Sex ratio variation in the bumblebee Bombus terrestris

Patterns of sex allocation in bumblebees have been enigmaticand difficult to interpret in either a Fisherian context orin a kin-selection perspective. We gathered data on severalhundred laboratory-reared colonies of the bumblebee Bombus terrestrisand analyzed sex allocation as a function of diapause durationand a series of variables describing colony development. Ouranalyses addressed both sex allocation patterns across differentcohorts of laboratory colonies reared at different times andsex allocation patterns across individual colonies within thesecohorts. We used path analysis to test a hypothetical modellinking a sequence of colony-development variables to the crucialreproductive parameters at the end of the colony life cycle.We show that (1) population-wide patterns of sex allocationshow equal investment in the sexes and are thus consistent withqueen control, but not with worker control. (2) A significantpart of the colony-level and cohort-specific variation in sexallocation is related to the hibernation conditions of foundingqueens: Queens with longer than average winter diapause producelarger cohorts of first and second brood workers, switch tohaploid eggs early, and produce colonies that raise mostly malesand few new queens and vice versa. (3) Colony-level sex allocationis significantly related to the time span between the switchpoint (date of first haploid egg laid by the queen) and thecompetition point (date of first haploid egg laid by one ofthe workers): the longer this period, the more male biased thesex ratio. (4) The breeding constraints of an annual life cycle,the short reproductive season, and the presumably high premiumon early produced males imply that bumblebee workers have norealistic options to capitalize on their relatedness asymmetrytoward the different kinds of reproductive brood by biasingthe sex ratio.     

Unexpected consequences of polyandry for parasitism and fitness in the bumblebee, Bombus terrestris

Multiple mating by females (polyandry) is taxonomically widespread but the evolution of such behaviors is not clearly understood given potential costs of polyandry such as time, energy, or predation risk. The genetic variability versus parasites hypothesis predicts a reduction of parasitism due to increased genetic variability among offspring and an associated fitness gain. We tested this hypothesis with a field experiment in the bumblebee, Bombus terrestris L. Worker heterogeneity within the colony was experimentally altered by artificially inseminating queens with sperm from one male, four brothers, two males, or four unrelated males. We found genetic variability to be effective, because intensity and prevalence of the most common parasite, Crithidia bombi, a trypanosome, decreased with increasing levels of colony heterogeneity. Fitness differed between treatments but did not increase in a simple way, with increasing genetic heterogeneity among colony workers. Instead, fitness followed a U-shaped function with a minimum for small amounts of genetic heterogeneity. We therefore suggest that polyandry also induces a cost, perhaps as a result of the social structure within the colony. In evolutionary terms, singly mated females appear to be constrained by an adaptive valley that needs to be crossed before high degrees of mating frequency can be reached. This may help to explain why B. terrestris and most other social insects are often monandrous.     

A larval hunger signal in the bumblebee Bombus terrestris

Larvae of Bombus terrestris, a pollen-storing bumblebee, are dependent on progressive provisioning by workers. We test the hypothesis that larval cuticular chemicals can act as a hunger signal. We first show with a new classical conditioning experiment, using a Y-shaped tube, that workers can be trained to prefer the extracts of normally fed larvae over those of starved larvae. This proves the ability of workers to discriminate between larval extracts. Second, we show in a bioassay that workers also use these perceived differences to feed larvae according to their nutritional status. Larval broods sprayed with the extracts of the starved larvae were fed significantly more than larval broods sprayed with the extracts of normally fed larvae or with the solvent (n-pentane) only. We therefore conclude that B. terrestris larvae signal their need for food via their cuticular chemicals, and discuss the extent to which this form of communication could give larvae some control over their development. Received 2 September 2005; revised 11 April; accepted 24 April 2006.     

Heritability of sperm length in the bumblebee Bombus terrestris

Sperm length is highly variable, both between and within species, but the evolutionary significance of this variation is poorly understood. Sexual selection on sperm length requires a significant additive genetic variance, but few studies have actually measured this. Here we present the first estimates of narrow sense heritability of sperm length in a social insect, the bumblebee Bombus terrestris. In spite of a balanced and straightforward rearing design of colonies, and the possibility to replicate measurements of sperm within single males nested within colonies, the analysis proved to be complex. Several appropriate statistical models were derived, each depending on different assumptions. The heritability estimates obtained ranged from h ² = 0.197 ± 0.091 to h ² = 0.429 ± 0.154. All our estimates were substantially lower than previous estimates of sperm length heritability in non-social insects and vertebrates.     

