Clustered male production by workers in the stingless bee Melipona subnitida Ducke (Apidae, Meliponinae)

Summary: In stingless bees brood cells are sequentially filled with liquid larval food (mass-provisioning), upon which the queen lays an egg. Thereafter the cell is closed by a worker. This study showed that during these processes workers of Melipona subnitida regularly laid eggs that served as food for the queen. Occasionally cells were oviposited in and immediately closed by a worker. These cells always rendered males. Some of these reproductive workers were seen to lay a trophic egg as well. Cells which were exclusively oviposited in by the physogastric queen gave rise to workers and queens only. In one colony it could be verified that three workers alone, which differed in age by one day, laid 15 male-producing eggs within a period of two successive weeks. Among them the number of ovipositions was positively related to the order in which workers eclosed - the oldest worker laying most eggs - and inversely related to the number of times they closed cells oviposited in exclusively by the queen. Apparently the physogastric queen was not able to stop certain workers from reproducing. We therefore conclude that some workers in M. subnitida temporarily dominated their queens in egg-laying.     

Queen execution in a monogynous ant

Workers in many species of social insects are capable of laying unfertilized eggs, which can develop into haploid males. This causes a conflict about male parentage between queens and workers. In a few species, this may result in matricide, that is, workers kill the colony''s queen. Queen killing has so far been observed mainly in multi‐queen colonies or in annual species, when the queen''s fecundity declines at the end of the reproductive period. Here, we report queen expulsion and matricide in a monogynous, monandrous ant with perennial societies. Workers were seen to aggressively expel both related and unrelated queens from their nest shortly after the end of hibernation. Queen expulsion and matricide led to a significant decrease in the number of workers and brood, but eventually increased the direct fitness of workers through significant male production. Long‐term observations revealed a short lifespan of queens, while workers in orphaned colonies survived and produced male offspring over several years.     

Male parentage and queen mating frequency in the bumblebee <Emphasis Type="Italic">Bombus ignitus</Emphasis> (Hymenoptera: bombinae)

Kin selection theory predicts conflict between queens and workers in the social insect colony with respect to male production. This conflict arises from the haplodiploid system of sex determination in Hymenoptera that creates relatedness asymmetries in which workers are more closely related to the sons of other workers than to those of the queen. In annual hymenopteran societies that are headed by a single queen, the mating frequency of the queen is the only factor that affects the colony kin structure. Therefore, we examined the mating structure of queens and the parentage of males in a monogynous bumblebee, Bombus ignitus, using DNA microsatellites. In the seven colonies that were studied, B. ignitus queens mated once, thereby leading to the prediction of conflict between the queen and workers regarding male production. In each of the five queen-right colonies, the majority of the males (95%) were produced by the colony’s queen. In contrast, workers produced approximately 47% of all the males in two queenless colonies. These results suggest that male production in B. ignitus is a conflict between queen and workers.     

Queen-worker conflicts over male production and sex allocation in a primitively eusocial wasp

Abstract In a colony headed by a single monandrous foundress, theories predict that conflicts between a queen and her workers over both sex ratio and male production should be intense. If production of males by workers is a function of colony size, this should affect sex ratios, but few studies have examined how queens and workers resolve both conflicts simultaneously. We conducted field and laboratory studies to test whether sex-ratio variation can be explained by conflict over male production between queen and workers in the primitively eusocial wasp Polistes chinensis antennalis.
Worker oviposition rate increased more rapidly with colony size than did queen oviposition. Allozyme and micro-satellite markers revealed that the mean frequency of workers' sons among male adults in queen-right colonies was 0.39 ± 0.08 SE (n = 22). Genetic relatedness among female nestmates was high (0.654–0.796), showing that colonies usually had a single, monandrous queen. The mean sex allocation ratio (male investment/male and gyne investments) of 46 queen-right colonies was 0.47 ± 0.02, and for 25 orphaned colonies was 0.86 ± 0.04. The observed sex allocation ratio was likely to be under queen control. For queen-right colonies, the larger colonies invested more in males and produced reproductives protandrously and/or simultaneously, whereas the smaller colonies invested more in females and produced reproductives protogynously. Instead of positive relationships between colony size and worker oviposition rate, the frequency of workers' sons within queen-right colonies did not increase with colony size. These results suggest that queens control colony investment, even though they allow worker oviposition in queen-right colonies. Eggs laid by workers may be policed by the queen and/or fellow workers. Worker oviposition did not influence the outcome of sex allocation ratio as a straightforward function of colony size.     

