Flow cytometry as a rapid and reliable method to quantify sperm viability in the honeybeeApis mellifera

Establishment of a rapid, cost-effective, and accurate method for assessing insect sperm viability

High-quality sperm cells are crucial to reproductive success for both males and post-mating females in animals. Sperm viability, defined as the proportion of viable sperm cells, is used as a sperm quality index and this method has provided new insights into research on reproductive strategies. However, current staining protocols could potentially underestimate viability due to cell damage caused by cell treatments such as high dye concentration and long time for post-mounting. In this study, we established a method that enables rapid sperm viability assessment, has low sperm cell toxicity, and provides precise results regardless of operator expertise, and cost-effective using sperm cells from an ant, Crematogaster osakensis (Hymenoptera). First, to shorten the time for observation of a sufficient number of sperm cells, the volume per field of view was increased by height elevation between the glass slide and the coverslip, thereby we increased the number of sperm cells in a field of view. Second, to reduce sperm cell toxicity, we optimized the minimum dye concentration and incubation time using acridine orange (AO) and Hoechst in addition to SYBR14 and propidium iodide (PI), which has been used in most previous studies. We determined the optimal protocol to be 1 µg/mL AO and 150 µM PI without incubation. Besides, we automated counting sperm cells with ImageJ software and combined with manual correction for more accurate results. We employed the improved method for sperm samples from mealworm beetles (Tenebrio molitor) and silkmoths (Bombyx mori). This method, established through our study, will advance research on reproductive strategies, including sperm competition and sperm quality maintenance in females.

Validation of Calcein Violet as a New Marker of Semen Membrane Integrity in Domestic Animals

Many fluorochromes routinely used in semen quality analysis emit in the green and red channels, limiting their possible combination for multiple parameter analysis. The use of fluorophores emitting in different light channels broadens the possibilities of combination to expand the range of simultaneously evaluated criteria. This is of great interest in cases of small ejaculated volumes, such as those naturally occurring in roosters, small dog breeds and drones (Apis mellifera). The purpose of this experiment is to establish Calcein Violet (CaV), a blue fluorochrome, as a marker of viability and acrosomal integrity in domestic animals in order to free the red and green channels. SYBR®14/Propidium Iodide (PI) was used as reference dye, heat-treated samples as negative controls, serial staining combination for validation and epifluorescence microscopy for observation. Dead spermatozoa marked in red with PI showed no blue fluorescence either from the head or the tail. Live spermatozoa showed a decreasing blue emission from head to tail when single stained with CaV. Unreacted acrosomes showed intense blue fluorescence irrespective of plasma membrane integrity. This needs to be further confirmed for species with small and difficult to observe heads. Establishment of CaV as a marker of membrane integrity by fluorescence microscopy is a decisive first step towards further technical development and use with flow cytometry.

Sperm competition increases sperm production and quality in Cataglyphis desert ants

Sperm competition is a pervasive evolutionary force that shapes sperm traits to maximize fertilization success. Indeed, it has been shown to increase sperm production in both vertebrates and invertebrates. However, sperm production is energetically costly, which may result in trade-offs among sperm traits. In eusocial hymenopterans, such as ants, mating dynamics impose unique selective pressures on ejaculate. Males are sperm limited: they enter adulthood with a fixed amount of sperm that will not be renewed. We explored whether sperm competition intensity was associated with sperm quantity and quality (i.e. sperm viability and DNA fragmentation) in nine Cataglyphis desert ants. Our results provide phylogenetically robust evidence that sperm competition is positively correlated with sperm production and sperm viability. However, it was unrelated to sperm DNA integrity, indicating the absence of a trade-off involving this trait. These findings underscore that sperm competition may strongly mould sperm traits and drive reproductive performance in eusocial Hymenoptera.

