Differential effects of insemination volume and substance on reproductive changes in honey bee queens (Apis mellifera L.)

The number of drones to inseminate a queen with has little potential for optimization of honeybee breeding programs

BACKGROUND

Mating control is a crucial aspect of honeybee breeding. Instrumental insemination of queens gives the breeder maximum control over the genetic origin of the involved drones. However, in addition to the drones' descent, the breeder's control also extends over the number of drones to use for inseminations. Thus far, this aspect has largely been ignored in attempts to optimize honeybee breeding schemes. The literature provides some comparisons between single drone inseminations (SDI) and multi drone inseminations (MDI) but it is unclear whether the number of drones used in MDI is a relevant parameter for the optimization of honeybee breeding programs.

METHODS

By computer simulations, we investigated the effect of the number of drones per inseminated queen in breeding programs that relied on best linear unbiased prediction (BLUP) breeding values. We covered a range of 1 to 50 drones per queen and observed the developments of genetic gain and inbreeding over a period of 20 years. Hereby, we focused on insemination schemes that take the drones for one queen from a single colony.

RESULTS

SDI strategies led to 5.46% to 14.19% higher genetic gain than MDI at the cost of 6.1% to 30.2% higher inbreeding rates. The number of drones used in MDI settings had only a negligible impact on the results. There was a slight tendency that more drones lead to lower genetic gain and lower inbreeding rates but whenever more than five drones were used for inseminations, no significant differences could be observed.

CONCLUSION

The opportunities to optimize breeding schemes via the number of drones used in inseminations are very limited. SDI can be a viable strategy in situations where breeders are interested in genetically homogeneous offspring or precise pedigree information. However, such strategies have to account for the fact that the semen from a single drone is insufficient to fill a queen's spermatheca, whence SDI queens will not build full-strength colonies. When deciding for MDI, breeders should focus on collecting enough semen for a succesful insemination, regardless of how many drones they need for this purpose.

High-Quality Queens Produce High-Quality Offspring Queens

Honey bees, rather than rear queens with eggs and larvae from worker cells, prefer to rear new queens with eggs form queen cells, if available. This may be a result of long-term evolutionary process for honey bee colonies. However, the exact mechanism of this phenomenon is unclear. In this study, queens were reared with eggs from queen cells (F1-QE), eggs from worker cells (F1-WE), and two-day-old larvae from worker cells (F1-2L). Physiological indexes and the expression of the development-related genes ((Hexamerin (Hex110, Hex70b), Transferrin (Trf), and Vitellogenin (Vg)) of reared F1 generation queens were measured and compared. Furthermore, F2 generation queens were reared with one-day-old larvae from F1 queens, and the weight and ovariole count of reared F2 generation daughter queens were examined. Meanwhile, the expression of the development- and reproduction-related genes (Hex110, Hex70b, Trf, Vg, and Juvenile Hormone (Jh)) and immune detoxication-related genes (Hymenoptaecin, Abeacin, and CytP450) of reared F2 queens were further explored. We found that the F1-QE queens had the highest physiological indexes and higher Hex110 and Trf expression levels, while no significant difference was found in the expression of Hex70b and Vg among the three groups of F1 queens. In addition, the reared queens of F2-QE had the highest quality, with the highest development, reproduction, immune-detoxication genes' expression levels. Our results revealed that the quality of reared offspring queens from high-quality mother queens was also high. These findings inform methods for rearing high-quality queens and highlight that a high-quality queen is essential for offspring colony growth and survival.

Investigating Genetic and Phenotypic Variability of Queen Bees: Morphological and Reproductive Traits

The quality of the honeybee queen has an important effect on a colony's development, productivity, and survival. Queen failure or loss is considered a leading cause for colonies' mortality worldwide. The queen's quality, resulting from her genetic background, developmental conditions, mating success, and environment, can be assessed by some morphological measures. The study aims to investigate variability for traits that could assess the quality of the queen. Related animals were enrolled in this study. Variance components were estimated fitting a mixed animal model to collected data. Heritabilities of body and tagmata weights ranged from 0.46 to 0.54, whereas lower estimates were found for the tagmata width and wing length. Heritabilities estimated for the spermatheca diameter and volume, number of ovarioles, and number of sperms were 0.17, 0.88, 0.70, and 0.57, respectively. Many phenotypic correlations related to size were high and positive, while weak correlations were found between morphology and reproductive traits. Introducing a queen's traits in a selection program could improve colonies' survivability. Further research should focus on better defining the correlations between the individual qualities of a queen and her colony's performance.