Kin structure and colony male reproduction in the hornet<Emphasis Type="Italic"> Vespa crabro</Emphasis> (Hymenoptera: Vespidae)

We estimated queen mating frequency, genetic relatedness among workers, and worker reproduction in Vespa crabro flavofasciata using microsatellite DNA markers. Of 20 colonies examined, 15 contained queens inseminated by a single male, 3 colonies contained queens inseminated by two males, and 2 colonies contained queens inseminated by three males. The genetic relatedness among workers was estimated to be 0.73±0.003 (mean±SE). For this high relatedness, kin selection theory predicts a potential conflict between queens and workers over male production. To verify whether males are derived from queens or workers, 260 males from 13 colonies were genotyped at four microsatellite loci. We found that all of the males were derived from the queens. This finding was further supported by the fact that only 33 of 2,990 workers dissected had developed ovaries. These workers belonged to 2 of the 20 colonies. There was no relationship between queen mating frequency and worker reproduction, and no workers produced male offspring in any of the colonies. These results suggest that male production dominated by queens in V. crabro flavofasciata is possibly due to worker policing.     

It's good to be queen: classically eusocial colony structure and low worker fitness in an obligately social sweat bee

Lasioglossum malachurum, a bee species common across much of Europe, is obligately eusocial across its range but exhibits clear geographic variation in demography and social behaviour. This variation suggests that social interactions between queens and workers, opportunities for worker oviposition, and patterns of relatedness among nest mates may vary considerably, both within and among regions. In this study, we used three microsatellite loci with 12-18 alleles each to examine the sociogenetic structure of colonies from a population at Agios Nikolaos Monemvasias in southern Greece. These analyses reveal that the majority of colonies exhibit classical eusocial colony structure in which a single queen mated to a single male monopolizes oviposition. Nevertheless, we also detect low rates of multiqueen nest founding, occasional caste switching by worker-destined females, and worker oviposition of both gyne and male-producing eggs in the final brood. Previous evidence that the majority of workers show some ovarian development and a minority (17%) have at least one large oocyte contrasts with the observation that only 2-3% of gynes and males (the so-called reproductive brood) are produced by workers. An evaluation of the parameters of Hamilton's Rule suggests that queens benefit greatly from the help provided by workers but that workers achieve greater fitness by provisioning and laying their own eggs rather than by tending to the queen's eggs. This conflict of interest between the queen and her workers suggests that the discrepancy between potential and achieved worker oviposition is due to queen interference. Comparison of relatedness and maternity patterns in the Agios Nikolaos Monemvasias population with those from a northern population near Tübingen, Germany, points to a north-south cline of increasingly effective queen control of worker behaviour.     

No evidence that reproductive bumblebee workers reduce the production of new queens

Kin selection theory predicts potential conflict between queen and workers over male parentage in hymenopteran societies headed by one, singly mated queen, because each party is more closely related to its own male offspring. In ‘late-switching’ colonies of the bumblebee Bombus terrestris, i.e. colonies whose queens lay haploid eggs relatively late in the colony cycle, workers start to lay male eggs shortly after the queen lays the female eggs that will develop into new queens. It has been hypothesized that this occurs because workers recognize, via a signal given by the queen instructing female larvae to commence development as queens, that egg laying is now in their kin-selected interest. This hypothesis assumes that aggressive behaviour in egg-laying workers does not substantially reduce the production of new queens, which would decrease the workers' fitness payoff from producing males. We tested the hypothesis that reproductive activity inB. terrestris workers does not reduce the production of new queens. We used microsatellite genotyping to sex eggs and hence to select eight size-matched pairs of ‘late-switching’ colonies from a set of commercial colonies. From one colony of each pair we removed every egg-laying or aggressive worker observed. From the other colony, we simultaneously removed a nonegg-laying, nonaggressive worker. Removed workers were replaced with young workers from separate colonies at equal frequencies within the pair. There was no significant difference in queen productivity between colonies with reduced or normal levels of egg-laying or aggressive workers. