Seminal proteins but not sperm induce morphological changes in the Drosophila melanogaster female reproductive tract during sperm storage

Regional specialization, polyploidy, and seminal fluid transcripts in the Drosophila female reproductive tract

Sexual reproduction requires the choreographed interaction of female cells and molecules with sperm and seminal fluid. In internally fertilizing animals, these interactions are managed by specialized tissues within the female reproductive tract (FRT), such as a uterus, glands, and sperm storage organs. However, female somatic reproductive tissues remain understudied, hindering insight into the molecular interactions that support fertility. Here, we report the identification, molecular characterization, and analysis of cell types throughout the somatic FRT in the premier Drosophila melanogaster model system. We find that the uterine epithelia is composed of 11 distinct cell types with well-delineated spatial domains, likely corresponding to functionally specialized surfaces that interact with gametes and reproductive fluids. Polyploidy is pervasive: More than half of lower reproductive tract cells are ≥4C. While seminal fluid proteins (SFPs) are typically thought of as male products that are transferred to females, we find that specialized cell types in the sperm storage organs heavily invest in expressing SFP genes. Rates of amino acid divergence between closely related species indicate heterogeneous evolutionary processes acting on male-limited versus female-expressed seminal fluid genes. Together, our results emphasize that more than 40% of annotated seminal fluid genes are better described as shared components of reproductive transcriptomes, which may function cooperatively to support spermatozoa. More broadly, our work provides the molecular foundation for improved technologies to catalyze the functional characterization of the FRT.

Neurobiology of egg-laying behavior in Drosophila: neural control of the female reproductive system

Egg-laying is one of the key aspects of female reproductive behavior in insects. Egg-laying has been studied since the dawn of Drosophila melanogaster as a model organism. The female's internal state, hormones, and external factors, such as nutrition, light, and social environment, affect egg-laying output. However, only recently, neurobiological features of egg-laying behavior have been studied in detail. fruitless and doublesex, two key players in the sex determination pathway, have become focal points in identifying neurons of reproductive significance in both central and peripheral nervous systems. The reproductive tract and external terminalia house sensory neurons that carry the sensory information of egg maturation, mating and egg-laying. These sensory signals include the presence of male accessory gland products and mechanical stimuli. The abdominal neuromere houses neurons that receive information from the reproductive tract, including sex peptide abdominal ganglion neurons (SAGs), and send their information to the brain. In the brain, neuronal groups like aDNs and pC1 clusters modulate egg-laying decision-making, and other neurons like oviINs and oviDNs are necessary for egg-laying itself. Lastly, motor neurons involved in egg-laying, which are mostly octopaminergic, reside in the abdominal neuromere and orchestrate the muscle movements required for laying the egg. Egg-laying neuronal control is important in various evolutionary processes like cryptic female choice, and using different Drosophila species can provide intriguing avenues for the future of the field.

Cell cycle variants during Drosophila male accessory gland development

The Drosophila melanogaster male accessory gland (AG) is a functional analog of the mammalian prostate and seminal vesicles containing two secretory epithelial cell types, termed main and secondary cells. This tissue is responsible for making and secreting seminal fluid proteins and other molecules that contribute to successful reproduction. The cells of this tissue are binucleate and polyploid, due to variant cell cycles that include endomitosis and endocycling during metamorphosis. Here, we provide evidence of additional cell cycle variants in this tissue. We show that main cells of the gland are connected by ring canals that form after the penultimate mitosis, and we describe an additional post-eclosion endocycle required for gland maturation that is dependent on juvenile hormone signaling. We present evidence that the main cells of the D. melanogaster AG undergo a unique cell cycle reprogramming throughout organ development that results in step-wise cell cycle truncations culminating in cells containing two octoploid nuclei with under-replicated heterochromatin in the mature gland. We propose this tissue as a model to study developmental and hormonal temporal control of cell cycle variants in terminally differentiating tissues.

Oncogenic signaling in the adult Drosophila prostate-like accessory gland leads to activation of a conserved pro-tumorigenic program, in the absence of proliferation

Drosophila models for tumorigenesis and metastasis have revealed conserved mechanisms of signaling that are also involved in mammalian cancer. Many of these models use the proliferating tissues of the larval stages of Drosophila development, when tissues are highly mitotically active, or stem cells are abundant. Fewer Drosophila tumorigenesis models use adult animals to initiate tumor formation when many tissues are largely terminally differentiated and postmitotic. The Drosophila accessory glands are prostate-like tissues and a model for some aspects of prostate tumorigenesis using this tissue has been explored. In this model, oncogenic signaling was induced during the proliferative stage of accessory gland development, raising the question of how oncogenic activity would impact the terminally differentiated and postmitotic adult tissue. Here, we show that oncogenic signaling in the adult Drosophila accessory gland leads to activation of a conserved pro-tumorigenic program, similar to that observed in mitotic larval tissues, but in the absence of proliferation. Oncogenic signaling in the adult postmitotic gland leads to tissue hyperplasia with nuclear anaplasia and aneuploidy through endoreduplication, which increases polyploidy and occasionally results in non-mitotic neoplastic-like extrusions. We compare gene expression changes in our Drosophila model with that of endocycling prostate cancer cells induced by chemotherapy, which potentially mediate tumor recurrence after treatment. Similar signaling pathways are activated in the Drosophila gland and endocycling cancer cells, suggesting the adult accessory glands provide a useful model for aspects of prostate cancer progression that do not involve cellular proliferation.

Three-dimensional correlative microscopy of the Drosophila female reproductive tract reveals modes of communication in seminal receptacle sperm storage

In many taxa, females store sperm in specialized storage organs. Most insect sperm storage organs have a tubular structure, typically consisting of a central lumen surrounded by epithelial cells. These specialized tubules perform the essential tasks of transporting sperm through the female reproductive tract and supporting long-term sperm survival and function. Little is known about the way in which female sperm storage organs provide an environment conducive to sperm survival. We address this using a combined light microscopy, micro computed tomography (microCT), and Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) approach for high-resolution correlative three-dimensional imaging to advance our understanding of sperm-female interactions in Drosophila melanogaster. Using this multimodal approach, we were able to scan the lower female reproductive tract and distal portion of the seminal receptacle at low magnification, and to subsequently zoom in for further analysis on an ultrastructural level. Our findings highlight aspects of the way in which the seminal receptacle keeps sperm viable in the lumen, and set the stage for further studies. The methods developed are suitable not only for Drosophila but also for other organisms with soft, delicate tissues.

The Drosophila melanogaster Y-linked gene, WDY, is required for sperm to swim in the female reproductive tract

Unique patterns of inheritance and selection on Y chromosomes have led to the evolution of specialized gene functions. We report CRISPR mutants in Drosophila of the Y-linked gene, WDY, which is required for male fertility. We demonstrate that the sperm tails of WDY mutants beat approximately half as fast as those of wild-type and that mutant sperm do not propel themselves within the male ejaculatory duct or female reproductive tract. Therefore, although mature sperm are produced by WDY mutant males, and are transferred to females, those sperm fail to enter the female sperm storage organs. We report genotype-dependent and regional differences in sperm motility that appear to break the correlation between sperm tail beating and propulsion. Furthermore, we identify a significant change in hydrophobicity at a residue at a putative calcium-binding site in WDY orthologs at the split between the melanogaster and obscura species groups, when WDY first became Y-linked. This suggests that a major functional change in WDY coincided with its appearance on the Y chromosome. Finally, we show that mutants for another Y-linked gene, PRY, also show a sperm storage defect that may explain their subfertility. Overall, we provide direct evidence for the long-held presumption that protein-coding genes on the Drosophila Y regulate sperm motility.