Transcriptomic analysis of the honey bee (Apis mellifera) queen spermathecae reveals genes that may be involved in sperm storage after mating

Honey bee (Apis mellifera) queens have a remarkable organ, the spermatheca, which successfully stores sperm for years after a virgin queen mates. This study uniquely characterized and quantified the transcriptomes of the spermathecae from mated and virgin honey bee queens via RNA sequencing to identify differences in mRNA levels based on a queen's mating status. The transcriptome of drone semen was analyzed for comparison. Samples from three individual bees were independently analyzed for mated queen spermathecae and virgin queen spermathecae, and three pools of semen from ten drones each were collected from three separate colonies. In total, the expression of 11,233 genes was identified in mated queen spermathecae, 10,521 in virgin queen spermathecae, and 10,407 in drone semen. Using a cutoff log2 fold-change value of 2.0, we identified 212 differentially expressed genes between mated and virgin spermathecal queen tissues: 129 (1.4% of total) were up-regulated and 83 (0.9% of total) were down-regulated in mated queen spermathecae. Three genes in mated queen spermathecae, three genes in virgin queen spermathecae and four genes in drone semen that were more highly expressed in those tissues from the RNA sequencing data were further validated by real time quantitative PCR. Among others, expression of Kielin/chordin-like and Trehalase mRNAs was highest in the spermathecae of mated queens compared to virgin queen spermathecae and drone semen. Expression of the mRNA encoding Alpha glucosidase 2 was higher in the spermathecae of virgin queens. Finally, expression of Facilitated trehalose transporter 1 mRNA was greatest in drone semen. This is the first characterization of gene expression in the spermathecae of honey bee queens revealing the alterations in mRNA levels within them after mating. Future studies will extend to other reproductive tissues with the purpose of relating levels of specific mRNAs to the functional competence of honey bee queens and the colonies they head.

Injection of seminal fluid into the hemocoel of honey bee queens (Apis mellifera) can stimulate post-mating changes

Honey bee queens undergo dramatic behavioral (e.g., reduced sexual receptivity), physiological (e.g., ovary activation, ovulation, and modulation of pheromone production) and transcriptional changes after they complete mating. To elucidate how queen post-mating changes are influenced by seminal fluid, the non-spermatozoa-containing component of semen, we injected queens with semen or seminal fluid alone. We assessed queen sexual receptivity (as measured by likelihood to take mating flights), ovary activation, worker retinue response (which is influenced by queen pheromone production), and transcriptional changes in queen abdominal fat body and brain tissues. Injection with either seminal fluid or semen resulted in decreased sexual receptivity, increased attractiveness of queens to workers, and altered expression of several genes that are also regulated by natural mating in queens. The post-mating and transcriptional changes of queens receiving seminal fluid were not significantly different from queens injected with semen, suggesting that components in seminal fluid, such as seminal fluid proteins, are largely responsible for stimulating post-mating changes in queens.

Seminal fluid compromises visual perception in honeybee queens reducing their survival during additional mating flights

Queens of social insects make all mate-choice decisions on a single day, except in honeybees whose queens can conduct mating flights for several days even when already inseminated by a number of drones. Honeybees therefore appear to have a unique, evolutionarily derived form of sexual conflict: a queen’s decision to pursue risky additional mating flights is driven by later-life fitness gains from genetically more diverse worker-offspring but reduces paternity shares of the drones she already mated with. We used artificial insemination, RNA-sequencing and electroretinography to show that seminal fluid induces a decline in queen vision by perturbing the phototransduction pathway within 24–48 hr. Follow up field trials revealed that queens receiving seminal fluid flew two days earlier than sister queens inseminated with saline, and failed more often to return. These findings are consistent with seminal fluid components manipulating queen eyesight to reduce queen promiscuity across mating flights.

