Microscopic and molecular characterization of ovarian follicle atresia in Rhodnius prolixus Stahl under immune challenge

Functional Role of AsAP in the Reproduction of Adelphocoris suturalis (Hemiptera: Miridae)

Adelphocoris suturalis Jakovlev (Hemiptera: Miridae) is an omnivorous agricultural pest that has severe economic impacts on a diverse range of agricultural crops. Although the targeted disruption of reproductive development among insects has been proposed as a novel control strategy for pest species, the current understanding of the physiology and molecular mechanisms of A. suturalis reproduction is very limited. In this study, we isolated a putative A. suturalisaspartic protease (AsAP) gene that is highly expressed in the fat body and ovaries of sexually mature females. The double-stranded RNA (dsRNA)-mediated knockdown of AsAP suppressed ovarian development and negatively impacted female fertility, which suggested that it plays an essential role in A. suturalis reproduction. The results of this study could help to expand our understanding of A. suturalis reproductive development and have the potential to facilitate the development of effective strategies for the better control of this pest species.

Development of efficient RNAi methods in the corn leafhopper Dalbulus maidis, a promising application for pest control

BACKGROUND

The corn leafhopper Dalbulus maidis is the main vector of important stunting pathogens that affect maize production. Currently, there are no effective methods available to manage this pest without adverse impact on the environment. In this context, genomic-based technologies such as RNA interference (RNAi) provide a more environmentally friendly pest control strategy. Therefore, we aimed to assess the application of RNAi in D. maidis and determine the function of a candidate gene related to insect reproduction and propagation.

RESULTS

We have characterized the core RNAi genes and evaluated the functionality of the RNAi machinery. We assessed the potential of RNAi technology in D. maidis via injection or ingestion of double-stranded RNA (dsRNA) to adult females. We chose Bicaudal C (BicC) as a target gene due to its important role during insect oogenesis. Administration of dsRNA^BicC caused significant reductions in the transcript levels (fold changes up to 170 times) and ovipositions. Phenotypic analysis of the ovaries revealed alterations in oocyte development, providing additional confirmation for our results and supporting the idea that Dmai-BicC is a key player of D. maidis oogenesis.

CONCLUSION

This is, to our knowledge, the first report of efficient RNAi in D. maidis. We believe our findings provide a starting point for future control strategies against one of the most important maize pests in the Americas. © 2022 Society of Chemical Industry.

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.

Silencing of RpATG8 impairs the biogenesis of maternal autophagosomes in vitellogenic oocytes, but does not interrupt follicular atresia in the insect vector Rhodnius prolixus

Follicular atresia is the mechanism by which the oocyte contents are degraded during oogenesis in response to stress conditions, allowing the energetic resources stored in the developing oocytes to be reallocated to optimize female fitness. Autophagy is a conserved intracellular degradation pathway where double-membrane vesicles are formed around target organelles leading to their degradation after lysosome fusion. The autophagy-related protein 8 (ATG8) is conjugated to the autophagic membrane and has a key role in the elongation and closure of the autophagosome. Here we identified one single isoform of ATG8 in the genome of the insect vector of Chagas Disease Rhodnius prolixus (RpATG8) and found that it is highly expressed in the ovary during vitellogenesis. Accordingly, autophagosomes were detected in the vitellogenic oocytes, as seen by immunoblotting and electron microscopy. To test if autophagosomes were important for follicular atresia, we silenced RpATG8 and elicited atresia in vitellogenic females by Zymosan-A injections. We found that silenced females were still able to trigger the same levels of follicle atresia, and that their atretic oocytes presented a characteristic morphology, with accumulated brown aggregates. Regardless of the difference in morphology, RpATG8-silenced atretic oocytes presented the same levels of protein, TAG and PolyP, as detected in control atretic oocytes, as well as the same levels of acidification of the yolk organelles. Because follicular atresia has the ultimate goal of restoring female fitness, we tested if RpATG8-silenced atresia would result in female physiology and behavior changes. Under insectarium conditions, we found that atresia-induced control and RpATG8-silenced females present no changes in blood meal digestion, survival, oviposition, TAG content in the fat body, haemolymph amino acid levels and overall locomotor activity. Altogether, we found that autophagosomes are formed during oogenesis and that the silencing of RpATG8 impairs autophagosome biogenesis in the oocytes. Nevertheless, regarding major macromolecule degradation and adaptations to the fitness costs imposed by triggering an immune response, we found that autophagic organelles are not essential for follicle atresia in R. prolixus.