Transcriptional programs are activated and microRNAs are repressed within minutes after mating in the Drosophila melanogaster female reproductive tract

BACKGROUND

The female reproductive tract is exposed directly to the male's ejaculate, making it a hotspot for mating-induced responses. In Drosophila melanogaster, changes in the reproductive tract are essential to optimize fertility. Many changes occur within minutes after mating, but such early timepoints are absent from published RNA-seq studies. We measured transcript abundances using RNA-seq and microRNA-seq of reproductive tracts of unmated and mated females collected at 10-15 min post-mating. We further investigated whether early transcriptome changes in the female reproductive tract are influenced by inhibiting BMPs in secondary cells, a condition that depletes exosomes from the male's ejaculate.

RESULTS

We identified 327 differentially expressed genes. These were mostly upregulated post-mating and have roles in tissue morphogenesis, wound healing, and metabolism. Differentially abundant microRNAs were mostly downregulated post-mating. We identified 130 predicted targets of these microRNAs among the differentially expressed genes. We saw no detectable effect of BMP inhibition in secondary cells on transcript levels in the female reproductive tract.

CONCLUSIONS

Our results indicate that mating induces early changes in the female reproductive tract primarily through upregulation of target genes, rather than repression. The upregulation of certain target genes might be mediated by the mating-induced downregulation of microRNAs. Male-derived exosomes and other BMP-dependent products were not uniquely essential for this process. Differentially expressed genes and microRNAs provide candidates that can be further examined for their participation in the earliest alterations of the reproductive tract microenvironment.

The Male Reproductive System of the Kissing Bug, Rhodnius prolixus Stål, 1859 (Hemiptera: Reduviidae: Triatominae): Arrangements of the Muscles and the Myoactivity of the Selected Neuropeptides

The gross anatomy of the male reproductive structures and their associated musculature are described in the blood-gorging vector of Chagas disease, Rhodnius prolixus. The male reproductive system is composed of muscular tissues each performing contractions that aid in the movement of sperm out of the testis into the vas deferens, seminal vesicle and then into the ejaculatory duct, along with proteins and lipids from the transparent and opaque accessory glands. Phalloidin staining shows the various patterns of muscle fiber layers, from thin circular to more complex crisscross patterns, implying subtle differences in the form of the contractions and movement of each of the structures, allowing for waves of contractions or twisting patterns. The transcripts for the receptors for proctolin, myosuppressin (RhoprMS) and for the extended FMRFamides are expressed in the various regions of the reproductive system, and the nerve processes extending over the reproductive structures are positive for FMRFamide-like immunoreactivity, as are neurosecretory cells lying on the nerves. Proctolin and AKDNFIRFamide are strong stimulators for the frequency of the contractions, and RhoprMS can inhibit the proctolin-induced contractions. Taken together, this work implicates these two families of peptides in coordinating the male reproductive structures for the successful transfer of sperm and the associated accessory gland fluid to the female during copulation.

The Drosophila melanogaster Y-linked gene, WDY, is required for sperm to swim in the female reproductive tract

Unique patterns of inheritance and selection on Y chromosomes lead to the evolution of specialized gene functions. Yet characterizing the function of genes on Y chromosomes is notoriously difficult. We report CRISPR mutants in Drosophila of the Y-linked gene, WDY, which is required for male fertility. WDY mutants produce mature sperm with beating tails that can be transferred to females but fail to enter the female sperm storage organs. We demonstrate that the sperm tails of WDY mutants beat approximately half as fast as wild-type sperm's and that the mutant sperm do not propel themselves within the male ejaculatory duct or female reproductive tract (RT). These specific motility defects likely cause the sperm storage defect and sterility of the mutants. Regional and genotype-dependent differences in sperm motility suggest that sperm tail beating and propulsion do not always correlate. Furthermore, we find significant differences in the hydrophobicity of key residues of a putative calcium-binding domain between orthologs of WDY that are Y-linked and those that are autosomal. Given that WDY appears to be evolving under positive selection, our results suggest that WDY's functional evolution coincides with its transition from autosomal to Y-linked in Drosophila melanogaster and its most closely related species. Finally, we show that mutants for another Y-linked gene, PRY, also show a sperm storage defect that may explain their subfertility. In contrast to WDY, PRY mutants do swim in the female RT, suggesting they are defective in yet another mode of motility, navigation, or a necessary interaction with the female RT. Overall, we provide direct evidence for the long-held presumption that protein-coding genes on the Drosophila Y regulate sperm motility.

Sperm storage by females across the animal phyla: A survey on the occurrence and biomolecules involved in sperm storage

Long-term sperm storage by females in various regions of the oviduct is documented across many invertebrate and vertebrate species. Although, many reports emphasize on the histology, histochemistry and ultrastructural features of sperm storage, very little is known about the mechanisms underlying the sperm storage. The current review documents the occurrence of sperm storage by females in a wide array of invertebrate and vertebrate species. This review also provides an insight on the presence of various molecular factors of the sperm storage tubules presumably responsible for the prolonged sperm storage with an emphasis on a model reptile, the Indian garden lizard, Calotes versicolor which contains a unique approximately 55-kDa protein in its utero-vaginal lavage and found to inhibit washed epididymal sperm motility in a concentration and time-dependent manner in a reversible fashion.

No evidence for asymmetric sperm deposition in a species with asymmetric male genitalia

Background

Asymmetric genitalia have repeatedly evolved in animals, yet the underlying causes for their evolution are mostly unknown. The fruit fly Drosophila pachea has asymmetric external genitalia and an asymmetric phallus with a right-sided phallotrema (opening for sperm release). The complex of female and male genitalia is asymmetrically twisted during copulation and males adopt a right-sided copulation posture on top of the female. We wished to investigate if asymmetric male genital morphology and a twisted gentitalia complex may be associated with differential allocation of sperm into female sperm storage organs.

Methods

We examined the internal complex of female and male reproductive organs by micro-computed tomography and synchrotron X-ray tomography before, during and after copulation. In addition, we monitored sperm aggregation states and timing of sperm transfer during copulation by premature interruption of copulation at different time-points.

Results

The asymmetric phallus is located at the most caudal end of the female abdomen during copulation. The female reproductive tract, in particular the oviduct, re-arranges during copulation. It is narrow in virgin females and forms a broad vesicle at 20 min after the start of copulation. Sperm transfer into female sperm storage organs (spermathecae) was only in a minority of examined copulation trials (13/64). Also, we found that sperm was mainly transferred early, at 2-4 min after the start of copulation. We did not detect a particular pattern of sperm allocation in the left or right spermathecae. Sperm adopted a granular or filamentous aggregation state in the female uterus and spermathecae, respectively.

Discussion

No evidence for asymmetric sperm deposition was identified that could be associated with asymmetric genital morphology or twisted complexing of genitalia. Male genital asymmetry may potentially have evolved as a consequence of a complex internal alignment of reproductive organs during copulation in order to optimize low sperm transfer rates.