For social insects like honeybees it is beneficial if their queens mate with many males, because genetic diversity can protect the hive against parasites. Early in life, a honeybee queen has a short period of time in which she can fly out to mate with males before returning to the hive with all the sperm needed to last for a lifetime. Queens that have mated on their first flight may embark on additional mating flights over a few consecutive days to further increase genetic variability in their offspring. This is problematic for a male that has already mated because the more males that inseminate the queen the fewer offspring will carry on his specific genes. This results in sexual conflict between males and queens over the number of mating flights.

In many animals, males manipulate females using molecules in seminal fluid to reduce the chances of the female mating again and honeybee males may use a similar strategy. Previous studies revealed that insemination alters the activity of genes related to vision in a honeybee queen’s brain. This could be one way for the males to prevent queens from embarking on additional mating flights.

Now, Liberti et al. find support for this idea by showing that seminal fluid can indeed trigger changes in the activity of vision-related genes in the brains of honeybee queens, which in turn reduce a queen’s opportunity to complete additional mating flights. Queens inseminated with seminal fluid were less responsive to light compared to queens that were exposed to saline instead. Electronic tracking devices affixed to queens showed that the seminal fluid-exposed queens left for mating flights sooner but were more likely to get lost and to not return to their hives compared to the saline-exposed queens.

The experiments support the idea of a sexual arms race in honeybees. Males use seminal fluid to cause rapid deteriorating vision in queens, thus reducing their likelihood of leaving the hive to mate again and to find males when they do fly again. The queens try to counteract these effects by leaving for mating flights sooner, thereby increasing offspring genetic diversity and the success of their colonies. Further studies will be needed to find out how the honeybee sexual arms race varies across seasons, bee races, and geographic ranges. Such information will be useful for honeybee breeding programs, which rely on queen mating success and hive genetic diversity to ensure hive health.

Mating triggers an up-regulation of vitellogenin and defensin in ant queens

Mating induces a range of physiological changes in female insects. In species that mate during several reproductive bouts throughout their life, mating causes an increase in oviposition, affects immune function, and decreases female lifespan and receptivity to further mating. Social Hymenoptera (ants, social bees, and wasps) are unique, since queens mate during a single reproductive effort at the beginning of their life. Their reproductive strategy is thus fundamentally different from that of other insects and one might expect the effects of mating on social Hymenoptera queens to be altered. We tested the effect of mating and multiple mating on the expression of six genes likely to be involved in post-mating changes, in queens of the ant Lasius niger L. We show that mating induces oviposition, and is followed by an up-regulation of vitellogenin and defensin expression. The expression of juvenile hormone esterase, insulin receptor 2, Cu-Zn superoxide dismutase 1, and prophenoloxidase is not significantly affected by mating. Queen-mating frequency did not affect the expression of the tested genes. Altogether, our results indicate that certain effects of mating on female insect physiology are generalized across species independent of their mating strategies, while others seem species specific.

Putative Drone Copulation Factors Regulating Honey Bee (Apis mellifera) Queen Reproduction and Health: A Review

Honey bees are major pollinators of agricultural and non-agricultural landscapes. In recent years, honey bee colonies have exhibited high annual losses and commercial beekeepers frequently report poor queen quality and queen failure as the primary causes. Honey bee colonies are highly vulnerable to compromised queen fertility, as each hive is headed by one reproductive queen. Queens mate with multiple drones (male bees) during a single mating period early in life in which they obtain enough spermatozoa to fertilize their eggs for the rest of their reproductive life span. The process of mating initiates numerous behavioral, physiological, and molecular changes that shape the fertility of the queen and her influence on the colony. For example, receipt of drone semen can modulate queen ovary activation, pheromone production, and subsequent worker retinue behavior. In addition, seminal fluid is a major component of semen that is primarily derived from drone accessory glands. It also contains a complex mixture of proteins such as proteases, antioxidants, and antimicrobial proteins. Seminal fluid proteins are essential for inducing post-mating changes in other insects such as Drosophila and thus they may also impact honey bee queen fertility and health. However, the specific molecules in semen and seminal fluid that initiate post-mating changes in queens are still unidentified. Herein, we summarize the mating biology of honey bees, the changes queens undergo during and after copulation, and the role of drone semen and seminal fluid in post-mating changes in queens. We then review the effects of seminal fluid proteins in insect reproduction and potential roles for honey bee drone seminal fluid proteins in queen reproduction and health. We finish by proposing future avenues of research. Further elucidating the role of drone fertility in queen reproductive health may contribute towards reducing colony losses and advancing honey bee stock development.

