Changes in follicular cells architecture during vitellogenin transport in the ovary of social Hymenoptera

Bicaudal C is required for the function of the follicular epithelium during oogenesis in Rhodnius prolixus

The morphology and physiology of the oogenesis have been well studied in the vector of Chagas disease Rhodnius prolixus. However, the molecular interactions that regulate the process of egg formation, key for the reproductive cycle of the vector, is still largely unknown. In order to understand the molecular and cellular basis of the oogenesis, we examined the function of the gene Bicaudal C (BicC) during oogenesis and early development of R. prolixus. We show that R. prolixus BicC (Rp-BicC) gene is expressed in the germarium, with cytoplasmic distribution, as well as in the follicular epithelium of the developing oocytes. RNAi silencing of Rp-BicC resulted in sterile females that lay few, small, non-viable eggs. The ovaries are reduced in size and show a disarray of the follicular epithelium. This indicates that Rp-BicC has a central role in the regulation of oogenesis. Although the follicular cells are able to form the chorion, the uptake of vitelline by the oocytes is compromised. We show evidence that the polarity of the follicular epithelium and the endocytic pathway, which are crucial for the proper yolk deposition, are affected. This study provides insights into the molecular mechanisms underlying oocyte development and show that Rp-BicC is important for de developmental of the egg and, therefore, a key player in the reproduction of this insect.

Evidence for a transcellular route for vitellogenin transport in the telotrophic ovary of Podisus nigrispinus (Hemiptera: Pentatomidae)

Vitellogenin is the main yolk precursor protein in insect oocytes. It is synthesized in the fat body and released into the hemolymph. To reach the oocyte surface, vitellogenin must cross a single layer of follicular epithelium cells. The transport of vitellogenin across the follicular epithelium has been suggested to occur through the enlarged intercellular spaces (patency) by a paracellular route or by endocytosis by follicular cells and release onto oocyte surface in a transcelluar route. In this study, we investigated whether vitellogenin transport in the meroistic telotrophic ovary of Podisus nigrispinus (Hemiptera) occurs via a paracellular or transcellular route. Light and transmission electron microscopies showed that short cell–cell contacts with well-developed occluding septate junctions were present in follicular cells with patency. Immunofluorescence microscopy revealed the presence of vitellogenin receptors in the plasma membrane and of vitellogenin in the cytoplasm of follicular cells. Data suggest that cell–cell contacts serve as a barrier to large vitellogenin molecules and that this protein is transported via a transcellular route of receptor-mediated endocytosis.

Morphology of ovary and spermathecae of the parasitoid Eibesfeldtphora tonhascai Brown (Diptera: Phoridae)

Eibesfeldtphora tonhascai (Diptera: Phoridae) is a parasitoid of leaf-cutting ants and a potential biological control agent against these insect pests. This study describes the morphology of the ovary and spermatheca of E. tonhascai. The female reproductive tract of this parasitoid has a pair of meroistic polytrophic ovaries, two lateral oviducts that open into a common oviduct, an elongated accessory gland, and two spermathecae. Young oocytes are small and spherical, and their size increases as yolk is stored in the cytoplasm. This process is followed by chorion production by follicular cells. Mature oocytes are elliptical or torpedo-shaped. The reservoir wall of the spermatheca has type III glandular cells with cytoplasm rich in free ribosomes, rough endoplasmic reticulum, and secretory vesicles. The apical surface of these cells has microvilli associated with mitochondria. The reservoir lumen is lined by a cuticle and filled with spermatozoa. This is the first report of the ovary and spermatheca morphology of E. tonhascai and contributes to the comprehension of the reproductive biology of this parasitoid of leaf-cutting ants.

Vitellogenin transcytosis in follicular cells of the honeybee Apis mellifera and the wasp Polistes simillimus

Vitellogenin receptor (VgR) is a low-density lipoprotein receptor responsible for the mediated endocytosis of vitellogenin (Vg) during egg formation in insects. The maturing oocyte is enveloped by a follicular epithelium, which has large intercellular spaces during Vg accumulation (patency). However, Vg has been reported in the cytoplasm of follicular cells, indicating that there may be a transcellular route for its transport. This study verified the presence of VgR in the follicular cells of the ovaries of the honeybee Apis mellifera and the wasp Polistes simillimus in order to evaluate if Vg is transported via transcytosis in these insects. Antibodies specific for vitellogenin receptor (anti-VgR), vitellogenin (anti-Vg), and clathrin (anti-Clt) were used for immunolocalization. The results showed the presence of VgR on the apical and basal plasma membranes of follicular cells of the vitellogenic follicles in both species, indicating that VgR may have been transported from the basal to the apical cell domain, followed by its release into the perivitelline space, evidenced by the presence of apical plasma membrane projections containing VgR. Co-localization proved that Vg bind to VgR and that the transport of this protein is mediated by clathrin. These data suggest that, in these social insects, Vg is transported via clathrin-mediated VgR transcytosis in follicular cells.

Juvenile hormone downregulates vitellogenin production in Ectatomma tuberculatum (Hymenoptera: Formicidae) sterile workers

In the ant Ectatomma tuberculatum (Olivier, 1792), workers have active ovaries and lay trophic eggs that are eaten by the queen and larvae. Vitellogenins are the main proteins found in the eggs of insects and are the source of nutrients for the embryo in the fertilized eggs and for adults when in the trophic eggs. In social insects, vitellogenin titers vary between castes and affect reproductive social status, nursing, foraging, longevity, somatic maintenance, and immunity. In most insects, vitellogenin synthesis is mainly regulated by juvenile hormone. However, in non-reproductive worker ants, this relationship is poorly characterized. This study determined the effects of juvenile hormone on vitellogenin synthesis in non-reproductive E. tuberculatum workers. Juvenile hormone was topically applied onto workers, and the effect on vitellogenin synthesis in the fat body and vitellogenin titers in the haemolymph were analyzed by ELISA and qPCR. Juvenile hormone downregulated protein synthesis and reduced vitellogenin titers in the haemolymph, suggesting that in workers E. tuberculatum, juvenile hormone loses its gonadotrophic function.