Physiological and Environmental Factors Affecting the Composition of the Ejaculate in Mosquitoes and Other Insects

The effects of female age on blood-feeding, insemination, sperm storage, and fertility in the dengue vector mosquito Aedes aegypti (Diptera: Culicidae)

Mating induces behavioral and physiological changes in female insects-collectively referred to as the female post-mating response (PMR)-that facilitate the production of progeny. PMRs are elicited by transfer of male-derived seminal components during mating, but are altered by other factors, including adult age. Increased female age is often accompanied by declines in fertility. However, mating shortly after emergence also impacts fertility in the insect model Drosophila melanogaster. Here, we determined the age post-emergence when females of the vector mosquito Aedes aegypti can be inseminated and blood-feed. We next examined fecundity, fertility, and the storage of sperm in the female reproductive tract in "young" (30-41 hours-old) and "old" (2- and 3-week-old) females, finding that blood-feeding began at 14 hours, and mating at ∼24 hours post-emergence. Although young females consumed smaller blood quantities and stored fewer sperm, they were similarly fertile to 4-day-old controls. Old females, however, suffered significant declines in fecundity by 2 weeks of age. Our results show that female Ae. aegypti start to become sexually receptive 1 day after their emergence, but can ingest blood much sooner, suggesting that mating is not a prerequisite to blood-feeding, and that females can ingest an arbovirus infected blood-meal shortly after emergence.

A testis-expressing heme peroxidase HPX12 regulates male fertility in the mosquito Anopheles stephensi

In vertebrates dysregulation of the antioxidant defense system has a detrimental impact on male fertility and reproductive physiology. However, in insects, especially mosquitoes the importance of sperm quality has been poorly studied. Since long-term storage of healthy and viable sperm earmarks male reproductive competency, we tested whether the heme peroxidase, a member of antioxidant enzyme family proteins, and abundantly expressed in the testis, also influence male fertility in the mosquito An. stephensi. Here, we show that a heme peroxidase 12 (HPX12), is an important cellular factor to protect the sperms from oxidative stress, and maintains semen quality in the male mosquito reproductive organ. We demonstrate that knockdown of the HPX12 not only impairs the sperm parameters such as motility, viability but also causes a significant down-regulation of MAG expressing transcripts such as ASTEI02706, ASTEI00744, ASTEI10266, likely encoding putative Accessory gland proteins. Mating with HPX12 knockdown male mosquitoes, resulted in ~ 50% reduction in egg-laying, coupled with diminished larval hatchability of a gravid female mosquito. Our data further outlines that increased ROS in the HPX12 mRNA depleted mosquitoes is the ultimate cause of sperm disabilities both qualitatively as well as quantitatively. Our data provide evidence that testis expressing AsHPX12 is crucial for maintaining optimal homeostasis for storing and protecting healthy sperms in the male mosquito's reproductive organs. Since, high reproductive capacity directly influences the mosquito population, manipulating male mosquito reproductive physiology could be an attractive tool to combat vector-borne diseases.

Effects of Mating on Gene Expression in Female Insects: Unifying the Field

Simple Summary

Insects play many important roles including in ecosystems, food production, pathogen transmission, and production of materials. As a result, humans are interested in understanding how to control insect population sizes for control, propagation, or conservation efforts. In many insect species, female reproductive output is promoted by mating and components of the ejaculate. Beyond just the impact of receiving sperm, mating and ejaculate components can result in increased rate of oocyte development, ovulation, and oviposition as well as other changes such as reduced mating receptivity. To understand how mating causes these changes, researchers have investigated changes in female gene expression that occur after mating. In this review, we summarize the current state of knowledge on mating-induced gene expression changes in female insects and the methods used for conducting such studies. We find that genes related to immune response, chemosensation, and metabolism are commonly regulated across species. We suggest future research paths to facilitate the comparison of studies on mating-regulated gene expression across insect species.

Abstract

There is intense interest in controlling insect reproductive output. In many insect species, reproductive output is profoundly influenced by mating, including the receipt of sperm and seminal fluid molecules, through physiological and behavior changes. To understand these changes, many researchers have investigated post-mating gene expression regulation. In this review, we synthesize information from studies both across and within different species about the impact of mating, or components of mating, on female gene expression patterns. We found that genes related to the roles of metabolism, immune-response, and chemosensation are regulated by mating across many different insect species. We highlight the few studies that have taken the important next step of examining the functional consequences of gene expression regulation which is crucial in order to understand the mechanisms underlying the mating-regulated control of female lifespan and reproduction and to make use of such knowledge to propagate or control insect populations. The potential of cross-study comparisons is diminished by different studies using different methods. Thus, we also include a consideration of how future studies could be designed to facilitate cross-study comparisons and a call for collaboration across researchers studying different insect species and different aspects of insect biology.