Follicular atresia is a phenomenon in response to environmental and physiological conditions in which female insects are able to signal the degeneration and resorption of their oocytes. It is crucial for the maintenance of female survival, as the energy stored in the developing oocytes can be reallocated allowing them to adapt to a stress condition. In the context of insect vectors of human diseases, such as flies, bugs and mosquitoes, the ability of the hematophagous female to interrupt oogenesis and reallocate its energy resources is strategic for safeguarding vector fitness. The cellular and molecular mechanisms that govern the oocytes degradation during atresia are mostly unknown. In this work, we found that a special degradation organelle, named autophagosome, is formed in the oocytes, and that these organelles are not needed for the oocytes to be degenerated during atresia in this insect. These findings are important in the context of vector population control as they provide us with knowledge regarding the vector’s specific molecular biology. Information such as these are important, as they can be used for the elaboration and design of novel population control strategies.

Natural infection by the protozoan Leptomonas wallacei impacts the morphology, physiology, reproduction, and lifespan of the insect Oncopeltus fasciatus

Trypanosomatids are protozoan parasites that infect thousands of globally dispersed hosts, potentially affecting their physiology. Several species of trypanosomatids are commonly found in phytophagous insects. Leptomonas wallacei is a gut-restricted insect trypanosomatid only retrieved from Oncopeltus fasciatus. The insects get infected by coprophagy and transovum transmission of L. wallacei cysts. The main goal of the present study was to investigate the effects of a natural infection by L. wallacei on the hemipteran insect O. fasciatus, by comparing infected and uninfected individuals in a controlled environment. The L. wallacei-infected individuals showed reduced lifespan and morphological alterations. Also, we demonstrated a higher infection burden in females than in males. The infection caused by L. wallacei reduced host reproductive fitness by negatively impacting egg load, oviposition, and eclosion, and promoting an increase in egg reabsorption. Moreover, we associated the egg reabsorption observed in infected females, with a decrease in the intersex gene expression. Finally, we suggest alterations in population dynamics induced by L. wallacei infection using a mathematical model. Collectively, our findings demonstrated that L. wallacei infection negatively affected the physiology of O. fasciatus, which suggests that L. wallacei potentially has a vast ecological impact on host population growth.

Long-chain acyl-CoA synthetase 2 knockdown leads to decreased fatty acid oxidation in fat body and reduced reproductive capacity in the insect Rhodnius prolixus

Long-chain acyl-CoA esters are important intermediates in lipid metabolism and are synthesized from fatty acids by long-chain acyl-CoA synthetases (ACSL). The hematophagous insect Rhodnius prolixus, a vector of Chagas' disease, produces glycerolipids in the midgut after a blood meal, which are stored as triacylglycerol in the fat body and eggs. We identified twenty acyl-CoA synthetase genes in R. prolixus, two encoding ACSL isoforms (RhoprAcsl1 and RhoprAcsl2). RhoprAcsl1 transcripts increased in posterior midgut on the second day after feeding, and RhoprAcsl2 was highly transcribed on the tenth day. Both enzymes were expressed in Escherichia coli. Recombinant RhoprACSL1 and RhoprACSL2 had broad pH optima (7.5-9.5 and 6.5-9.5, respectively), were inhibited by triacsin C, and were rosiglitazone-insensitive. Both showed similar apparent Km for palmitic and oleic acid (2-6 μM), but different Km for arachidonic acid (0.5 and 6 μM for RhoprACSL1-Flag and RhoprACSL2-Flag, respectively). The knockdown of RhoprAcsl1 did not result in noticeable phenotypes. However, RhoprACSL2 deficient insects exhibited a 2.5-fold increase in triacylglycerol content in the fat body, and 90% decrease in fatty acid β-oxidation. RhoprAcsl2 knockdown also resulted in 20% increase in lifespan, delayed digestion, 30% reduced oviposition, and 50% reduction in egg hatching. Laid eggs and hatched nymphs showed remarkable alterations in morphology. In summary, R. prolixus ACSL isoforms have distinct roles on lipid metabolism. Although RhoprACSL1 functions remain unclear, we propose that RhoprACSL2 is the main contributor for the formation of the intracellular acyl-CoA pool channeled for β-oxidation in the fat body, and is also required for normal reproduction.