Recent progress in cytometric technologies and their applications in ecotoxicology and environmental risk assessment

Environmental toxicology focuses on identifying and predicting impact of potentially toxic anthropogenic chemicals on biosphere at various levels of biological organization. Presently there is a significant drive to gain deeper understanding of cellular and sub-cellular mechanisms of ecotoxicity. Most notable is increased focus on elucidation of cellular-response networks, interactomes, and greater implementation of cell-based biotests using high-throughput procedures, while at the same time decreasing the reliance on standard animal models used in ecotoxicity testing. This is aimed at discovery and interpretation of molecular pathways of ecotoxicity at large scale. In this regard, the applications of cytometry are perhaps one of the most fundamental prospective analytical tools for the next generation and high-throughput ecotoxicology research. The diversity of this modern technology spans flow, laser-scanning, imaging, and more recently, Raman as well as mass cytometry. The cornerstone advantages of cytometry include the possibility of multi-parameter measurements, gating and rapid analysis. Cytometry overcomes, thus, limitations of traditional bulk techniques such as spectrophotometry or gel-based techniques that average the results from pooled cell populations or small model organisms. Novel technologies such as cell imaging in flow, laser scanning cytometry, as well as mass cytometry provide innovative and tremendously powerful capabilities to analyze cells, tissues as well as to perform in situ analysis of small model organisms. In this review, we outline cytometry as a tremendously diverse field that is still vastly underutilized and often largely unknown in environmental sciences. The main motivation of this work is to highlight the potential and wide-reaching applications of cytometry in ecotoxicology, guide environmental scientists in the technological aspects as well as popularize its broader adoption in environmental risk assessment.

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.

Sperm viability varies with buffer and genotype in Drosophila melanogaster

Sperm quality, an important male fitness trait, is commonly compared between studies. However, few studies consider how genetic and environmental variation affect sperm quality, even in the genetic model Drosophila melanogaster. Here we show that sperm viability, the proportion of live sperm, differed across the genotypes Oregon-R, Dahomey, and Canton-S by more than 15%, and across buffers (phosphate-buffered saline (PBS), Grace's Medium and Drosophila Ringer solution) by more than 20%. In terms of genotype-buffer pair comparisons, nearly half of the comparisons would produce significant differences in sperm viability (15 in 36), or its temporal decrease in a stress medium (19 in 36). Grace's medium produced the longest-lived sperm in vitro and the smallest differences between genotypes, Drosophila Ringer Solution produced the shortest lifespan and the largest differences. Our results suggest that fly and other sperm researchers would benefit from a standardized protocol of measuring sperm viability.

Sperm Quality Assessment in Honey Bee Drones

The quality of honey bee drone semen is relevant in different contexts, ranging from colony productivity to pathology, toxicology and biodiversity preservation. Despite its importance, considerably less knowledge is available on this subject for the honey bee when compared to other domestic animal species. A proper assessment of sperm quality requires a multiple testing approach which discriminates between the different aspects of sperm integrity and functionality. Most studies on drone semen quality have only assessed a few parameters, such as sperm volume, sperm concentration and/or sperm plasma membrane integrity. Although more recent studies have focused on a broader variety of aspects of semen quality, some techniques currently used in vertebrates, such as computer-assisted sperm analysis (CASA) or multiparametric sperm quality testing, still remain to be developed in the honey bee. This may be attributed to the particular sperm morphology and physiology in this species, requiring the development of technologies specifically adapted to it. This article reviews the present knowledge of sperm quality in honey bee drones, highlighting its peculiarities and proposing future lines of research.

Quercetin supplementation to the thawing and incubation media of boar sperm improves post-thaw sperm characteristics and the in vitro production of pig embryos

Elevated levels of reactive oxygen species can cause oxidative stress, which could lead to membrane damage, decreased fertility, and spermatozoan morphological deformities. Antioxidants can be supplemented to reduce the impacts of oxidative stress. The objective of this study was to determine the effects of supplementing quercetin (0.25, 0.50, 0.75 mM) during the thawing and incubation of frozen-thawed boar semen on spermatozoan characteristics, IVF kinetics (n = 400) and subsequent embryonic development (n = 1340). Spermatozoa were evaluated for motility, viability, and membrane lipid peroxidation levels at 0, 2, 4, 6, 8, and 10 h after thawing. Embryos were evaluated for IVF kinetics 12 h after IVF (penetration, polyspermy, male pronucleus formation, IVF efficiency) and cleavage and blastocyst formation at 48 h and 144 h after IVF, respectively. Spermatozoa supplemented with 0.25 mM quercetin had significantly higher (P < 0.05) motility (51.67±8.50 %) and percent of viable cells (61.21 ± 2.44 %) compared to all other treatments at 10 h after thawing, in addition to having significantly (P < 0.05) lower levels of hydroperoxide (3.38 ± 0.88 μM/10⁷cells). There were no differences in penetration rates and male pronucleus formation between treatment groups. Supplementation of quercetin significantly decreased (P < 0.05) polyspermy and significantly increased (P < 0.05) the percentage of embryos reaching blastocyst stage of development by 144 h after IVF compared to no supplementation. Results indicated that supplementing frozen-thawed boar semen with 0.25 mM quercetin improves sperm characteristics up to 10 h after thawing and decreases polyspermy while improving early embryonic development in pigs.