Beyond Sperm and Male Accessory Gland Proteins: Exploring Insect Reproductive Metabolomes

Insect seminal fluid, the non-sperm component of the ejaculate, comprises a variegated set of molecules, including, but not limited to, lipids, proteins, carbohydrates, salts, hormones, nucleic acids, and vitamins. The identity and functional role of seminal fluid proteins (SFPs) have been widely investigated, in multiple species. However, most of the other small molecules in insect ejaculates remain uncharacterized. Metabolomics is currently adopted to deepen our understanding of complex biological processes and in the last 15years has been applied to answer different physiological questions. Technological advances in high-throughput methods for metabolite identification such as mass spectrometry and nuclear magnetic resonance (NMR) are now coupled to an expanded bioinformatics toolbox for large-scale data analysis. These improvements allow for the processing of smaller-sized samples and for the identification of hundreds to thousands of metabolites, not only in Drosophila melanogaster but also in disease vectors, animal, and agricultural pests. In this review, we provide an overview of the studies that adopted metabolomics-based approaches in insects, with a particular focus on the reproductive tract (RT) of both sexes and the ejaculate. Progress in the field of metabolomics will contribute not only to achieve a deeper understanding of the composition of insect ejaculates and how they are affected by endogenous and exogenous factors, but also to provide increasingly powerful tools to decipher the identity and molecular interactions between males and females during and after mating.

Composition, seasonal abundance, and public health importance of mosquito species in the regional unit of Thessaloniki, Northern Greece

Mosquitoes (Diptera: Culicidae) are the largest group of blood-feeding insects that disturb not only humans but also other mammals and birds. This study reports the presence of native mosquito species in the regional unit of Thessaloniki and the monitoring of their population. In total, 13 mosquito species belonging to four genera were identified. The most dominant species was Culex pipiens, followed by Aedes caspius. In the present study, we report for the first time the presence of Ae. vittatus in Greece and of Anopheles plumbeus in the regional unit of Thessaloniki. Regarding the seasonal variation, species of the genus Aedes were the ones that first appeared in late March, followed by Culex species at the end of April and finally species of the genus Anopheles in July. Species of the Aedes genus were found to be the most abundant in the first quarter of the year (late March to early April). Population of Cx. pipiens remained at high levels from late April to late September. Species of the genus Anopheles were found in high densities from early August to October. The current study contributes to the knowledge of the mosquito species composition and their relative abundance in an area where West Nile virus caused severe epidemic outbreaks.

Octopamine signaling is involved in the female postmating state in Nilaparvata lugens Stål (Hemiptera: Delphacidae)

Mating triggers physiological and behavioral changes in female insects. In many species, females experience postmating behavioral and physiological changes that define a post-mated state. These changes are comprised of several conditions, including long-term refractoriness to re-mating and increased production and laying of eggs. Here, we report that mating led to several changes in brown planthopper (BPH) females, including increased octopamine (OA), cAMP concentrations, and activities of several enzymes. Mating also led to changes in the expression of several genes acting in female physiology, including those in the cAMP/PKA signal transduction pathway. OA injections into virgin females led to similar changes. RNAi silencing of the gene encoding tyramine β-hydroxylase, involved in the final step in OA synthesis, led to decreased expression of these genes, and reduced the cAMP/PKA signaling. At the whole-organism level, the RNAi treatments led to reduced fecundity, body weights, and longevity. RNAi silencing of genes acting in OA signaling led to truncated ovarian development, egg maturation, and ovarian vitellogenin (Vg) uptake. The impact of these decreases is also registered at the population level, seen as decreased population growth. We infer that OA signaling modulates the postmating state in female BPH and possibly other hemipterans.

Real-time PCR-based Y-specific sperm quantification assay in Queensland fruit fly: Insights to patterns of sperm storage

Studies of reproductive biology in insects often require quantification of sperm production, transfer or storage. Here, we develop a quantitative real-time PCR-based assay using a Y-specific marker for quantification of sperm from spermathecae of female Queensland fruit fly ('Q-fly'), overcoming constraints typical of traditional sperm quantification methods. The assay enables accurate and reliable quantification of as few as 50 sperms and provides a means to analyse large numbers of samples with flexible timing. The real-time PCR method enables revised understanding of how many sperms are stored by female Q-flies, the distribution of storage between the two spermathecae and the relationship between copula duration and sperm storage. Real-time PCR assays based on Y-specific markers provide an effective solution for sperm quantification in tephritid flies, as well as in other insects and potentially other animals with sperm storage organs.

