Organelle assemblages implicated in the transfer of oocyte components to the embryo: an insect perspective

Kinesin KIF3A regulates meiotic progression and spindle assembly in oocyte meiosis

Kinesin family member 3A (KIF3A) is a microtubule-oriented motor protein that belongs to the kinesin-2 family for regulating intracellular transport and microtubule movement. In this study, we characterized the critical roles of KIF3A during mouse oocyte meiosis. We found that KIF3A associated with microtubules during meiosis and depletion of KIF3A resulted in oocyte maturation defects. LC-MS data indicated that KIF3A associated with cell cycle regulation, cytoskeleton, mitochondrial function and intracellular transport-related molecules. Depletion of KIF3A activated the spindle assembly checkpoint, leading to metaphase I arrest of the first meiosis. In addition, KIF3A depletion caused aberrant spindle pole organization based on its association with KIFC1 to regulate expression and polar localization of NuMA and γ-tubulin; and KIF3A knockdown also reduced microtubule stability due to the altered microtubule deacetylation by histone deacetylase 6 (HDAC6). Exogenous Kif3a mRNA supplementation rescued the maturation defects caused by KIF3A depletion. Moreover, KIF3A was also essential for the distribution and function of mitochondria, Golgi apparatus and endoplasmic reticulum in oocytes. Conditional knockout of epithelial splicing regulatory protein 1 (ESRP1) disrupted the expression and localization of KIF3A in oocytes. Overall, our results suggest that KIF3A regulates cell cycle progression, spindle assembly and organelle distribution during mouse oocyte meiosis.

Morphology and ultrastructure of the Balbiani body in the oocytes of closely related bush cricket species. Shared features reveal important aspect of functioning

Balbiani bodies (Bbs) are female germline-specific organelle assemblages usually composed of mitochondria, Golgi complexes, elements of endoplasmic reticulum and accumulations of fine granular material, termed the nuage. Here we present results of morphological and ultrastructural analysis of the Bb of four bush crickets nested in four subfamilies of the family Tettigonidae. This study has revealed that Bbs of closely related species (belonging to the defined evolutionary line) are morphologically rather different. In two species (Meconema meridionale and Pholidoptera griseoaptera) the Bb has the form of a hollow hemisphere that covers a part of the germinal vesicle surface. In contrast, the Bb of Conocephalus fuscus and Leptophyes albovittata is less distinct and surrounds the whole or the majority of the germinal vesicle surface. Aside from this difference, the Bbs of all four studied species are built of identical sets of organelles and, most importantly, share one significant feature: close association of mitochondria and nuage accumulations. We show additionally that mitochondria remaining in direct contact with the nuage are characterized by distinct morphologies e.g. elongated, dumbbell shaped or bifurcated. In the light of our results and literature survey, the ancestral function of the Bb is discussed.

The KNRL nuclear receptor controls hydrolase-mediated vitellin breakdown during embryogenesis in the brown planthopper, Nilaparvata lugens

Vitellin (Vn) homeostasis is central to the fecundity of oviparous insects. Most studies have focused on the synthesis and transportation of Vn as a building block for developing eggs during vitellogenesis; however, less is known about how the utilization of this nutrient reserve affects embryonic development. Here, we show that the single ortholog of the knirps and knirps-like nuclear receptors, KNRL, negatively regulates Vn breakdown by suppressing the expression of hydrolase genes in the brown planthopper, Nilaparvata lugens. KNRL was highly expressed in the ovary of adult females, and knockdown of KNRL by RNA interference resulted in the acceleration of Vn breakdown and the inhibition of embryonic development. Transcriptome sequencing analysis revealed that numerous hydrolase genes, including cathepsins and trypsins were up-regulated after KNRL knockdown. At least eight of the nine significantly enriched Gene Ontology terms for the up-regulated genes were in proteolysis-related categories. The expression levels of five selected trypsin genes and the enzymatic activities of trypsin in the embryos were significantly increased after KNRL knockdown. Moreover, trypsin injection prolonged egg duration, delayed embryonic development, accelerated Vn breakdown and severely reduced egg hatchability, a pattern similar to that observed in KNRL-silenced N. lugens. These observations suggest that KNRL controls Vn breakdown in embryos via the transcriptional inhibition of hydrolases. Generally, this study provides a foundation for understanding how embryo nutrient reserves are mobilized during embryogenesis and identifies several genes and pathways that may prove valuable targets for pest control.