Impact of immune activation on stored sperm viability in ant queens

Ant queens mate on a single occasion early in life and store millions of sperm cells in their spermatheca. By carefully using stored sperm to fertilize eggs, they can produce large colonies of thousands of individuals. Queens can live for decades and their lifetime reproductive success is dependent on their ability to keep stored sperm alive. Maintaining high sperm viability requires metabolic energy which could trade-off with other costly processes such as immunity. We tested the impact of immune activation on the survival of stored sperm by prompting Lasius niger ant queens to mount a melanization response and subsequently measuring sperm viability in their spermatheca. Since queens face different challenges that influence energy allocation depending on the life stage of their colony, we measured sperm viability after immune activation in both newly mated queens (incipient) and in queens 1 year after mating (established). We found that immune activation reduced sperm viability in established queens but not in incipient queens, showing that the cost of immunity on sperm preservation depends on the life stage. Unexpectedly, established queens had significantly higher sperm viability in their spermatheca compared to incipient queens suggesting that ant queens are able to remove dead sperm from their spermatheca.

Protein restriction affects sperm number but not sperm viability in male ants

Sperm cells are costly to produce; diet should therefore affect sperm number and/or viability. In non-social insects and vertebrates, there is compelling evidence that diet influences sperm production. Less is known about this relationship in eusocial hymenopterans (all ants and some bees and wasps), whose mating systems impose unique selective pressures on sperm production. Males face physiological constraints: they acquire all of the resources they will use in future reproductive efforts as larvae and emerge from the pupal stage with a fixed, non-renewable amount of sperm. Furthermore, males die shortly after copulation, but their genetic material persists for years since their spermatozoa are stored in their mates' spermathecae. We examined the effects of protein restriction during larval development on sperm number and viability in the Argentine ant Linepithema humile. We also looked at its impact on male development, adult mass, and adult fluctuating asymmetry. We found that protein restriction during larval development significantly reduced sperm production, but not sperm viability. It did not affect the number of males reared, male mass, or male asymmetry. However, males from protein-restricted nests developed much more slowly than males from protein-supplemented nests. These results suggest investing in sperm quality and in somatic growth, which enhances a male's ability to disperse and find a mate, are critical to successful male reproduction.

Improving the future of honey bee breeding programs by employing recent scientific advances

A recent abundance of studies investigating causes of honey bee (Apis mellifera) colony losses has led to enhanced recommendations in management practices with particular emphasis on breeding for resistant bee stocks. Here we review the latest advances in research which could improve the future of breeding programs. We discuss diversity in colonies particularly in breeding programs, giving special emphasis to recent improvement in cryopreservation of honey bee germplasm. We also review factors that affect the health and reproductive quality of queens and drones. We briefly discuss how techniques developed by scientists are finding more regular usage with breeders in the assessment of reproductive caste health and quality and in determining best management practices for breeding programs.

Consequences of Nosema apis infection for male honey bees and their fertility

The queens of eusocial bees, ants and wasps mate only during a very short period early in life and males therefore produce ejaculates consisting of large numbers of high quality sperm. Such extreme selection for high fecundity resulted in males investing minimally into their somatic survival, including their immune system. However, if susceptible males are unable to protect their reproductive tissue from infections, they compromise queen fitness if they transfer pathogens during mating. We used the honey bee Apis mellifera and investigated the course of infection of the sexually transmitted pathogen Nosema apis. We predicted that honey bee males are susceptible but protect their reproductive tissues from infections. We investigated the effects of N. apis infections on the midgut, the accessory glands and the accessory testes and quantified the consequences of infection on male survival and fecundity. We found that N. apis is able to infect males, and as infections progressed, it significantly impacted fertility and survival in older males. Even though we confirm males to be able to minimize N. apis infections of their reproductive tissues, the parasite is present in ejaculates of older males. Consequently N. apis evolved alternative routes to successfully infect ejaculates and get sexually transmitted.