YA is needed for proper nuclear organization to transition between meiosis and mitosis in Drosophila

A genetic basis for facultative parthenogenesis in Drosophila

Facultative parthenogenesis enables sexually reproducing organisms to switch between sexual and asexual parthenogenetic reproduction. To gain insights into this phenomenon, we sequenced the genomes of sexually reproducing and parthenogenetic strains of Drosophila mercatorum and identified differences in the gene expression in their eggs. We then tested whether manipulating the expression of candidate gene homologs identified in Drosophila mercatorum could lead to facultative parthenogenesis in the non-parthenogenetic species Drosophila melanogaster. This identified a polygenic system whereby increased expression of the mitotic protein kinase polo and decreased expression of a desaturase, Desat2, caused facultative parthenogenesis in the non-parthenogenetic species that was enhanced by increased expression of Myc. The genetically induced parthenogenetic Drosophila melanogaster eggs exhibit de novo centrosome formation, fusion of the meiotic products, and the onset of development to generate predominantly triploid offspring. Thus, we demonstrate a genetic basis for sporadic facultative parthenogenesis in an animal.

Activating embryonic development in Drosophila

The transition from oocyte to embryo marks the onset of development. This process requires complex regulation to link developmental signals with profound changes in mRNA translation, cell cycle control, and metabolism. This control is beginning to be understood for most organisms, and research in the fruit fly Drosophila melanogaster has generated new insights. Recent findings have increased our understanding of the roles played by hormone and Ca2+ signaling events as well as metabolic remodeling crucial for this transition. Specialized features of the structure and assembly of the meiotic spindle have been identified. The changes in protein levels, mRNA translation, and polyadenylation that occur as the oocyte becomes an embryo have been identified together with key aspects of their regulation. Here we highlight these important developments and the insights they provide on the intricate regulation of this dramatic transition.

Maternal Proteins That Are Phosphoregulated upon Egg Activation Include Crucial Factors for Oogenesis, Egg Activation and Embryogenesis in Drosophila melanogaster

Egg activation is essential for the successful transition from a mature oocyte to a developmentally competent egg. It consists of a series of events including the resumption and completion of meiosis, initiation of translation of some maternal mRNAs and destruction of others, and changes to the vitelline envelope. This major change of cell state is accompanied by large scale alteration in the oocyte’s phosphoproteome. We hypothesize that the cohort of proteins that are subject to phosphoregulation during egg activation are functionally important for processes before, during, or soon after this transition, potentially uniquely or as proteins carrying out essential cellular functions like those they do in other (somatic) cells. In this study, we used germline-specific RNAi to examine the function of 189 maternal proteins that are phosphoregulated during egg activation in Drosophila melanogaster. We identified 53 genes whose knockdown reduced or abolished egg production and caused a range of defects in ovarian morphology, as well as 51 genes whose knockdown led to significant impairment or abolishment of the egg hatchability. We observed different stages of developmental arrest in the embryos and various defects in spindle morphology and aberrant centrosome activities in the early arrested embryos. Our results, validated by the detection of multiple genes with previously-documented maternal effect phenotypes among the proteins we tested, revealed 15 genes with newly discovered roles in egg activation and early embryogenesis in Drosophila. Given that protein phosphoregulation is a conserved characteristic of this developmental transition, we suggest that the phosphoregulated proteins may provide a rich pool of candidates for the identification of important players in the egg-to-embryo transition.

The intimate genetics of Drosophila fertilization

The union of haploid gametes at fertilization initiates the formation of the diploid zygote in sexually reproducing animals. This founding event of embryogenesis includes several fascinating cellular and nuclear processes, such as sperm-egg cellular interactions, sperm chromatin remodelling, centrosome formation or pronuclear migration. In comparison with other aspects of development, the exploration of animal fertilization at the functional level has remained so far relatively limited, even in classical model organisms. Here, we have reviewed our current knowledge of fertilization in Drosophila melanogaster, with a special emphasis on the genes involved in the complex transformation of the fertilizing sperm nucleus into a replicated set of paternal chromosomes.

Phospho-regulation pathways during egg activation in Drosophila melanogaster

Egg activation is the series of events that transition a mature oocyte to an egg capable of supporting embryogenesis. Increasing evidence points toward phosphorylation as a critical regulator of these events. We used Drosophila melanogaster to investigate the relationship between known egg activation genes and phosphorylation changes that occur upon egg activation. Using the phosphorylation states of four proteins-Giant Nuclei, Young Arrest, Spindly, and Vap-33-1-as molecular markers, we showed that the egg activation genes sarah, CanB2, and cortex are required for the phospho-regulation of multiple proteins. We show that an additional egg activation gene, prage, regulates the phosphorylation state of a subset of these proteins. Finally, we show that Sarah and calcineurin are required for the Anaphase Promoting Complex/Cyclosome (APC/C)-dependent degradation of Cortex following egg activation. From these data, we present a model in which Sarah, through the activation of calcineurin, positively regulates the APC/C at the time of egg activation, which leads to a change in phosphorylation state of numerous downstream proteins.