Octopaminergic/tyraminergic Tdc2 neurons regulate biased sperm usage in female Drosophila melanogaster

In polyandrous internally fertilizing species, a multiply-mated female can use stored sperm from different males in a biased manner to fertilize her eggs. The female's ability to assess sperm quality and compatibility is essential for her reproductive success, and represents an important aspect of postcopulatory sexual selection. In Drosophila melanogaster, previous studies demonstrated that the female nervous system plays an active role in influencing progeny paternity proportion, and suggested a role for octopaminergic/tyraminergic Tdc2 neurons in this process. Here, we report that inhibiting Tdc2 neuronal activity causes females to produce a higher-than-normal proportion of first-male progeny. This difference is not due to differences in sperm storage or release, but instead is attributable to the suppression of second-male sperm usage bias that normally occurs in control females. We further show that a subset of Tdc2 neurons innervating the female reproductive tract is largely responsible for the progeny proportion phenotype that is observed when Tdc2 neurons are inhibited globally. On the contrary, overactivation of Tdc2 neurons does not further affect sperm storage, release or progeny proportion. These results suggest that octopaminergic/tyraminergic signaling allows a multiply-mated female to bias sperm usage, and identify a new role for the female nervous system in postcopulatory sexual selection.

The Effects of Male Seminal Fluid Proteins on Gut/Gonad Interactions in Drosophila

Mating initiates broad physiological changes encompassing multiple organ systems in females. Elucidating the complex inter- and intra-organ signaling events that coordinate these physiological changes is an important goal in the field of reproductive biology. Further characterization of these complex molecular and physiological interactions is key to understanding how females meet the energetic demands of offspring production. Many recent studies of the fruit fly, Drosophila melanogaster, have described the mechanisms of post-mating changes within the female reproductive tract and digestive system. Additionally, other studies have described post-mating signaling crosstalk between these systems. Interestingly, male seminal fluid proteins have been linked to post-mating responses within the female reproductive tract and gut, and to signaling events between the two organ systems. However, information about the hormonal and neuronal signaling pathways underlying the post-mating signaling events within and between the reproductive tract and digestive systems that are triggered by seminal fluid proteins has yet to be combined into a single view. In this article, we summarize and integrate these studies into a single "network schematic" of the known signaling events within and between the reproductive and digestive systems downstream of male seminal fluid proteins. This synthesis also draws attention to the incomplete parts of these pathways, so that outstanding questions may be addressed in future studies.

A standardized nomenclature and atlas of the female terminalia of Drosophila melanogaster

The model organism Drosophila melanogaster has become a focal system for investigations of rapidly evolving genital morphology as well as the development and functions of insect reproductive structures. To follow up on a previous paper outlining unifying terminology for the structures of the male terminalia in this species, we offer here a detailed description of the female terminalia of D. melanogaster. Informative diagrams and micrographs are presented to provide a comprehensive overview of the external and internal reproductive structures of females. We propose a collection of terms and definitions to standardize the terminology associated with the female terminalia in D. melanogaster and we provide a correspondence table with the terms previously used. Unifying terminology for both males and females in this species will help to facilitate communication between various disciplines, as well as aid in synthesizing research across publications within a discipline that has historically focused principally on male features. Our efforts to refine and standardize the terminology should expand the utility of this important model system for addressing questions related to the development and evolution of animal genitalia, and morphology in general.

Two Male-Specific Antimicrobial Peptides SCY2 and Scyreprocin as Crucial Molecules Participated in the Sperm Acrosome Reaction of Mud Crab Scylla paramamosain

Antimicrobial peptides (AMPs) identified in the reproductive system of animals have been widely studied for their antimicrobial activity, but only a few studies have focused on their physiological roles. Our previous studies have revealed the in vitro antimicrobial activity of two male gonadal AMPs, SCY2 and scyreprocin, from mud crab Scylla paramamosain. Their physiological functions, however, remain a mystery. In this study, the two AMPs were found co-localized on the sperm apical cap. Meanwhile, progesterone was confirmed to induce acrosome reaction (AR) of mud crab sperm in vitro, which intrigued us to explore the roles of the AMPs and progesterone in AR. Results showed that the specific antibody blockade of scyreprocin inhibited the progesterone-induced AR without affecting intracellular Ca2+ homeostasis, while the blockade of SCY2 hindered the influx of Ca2+. We further showed that SCY2 could directly bind to Ca2+. Moreover, progesterone failed to induce AR when either scyreprocin or SCY2 function was deprived. Taken together, scyreprocin and SCY2 played a dual role in reproductive immunity and sperm AR. To our knowledge, this is the first report on the direct involvement of AMPs in sperm AR, which would expand the current understanding of the roles of AMPs in reproduction.

Population differences and domestication effects on mating and remating frequencies in Queensland fruit fly

Females of many insect species are unreceptive to remating for a period following their first mating. This inhibitory effect may be mediated by either the female or her first mate, or both, and often reflects the complex interplay of reproductive strategies between the sexes. Natural variation in remating inhibition and how this phenotype responds to captive breeding are largely unexplored in insects, including many pest species. We investigated genetic variation in remating propensity in the Queensland fruit fly, Bactrocera tryoni, using strains differing in source locality and degree of domestication. We found up to threefold inherited variation between strains from different localities in the level of intra-strain remating inhibition. The level of inhibition also declined significantly during domestication, which implied the existence of genetic variation for this trait within the starting populations as well. Inter-strain mating and remating trials showed that the strain differences were mainly due to the genotypes of the female and, to a lesser extent, the second male, with little effect of the initial male genotype. Implications for our understanding of fruit fly reproductive biology and population genetics and the design of Sterile Insect Technique pest management programs are discussed.

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.

Silencing the triacylglycerol lipase (TGL) gene decreases the number of apyrene sperm and inhibits oviposition in Sitotroga cerealella

Triacylglycerol lipase (TGL) is an essential lipid metabolism enzyme that also plays a critical role in energy metabolism; however, how it regulates other life processes is unknown. To investigate the functional role of TGL in moth reproduction, males Sitotroga cerealella were used as a model. The TGL gene was cloned and analysed. The results showed that the open reading frame of TGL was 1968 bp long and contained three conserved regions. TGL gene expression was higher in the larval and early adult stages than in the pupal stage, with the highest levels observed in the fat body, testis and accessory glands during the early adult stage. Moreover, after TGL in male adults was silenced through RNAi, the protein content in male accessory glands remained unchanged, and the spermatophore transferred into females mated with TGL-silenced males became small and empty; meanwhile, the number of apyrene sperm in the spermatophore was significantly reduced due to the reduction of apyrene sperm in males, which eventually led to the significant reduction of egg-laying amount. All of the findings suggest that TGL regulates the amount of sperm in male moths as well as the morphology and quality of spermatophores transferred to females after mating with treated males, implying that TGL is critical for Sitotroga cerealella's reproductive process.

Developmental mechanisms of sex differences: from cells to organisms

Male-female differences in many developmental mechanisms lead to the formation of two morphologically and physiologically distinct sexes. Although this is expected for traits with prominent differences between the sexes, such as the gonads, sex-specific processes also contribute to traits without obvious male-female differences, such as the intestine. Here, we review sex differences in developmental mechanisms that operate at several levels of biological complexity - molecular, cellular, organ and organismal - and discuss how these differences influence organ formation, function and whole-body physiology. Together, the examples we highlight show that one simple way to gain a more accurate and comprehensive understanding of animal development is to include both sexes.