Determining the effects of nutrition on the reproductive physiology of male mosquitoes

Nutrition affects multiple aspects of insect physiology such as body size and fecundity, but we lack a detailed understanding of how nutrition influences the reproductive physiology of male insects such as mosquitoes. Given that female mosquitoes are vectors of many deadly diseases and can quickly proliferate, understanding how male nutrition impacts female fecundity could be of critical importance. To uncover the relationship between nutrition in adult male mosquitoes and its impacts on reproductive physiology, we reared larvae of the Northern house mosquito, Culex pipiens, on a standard lab diet and divided adult males among three different dietary treatments: low (3%), moderate (10%), and high (20%) sucrose. We found that although overall body size did not differ among treatments, one-week-old males raised on the 3% sucrose diet had significantly smaller male accessory glands (MAGs) compared to males that consumed the 10% and the 20% sucrose diets. Diet affected whole-body lipid content but did not affect whole-body protein content. Using nuclear magnetic resonance (NMR) spectroscopy, we found that diet altered the metabolic composition of the MAGs, including changes in lactic acid, formic acid, and glucose. We also observed changes in protein and lipid abundance and composition in MAGs. Females who mated with males on the 3% diet were found to produce significantly fewer larvae than females who had mated with males on the 10% diet. Taken together, our results demonstrate that the diet of adult male mosquitoes clearly affects male reproductive physiology and female fecundity.

Mating and blood-feeding induce transcriptome changes in the spermathecae of the yellow fever mosquito Aedes aegypti

Aedes aegypti mosquitoes are the primary vectors of numerous viruses that impact human health. As manipulation of reproduction has been proposed to suppress mosquito populations, elucidation of biological processes that enable males and females to successfully reproduce is necessary. One essential process is female sperm storage in specialized structures called spermathecae. Aedes aegypti females typically mate once, requiring them to maintain sperm viably to fertilize eggs they lay over their lifetime. Spermathecal gene products are required for Drosophila sperm storage and sperm viability, and a spermathecal-derived heme peroxidase is required for long-term Anopheles gambiae fertility. Products of the Ae. aegypti spermathecae, and their response to mating, are largely unknown. Further, although female blood-feeding is essential for anautogenous mosquito reproduction, the transcriptional response to blood-ingestion remains undefined in any reproductive tissue. We conducted an RNAseq analysis of spermathecae from unfed virgins, mated only, and mated and blood-fed females at 6, 24, and 72 h post-mating and identified significant differentially expressed genes in each group at each timepoint. A blood-meal following mating induced a greater transcriptional response in the spermathecae than mating alone. This study provides the first view of elicited mRNA changes in the spermathecae by a blood-meal in mated females.

Structure of the reproductive system of the sexual generation of the endemic Arctic species Acyrthosiphon svalbardicum and its temperate counterpart Acyrthosiphon pisum (Hemiptera, Aphididae)

The Arctic aphids live briefly and must breed quickly to survive. Shortened life cycle, with only two generations: the stem mother and sexuales-oviparous females and males is an adaptation for optimal use of the short breeding period, which lasts from late July to the end of August. Using Acyrthosiphon svalbardicum, an endemic High Arctic aphid species, we describe the structure of the reproductive system of sexual morphs and compare with its temperate counterparts, in particular the model organism the pea aphid Acyrthosiphon pisum. Generally, the histological composition and ultrastructure of reproductive system of sexuales of A. svalbardicum is broadly similar to the reproductive systems described already in other species of aphids. The unique characters include in both oviparous females and males an enormous layer of the fat body, adhering to the structures of the internal reproductive system. The greatly enlarged accessory glands of males accumulate a heterogenous secretion composed of irregularly organized bunches of spicule-like structures of high electron density embedded in fine and coarse granular material. This material, unknown among temperate counterparts of A. svalbardicum, during mating is transported from the accessory glands of the male to its ejaculatory duct, where it is mixed with the ejaculate, and then is transferred to the spermatheca of the oviparous female.