Natural variation in the genetic architecture of a host-parasite interaction in the bumblebee Bombus terrestris

The genetic architecture of fitness-relevant traits in natural populations is a topic that has remained almost untouched by quantitative genetics. Given the importance of parasitism for the host's fitness, we used QTL mapping to study the genetic architecture of traits relevant for host-parasite interactions in the trypanosome parasite, Crithidia bombi and its host, Bombus terrestris. The three traits analysed were the parasite's infection intensity, the strength of the general immune response (measured as the encapsulation of a novel antigen) and body size. The genetic architecture of these traits was examined in three natural, unmanipulated mapping populations of B. terrestris. Our results indicate that the intracolonial phenotypic variation of all three traits is based on a network of QTLs and epistatic interactions. While these networks are similar between mapping populations in complexity and number of QTLs, as well as in their epistatic interactions, the variability in the position of QTL and the interacting loci was high. Only one QTL for body size was plausibly found in at least two populations. QTLs for encapsulation and Crithidia infection intensity were located on the same linkage groups.     

An immune response in the bumblebee, Bombus terrestris leads to increased food consumption

Background  

The concept of a costly immune system that must be traded off against other important physiological systems is fundamental to the burgeoning field of ecological immunity. Bumblebees have become one of the central models in this field. Although previous work has demonstrated costs of immunity in numerous life history traits, estimates of the more direct costs of bumblebee immunity have yet to be made.     

Age- and task-dependent foraging gene expression in the bumblebee Bombus terrestris

In eusocial insects, the division of labor within a colony, based on either age or size, is correlated with a differential foraging (for) gene expression and PKG activity. This article presents in the first part a study on the for gene, encoding a cGMP-dependent protein kinase (PKG) in the bumblebee Bombus terrestris. Cloning of the open reading frame allowed phylogenetic tracing, which showed conservation of PKGs among social insects. Our results confirm the proposed role for PKGs in division of labor. Btfor gene expression is significantly higher in the larger foragers compared with the smaller sized nurses. More importantly, we discovered an age-related decrease in Btfor expression in both nursing and foraging bumblebees. We therefore speculate that the presence of BtFOR is required for correct adaptation to new external stimuli and rapid learning for foraging. In a second series of experiments, worker bumblebees of B. terrestris were treated with two insecticides imidacloprid and kinoprene, which have shown to cause impaired foraging behavior. Compared with controls, only the latter treatment resulted in a decreased Btfor expression, which concurs with a stimulation of ovarian growth and a shift in labor toward nest-related tasks. The data are discussed in relation to Btfor expression in the complex physiological event of foraging and side-effects by pesticides.     

The control of nest climate in bumblebee (Bombus terrestris) colonies: interindividual variability and self reinforcement in fanning response

Interindividual variability in response to environmental stimuliis believed to have a major impact on collective behaviors insocial insects. The present study presents a detailed investigationof the variability in individual fanning behavior underlyingthe collective control of nest climate in bumblebee (Bombusterrestris) colonies. Four colonies were repeatedly exposedto increasing temperature and CO₂ levels. The response thresholdof each worker (defined as the mean stimulus intensity at whicha worker responded by fanning) was determined. Temperature responsethresholds of 118 workers and CO₂ response thresholds of 88workers were analyzed. Workers differed in their response thresholds.Some consistently responded to low stimulus intensities, othersconsistently responded to high stimulus intensities. No consistentcorrelation between temperature and CO₂ thresholds was foundwithin individuals. Response thresholds of fanning bees decreasedover successive trials, providing empirical support for theidea of specialization through individual threshold reinforcement.In addition to variability in individual response thresholds,workers of a colony differed in two other parameters of responsiveness:response probability (the probability of responding to a stimulusonce it exceeded an individual's response threshold) and responseduration (the persistency with which fanning was performed oncean individual responded). The results of the present study suggestthat response threshold, response probability and response durationare important independent parameters of individual responsivenessin the collective control of nest climate in bumblebee colonies.     

Thermoregulation dynamics in commercially reared colonies of the bumblebee Bombus terrestris

Thermoregulation, that is, the active control of temperature, is key to ensure proper brood development in both wild and captive bumblebee nests. In this study, thermoregulation dynamics were assessed relative to colony age and ambient temperature using commercially reared Bombus terrestris L. (Hymenoptera, Apidae, Bombus) colonies. We observed a positive relationship between brood and nest temperatures in response to ambient temperature. Thermoregulation investment (by either brooding or fanning) was lowest at brood surface temperatures between 33 and 34 °C and ambient temperatures between 28 and 32 °C. Brood temperature was less stable and thermoregulation investment higher in younger colonies, especially at lower ambient temperatures. Furthermore, queens initiated colonies sooner and colonies developed faster when kept at an ambient temperature of 29 °C as compared to 24 °C. Our results suggest that ambient temperatures are ideally kept between 29 and 31 °C.     