Post-mating gene expression of Mexican fruit fly females: disentangling the effects of the male accessory glands

Mating has profound physiological and behavioural consequences for female insects. During copulation, female insects typically receive not only sperm, but a complex ejaculate containing hundreds of proteins and other molecules from male reproductive tissues, primarily the reproductive accessory glands. The post-mating phenotypes affected by male accessory gland (MAG) proteins include egg development, attraction to oviposition hosts, mating, attractiveness, sperm storage, feeding and lifespan. In the Mexican fruit fly, Anastrepha ludens, mating increases egg production and the latency to remating. However, previous studies have not found a clear relationship between injection of MAG products and oviposition or remating inhibition in this species. We used RNA-seq to study gene expression in mated, unmated and MAG-injected females to understand the potential mating- and MAG-regulated genes and pathways in A. ludens. Both mating and MAG-injection regulated transcripts and pathways related to egg development. Other transcripts regulated by mating included those with orthologs predicted to be involved in immune response, musculature and chemosensory perception, whereas those regulated by MAG-injection were predicted to be involved in translational control, sugar regulation, diet detoxification and lifespan determination. These results suggest new phenotypes that may be influenced by seminal fluid molecules in A. ludens. Understanding these influences is critical for developing novel tools to manage A. ludens.

Prospects in Connecting Genetic Variation to Variation in Fertility in Male Bees

Bees are economically and ecologically important pollinating species. Managed and native bee species face increasing pressures from human-created stressors such as habitat loss, pesticide use, and introduced pathogens. There has been increasing attention towards how each of these factors impacts fertility, especially sperm production and maintenance in males. Here, we turn our attention towards another important factor impacting phenotypic variation: genetics. Using honey bees as a model, we explore the current understanding of how genetic variation within and between populations contributes to variation in sperm production, sperm maintenance, and insemination success among males. We conclude with perspectives and future directions in the study of male fertility in honey bees and non-Apis pollinators more broadly, which still remain largely understudied.

Drosophila female reproductive tract gene expression reveals coordinated mating responses and rapidly evolving tissue-specific genes

Sexual reproduction in internally fertilizing species requires complex coordination between female and male reproductive systems and among the diverse tissues of the female reproductive tract (FRT). Here, we report a comprehensive, tissue-specific investigation of Drosophila melanogaster FRT gene expression before and after mating. We identified expression profiles that distinguished each tissue, including major differences between tissues with glandular or primarily nonglandular epithelium. All tissues were enriched for distinct sets of genes possessing secretion signals that exhibited accelerated evolution, as might be expected for genes participating in molecular interactions between the sexes within the FRT extracellular environment. Despite robust transcriptional differences between tissues, postmating responses were dominated by coordinated transient changes indicative of an integrated systems-level functional response. This comprehensive characterization of gene expression throughout the FRT identifies putative female contributions to postcopulatory events critical to reproduction and potentially reproductive isolation, as well as the putative targets of sexual selection and conflict.

Expression of Mst89B and CG31287 is Needed for Effective Sperm Storage and Egg Fertilization in Drosophila

In Drosophila, male reproductive fitness can be affected by any number of processes, ranging from development of gametes, transfer to and storage of mature sperm within the female sperm storage organs, and utilization of sperm for fertilization. We have previously identified the 89B cytogenetic map position of D. melanogaster as a hub for genes that effect male paternity success when disturbed. Here, we used RNA interference to test 11 genes that are highly expressed in the testes and located within the 89B region for their role in sperm competition and male fecundity when their expression is perturbed. Testes-specific knockdown (KD) of bor and CSN5 resulted in complete sterility, whereas KD of CG31287, Manf and Mst89B, showed a breakdown in sperm competitive success when second to mate (P2 < 0.5) and reduced fecundity in single matings. The low fecundity of Manf KD is explained by a significant reduction in the amount of mature sperm produced. KD of Mst89B and CG31287 does not affect sperm production, sperm transfer into the female bursa or storage within 30 min after mating. Instead, a significant reduction of sperm in female storage is observed 24 h after mating. Egg hatchability 24 h after mating is also drastically reduced for females mated to Mst89B or CG31287 KD males, and this reduction parallels the decrease in fecundity. We show that normal germ-line expression of Mst89B and CG31287 is needed for effective sperm usage and egg fertilization.

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.

Does the Act of Copulation per se, without Considering Seminal Deposition, Change the Expression of Genes in the Porcine Female Genital Tract?

Semen—through its specific sperm and seminal plasma (SP) constituents—induces changes of gene expression in the internal genital tract of pigs, particularly in the functional sperm reservoir at the utero-tubal junction (UTJ). Although seminal effects are similarly elicited by artificial insemination (AI), major changes in gene expression are registered after natural mating, a fact suggesting the act of copulation induces per se changes in genes that AI does not affect. The present study explored which pathways were solely influenced by copulation, affecting the differential expression of genes (DEGs) of the pre/peri-ovulatory genital tract (cervix, distal uterus, proximal uterus and UTJ) of estrus sows, 24 h after various procedures were performed to compare natural mating with AI of semen (control 1), sperm-free SP harvested from the sperm-peak fraction (control 2), sperm-free SP harvested from the whole ejaculate (control 3) or saline-extender BTS (control 4), using a microarray chip (GeneChip^® porcine gene 1.0 st array). Genes related to neuroendocrine responses (ADRA1, ADRA2, GABRB2, CACNB2), smooth muscle contractility (WNT7A), angiogenesis and vascular remodeling (poFUT1, NTN4) were, among others, overrepresented with distal and proximal uterine segments exhibiting the highest number of DEGs. The findings provide novel evidence that relevant transcriptomic changes in the porcine female reproductive tract occur in direct response to the specific act of copulation, being semen-independent.

Reproductive isolation caused by azoospermia in sterile male hybrids of Drosophila

Recently diverged populations in the early stages of speciation offer an opportunity to understand mechanisms of isolation and their relative contributions. Drosophila willistoni is a tropical species with broad distribution from Argentina to the southern United States, including the Caribbean islands. A postzygotic barrier between northern populations (North America, Central America, and the northern Caribbean islands) and southern populations (South American and the southern Caribbean islands) has been recently documented and used to propose the existence of two different subspecies. Here, we identify premating isolation between populations regardless of their subspecies status. We find no evidence of postmating prezygotic isolation and proceeded to characterize hybrid male sterility between the subspecies. Sterile male hybrids transfer an ejaculate that is devoid of sperm but causes elongation and expansion of the female uterus. In sterile male hybrids, bulging of the seminal vesicle appears to impede the movement of the sperm toward the sperm pump, where sperm normally mixes with accessory gland products. Our results highlight a unique form of hybrid male sterility in Drosophila that is driven by a mechanical impediment to transfer sperm rather than by an abnormality of the sperm itself. Interestingly, this form of sterility is reminiscent of a form of infertility (azoospermia) that is caused by lack of sperm in the semen due to blockages that impede the sperm from reaching the ejaculate.