Long-term interaction between Drosophila sperm and sex peptide is mediated by other seminal proteins that bind only transiently to sperm

Seminal fluid proteins elicit several post-mating physiological changes in mated Drosophila melanogaster females. Some of these changes persist for over a week after mating because the seminal protein that causes these changes, the Sex Peptide (SP), binds to sperm that are stored in the female reproductive tract. SP's sperm binding is mediated by a network of at least eight seminal proteins. We show here that some of these network proteins (CG1656, CG1652, CG9997 and Antares) bind to sperm within 2 h of mating, like SP. However, while SP remains bound to sperm at 4 days post-mating, none of the other network proteins are detectable at this time. We also observed that the same network proteins are detectable at 2 h post-mating in seminal receptacle tissue from which sperm have been removed, but are no longer detectable there by 4 days post-mating, suggesting short-term retention of these proteins in this female sperm storage organ. Our results suggest that these network proteins act transiently to facilitate the conditions for SP's binding to sperm, perhaps by modifying SP or the sperm surface, but are not part of a long-acting complex that stably attaches SP to sperm.

Queen Quality and the Impact of Honey Bee Diseases on Queen Health: Potential for Interactions between Two Major Threats to Colony Health

Western honey bees, Apis mellifera, live in highly eusocial colonies that are each typically headed by a single queen. The queen is the sole reproductive female in a healthy colony, and because long-term colony survival depends on her ability to produce a large number of offspring, queen health is essential for colony success. Honey bees have recently been experiencing considerable declines in colony health. Among a number of biotic and abiotic factors known to impact colony health, disease and queen failure are repeatedly reported as important factors underlying colony losses. Surprisingly, there are relatively few studies on the relationship and interaction between honey bee diseases and queen quality. It is critical to understand the negative impacts of pests and pathogens on queen health, how queen problems might enable disease, and how both factors influence colony health. Here, we review the current literature on queen reproductive potential and the impacts of honey bee parasites and pathogens on queens. We conclude by highlighting gaps in our knowledge on the combination of disease and queen failure to provide a perspective and prioritize further research to mitigate disease, improve queen quality, and ensure colony health.

Neonicotinoid pesticides severely affect honey bee queens

Queen health is crucial to colony survival of social bees. Recently, queen failure has been proposed to be a major driver of managed honey bee colony losses, yet few data exist concerning effects of environmental stressors on queens. Here we demonstrate for the first time that exposure to field-realistic concentrations of neonicotinoid pesticides during development can severely affect queens of western honey bees (Apis mellifera). In pesticide-exposed queens, reproductive anatomy (ovaries) and physiology (spermathecal-stored sperm quality and quantity), rather than flight behaviour, were compromised and likely corresponded to reduced queen success (alive and producing worker offspring). This study highlights the detriments of neonicotinoids to queens of environmentally and economically important social bees, and further strengthens the need for stringent risk assessments to safeguard biodiversity and ecosystem services that are vulnerable to these substances.

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.

RNA-sequencing elucidates the regulation of behavioural transitions associated with the mating process in honey bee queens

Background

Mating is a complex process, which is frequently associated with behavioural and physiological changes. However, understanding of the genetic underpinnings of these changes is limited. Honey bees are both a model system in behavioural genomics, and the dominant managed pollinator of human crops; consequently understanding the mating process has both pure and applied value. We used next-generation transcriptomics to probe changes in gene expression in the brains of honey bee queens, as they transition from virgin to mated reproductive status. In addition, we used CO₂-narcosis, which induces oviposition without mating, to isolate the process of reproductive maturation.