Behavioural interactions, kin and disease susceptibility in the bumblebee Bombus terrestris

Behavioural interactions are often analysed in terms of their costs and benefits to the actors [Hamilton, (1964) J. Theor. Biol.7 1-16; Gadagkar, (1993) Trends Ecol. Evol.8 232-234; Foster et al., (2001) Ann. Zool. Fenn.38 229-238]. Using the bumblebee Bombus terrestris, we wish to distinguish between two possible determinants of interaction behaviour between conspecifics, namely kin-directed behaviour that reflects genetic distance between individuals, or, alternatively, interactions guided by a functional distance between individuals, specifically, with respect to disease susceptibility. We find no relationship between contact rate of individuals and the genetic distance of their respective colonies. Interestingly, we do find a significant negative correlation between contact rate and the distance between the two colonies in susceptibility to a spectrum of parasite strains. This cannot be explained by either of the a priori alternatives so we propose two further testable hypotheses to explain our results.     

Specific versus nonspecific immune defense in the bumblebee,Bombus terrestris L

Abstract.— Hosts vary in both their strength of response to a general immunological insult and in their specific susceptibility to different parasite species or different strains of the same parasite. The variation in the general immune response is considered a result of the costs imposed by selection on defended individuals. The variation in the specific response may originate from variation in host and parasite genotypes and is a requirement for frequency-dependent selection. The relationship between these two fundamental aspects of defense has only rarely been studied. Using the bumblebee Bombus terrestris and its gut trypanosomal parasite Crithidia bombi we found that the host's specific response profile toward different strains correlates negatively with its level of response to a general insult. This is the opposite result one would expect if the level of general response were simply a measure of immunological quality (immunocompetence). Rather, it suggests that there is some form of a trade-off between these two fundamental aspects of the immune system. These results, therefore, shed an important light on the possible constraints that affect the evolution of the immune system and particularly the trade-off between different arms of the immune system.     

Effect of worker number and diapause duration on colony parameters of the bumblebee,Bombus terrestris (Hymenoptera: Apidae)

Bumblebee, Bombus terrestris queens undergo winter diapause and show a great difference in diapause duration in natural conditions. Queens emerged from diapause initiate colonies by producing a batch of diploid (fertilised) eggs that develop into workers. In this study we investigated the effects of both the duration of queen diapause (2, 3, 4, or 5 months) and colony size (artificially limited to 50, 100, 150, and 200 workers) on the number of sexuals (males or new queens — gynes) produced, when gynes are produced and the longevity of both the foundress queen and the colony. Both worker population and diapause duration showed significant effect on sexual gyne production, foundress queen longevity and colony longevity but their interaction effect was insignificant. The worker number and diapause duration, respectively showed significant effect on sexual male production and gyne emergence period, but their interaction effects were insignificant.     

Population-level variation of digestive physiology costs of mounting an immune response in damselflies

1. Trade-offs are often predicted to occur between energetically costly activities, such as somatic growth and eliciting immune responses to parasites. Although parasitism frequently reduces growth via lowered consumption, it remains unclear if the energetic demands of generating immune responses also affect the digestive physiological processes necessary for growth. Moreover, as local environmental conditions affect energetic investment towards growth and immune responses, the extent of any digestive–immune response trade-offs may vary among populations and not be fixed at the species-level. 2. To test these ideas, melanisation – a general innate immune response – was first induced in damselfly larvae (Enallagma vesperum) from two populations. The study then quantified growth and consumption rates, assimilation and production efficiencies, and daily metabolic rates to determine if digestive–immune response trade-offs were present and, if so, whether they differed between populations. 3. There was no evidence of any trade-offs between immune responses and digestive physiology components in either population. However, the results did show that populations differentially allocated energy towards different digestive physiology components after an immune response was elicited: one population increased their relative consumption and daily metabolic rates, while the other population had lower assimilation efficiencies and consumption rates. 4. Although researchers lack a mechanistic understanding of the observed population-level differences, these results suggest that accounting for population-level variation in digestive physiology and immune responses is critical to inferences about how immunological defences to parasitism may affect the ability for organisms to both acquire and utilise resources.     

The composition of larval food and the significance of exocrine secretions in the bumblebee Bombus terrestris

Summary: This paper describes a study on the relation between the composition of larval food and the development of female castes in bumblebees. The first aim was to evaluate the significance of glandular secretions in the larval diet as a possible factor involved in larval feeding and caste differentiation. Small amounts of proteinaceous secretions were found to be added during the ingestion of sucrose but not while discharging food to the larvae. It is discussed that these secretions are digestive in function rather than food additives that would possibly play a role in the process of caste differentiation.¶Secondly, a comparative analysis was made of the general composition of food samples obtained from larvae of different castes and ages and from various periods in the social development of the colony. No significant differences in the total amount of pollen, sucrose and protein were detected between the castes or different age groups. Unlike honeybee workers, individual bumblebee workers did not change the composition of the diet they supplied to the brood in relation to their own age, nor to the social development of the colony. These findings suggest that all larvae receive the same nourishment during their total development and indicate that differences in development between queen larvae and worker larvae are neither caused by variations nor by a qualitative modification of their food with respect to the amount of pollen, protein and carbohydrates.     