Morphogenesis of the Balbiani body in developing oocytes of an orthopteran, Metrioptera brachyptera, and multiplication of female germline mitochondria

Balbiani body (Bb) is a female germline specific organelle complex. Although the morphology and morphogenesis of the Bb have been analyzed in numerous vertebrate and invertebrate species, the role and ultimate fate of this organelle assemblage are still under debate. As a result, various functions have been attributed to the Bb in given animal lineages or even species. Our analyses showed that in the bush cricket, Metrioptera brachyptera, the Bb is an elaborate and highly dynamic structure positioned at one side of the oocyte nucleus. It forms in early previtellogenic oocytes and consists of two compartments: perinuclear and cytoplasmic. In the cytoplasmic compartment, characteristic complexes of nuage and polymorphous mitochondria are present. Computer-aided 3D reconstructions revealed that mitochondria clustered around neighboring nuage accumulations remain in a physical contact and form an extensive, though dispersed network. As oogenesis progresses, nuage/mitochondria complexes are partitioned into progressively smaller entities that become separated from each other. Concurrently, the mitochondrial network splits into small individual mitochondria populating the whole ooplasm. Immunohistochemical analysis showed that the latter process involves dynamin-related protein 1 (Drp1). Collectively, our findings suggest that in basal insect species, the Bb might be responsible for the selection as well as multiplication of the oocyte mitochondria.

Morphology of Mitochondria in Syncytial Annelid Female Germ-Line Cyst Visualized by Serial Block-Face SEM

Mitochondria change their morphology and distribution depending on the metabolism and functional state of a cell. Here, we analyzed the mitochondria and selected structures in female germ-line cysts in a representative of clitellate annelids – the white worm Enchytraeus albidus in which each germ cell has one cytoplasmic bridge that connects it to a common cytoplasmic mass. Using serial block-face scanning electron microscopy (SBEM), we prepared three-dimensional ultrastructural reconstructions of the entire selected compartments of a cyst at the advanced stage of oogenesis, i.e. the nurse cell, cytophore, and cytoplasmic bridges of all 16 cells (15 nurse cells and oocyte). We revealed extensive mitochondrial networks in the nurse cells, cytophore and mitochondria that pass through the cytoplasmic bridges, which indicates that a mitochondrial network can extend throughout the entire cyst. The dynamic hyperfusion state was suggested for such mitochondrial aggregations. We measured the mitochondria distribution and revealed their polarized distribution in the nurse cells and more abundant accumulation within the cytophore compared to the nurse cell. A close association of mitochondrial networks with dispersed nuage material, which seems to be the structural equivalent of a Balbiani body, not described in clitellate annelids so far, was also revealed.

Transmission of Functional, Wild-Type Mitochondria and the Fittest mtDNA to the Next Generation: Bottleneck Phenomenon, Balbiani Body, and Mitophagy

The most important role of mitochondria is to supply cells with metabolic energy in the form of adenosine triphosphate (ATP). As synthesis of ATP molecules is accompanied by the generation of reactive oxygen species (ROS), mitochondrial DNA (mtDNA) is highly vulnerable to impairment and, consequently, accumulation of deleterious mutations. In most animals, mitochondria are transmitted to the next generation maternally, i.e., exclusively from female germline cells (oocytes and eggs). It has been suggested, in this context, that a specialized mechanism must operate in the developing oocytes enabling escape from the impairment and subsequent transmission of accurate (devoid of mutations) mtDNA from one generation to the next. Literature survey suggest that two distinct and irreplaceable pathways of mitochondria transmission may be operational in various animal lineages. In some taxa, the mitochondria are apparently selected: functional mitochondria with high inner membrane potential are transferred to the cells of the embryo, whereas those with low membrane potential (overloaded with mutations in mtDNA) are eliminated by mitophagy. In other species, the respiratory activity of germline mitochondria is suppressed and ROS production alleviated leading to the same final effect, i.e., transmission of undamaged mitochondria to offspring, via an entirely different route.

Viviparity in Two Closely Related Epizoic Dermapterans Relies on Disparate Modifications of Reproductive Systems and Embryogenesis

Nutritional modes operating during embryonic/larval development of viviparous species range from "pure" lecitothrophy in which embryos rely solely on reserve materials (yolk spheres, lipid droplets, and glycogen particles) accumulated in the egg cytoplasm to matrotrophy in which embryos are continuously supplied with nutrients from a parental organism. Interestingly, a wide spectrum of diverse "mixed" modes employed in the embryo nourishment have also been described among viviparous species. Here, we summarize results of histochemical, ultrastructural, and biochemical analyses of reproductive systems as well as developing embryos of two closely related viviparous species of earwigs (Dermaptera), Hemimerus talpoides and Arixenia esau. These analyses clearly indicate that morphological as well as physiological modifications (adaptations) supporting viviparity and matrotrophy in Hemimerus and Arixenia, with the exception of a complex biphasic respiration, are markedly different. Most importantly, Hemimerus embryos complete their development inside terminal (largest) ovarian follicles, whereas Arixenia embryos, after initial developmental stages, are transferred to highly modified lateral oviducts, that is the uterus, where they develop until the release (birth) of larvae. The obtained results strongly suggest that viviparity in hemimerids and arixeniids had evolved independently and might therefore serve as an example of evolutionary parallelism as well as remarkable functional plasticity of insect reproduction and embryonic development.