Results

The mating process produced significant changes in the expression of vision, chemo-reception, metabolic, and immune-related genes. Differential expression of these genes maps clearly onto known behavioural and physiological changes that occur during the transition from being a virgin queen to a newly-mated queen. A subset of these changes in gene expression were also detected in CO₂-treated queens, as predicted from previous physiological studies. In addition, we compared our results to previous studies that used microarray techniques across a range of experimental time-points. Changes in expression of immune- and vision-related genes were common to all studies, supporting an involvement of these groups of genes in the mating process.

Conclusions

Our study is an important step in understanding the molecular mechanisms regulating post-mating behavioural transitions in a natural system. The weak overlap in patterns of gene expression with previous studies demonstrates the high sensitivity of genome-wide approaches. Thus, while we build on previous microarray studies that explored post-mating changes in honey bees, the broader experimental design, use of RNA-sequencing, and focus on Australian honey bees, which remain free from the devastating parasite Varroa destructor, in the current study, provide unique insights into the biology of the mating process in honey bees.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1750-7) contains supplementary material, which is available to authorized users.

Transcriptome comparison between inactivated and activated ovaries of the honey bee Apis mellifera L

Ovarian activity not only influences fertility, but is also involved with the regulation of division of labour between reproductive and behavioural castes of female honey bees. In order to identify candidate genes associated with ovarian activity, we compared the gene expression patterns between inactivated and activated ovaries of queens and workers by means of high-throughput RNA-sequencing technology. A total of 1615 differentially expressed genes (DEGs) was detected between ovaries of virgin and mated queens, and more than 5300 DEGs were detected between inactivated and activated worker ovaries. Intersection analysis of DEGs amongst five libraries revealed that a similar set of genes (824) participated in the ovary activation of both queens and workers. A large number of these DEGs were predominantly related to cellular, cell and cell part, binding, biological regulation and metabolic processes. In addition, over 1000 DEGs were linked to more than 230 components of Kyoto Encyclopedia of Genes and Genomes pathways, including 25 signalling pathways. The reliability of the RNA-sequencing results was confirmed by means of quantitative real-time PCR. Our results provide new insights into the molecular mechanisms involved in ovary activation and reproductive division of labour.

Exploring the role of juvenile hormone and vitellogenin in reproduction and social behavior in bumble bees

Background

The genetic and physiological pathways regulating behavior in solitary species are hypothesized to have been co-opted to regulate social behavior in social species. One classic example is the interaction between vitellogenin (an egg-yolk and storage protein) and juvenile hormone, which are positively correlated in most insect species but have modified interactions in highly eusocial insects. In some of these species (including some termites, ants, and the honey bee), juvenile hormone and vitellogenin levels are negatively correlated and juvenile hormone has shifted its role from a gonadotropin to a regulator of maturation and division of labor in the primarily sterile workers. The function of vitellogenin also seems to have broadened to encompass similar roles. Thus, the functions and molecular interactions of juvenile hormone and vitellogenin are hypothesized to have undergone changes during the evolution of eusociality, but the mechanisms underlying these changes are unknown.

Bumble bees offer an excellent model system for testing how the relationship between juvenile hormone and vitellogenin evolved from solitary to social species. Bumble bee colonies are primitively eusocial and comprised of a single reproductive queen and facultatively sterile workers. In Bombus terrestris, juvenile hormone retains its ancestral role as a gonadotropin and is also hypothesized to regulate aggressive behavior. However, the function of vitellogenin and its interactions with juvenile hormone have not yet been characterized.

Results

By characterizing vitellogenin RNA expression levels (vg) in B. terrestris we show that vg is not associated with task and only partially associated with worker age, queen presence, and caste (queen vs worker). The correlations of vg with ovarian activation were not consistent across experiments, but both vg and ovarian activation were significantly associated with levels of aggression experienced by workers. Treatment with juvenile hormone did not affect vg levels in queenless groups.

Conclusions

We suggest that social interactions affect vg levels more strongly than a worker’s reproductive physiological state, and that juvenile hormone and vg are uncoupled in this species. Thus, although juvenile hormone maintains its traditional role as gonadotropin in B. terrestris, vg has already been co-opted into a novel role, consistent with the model that Bombus represents an intermediate stage in the evolution of eusociality.