Social life-history response to individual immune challenge of workers of Bombus terrestris L.: a possible new cooperative phenomenon

Parasites typically reduce host survival or fecundity. To minimize fitness loss, hosts can make temporal adjustments of their reproductive effort. To date such plastic shifts of life‐history traits in response to parasitism are only known from solitary organisms where infected individuals can react by themselves. In the case of social insects, where brood care and reproductive effort is shared between reproductive individuals (typically the queen) and workers, adjustments of the reproductive effort would depend on collective decision‐making. We tested for this possibility by experimentally activating the immune response of individual workers in colonies of the bumblebee, Bombus terrestris L. This induction resulted, in combination with environmental conditions, in a reduction of fitness of the social unity (i.e. colony success, measured by number and biomass of offspring) and a collective response towards earlier reproduction. As both phenomena are expressed at the level of the colony, the result suggests that key elements of the use of immune defence have been maintained through the evolutionary transition to sociality.     

Photoperiodic influence on the body mass of bumblebee, Bombus terrestris and its copulation duration

Abstract:  The body mass of Bombus terrestris individuals is an important trait for their behavioural performance and colony organization. In this study, colonies were reared under four different photoperiodic regimes, viz. 0 : 24, 8 : 16, 16 : 8 and 24 : 0 h light : darkness (L : D) at 28°C and 50% relative humidity. The changes in body mass were observed at the stages of larvae, pupae and on the day of adult eclosion. Both the wet and dry mass of sexuals gradually decreased with increasing day length. The relationship between body mass and copulation duration revealed that copulation duration was negatively correlated with male body mass, but positively with queen body mass. Higher number of matings by males resulted in significantly higher duration of copulation.     

Low parasite loads accompany the invading population of the bumblebee, Bombus terrestris in Tasmania

In its native Europe, the bumblebee, Bombus terrestris (L.) has co-evolved with a large array of parasites whose numbers are negatively linked to the genetic diversity of the colony. In Tasmania B. terrestris was first detected in 1992 and has since spread over much of the state. In order to understand the bee’s invasive success and as part of a wider study into the genetic diversity of bumblebees across Tasmania, we screened bees for co-invasions of ectoparasitic and endoparasitic mites, nematodes and micro-organisms, and searched their nests for brood parasites. The only bee parasite detected was the relatively benign acarid mite Kuzinia laevis (Dujardin) whose numbers per bee did not vary according to region. Nests supported no brood parasites, but did contain the pollen-feeding life stages of K. laevis. Upon summer-autumn collected drones and queens, mites were present on over 80% of bees, averaged ca. 350–400 per bee and were more abundant on younger bees. Nest searching spring queens had similar mite numbers to those collected in summer-autumn but mite numbers dropped significantly once spring queens began foraging for pollen. The average number of mites per queen bee was over 30 fold greater than that reported in Europe. Mite incidence and mite numbers were significantly lower on worker bees than drones or queens, being present on just 51% of bees and averaging 38 mites per bee. Our reported incidence of worker bee parasitism by this mite is 5–50 times higher than reported in Europe. That only one parasite species co-invaded Tasmania supports the notion that a small number of queens founded the Tasmanian population. However, it is clearly evident that both the bee in the absence of parasites, and the mite have been extraordinarily successful invaders. Received 12 April 2006; revised 10 November 2006; accepted 15 November 2006.     

Selection for non-diapause in the bumblebee Bombus terrestris, with notes on the effect of inbreeding

Bumblebees are generally believed to be annual insects. However, here we will show that under laboratory conditions the bumblebee Bombus terrestris (L.) can produce a second generation without a period of cold storage (diapause) or CO₂ narcosis (a method to break diapause). It is also shown that this so-called non-diapause trait can be selected for. The percentage of non-diapausing queens increased from 8% (minimum) to 97% (maximum) in two generations of selection. However, it was not possible to maintain isofemale non-diapause lines. Colonies of the fourth and fifth generation remained small (expressed in worker number) and produced a small number of queens. Also the percentage of queens that started laying eggs (defined as the percentage non-diapause) decreased in the fourth and fifth generations. To study whether this decline of the non-diapause lines was caused by inbreeding, a control experiment was conducted. In this control experiment queens were mated with their brothers (full-sib mating) for several generations and the number of queens that start egg laying was measured. This revealed that inbreeding can have a negative effect on the egg-laying capacities of queens thus causing the decline of inbred (non-diapause) lines.