Parents Without Partners: Drosophila as a Model for Understanding the Mechanisms and Evolution of Parthenogenesis

Of 40 Drosophila species screened to date, a majority have shown some ability to at least initiate parthenogenetic development. In one case, Drosophila mangebeirai, natural populations are entirely female, making it the only obligate parthenogenetic species of Drosophila. Only a few of the species that exhibit the ability to undergo early embryonic development of unfertilized eggs successfully respond to selection for parthenogenetic production of adult flies. Laboratory strains of parthenogenetic Drosophila mercatorum have been created by artificial selection on multiple occasions, but the proportion of eggs undergoing development to adulthood has never exceeded 8%. Selection produces gains in the number of unfertilized eggs undergoing early development, but the majority arrest at the embryonic or first larval instar stages. Four components to successful parthenogenesis include (1) a female’s propensity to lay unfertilized eggs, (2) the ability of the eggs to restore diploidy, (3) the ability of the parthenogenetically produced diploid embryo to complete larval development and pupation, and (4) the existence of genetic variability within and among Drosophila species in the frequency of parthenogenesis suggests the existence of multiple steps in its evolution and offers a way to explore the genetics of this unusual reproductive strategy.

Protein phosphorylation changes reveal new candidates in the regulation of egg activation and early embryogenesis in D. melanogaster

Egg activation is the series of events that must occur for a mature oocyte to become capable of supporting embryogenesis. These events include changes to the egg's outer coverings, the resumption and completion of meiosis, the translation of new proteins, and the degradation of specific maternal mRNAs. While we know some of the molecules that direct the initial events of egg activation, it remains unclear how multiple pathways are coordinated to change the cellular state from mature oocyte to activated egg. Using a proteomic approach we have identified new candidates for the regulation and progression of egg activation. Reasoning that phosphorylation can simultaneously and rapidly modulate the activity of many proteins, we identified proteins that are post-translationally modified during the transition from oocyte to activated egg in Drosophila melanogaster. We find that at least 311 proteins change in phosphorylation state between mature oocytes and activated eggs. These proteins fall into various functional classes related to the events of egg activation including calcium binding, proteolysis, and protein translation. Our set of candidates includes genes already associated with egg activation, as well as many genes not previously studied during this developmental period. RNAi knockdown of a subset of these genes revealed a new gene, mrityu, necessary for embryonic development past the first mitosis. Thus, by identifying phospho-modulated proteins we have produced a focused candidate set for future genetic studies to test their roles in egg activation and the initiation of embryogenesis.

Developmental control of oocyte maturation and egg activation in metazoan models

Production of functional eggs requires meiosis to be coordinated with developmental signals. Oocytes arrest in prophase I to permit oocyte differentiation, and in most animals, a second meiotic arrest links completion of meiosis to fertilization. Comparison of oocyte maturation and egg activation between mammals, Caenorhabditis elegans, and Drosophila reveal conserved signaling pathways and regulatory mechanisms as well as unique adaptations for reproductive strategies. Recent studies in mammals and C. elegans show the role of signaling between surrounding somatic cells and the oocyte in maintaining the prophase I arrest and controlling maturation. Proteins that regulate levels of active Cdk1/cyclin B during prophase I arrest have been identified in Drosophila. Protein kinases play crucial roles in the transition from meiosis in the oocyte to mitotic embryonic divisions in C. elegans and Drosophila. Here we will contrast the regulation of key meiotic events in oocytes.

Genome-wide analysis of mRNA decay patterns during early Drosophila development

BACKGROUND

The modulation of mRNA levels across tissues and time is key for the establishment and operation of the developmental programs that transform the fertilized egg into a fully formed embryo. Although the developmental mechanisms leading to differential mRNA synthesis are heavily investigated, comparatively little attention is given to the processes of mRNA degradation and how these relate to the molecular programs controlling development.

RESULTS

Here we combine timed collection of Drosophila embryos and unfertilized eggs with genome-wide microarray technology to determine the degradation patterns of all mRNAs present during early fruit fly development. Our work studies the kinetics of mRNA decay, the contributions of maternally and zygotically encoded factors to mRNA degradation, and the ways in which mRNA decay profiles relate to gene function, mRNA localization patterns, translation rates and protein turnover. We also detect cis-regulatory sequences enriched in transcripts with common degradation patterns and propose several proteins and microRNAs as developmental regulators of mRNA decay during early fruit fly development. Finally, we experimentally validate the effects of a subset of cis-regulatory sequences and trans-regulators in vivo.

CONCLUSIONS

Our work advances the current understanding of the processes controlling mRNA degradation during early Drosophila development, taking us one step closer to the understanding of mRNA decay processes in all animals. Our data also provide a valuable resource for further experimental and computational studies investigating the process of mRNA decay.