Sociogenomics of cooperation and conflict during colony founding in the fire ant Solenopsis invicta

One of the fundamental questions in biology is how cooperative and altruistic behaviors evolved. The majority of studies seeking to identify the genes regulating these behaviors have been performed in systems where behavioral and physiological differences are relatively fixed, such as in the honey bee. During colony founding in the monogyne (one queen per colony) social form of the fire ant Solenopsis invicta, newly-mated queens may start new colonies either individually (haplometrosis) or in groups (pleometrosis). However, only one queen (the “winner”) in pleometrotic associations survives and takes the lead of the young colony while the others (the “losers”) are executed. Thus, colony founding in fire ants provides an excellent system in which to examine the genes underpinning cooperative behavior and how the social environment shapes the expression of these genes. We developed a new whole genome microarray platform for S. invicta to characterize the gene expression patterns associated with colony founding behavior. First, we compared haplometrotic queens, pleometrotic winners and pleometrotic losers. Second, we manipulated pleometrotic couples in order to switch or maintain the social ranks of the two cofoundresses. Haplometrotic and pleometrotic queens differed in the expression of genes involved in stress response, aging, immunity, reproduction and lipid biosynthesis. Smaller sets of genes were differentially expressed between winners and losers. In the second experiment, switching social rank had a much greater impact on gene expression patterns than the initial/final rank. Expression differences for several candidate genes involved in key biological processes were confirmed using qRT-PCR. Our findings indicate that, in S. invicta, social environment plays a major role in the determination of the patterns of gene expression, while the queen's physiological state is secondary. These results highlight the powerful influence of social environment on regulation of the genomic state, physiology and ultimately, social behavior of animals.

The characterization of the genomic basis for complex behaviors is one of the major goals of biological research. The genomic state of an individual results from the interplay between its internal condition (the “nature”) and the external environment (the “nurture”), which may include the social environment. Colony founding in the fire ant Solenopsis invicta is a complex process that serves as a useful model for investigating how the interplay between genes and social environment shapes social behavior. Unrelated, newly mated S. invicta queens may start a new colony as a group, but ultimately only one queen will survive and gain full reproductive dominance. By uncovering the genetic basis for founding behavior in fire ants we therefore provide useful insights into how cooperative behavior evolved in a context that might be considered primitively eusocial, because newly mated queens in a founding association are morphologically, physiologically and genetically very similar and display no evident division of labor. Our results suggest that social environment (founding singly or in pairs, switching dominance rank vs. maintaining rank) is a much greater driver of gene expression changes than social rank itself, suggesting that social environment, and not reproductive state, is a key regulator of gene expression, physiology and ultimately, behavior.

Influence of pollen nutrition on honey bee health: do pollen quality and diversity matter?

Honey bee colonies are highly dependent upon the availability of floral resources from which they get the nutrients (notably pollen) necessary to their development and survival. However, foraging areas are currently affected by the intensification of agriculture and landscape alteration. Bees are therefore confronted to disparities in time and space of floral resource abundance, type and diversity, which might provide inadequate nutrition and endanger colonies. The beneficial influence of pollen availability on bee health is well-established but whether quality and diversity of pollen diets can modify bee health remains largely unknown. We therefore tested the influence of pollen diet quality (different monofloral pollens) and diversity (polyfloral pollen diet) on the physiology of young nurse bees, which have a distinct nutritional physiology (e.g. hypopharyngeal gland development and vitellogenin level), and on the tolerance to the microsporidian parasite Nosemaceranae by measuring bee survival and the activity of different enzymes potentially involved in bee health and defense response (glutathione-S-transferase (detoxification), phenoloxidase (immunity) and alkaline phosphatase (metabolism)). We found that both nurse bee physiology and the tolerance to the parasite were affected by pollen quality. Pollen diet diversity had no effect on the nurse bee physiology and the survival of healthy bees. However, when parasitized, bees fed with the polyfloral blend lived longer than bees fed with monofloral pollens, excepted for the protein-richest monofloral pollen. Furthermore, the survival was positively correlated to alkaline phosphatase activity in healthy bees and to phenoloxydase activities in infected bees. Our results support the idea that both the quality and diversity (in a specific context) of pollen can shape bee physiology and might help to better understand the influence of agriculture and land-use intensification on bee nutrition and health.