin redistribution and is able to initiate the entire process of cytoskeletal remodelling, thereby defining it as a major regulator of epidermal differentiation.

Genetic control of germline sexual dimorphism in Drosophila

Females produce eggs and males produce sperm. Work in Drosophila is helping to elucidate how this sex-specific germline differentiation is genetically encoded. While important details remain somewhat controversial, it is clear that signals generated by somatic cells, probably in the embryonic gonads, are required as extrinsic factors for germline sex determination. It is equally clear that the sex chromosome karyotype of the germ cell is an intrinsic factor for germline sex determination. There is also extensive somatic signaling required for differentiation of germline cells in the adult gonads. Mismatched germline and somatic line sexual identities place germ cells in an inappropriate signaling milieu, which results in either failed maintenance of germline stems cells when female germ cells are in a male soma or overproliferation of germline cells when male germ cells are in a female soma. The well-studied somatic sex determination genes including transformer, transformer-2, and doublesex are clearly involved in the nonautonomous signaling from somatic cells, while the autonomous functions of genes including ovo, ovarian tumor, and Sex-lethal are involved in the germline. The integration of these two pathways is not yet clear.

Ovol2, a mammalian homolog of Drosophila ovo: gene structure, chromosomal mapping, and aberrant expression in blind-sterile mice

The ovo gene family consists of evolutionarily conserved genes including those cloned from Caenorhabditis elegans, Drosophila melanogaster, mouse, and human. Here we report the isolation and characterization of mouse Ovol2 (also known as movol2 or movo2) and provide evidence supporting the existence of multiple Ovol2 transcripts. These transcripts are produced by alternative promoter usage and alternative splicing and encode long and short OVOL2 protein isoforms, whose sequences differ from those previously reported. Mouse and human OVOL2 genes are expressed in overlapping tissues including testis, where Ovol2 expression is developmentally regulated and correlates with the meiotic/postmeiotic stages of spermatogenesis. Mouse Ovol2 maps to chromosome 2 in a region containing blind-sterile (bs), a spontaneous mutation that causes spermatogenic defects and germ cell loss. No mutation has been detected in the coding region of Ovol2 from bs mice, but Ovol2 transcription was dramatically reduced in testes from these mice, suggesting that Ovol2 is expressed in male germ cells.

A germline-specific splicing generates an extended ovo protein isoform required for Drosophila oogenesis

Most regulatory genes are employed multiple times to control different processes during development. The Drosophila Ovo/Shavenbaby (Svb) transcription factor is required both for germline and epidermal differentiation, two roles also found for its ortholog m-ovo1 in mice. In Drosophila, these two distinct functions are contributed by separate control regions directing the expression of Ovo/Svb in the germline (ovo) and soma (svb), respectively. We report here that alternative splicing represents an additional level of the regulation of Ovo/Svb functional specificity. Characterization of the ovo(D1rv23) mutation revealed that the intragenic insertion of a novel retrotransposon, romano, inactivates ovo without altering svb. We provide evidence that this insertion disrupts a germline-specific alternative exon, exon 2b, which encodes a 178-amino-acid internal extension (2B). While both isoforms, Ovo+2B and Ovo-2B, accumulate during oogenesis, only Ovo+2B is able to fulfill germinal ovo functions. Ovo-2B is unable, even when overexpressed, to fully rescue oogenic defects resulting from the absence of wild type ovo product. By contrast, either Ovo+2B or Ovo-2B germline protein can substitute for Svb in the epidermis. Our results emphasize the specific features of splicing in the germline, and reveal its functional importance for the control of ovo/svb-dependent ovarian and epidermal differentiation.

The LEF1/beta -catenin complex activates movo1, a mouse homolog of Drosophila ovo required for epidermal appendage differentiation

Drosophila ovo/svb (dovo) is required for epidermal cuticle/denticle differentiation and is genetically downstream of the wg signaling pathway. Similarly, a mouse homolog of dovo, movo1, is required for the proper formation of hair, a mammalian epidermal appendage. Here, we provide biochemical evidence that movo1 encodes a nuclear DNA binding protein (mOvo1a) that binds to DNA sequences similar to those that dOvo binds to, further supporting the notion that mOvo1a and dOvo are genetically and biochemically homologous proteins. Additionally, we show that the movo1 promoter is activated by the lymphoid enhancer factor 1 (LEF1)/beta-catenin complex, a transducer of wnt signaling. Collectively, our findings suggest that movo1 is a developmental target of wnt signaling during hair morphogenesis in mice, and that the wg/wnt-ovo link in epidermal appendage regulatory pathways has been conserved between mice and flies.

