HorkaD, a chromosome instability-causing mutation in Drosophila, is a dominant-negative allele of Lodestar

A dual-function SNF2 protein drives chromatid resolution and nascent transcripts removal in mitosis

Mitotic chromatin is largely assumed incompatible with transcription due to changes in the transcription machinery and chromosome architecture. However, the mechanisms of mitotic transcriptional inactivation and their interplay with chromosome assembly remain largely unknown. By monitoring ongoing transcription in Drosophila early embryos, we reveal that eviction of nascent mRNAs from mitotic chromatin occurs after substantial chromosome compaction and is not promoted by condensin I. Instead, we show that the timely removal of transcripts from mitotic chromatin is driven by the SNF2 helicase-like protein Lodestar (Lds), identified here as a modulator of sister chromatid cohesion defects. In addition to the eviction of nascent transcripts, we uncover that Lds cooperates with Topoisomerase 2 to ensure efficient sister chromatid resolution and mitotic fidelity. We conclude that the removal of nascent transcripts upon mitotic entry is not a passive consequence of cell cycle progression and/or chromosome compaction but occurs via dedicated mechanisms with functional parallelisms to sister chromatid resolution.

Snf2 Proteins Are Required to Generate Gamete Pronuclei in Tetrahymena thermophila

During sexual reproduction/conjugation of the ciliate Tetrahymena thermophila, the germinal micronucleus undergoes meiosis resulting in four haploid micronuclei (hMICs). All hMICs undergo post-meiotic DNA double-strand break (PM-DSB) formation, cleaving their genome. DNA lesions are subsequently repaired in only one ‘selected’ hMIC, which eventually produces gametic pronuclei. DNA repair in the selected hMIC involves chromatin remodeling by switching from the heterochromatic to the euchromatic state of its genome. Here, we demonstrate that, among the 15 Tetrahymena Snf2 family proteins, a core of the ATP-dependent chromatin remodeling complex in Tetrahymena, the germline nucleus specific Iswi in Tetrahymena IswiGTt and Rad5Tt is crucial for the generation of gametic pronuclei. In either gene knockout, the selected hMIC which shows euchromatin markers such as lysine-acetylated histone H3 does not appear, but all hMICs in which markers for DNA lesions persist are degraded, indicating that both IswiGTt and Rad5Tt have important roles in repairing PM-DSB DNA lesions and remodeling chromatin for the euchromatic state in the selected hMIC.

Nuclear export restricts Gdown1 to a mitotic function

Approximately half of purified mammalian RNA polymerase II (Pol II) is associated with a tightly interacting sub-stoichiometric subunit, Gdown1. Previous studies have established that Gdown1 inhibits transcription initiation through competitive interactions with general transcription factors and blocks the Pol II termination activity of transcription termination factor 2 (TTF2). However, the biological functions of Gdown1 remain poorly understood. Here, we utilized genetic, microscopic, and multi-omics approaches to functionally characterize Gdown1 in three human cell lines. Acute depletion of Gdown1 caused minimal direct effects on transcription. We show that Gdown1 resides predominantly in the cytoplasm of interphase cells, shuttles between the cytoplasm and nucleus, and is regulated by nuclear export. Gdown1 enters the nucleus at the onset of mitosis. Consistently, genetic ablation of Gdown1 is associated with partial de-repression of mitotic transcription, and Gdown1 KO cells present with evidence of aberrant mitoses coupled to p53 pathway activation. Evidence is presented demonstrating that Gdown1 modulates the combined functions of purified productive elongation factors PAF1C, RTF1, SPT6, DSIF and P-TEFb in vitro. Collectively, our findings support a model wherein the Pol II-regulatory function of Gdown1 occurs during mitosis and is required for genome integrity.

Genome and transcriptome analysis of the mealybug Maconellicoccus hirsutus: Correlation with its unique phenotypes

Mealybugs are aggressive pests with world-wide distribution and are suitable for the study of different phenomena like genomic imprinting and epigenetics. Genomic approaches facilitate these studies in absence of robust genetics in this system. We sequenced, de novo assembled, annotated Maconellicoccus hirsutus genome. We carried out comparative genomics it with four mealybug and eight other insect species, to identify expanded, specific and contracted gene classes that relate to pesticide and desiccation resistance. We identified horizontally transferred genes adding to the mutualism between the mealybug and its endosymbionts. Male and female transcriptome analysis indicates differential expression of metabolic pathway genes correlating with their physiology and the genes for sexual dimorphism. The significantly lower expression of endosymbiont genes in males relates to the depletion of endosymbionts in males during development.

Control of mating plug expelling and sperm storage in Drosophila: A gynandromorph- and mutation-based dissection

INTRODUCTION

In this study, we analyzed gynandromorphs with female terminalia, to dissect mating-related female behaviors in Drosophila.