Venereal Transmission of Vesicular Stomatitis Virus by Culicoides sonorensis Midges

Culicoides sonorensis biting midges are well-known agricultural pests and transmission vectors of arboviruses such as vesicular stomatitis virus (VSV). The epidemiology of VSV is complex and encompasses a broad range of vertebrate hosts, multiple routes of transmission, and diverse vector species. In temperate regions, viruses can overwinter in the absence of infected animals through unknown mechanisms, to reoccur the next year. Non-conventional routes for VSV vector transmission may help explain viral maintenance in midge populations during inter-epidemic periods and times of adverse conditions for bite transmission. In this study, we examined whether VSV could be transmitted venereally between male and female midges. Our results showed that VSV-infected females could venereally transmit virus to uninfected naïve males at a rate as high as 76.3% (RT-qPCR), 31.6% (virus isolation) during the third gonotrophic cycle. Additionally, VSV-infected males could venereally transmit virus to uninfected naïve females at a rate as high as 76.6% (RT-qPCR), 49.2% (virus isolation). Immunofluorescent staining of micro-dissected reproductive organs, immunochemical staining of midge histological sections, examination of internal reproductive organ morphology, and observations of mating behaviors were used to determine relevant anatomical sites for virus location and to hypothesize the potential mechanism for VSV transmission in C. sonorensis midges through copulation.

Male sexual history influences female fertility and re-mating incidence in the mosquito vector Aedes aegypti (Diptera: Culicidae)

Aedes aegypti is the primary vector of several arboviruses that impact human health including the dengue, Zika, and yellow fever viruses. The potential of Ae. aegypti females to transmit viruses is enhanced by mating-induced behavioral and physiological changes that increase female host-seeking behaviors, blood-feeding frequency and longevity. The mating-induced changes are due to female receipt of male seminal fluid proteins (SFPs) during copulation. SFPs also inhibit female re-mating-re-mating incidence is significantly reduced in the initial hours after mating and nearly absent after 24 h. Males, however, are not limited in the number of females they can inseminate and are able to mate with multiple females in succession. As successive mating depletes SFPs, we examined parameters of fertility and re-mating incidence in females after mating with recently mated males. Males of two Ae. aegypti strains (Colombian and Thai) were mated five consecutive times and fecundity, resulting larvae and hatch percentage in each female of the mating sequence was assessed. In both strains, we found that males can mate three times in succession without impacting fertility in their mates. However, significant declines in fecundity, resulting larvae, and hatch percentage were observed after a third mating. Male size influenced female fecundity and fertility as mates of small males showed further reductions compared to mates of big males after mating consecutively. Seven days after the consecutive mating assays, the re-mating rate of females mated fifth in succession was significantly increased (Colombian strain: 33%; Thai strain: 48%) compared to females mated first (0% in both strains). Re-mating incidence was further increased in small, Thai strain males where 82% of fifth mated females re-mated compared to 0% of first mated females. Finally, we show that regardless of male size, mates of experimental males were similarly fertile to mates of control males when mated for a sixth time 48 h after the consecutive mating assays, showing that males recover fertility after 2 d. Our results show that male sexual history influences fertility and re-mating incidence of Ae. aegypti females.

Modulation of seminal fluid molecules by males and females

In insects, seminal fluid molecules (SFMs) influence female post-mating phenotypes that affect reproductive success including egg development, sperm use, mating behavior, attractiveness, and lifespan. Yet, the magnitude of these effects can be quite variable, even within inbred strains. This variation is important because it could impact post-copulatory reproductive success of both males and females. One likely cause of this variation is modulation by males or females of the quantities or qualities (e.g. stability or activity state) of SFMs, or, in the case of females, of their sensitivity to SFMs. Here, I review opportunities for SFM modulation by males and females and propose that these processes could provide mechanisms by which information received before and during copulation influences post-copulatory reproductive success.

The reproductive system of the male and oviparous female of a model organism-the pea aphid, Acyrthosiphon pisum (Hemiptera, Aphididae)

The structure of the reproductive system of the sexual generation-males and oviparous females-of the pea aphid Acyrthosiphon pisum (Harris) (Hemiptera, Aphididae), a serious pest of cultivated plants of Fabaceae, was investigated. For the first time we describe the morphology, histology and ultrastructure of the reproductive system in both morphs of the sexual generation of aphids within one species, using light and fluorescent microscopy, as well as transmission and scanning electron microscopy. The results revealed that males have testes composed of three follicles fused by the upper ends of the vasa efferentia, the vasa deferentia run independently, the accessory glands are asymmetric and the ejaculatory duct shortened. Oviparous females have ovaries composed of seven ovarioles each. The lateral oviducts join to a short common oviduct connected with the unpaired spermatheca and paired accessory glands. Yolky eggs with an aggregation of symbiotic bacteria at the posterior pole are produced. Histologically, the components of genital tracts are broadly similar: the epithelial cells of the walls of the vasa deferentia and accessory glands of the male and oviparous female have secretory functions which correlate with the age of the studied morphs. We also found symbiotic bacteria within the vasa deferentia epithelial cells in males and within the cells of the lateral oviducts of females. Because the pea aphid is listed among the 14 species that are of the greatest economic importance, our results will be useful for managing aphid populations, protecting plants and ensuring global food security.