Genetic analysis of Drosophila melanogaster polytene chromosome region 44D-45F: loci required for viability and fertility

A genetic screen to identify mutations in genes in the 45A region on the right arm of chromosome 2 that are involved in oogenesis in Drosophila was undertaken. Several lethal but no female sterile mutations in the region had previously been identified in screens for P-element insertion or utilizing X rays or EMS as a mutagen. Here we report the identification of EMS-induced mutations in 21 essential loci in the 45D-45F region, including 13 previously unidentified loci. In addition, we isolated three mutant alleles of a newly identified locus required for fertility, sine prole. Mutations in sine prole disrupt spermatogenesis at or before individualization of spermatozoa and cause multiple defects in oogenesis, including inappropriate division of the germline cyst and arrest of oogenesis at stage 4.

Sex determination signals control ovo-B transcription in Drosophila melanogaster germ cells

Nonautonomous inductive signals from the soma and autonomous signals due to a 2X karyotype determine the sex of Drosophila melanogaster germ cells. These two signals have partially overlapping influences on downstream sex determination genes. The upstream OVO-B transcription factor is required for the viability of 2X germ cells, regardless of sexual identity, and for female germline sexual identity. The influence of inductive and autonomous signals on ovo expression has been controversial. We show that ovo-B is strongly expressed in the 2X germ cells in either a male or a female soma. This indicates that a 2X karyotype controls ovo-B expression in the absence of inductive signals from the female soma. However, we also show that female inductive signals positively regulate ovo-B transcription in the 1X germ cells that do not require ovo-B function. Genetic analysis clearly indicates that inductive signals from the soma are not required for ovo-B function in 2X germ cells. Thus, while somatic inductive signals and chromosome karyotype have overlapping regulatory influences, a 2X karyotype is a critical germline autonomous determinant of ovo-B function in the germline.

Germ cell selection in genetic mosaics in Drosophila melanogaster

Heritable mutations in the germ line lead to genetically heterogeneous, or mosaic, gonads. Many of the genes used in germ-line development also play roles in somatic development [Saffman, E. E. & Lasko, P. (1999) Cell. Mol. Life Sci. 55, 1141-1163]. Mutations in these genes may have cellular phenotypes throughout germ-line development leading to their differential elimination or survival, as has been observed in somatic cells [Morata, G. & Ripoll, P. (1975) Dev. Biol. 42, 211-221]. We investigate whether mutations in heterozygosis are subject to pregametic selection in the germ line. We initiated clones of wild-type homozygous cells at different stages of development in gonads heterozygous for eight different recessive chromosome deficiencies. Here we show that cell selection takes place in mosaic germ-line populations. This phenomenon represents a level of selection that precedes and conditions subsequent zygotic selection by affecting the genes available in the gametic population.

Protein determinants of insertional specificity for the Drosophila gypsy retrovirus

The gypsy retrovirus invades the germ line of Drosophila females, inserting with a high frequency into the ovo locus. Gypsy insertion sites in ovo are clustered within a region in the promoter of the ovo gene that contains multiple binding sites for the OvoA and OvoB proteins. We found that a 1.3-kb DNA fragment containing this region is able to confer gypsy insertional specificity independent of its genomic location. The frequency of gypsy insertions into the ovo gene is significantly lower in wild-type females than in ovoD1 females. In addition, gypsy insertions in ovoD1 females occur during most stages of germ-line development whereas insertions in wild-type females occur only in late stages. This pattern of temporally specific insertions, as well as the higher frequency of insertion in ovoD1 females, correlates with the presence of the OvoA or OvoD1 proteins. The results suggest that gypsy insertional specificity might be determined by the binding of the OvoA repressor isoform to the promoter region of the gene.

Drosophila OVO regulates ovarian tumor transcription by binding unusually near the transcription start site

Evolutionarily conserved ovo loci encode developmentally regulated, sequence-specific, DNA-binding, C(2)H(2)-zinc-finger proteins required in the germline and epidermal cells of flies and mice. The direct targets of OVO activity are not known. Genetic experiments suggest that ovo acts in the same regulatory network as ovarian tumor (otu), but the relative position of these genes in the pathway is controversial. Three OVO-binding sites exist in a compact regulatory region that controls germline expression of the otu gene. Interestingly, the strongest OVO-binding site is very near the otu transcription start, where basal transcriptional complexes must function. Loss-of-function, gain-of-function and promoter swapping constructs demonstrate that OVO binding near the transcription start site is required for OVO-dependent otu transcription in vivo. These data unambiguously identify otu as a direct OVO target gene and raise the tantalizing possibility that an OVO site, at the location normally occupied by basal components, functions as part of a specialized core promoter.