Evolution of Reproductive Behavior

Behaviors associated with reproduction are major contributors to the evolutionary success of organisms and are subject to many evolutionary forces, including natural and sexual selection, and sexual conflict. Successful reproduction involves a range of behaviors, from finding an appropriate mate, courting, and copulation, to the successful production and (in oviparous animals) deposition of eggs following mating. As a consequence, behaviors and genes associated with reproduction are often under strong selection and evolve rapidly. Courtship rituals in flies follow a multimodal pattern, mediated through visual, chemical, tactile, and auditory signals. Premating behaviors allow males and females to assess the species identity, reproductive state, and condition of their partners. Conflicts between the "interests" of individual males, and/or between the reproductive strategies of males and females, often drive the evolution of reproductive behaviors. For example, seminal proteins transmitted by males often show evidence of rapid evolution, mediated by positive selection. Postmating behaviors, including the selection of oviposition sites, are highly variable and Drosophila species span the spectrum from generalists to obligate specialists. Chemical recognition features prominently in adaptation to host plants for feeding and oviposition. Selection acting on variation in pre-, peri-, and postmating behaviors can lead to reproductive isolation and incipient speciation. Response to selection at the genetic level can include the expansion of gene families, such as those for detecting pheromonal cues for mating, or changes in the expression of genes leading to visual cues such as wing spots that are assessed during mating. Here, we consider the evolution of reproductive behavior in Drosophila at two distinct, yet complementary, scales. Some studies take a microevolutionary approach, identifying genes and networks involved in reproduction, and then dissecting the genetics underlying complex behaviors in D. melanogaster Other studies take a macroevolutionary approach, comparing reproductive behaviors across the genus Drosophila and how these might correlate with environmental cues. A full synthesis of this field will require unification across these levels.

Control of mating plug expelling and sperm storage in Drosophila: A gynandromorph- and mutation-based dissection

INTRODUCTION

In this study, we analyzed gynandromorphs with female terminalia, to dissect mating-related female behaviors in Drosophila.

MATERIALS AND METHODS

We used gynandromorphs, experimentally modified wild-type (Oregon-R) females, and mutant females that lacked different components of the female reproductive apparatus.

RESULTS

Many of the gynandromorphs mated but did not expel the mating plug (MP). Some of these - with thousands of sperm in the uterus - failed to take up sperm into the storage organs. There were gynandromorphs that stored plenty of sperm but failed to release them to fertilize eggs. Expelling the MP, sperm uptake into the storage organs, and the release of stored sperm along egg production are separate steps occurring during Drosophila female fertility. Cuticle landmarks of the gynandromorphs revealed that while the nerve foci that control MP expelling and also those that control sperm uptake reside in the abdominal, the sperm release foci derive from the thoracic region of the blastoderm.

DISCUSSION AND CONCLUSION

The gynandromorph study is confirmed by analyses of (a) mutations that cause female sterility: Fs(3)Avar (preventing egg deposition), Tm2^gs (removing germline cells), and iab-4^DB (eliminating gonad formation) and (b) by experimentally manipulated wild-type females: decapitated or cut through ventral nerve cord.

BMP signaling inhibition in Drosophila secondary cells remodels the seminal proteome and self and rival ejaculate functions

Seminal fluid proteins (SFPs) exert potent effects on male and female fitness. Rapidly evolving and molecularly diverse, they derive from multiple male secretory cells and tissues. In Drosophila melanogaster, most SFPs are produced in the accessory glands, which are composed of ∼1,000 fertility-enhancing "main cells" and ∼40 more functionally cryptic "secondary cells." Inhibition of bone morphogenetic protein (BMP) signaling in secondary cells suppresses secretion, leading to a unique uncoupling of normal female postmating responses to the ejaculate: refractoriness stimulation is impaired, but offspring production is not. Secondary-cell secretions might therefore make highly specific contributions to the seminal proteome and ejaculate function; alternatively, they might regulate more global-but hitherto undiscovered-SFP functions and proteome composition. Here, we present data that support the latter model. We show that in addition to previously reported phenotypes, secondary-cell-specific BMP signaling inhibition compromises sperm storage and increases female sperm use efficiency. It also impacts second male sperm, tending to slow entry into storage and delay ejection. First male paternity is enhanced, which suggests a constraint on ejaculate evolution whereby high female refractoriness and sperm competitiveness are mutually exclusive. Using quantitative proteomics, we reveal changes to the seminal proteome that surprisingly encompass alterations to main-cell-derived proteins, indicating important cross-talk between classes of SFP-secreting cells. Our results demonstrate that ejaculate composition and function emerge from the integrated action of multiple secretory cell types, suggesting that modification to the cellular make-up of seminal-fluid-producing tissues is an important factor in ejaculate evolution.

On the evolutionary origins of insect seminal fluid proteins

In most cases, proteins affect the phenotype of the individual in which they are produced. However, in some cases, proteins have evolved in such a way that they are able to influence the phenotype of another individual of the same or of a different species ("influential proteins"). Examples of interspecific influential proteins include venom proteins and proteins produced by parasites that influence their hosts' physiology or behavior. Examples of intraspecific influential proteins include those produced by both mothers and fetuses that mitigate maternal resource allocation and proteins transferred to females in the seminal fluid during mating that change female physiology and behavior. Although there has been much interest in the functions and evolutionary dynamics of these influential proteins, less is known about the origin of these proteins. Where does the DNA that encodes the proteins that can impact another individual's phenotype come from and how do the proteins acquire their influential abilities? In this mini-review, I use insect seminal fluid proteins as a case study to consider the origin of intraspecific influential proteins. The existing data suggest that influential insect seminal fluid proteins arise both through co-option of existing genes (both single copy genes and gene duplicates) and de novo evolution. Other mechanisms for the origin of new insect seminal fluid proteins (e.g., retrotransoposition and horizontal gene transfer) are plausible but have not yet been demonstrated. Additional gaps in our understanding of the origin of insect seminal fluid proteins include an understanding of the cis-regulatory elements that designate expression in the male reproductive tract and of the evolutionary steps by which individual proteins come to depend on other seminal fluid proteins for their activity within the mated female.

Female Genetic Contributions to Sperm Competition in Drosophila melanogaster

In many species, sperm can remain viable in the reproductive tract of a female well beyond the typical interval to remating. This creates an opportunity for sperm from different males to compete for oocyte fertilization inside the female's reproductive tract. In Drosophila melanogaster, sperm characteristics and seminal fluid content affect male success in sperm competition. On the other hand, although genome-wide association studies (GWAS) have demonstrated that female genotype plays a role in sperm competition outcome as well, the biochemical, sensory, and physiological processes by which females detect and selectively use sperm from different males remain elusive. Here, we functionally tested 26 candidate genes implicated via a GWAS for their contribution to the female's role in sperm competition, measured as changes in the relative success of the first male to mate (P1). Of these 26 candidates, we identified eight genes that affect P1 when knocked down in females, and showed that five of them do so when knocked down in the female nervous system. In particular, Rim knockdown in sensory pickpocket (ppk)+ neurons lowered P1, confirming previously published results, and a novel candidate, caup, lowered P1 when knocked down in octopaminergic Tdc2+ neurons. These results demonstrate that specific neurons in the female's nervous system play a functional role in sperm competition and expand our understanding of the genetic, neuronal, and mechanistic basis of female responses to multiple matings. We propose that these neurons in females are used to sense, and integrate, signals from courtship or ejaculates, to modulate sperm competition outcome accordingly.