The Role of DmCatD, a Cathepsin D-Like Peptidase, and Acid Phosphatase in the Process of Follicular Atresia in Dipetalogaster maxima (Hemiptera: Reduviidae), a Vector of Chagas' Disease

In this work, we have investigated the involvement of DmCatD, a cathepsin D-like peptidase, and acid phosphatase in the process of follicular atresia of Dipetalogaster maxima, a hematophagous insect vector of Chagas' disease. For the studies, fat bodies, ovaries and hemolymph were sampled from anautogenous females at representative days of the reproductive cycle: pre-vitellogenesis, vitellogenesis as well as early and late atresia. Real time PCR (qPCR) and western blot assays showed that DmCatD was expressed in fat bodies and ovaries at all reproductive stages, being the expression of its active form significantly higher at the atretic stages. In hemolymph samples, only the immunoreactive band compatible with pro-DmCatD was observed by western blot. Acid phosphatase activity in ovarian tissues significantly increased during follicular atresia in comparison to pre-vitellogenesis and vitellogenesis. A further enzyme characterization with inhibitors showed that the high levels of acid phosphatase activity in atretic ovaries corresponded mainly to a tyrosine phosphatase. Immunofluorescence assays demonstrated that DmCatD and tyrosine phosphatase were associated with yolk bodies in vitellogenic follicles, while in atretic stages they displayed a different cellular distribution. DmCatD and tyrosine phosphatase partially co-localized with vitellin. Moreover, their interaction was supported by FRET analysis. In vitro assays using homogenates of atretic ovaries as the enzyme source and enzyme inhibitors demonstrated that DmCatD, together with a tyrosine phosphatase, were necessary to promote the degradation of vitellin. Taken together, the results strongly suggested that both acid hydrolases play a central role in early vitellin proteolysis during the process of follicular atresia.

Bridging developmental boundaries: lifelong dietary patterns modulate life histories in a parthenogenetic insect

Determining the effects of lifelong intake patterns on performance is challenging for many species, primarily because of methodological constraints. Here, we used a parthenogenetic insect (Carausius morosus) to determine the effects of limited and unlimited food availability across multiple life-history stages. Using a parthenogen allowed us to quantify intake by juvenile and adult females and to evaluate the morphological, physiological, and life-history responses to intake, all without the confounding influences of pair-housing, mating, and male behavior. In our study, growth rate prior to reproductive maturity was positively correlated with both adult and reproductive lifespans but negatively correlated with total lifespan. Food limitation had opposing effects on lifespan depending on when it was imposed, as it protracted development in juveniles but hastened death in adults. Food limitation also constrained reproduction regardless of when food was limited, although decreased fecundity was especially pronounced in individuals that were food-limited as late juveniles and adults. Additional carry-over effects of juvenile food limitation included smaller adult size and decreased body condition at the adult molt, but these effects were largely mitigated in insects that were switched to ad libitum feeding as late juveniles. Our data provide little support for the existence of a trade-off between longevity and fecundity, perhaps because these functions were fueled by different nutrient pools. However, insects that experienced a switch to the limited diet at reproductive maturity seem to have fueled egg production by drawing down body stores, thus providing some evidence for a life-history trade-off. Our results provide important insights into the effects of food limitation and indicate that performance is modulated by intake both within and across life-history stages.

Ovarian nutritional resources during the reproductive cycle of the hematophagous Dipetalogaster maxima (Hemiptera: Reduviidae): focus on lipid metabolism