MATERIALS AND METHODS

We used gynandromorphs, experimentally modified wild-type (Oregon-R) females, and mutant females that lacked different components of the female reproductive apparatus.

RESULTS

Many of the gynandromorphs mated but did not expel the mating plug (MP). Some of these - with thousands of sperm in the uterus - failed to take up sperm into the storage organs. There were gynandromorphs that stored plenty of sperm but failed to release them to fertilize eggs. Expelling the MP, sperm uptake into the storage organs, and the release of stored sperm along egg production are separate steps occurring during Drosophila female fertility. Cuticle landmarks of the gynandromorphs revealed that while the nerve foci that control MP expelling and also those that control sperm uptake reside in the abdominal, the sperm release foci derive from the thoracic region of the blastoderm.

DISCUSSION AND CONCLUSION

The gynandromorph study is confirmed by analyses of (a) mutations that cause female sterility: Fs(3)Avar (preventing egg deposition), Tm2^gs (removing germline cells), and iab-4^DB (eliminating gonad formation) and (b) by experimentally manipulated wild-type females: decapitated or cut through ventral nerve cord.

An assay to detect in vivo Y chromosome loss in Drosophila wing disc cells

Loss of the Y chromosome in Drosophila has no impact on cell viability and therefore allows us to assay the impact of environmental agents and genetic alterations on chromosomal loss. To detect in vivo chromosome loss in cells of the developing Drosophila wing primordia, we first engineered a Y chromosome with an attP docking site. By making use of the ΦC31 integrase system, we site-specifically integrated a genomic transgene encompassing the multiple wing hair (mwh) locus into this attP site, leading to a mwh⁺Y chromosome. This chromosome fully rescues the mwh mutant phenotype, an excellent recessive wing cell marker mutation. Loss of this mwh⁺Y chromosome in wing primordial cells then leads to manifestation of the mwh mutant phenotype in mwh-homozygous cells. The forming mwh clones permit us to quantify the effect of agents and genetic alterations by assaying frequency and size of the mwh mosaic spots. To illustrate the use of the mwh⁺Y loss system, the effects of four known mutagens (X-rays, colchicine, ethyl methanesulfonate, and formaldehyde) and two genetic conditions (loss- and gain-of-function lodestar mutant alleles) are documented. The procedure is simple, sensitive, and inexpensive.

Sexual and asexual oogenesis require the expression of unique and shared sets of genes in the insect Acyrthosiphon pisum

Background

Although sexual reproduction is dominant within eukaryotes, asexual reproduction is widespread and has evolved independently as a derived trait in almost all major taxa. How asexuality evolved in sexual organisms is unclear. Aphids, such as Acyrthosiphon pisum, alternate between asexual and sexual reproductive means, as the production of parthenogenetic viviparous females or sexual oviparous females and males varies in response to seasonal photoperiodism. Consequently, sexual and asexual development in aphids can be analyzed simultaneously in genetically identical individuals.

Results

We compared the transcriptomes of aphid embryos in the stages of development during which the trajectory of oogenesis is determined for producing sexual or asexual gametes. This study design aimed at identifying genes involved in the onset of the divergent mechanisms that result in the sexual or asexual phenotype. We detected 33 genes that were differentially transcribed in sexual and asexual embryos. Functional annotation by gene ontology (GO) showed a biological signature of oogenesis, cell cycle regulation, epigenetic regulation and RNA maturation. In situ hybridizations demonstrated that 16 of the differentially-transcribed genes were specifically expressed in germ cells and/or oocytes of asexual and/or sexual ovaries, and therefore may contribute to aphid oogenesis. We categorized these 16 genes by their transcription patterns in the two types of ovaries; they were: i) expressed during sexual and asexual oogenesis; ii) expressed during sexual and asexual oogenesis but with different localizations; or iii) expressed only during sexual or asexual oogenesis.

Conclusions

Our results show that asexual and sexual oogenesis in aphids share common genetic programs but diverge by adapting specificities in their respective gene expression profiles in germ cells and oocytes.

Nonmammalian parent-of-origin effects

Chromosomes acquire different epigenetic marks during oogenesis and spermatogenesis. After fertilization, if retained and selected, these differences may result in imprinting effects. Rather than being an oddity, imprinting effects have been found in many sexually reproducing organisms. Interestingly, imprinting can result in disparate effects under different selective forces. At the same time, epigenetic mechanisms and selective pressures shared by sexually reproducing organisms could underlie common imprinting effects. Large-scale studies are revealing that parent-of-origin effects are more common than previously thought and supporting the important contribution of imprinting to many traits and diseases.