Sperm success and immunity

The moment of the fertilization of an egg by a spermatozoon-the point of "sperm success"-is a key milestone in the biology of sexually reproducing species and is a fundamental requirement for offspring production. Fertilization also represents the culmination of a suite of sexually selected processes in both sexes and is commonly used as a landmark to measure reproductive success. Sperm success is heavily dependent upon interactions with other key aspects of male and female biology, with the immune system among the most important. The immune system is vital to maintaining health in both sexes; however, immune reactions can also have antagonistic effects on sperm success. The effects of immunity on sperm success are diverse, and may include trade-offs in the male between investment in the production or protection of sperm, as well as more direct, hostile, immune responses to sperm within the female, and potentially the male, reproductive tract. Here, we review current understanding of where the biology of immunity and sperm meet, and identify the gaps in our knowledge.

Sexual conflict drives male manipulation of female postmating responses in Drosophila melanogaster

In species with males and females, reproduction requires contributions from both sexes and therefore some degree of cooperation. At the same time, antagonistic interactions can evolve because of the differing goals of males and females. We aligned the interests of the sexes in the naturally promiscuous fruit fly Drosophila melanogaster by enforcing randomized monogamy for more than 150 generations. Males repeatedly evolved to manipulate females less, a pattern visible in both the timing of female reproductive effort and gene expression changes after mating. Male investment in expression of genes encoding seminal fluid proteins, which shape the female postmating response, declined concurrently. Our results confirm the presence of sexually antagonistic selection on postcopulatory interactions that can be reversed by monogamy.

In many animals, females respond to mating with changes in physiology and behavior that are triggered by molecules transferred by males during mating. In Drosophila melanogaster, proteins in the seminal fluid are responsible for important female postmating responses, including temporal changes in egg production, elevated feeding rates and activity levels, reduced sexual receptivity, and activation of the immune system. It is unclear to what extent these changes are mutually beneficial to females and males or instead represent male manipulation. Here we use an experimental evolution approach in which females are randomly paired with a single male each generation, eliminating any opportunity for competition for mates or mate choice and thereby aligning the evolutionary interests of the sexes. After >150 generations of evolution, males from monogamous populations elicited a weaker postmating stimulation of egg production and activity than males from control populations that evolved with a polygamous mating system. Males from monogamous populations did not differ from males from polygamous populations in their ability to induce refractoriness to remating in females, but they were inferior to polygamous males in sperm competition. Mating-responsive genes in both the female abdomen and head showed a dampened response to mating with males from monogamous populations. Males from monogamous populations also exhibited lower expression of genes encoding seminal fluid proteins, which mediate the female response to mating. Together, these results demonstrate that the female postmating response, and the male molecules involved in eliciting this response, are shaped by ongoing sexual conflict.

Drosophila melanogaster tPlus3a and tPlus3b ensure full male fertility by regulating transcription of Y-chromosomal, seminal fluid, and heat shock genes

Spermatogenesis in Drosophila melanogaster is characterized by a specific transcriptional program during the spermatocyte stage. Transcription of thousands of genes is regulated by the interaction of several proteins or complexes, including a tTAF-containing TFIID variant, tMAC, Mediator, and chromatin interactors, e.g., bromodomain proteins. We addressed how distinct subsets of target genes are selected. We characterized the highly similar proteins tPlus3a and tPlus3b, which contain a Plus3 domain and are enriched in the testis, mainly in spermatocytes. In tPlus3a and tplus3b deletion mutants generated using the CRISPR/Cas9 system, fertility was severely reduced and sperm showed defects during individualization. tPlus3a and tPlus3b heterodimerized with the bromodomain protein tBRD-1. To elucidate the role of the tPlus3a and tPlus3b proteins in transcriptional regulation, we determined the transcriptomes of tplus3a-tplus3b and tbrd-1 deletion mutants using next-generation sequencing (RNA-seq) and compared them to that of the wild-type. tPlus3a and tPlus3b positively or negatively regulated the expression of nearly 400 genes; tBRD-1 regulated 1,500 genes. Nearly 200 genes were regulated by both tPlus3a and tPlus3b and tBRD-1. tPlus3a and tPlus3b activated the Y-chromosomal genes kl-3 and kl-5, which indicates that tPlus3a and tPlus3b proteins are required for the function of distinct classes of genes. tPlus3a and tPlus3b and tBRD-1 repress genes relevant for seminal fluid and heat shock. We hypothesize that tPlus3a and tPlus3b proteins are required to specify the general transcriptional program in spermatocytes.

Rab-mediated trafficking in the secondary cells of Drosophila male accessory glands and its role in fecundity

The male seminal fluid contains factors that affect female post‐mating behavior and physiology. In Drosophila, most of these factors are secreted by the two epithelial cell types that make up the male accessory gland: the main and secondary cells. Although secondary cells represent only ~4% of the cells of the accessory gland, their contribution to the male seminal fluid is essential for sustaining the female post‐mating response. To better understand the function of the secondary cells, we investigated their molecular organization, particularly with respect to the intracellular membrane transport machinery. We determined that large vacuole‐like structures found in the secondary cells are trafficking hubs labeled by Rab6, 7, 11 and 19. Furthermore, these organelles require Rab6 for their formation and many are essential in the process of creating the long‐term postmating behavior of females. In order to better serve the intracellular membrane and protein trafficking communities, we have created a searchable, online, open‐access imaging resource to display our complete findings regarding Rab localization in the accessory gland.

Genes Relocated Between Drosophila Chromosome Arms Evolve Under Relaxed Selective Constraints Relative to Non-Relocated Genes

Gene duplication creates a second copy of a gene either in tandem to the ancestral locus or dispersed to another chromosomal location. When the ancestral copy of a dispersed duplicate is lost from the genome, it creates the appearance that the gene was "relocated" from the ancestral locus to the derived location. Gene relocations may be as common as canonical dispersed duplications in which both the ancestral and derived copies are retained. Relocated genes appear to be under more selective constraints than the derived copies of canonical duplications, and they are possibly as conserved as single-copy non-relocated genes. To test this hypothesis, we combined comparative genomics, population genetics, gene expression, and functional analyses to assess the selection pressures acting on relocated, duplicated, and non-relocated single-copy genes in Drosophila genomes. We find that relocated genes evolve faster than single-copy non-relocated genes, and there is no evidence that this faster evolution is driven by positive selection. In addition, relocated genes are less essential for viability and male fertility than single-copy non-relocated genes, suggesting that relocated genes evolve fast because of relaxed selective constraints. However, relocated genes evolve slower than the derived copies of canonical dispersed duplicated genes. We therefore conclude that relocated genes are under more selective constraints than canonical duplicates, but are not as conserved as single-copy non-relocated genes.

Last male sperm precedence is modulated by female remating rate in Drosophila melanogaster

Following multiple matings, sperm from different males compete for fertilization within the female reproductive tract. In many species, this competition results in an unequal sharing of paternity that favors the most recent mate, termed last male sperm precedence (LMSP). Much of our understanding of LMSP comes from studies in Drosophila melanogaster that focus on twice‐mated females with standardized latencies between successive matings. Despite accumulating evidence indicating that females often mate with more than two males and exhibit variation in the latency between matings, the consequences of mating rate on LMSP are poorly understood. Here, we developed a paradigm utilizing D. melanogaster in which females remated at various time intervals with either two or three transgenic males that produce fluorescent sperm (green, red, or blue). This genetic manipulation enables paternity assessment of offspring and male‐specific sperm fate examination in female reproductive tracts. We found that remating latency had no relationship with LMSP in females that mated with two males. However, LMSP was significantly reduced in thrice‐mated females with short remating intervals; coinciding with reduced last‐male sperm storage. Thus, female remating rate influences the relative share of paternity, the overall clutch paternity diversity, and ultimately the acquisition of indirect genetic benefits to potentially maximize female reproductive success.