In this study, we have analyzed the changes of the ovarian nutritional resources in Dipetalogaster maxima at representative days of the reproductive cycle: previtellogenesis, vitellogenesis, as well as fasting-induced early and late atresia. As expected, the amounts of ovarian lipids, proteins, and glycogen increased significantly from previtellogenesis to vitellogenesis and then, diminished during atresia. However, lipids and protein stores found at the atretic stages were higher in comparison to those registered at previtellogenesis. Specific lipid staining of ovarian tissue sections evidenced remarkable changes in the shape, size, and distribution of lipid droplets throughout the reproductive cycle. The role of lipophorin (Lp) as a yolk protein precursor was analyzed by co-injecting Lp-OG (where OG is Oregon Green) and Lp-DiI (where DiI is 1,10-dioctadecyl-3,3,30,30-tetramethylindocarbocyanine) to follow the entire particle, demonstrating that both probes colocalized mainly in the yolk bodies of vitellogenic oocytes. Immunofluorescence assays also showed that Lp was associated to yolk bodies, supporting its endocytic pathway during vitellogenesis. The involvement of Lp in lipid delivery to oocytes was investigated in vivo by co-injecting fluorescent probes to follow the fate of the entire particle (Lp-DiI) and its lipid cargo (Lp-Bodipy-FA). Lp-DiI was readily incorporated by vitellogenic oocytes and no lipoprotein uptake was observed in terminal follicles of ovaries at atretic stages. Bodipy-FA was promptly transferred to vitellogenic oocytes and, to a much lesser extent, to previtellogenic follicles and to oocytes of ovarian tissue at atretic stages. Colocalization of Lp-DiI and Lp-Bodipy-FA inside yolk bodies indicated the relevance of Lp in the buildup of lipid and protein oocyte stores during vitellogenesis.

The natural insect peptide Neb-colloostatin induces ovarian atresia and apoptosis in the mealworm Tenebrio molitor

Background

The injection of Neb-colloostatin into T. molitor females causes gonadoinhibitory effects on ovarian development. This peptide inhibits intercellular space formation (patency) in follicular epithelium and results in slowed vitellogenesis, delayed ovulation, reduced number of eggs laid and presumably cell death in the terminal follicles. However, as does the form of cell death in the terminal follicle, the mode of action of Neb-colloostatin remains unknown.

Results

We tested Neb-colloostatin for a sterilizing effect on females of Tenebrio molitor. We report that injection of nanomolar doses of Neb-colloostatin induce ovarian follicle atresia in 4-day old females during their first gonadotropic cycle. Light microscope observations revealed morphological changes in the ovary: after Neb-colloostatin injection the terminal oocytes are significantly smaller and elicit massive follicle resorption, but the control terminal follicles possess translucent ooplasm in oocytes at different stages of vitellogenesis. A patency is visible in follicular epithelium of the control vitellogenic oocytes, whereas peptide injection inhibits intercellular space formation and, in consequence, inhibits vitellogenesis. Confocal and electron microscope examination showed that peptide injection causes changes in the morphology indicating death of follicular cells. We observed F-actin cytoskeleton disorganization, induction of caspase activity, changes in chromatin organization and autophagic vacuole formation. Moreover, the apical cytoplasm of follicular cells is filled with numerous free ribosomes, probably indicating a higher demand for protein biosynthesis, especially in preparation for autophagic vacuole formation. On the other hand, the process of polyribosomes formation is inhibited, indicating the contributing effect of this hormone.

Conclusion

Neb-colloostatin induces atresia in the mealworm ovary. Degeneration of T. molitor follicles includes changes in morphology and viability of follicular cells, and oosorption as a consequence of these changes.

Cell death mechanisms during follicular atresia in Dipetalogaster maxima, a vector of Chagas' disease (Hemiptera: Reduviidae)

In this work we have analyzed the involvement of cell death pathways during the process of follicular atresia in the hematophagous insect vector Dipetalogaster maxima. Standardized insect rearing conditions were established to induce a gradual follicular degeneration stage by depriving females of blood meal during post-vitellogenesis. We first characterized the morpho-histological and ultrastructural changes of the ovarian tissue at early and late follicular atresia by light and transmission electron microscopy. Apoptosis was investigated by DAPI nuclear staining, TUNEL labeling and the detection of active caspase-3 by immunofluorescence. Autophagy was assessed by the measurement of acid phosphatase activity in ovarian homogenates and monitored by the detection of the specific marker of autophagic compartments, LC3. High levels of acid phosphatase activity were detected at all atretic stages. However, follicular cells of follicles undergoing incipient degeneration in early atresia exhibited features of apoptosis such as chromatin condensation, DNA fragmentation and the presence of active caspase-3. The ultrastructural findings and the increased levels of LC3-II found at late follicular atresia supported the relevance of autophagy at this atretic stage, although the extent of autophagosome formation demonstrated that this cell death pathway also occurred at early atresia. In late atresia, follicular cells also displayed more drastic changes compatible with necrosis. Taken together, results showed that apoptosis, autophagy and necrosis were operative during follicular atresia in D. maxima. Moreover, it was shown that the relevance of these cell death mechanisms correlates with the time elapsed since the onset of the degenerative process.