Vitelline membrane biogenesis in Drosophila requires the activity of the alpha-methyl dopa hypersensitive gene (I(2)amd) in both the germline and follicle cells

Finishing the egg

Gamete development is a fundamental process that is highly conserved from early eukaryotes to mammals. As germ cells develop, they must coordinate a dynamic series of cellular processes that support growth, cell specification, patterning, the loading of maternal factors (RNAs, proteins, and nutrients), differentiation of structures to enable fertilization and ensure embryonic survival, and other processes that make a functional oocyte. To achieve these goals, germ cells integrate a complex milieu of environmental and developmental signals to produce fertilizable eggs. Over the past 50 years, Drosophila oogenesis has risen to the forefront as a system to interrogate the sophisticated mechanisms that drive oocyte development. Studies in Drosophila have defined mechanisms in germ cells that control meiosis, protect genome integrity, facilitate mRNA trafficking, and support the maternal loading of nutrients. Work in this system has provided key insights into the mechanisms that establish egg chamber polarity and patterning as well as the mechanisms that drive ovulation and egg activation. Using the power of Drosophila genetics, the field has begun to define the molecular mechanisms that coordinate environmental stresses and nutrient availability with oocyte development. Importantly, the majority of these reproductive mechanisms are highly conserved throughout evolution, and many play critical roles in the development of somatic tissues as well. In this chapter, we summarize the recent progress in several key areas that impact egg chamber development and ovulation. First, we discuss the mechanisms that drive nutrient storage and trafficking during oocyte maturation and vitellogenesis. Second, we examine the processes that regulate follicle cell patterning and how that patterning impacts the construction of the egg shell and the establishment of embryonic polarity. Finally, we examine regulatory factors that control ovulation, egg activation, and successful fertilization.

Palisade is required in the Drosophila ovary for assembly and function of the protective vitelline membrane

The innermost layer of the Drosophila eggshell, the vitelline membrane, provides structural support and positional information to the embryo. It is assembled in an incompletely understood manner from four major proteins to form a homogeneous, transparent extracellular matrix. Here we show that RNAi knockdown or genetic deletion of a minor constituent of this matrix, Palisade, results in structural disruptions during the initial synthesis of the vitelline membrane by somatic follicle cells surrounding the oocyte, including wide size variation among the precursor vitelline bodies and disorganization of follicle cell microvilli. Loss of Palisade or the microvillar protein Cad99C results in abnormal uptake into the oocyte of sV17, a major vitelline membrane protein, and defects in non-disulfide cross-linking of sV17 and sV23, while loss of Palisade has additional effects on processing and disulfide cross-linking of these proteins. Embryos surrounded by the abnormal vitelline membranes synthesized when Palisade is reduced are fertilized but undergo developmental arrest, usually during the first 13 nuclear divisions, with a nuclear phenotype of chromatin margination similar to that described for wild-type embryos subjected to anoxia. Our results demonstrate that Palisade is involved in coordinating assembly of the vitelline membrane and is required for functional properties of the eggshell.

Gene amplification as a developmental strategy: isolation of two developmental amplicons in Drosophila

Gene amplification is known to be critical for upregulating gene expression in a few cases, but the extent to which amplification is utilized in the development of diverse organisms remains unknown. By quantifying genomic DNA hybridization to microarrays to assay gene copy number, we identified two additional developmental amplicons in the follicle cells of the Drosophila ovary. Both amplicons contain genes which, following their amplification, are expressed in the follicle cells, and the expression of three of these genes becomes restricted to specialized follicle cells late in differentiation. Genetic analysis establishes that at least one of these genes, yellow-g, is critical for follicle cell function, because mutations in yellow-g disrupt eggshell integrity. Thus, during follicle cell differentiation the entire genome is overreplicated as the cells become polyploid, and subsequently specific genomic intervals are overreplicated to facilitate gene expression.

The nudel protease of Drosophila is required for eggshell biogenesis in addition to embryonic patterning

The dorsoventral axis of the Drosophila embryo is defined by a ventral signal that arises within the perivitelline space, an extracellular compartment between the embryo plasma membrane and the vitelline membrane layer of the eggshell. Production of the ventral signal requires four members of the serine protease family, including a large modular protein with a protease domain encoded by the nudel gene. Here we provide evidence that the Nudel protease has an integral role in eggshell biogenesis. Mutations in nudel that disrupt Nudel protease function produce eggs having vitelline membranes that are abnormally permeable to the dye neutral red. Permeability varies among mutant nudel alleles but correlates with levels of Nudel protease catalytic activity and function in embryonic dorsoventral patterning. These mutations also block cross-linking of vitelline membrane proteins that normally occurs upon egg activation, just prior to fertilization. In addition, Nudel protease autoactivation temporally coincides with vitelline membrane cross-linking and can be triggered in mature eggs in vitro by conditions that lead to egg activation. We discuss how the Nudel protease might be involved in both eggshell biogenesis and embryonic patterning.

The catecholamines up (Catsup) protein of Drosophila melanogaster functions as a negative regulator of tyrosine hydroxylase activity

We report the genetic, phenotypic, and biochemical analyses of Catecholamines up (Catsup), a gene that encodes a negative regulator of tyrosine hydroxylase (TH) activity. Mutations within this locus are semidominant lethals of variable penetrance that result in three broad, overlapping effective lethal phases (ELPs), indicating that the Catsup gene product is essential throughout development. Mutants from each ELP exhibit either cuticle defects or catecholamine-related abnormalities, such as melanotic salivary glands or pseudotumors. Additionally, Catsup mutants have significantly elevated TH activity that may arise from a post-translational modification of the enzyme. The hyperactivation of TH in Catsup mutants results in abnormally high levels of catecholamines, which can account for the lethality, visible phenotypes, and female sterility observed in these mutants. We propose that Catsup is a component of a novel system that downregulates TH activity, making Catsup the fourth locus found within the Dopa decarboxylase (Ddc) gene cluster that functions in catecholamine metabolism.

Evolutionary changes in the expression pattern of a developmentally essential gene in three Drosophila species

The hypothesis that morphological evolution may largely result from changes in gene regulation rather than gene structure has been difficult to test. Morphological differences among insects are often apparent in the cuticle structures produced. The dopa decarboxylase (Ddc) and alpha-methyldopa hypersensitive (amd) genes arose from an ancient gene duplication. In Drosophila, they have evolved nonoverlapping functions, including the production of distinct types of cuticle, and for Ddc, the production of the neurotransmitters, dopamine and serotonin. The amd gene is particularly active in the production of specialized flexible cuticles in the developing embryo. We have compared the pattern of amd expression in three Drosophila species. Several regions of expression conserved in all three species but, surprisingly, a unique domain of expression is found in Drosophila simulans that does occur in the closely related (2-5 million years) Drosophila melanogaster or in the more remote (40-50 million years) Drosophila virilis. The "sudden" appearance of a completely new and robust domain of expression provides a glimpse of evolutionary variation resulting from changes in regulation of structural gene expression.