Chemical Cues that Guide Female Reproduction in Drosophila melanogaster

Chemicals released into the environment by food, predators and conspecifics play critical roles in Drosophila reproduction. Females and males live in an environment full of smells, whose molecules communicate to them the availability of food, potential mates, competitors or predators. Volatile chemicals derived from fruit, yeast growing on the fruit, and flies already present on the fruit attract Drosophila, concentrating flies at food sites, where they will also mate. Species-specific cuticular hydrocarbons displayed on female Drosophila as they mature are sensed by males and act as pheromones to stimulate mating by conspecific males and inhibit heterospecific mating. The pheromonal profile of a female is also responsive to her nutritional environment, providing an honest signal of her fertility potential. After mating, cuticular and semen hydrocarbons transferred by the male change the female's chemical profile. These molecules make the female less attractive to other males, thus protecting her mate's sperm investment. Females have evolved the capacity to counteract this inhibition by ejecting the semen hydrocarbon (along with the rest of the remaining ejaculate) a few hours after mating. Although this ejection can temporarily restore the female's attractiveness, shortly thereafter another male pheromone, a seminal peptide, decreases the female's propensity to re-mate, thus continuing to protect the male's investment. Females use olfaction and taste sensing to select optimal egg-laying sites, integrating cues for the availability of food for her offspring, and the presence of other flies and of harmful species. We argue that taking into account evolutionary considerations such as sexual conflict, and the ecological conditions in which flies live, is helpful in understanding the role of highly species-specific pheromones and blends thereof, as well as an individual's response to the chemical cues in its environment.

The impact of ageing on male reproductive success in Drosophila melanogaster

Male reproductive ageing has been mainly explained by a reduction in sperm quality with negative effects on offspring development and quality. In addition to sperm, males transfer seminal fluid proteins (Sfps) at mating; Sfps are important determinants of male reproductive success. Receipt of Sfps leads to female post-mating changes including physiological changes, and affects sperm competition dynamics. Using the fruit fly Drosophila melanogaster we studied ageing males' ability to induce female post-mating responses and determined the consequences of male ageing on their reproductive success. We aged males for up to 7weeks and assayed their ability to: i) gain a mating, ii) induce egg-laying and produce offspring, iii) prevent females from remating and iv) transfer sperm and elicit storage after a single mating. We found that with increasing age, males were less able to induce post-mating responses in their mates; moreover ageing had negative consequences for male success in competitive situations. Our findings indicate that with advancing age male flies transferred less effective ejaculates and that Sfp composition might change over a male's lifetime in quantity and/or quality, significantly affecting his reproductive success.

The genetics of egg retention and fertilization success in Drosophila: One step closer to understanding the transition from facultative to obligate viviparity

Oviparous, facultative egg retention enables Drosophila females to withhold fertilized eggs in their reproductive tracts until circumstances favor oviposition. The propensity to retain fertilized eggs varies greatly between species, and is correlated with other reproductive traits, such as egg size and ovariole number. While previous studies have described the phenomenon, no study to date has characterized within-species variation or the genetic basis of the trait. Here, we develop a novel microscope-based method for measuring egg retention in Drosophila females and determine the range of phenotypic variation in mated female egg retention in a subset of 91 Drosophila Genetic Reference Panel (DGRP) lines. We inferred the genetic basis of egg retention using a genome-wide association study (GWAS). Further, the scoring of more than 95,000 stained, staged eggs enabled estimates of fertilization success for each line. We found evidence that ovary- and spermathecae-related genes as well as genes affecting olfactory behavior, male mating behavior, male-female attraction and sperm motility may play a crucial role in post-mating physiology. Based on our findings we also propose potential evolutionary routes toward obligate viviparity. In particular, we propose that the loss of fecundity incurred by viviparity could be offset by benefits arising from enhanced mate discrimination, resource specialization, or modified egg morphology.

Severe Fertility Effects of sheepish Sperm Caused by Failure To Enter Female Sperm Storage Organs in Drosophila melanogaster

In Drosophila, mature sperm are transferred from males to females during copulation, stored in the sperm storage organs of females, and then utilized for fertilization. Here, we report a gene named sheepish (shps) of Drosophila melanogaster that is essential for sperm storage in females. shps mutant males, although producing morphologically normal and motile sperm that are effectively transferred to females, produce very few offspring. Direct counts of sperm indicated that the primary defect was correlated to failure of shps sperm to migrate into the female sperm storage organs. Increased sperm motion parameters were seen in the control after transfer to females, whereas sperm from shps males have characteristics of the motion parameters different from the control. The few sperm that occasionally entered the female sperm storage organs showed no obvious defects in fertilization and early embryo development. The female postmating responses after copulation with shps males appeared normal, at least with respect to conformational changes of uterus, mating plug formation, and female remating rates. The shps gene encodes a protein with homology to amine oxidases, including as observed in mammals, with a transmembrane region at the C-terminal end. The shps mutation was characterized by a nonsense replacement in the third exon of CG13611, and shps was rescued by transformants of the wild-type copy of CG13611 Thus, shps may define a new class of gene responsible for sperm storage.

Cleavage of the Drosophila seminal protein Acp36DE in mated females enhances its sperm storage activity

Sperm storage in the mated female reproductive tract (RT) is required for optimal fertility in numerous species with internal fertilization. In Drosophila melanogaster, sperm storage is dependent on female receipt of seminal fluid proteins (SFPs) during mating. The seminal fluid protein Acp36DE is necessary for the accumulation of sperm into storage. In the female RT, Acp36DE localizes to the anterior mating plug and also to a site in the common oviduct, potentially "corralling" sperm near the entry sites into the storage organs. Genetic studies showed that Acp36DE is also required for a series of conformational changes of the uterus that begin at the onset of mating and are hypothesized to move sperm towards the entry sites of the sperm storage organs. After Acp36DE is transferred to the female RT, the protein is cleaved by the astacin-metalloprotease Semp1. However, the effect of this cleavage on Acp36DE's function in sperm accumulation into storage is unknown. We used mass spectrometry to identify the single cleavage site in Acp36DE. We then mutated this site and tested the effects on sperm storage. Mutations of Acp36DE's cleavage site that slowed or prevented cleavage of the protein slowed the accumulation of sperm into storage, although they did not affect uterine conformational changes in mated females. Moreover, the N-terminal cleavage product of Acp36DE was sufficient to mediate sperm accumulation in storage, and it did so faster than versions of Acp36DE that could not be cleaved or were only cleaved slowly. These results suggest that cleavage of Acp36E may increase the number of bioactive molecules within the female RT, a mechanism similar to that hypothesized for Semp1's other substrate, the seminal fluid protein ovulin.

Functional male accessory glands and fertility in Drosophila require novel ecdysone receptor

In many insects, the accessory gland, a secretory tissue of the male reproductive system, is essential for male fertility. Male accessory gland is the major source of proteinaceous secretions, collectively called as seminal proteins (or accessory gland proteins), which upon transfer, manipulate the physiology and behavior of mated females. Insect hormones such as ecdysteroids and juvenoids play a key role in accessory gland development and protein synthesis but little is known about underlying molecular players and their mechanism of action. Therefore, in the present study, we examined the roles of hormone-dependent transcription factors (Nuclear Receptors), in accessory gland development, function and male fertility of a genetically tractable insect model, Drosophila melanogaster. First, we carried out an RNAi screen involving 19 hormone receptors, individually and specifically, in a male reproductive tissue (accessory gland) for their requirement in Drosophila male fertility. Subsequently, by using independent RNAi/ dominant negative forms, we show that Ecdysone Receptor (EcR) is essential for male fertility due to its requirement in the normal development of accessory glands in Drosophila: EcR depleted glands fail to make seminal proteins and have dying cells. Further, our data point to a novel ecdysone receptor that does not include Ultraspiracle but is probably comprised of EcR isoforms in Drosophila male accessory glands. Our data suggest that this novel ecdysone receptor might act downstream of homeodomain transcription factor paired (prd) in the male accessory gland. Overall, the study suggests novel ecdysone receptor as an important player in the hormonal regulation of seminal protein production and insect male fertility.

Insects are the major contributors to biodiversity and have economic, agricultural and health importance. This unparalleled abundance of insects, in part, can be attributed to their high reproductive potential. In many insects, proteins derived from the accessory gland, the secretory tissue of male reproductive system, are critical for fertility. The production of these accessory gland proteins is regulated by insect hormones but the underlying mechanisms/molecular players remain poorly understood. Elucidation of the same has potential applications in designing pest control management strategies and to understand the effect of environmental chemicals on reproduction. In view of this, we analyzed the role, if any, of various insect hormone receptors in development and function of the male accessory gland in a genetically tractable insect model, Drosophila melanogaster. Here, we report the involvement of Ecdysone receptor (EcR with novel composition) in Drosophila male fertility. We show that the depletion of this receptor causes cell death in male accessory glands, which fail to produce seminal fluid proteins leading to sterility/sub-fertility of Drosophila males. These findings will find potential applications in designing insect pest control strategies.

Sperm-less males modulate female behaviour in Ceratitis capitata (Diptera: Tephritidae)

In the Mediterranean fruit fly, Ceratitis capitata (Wiedemann)(Diptera: Tephritidae), mating has a strong impact on female biology, leading to a decrease in sexual receptivity and increased oviposition and fecundity. Previous studies suggest that sperm transfer may play a role in inducing these behavioural changes. Here we report the identification of a medfly innexin gene, Cc-inx5, whose expression is limited to the germ-line of both sexes. Through RNA interference of this gene, we generated males without testes and, consequently, sperm, but apparently retaining all the other reproductive organs intact. These sperm-less males were able to mate and, like their wild-type counterparts, to induce in their partners increased oviposition rates and refractoriness to remating. Interestingly, matings to sperm-less males results in oviposition rates higher than those induced by copulation with control males. In addition, the observed female post-mating behavioural changes were congruent with changes in transcript abundance of genes known to be regulated by mating in this species. Our results suggest that sperm transfer is not necessary to reduce female sexual receptivity and to increase oviposition and fecundity. These data pave the way to a better understanding of the role/s of seminal components in modulating female post-mating responses. In the long term, this knowledge will be the basis for the development of novel approaches for the manipulation of female fertility, and, consequently, innovative tools to be applied to medfly control strategies in the field.

Don't pull the plug! the Drosophila mating plug preserves fertility

Mating plugs are hardened structures--typically a coagulation of seminal fluid components--that are transferred to, or formed within, the female reproductive tract of numerous animal species (both mammals and insects). Analysis of the role(s) of the mating plug in reproduction has been conducted in a wide array of diverse species. These structures have been proposed to have a multitude of functions, which include altering female re-mating rate, acting as a barrier to re-mating and being required for sperm storage or sperm movement to occur in mated females. A recent analysis of the Drosophila melanogaster mating plug has shown that proper formation of the structure is required for optimal fertility in flies: the Drosophila mating plug is required to retain the ejaculate within the female reproductive tract once mating has terminated. Here, we discuss the possible implications of the Drosophila mating plug in the fertility of this species in light of these new results.

Mating induces developmental changes in the insect female reproductive tract

In response to mating, the Drosophila female undergoes a series of rapid molecular, morphological, behavioral and physiological changes. Studies in Drosophila and other organisms have shown that stimuli received during courtship and copulation, sperm, and seminal fluid are needed for the full mating response and thus reproductive success. Very little is known, however, about how females respond to these male-derived stimuli/factors at the molecular level. More specifically, it is unclear what mechanisms regulate and mediate the mating response, how the signals received during mating are integrated and processed, and what network of molecules are essential for a successful mating response. Moreover, it is yet to be determined whether the rapid transition of the reproductive tract induced by mating is a general phenomenon in insects. This review highlights current knowledge and advances on the developmental switch that rapidly transitions the female from the 'unmated' to 'mated' state.

Evolution of reduced post-copulatory molecular interactions in Drosophila populations lacking sperm competition

In many species with internal fertilization, molecules transferred in the male ejaculate trigger and interact with physiological changes in females. It is controversial to what extent these interactions between the sexes act synergistically to mediate the female switch to a reproductive state or instead reflect sexual antagonism evolved as a by product of sexual selection on males. To address this question, we eliminated sexual selection by enforcing monogamy in populations of Drosophila melanogaster for 65 generations and then measured the expression of male seminal fluid protein genes and genes involved in the female response to mating. In the absence of sperm competition, male and female reproductive interests are perfectly aligned and any antagonism should be reduced by natural selection. Consistent with this idea, males from monogamous populations showed reduced expression of seminal fluid protein genes, 16% less on average than in polygamous males. Further, we identified 428 genes that responded to mating in females. After mating, females with an evolutionary history of monogamy exhibited lower relative expression of genes that were up regulated in response to mating and higher expression of genes that were down-regulated--in other words, their post-mating transcriptome appeared more virgin-like. Surprisingly, these genes showed a similar pattern even before mating, suggesting that monogamous females evolved to be less poised for mating and the accompanying receipt of male seminal fluid proteins. This reduced investment by both monogamous males and females in molecules involved in post-copulatory interactions points to a pervasive role of sexual conflict in shaping these interactions.

The song of the old mother: reproductive senescence in female drosophila

Among animals with multiple reproductive episodes, changes in adult condition over time can have profound effects on lifetime reproductive fitness and offspring performance. The changes in condition associated with senescence can be particularly acute for females who support reproductive processes from oogenesis through fertilization. The pomace fly Drosophila melanogaster is a well-established model system for exploring the physiology of reproduction and senescence. In this review, we describe how increasing maternal age in Drosophila affects reproductive fitness and offspring performance as well as the genetic foundation of these effects. Describing the processes underlying female reproductive senescence helps us understand diverse phenomena including population demographics, condition-dependent selection, sexual conflict, and transgenerational effects of maternal condition on offspring fitness. Understanding the genetic basis of reproductive senescence clarifies the nature of life-history trade-offs as well as potential ways to augment and/or limit female fertility in a variety of organisms.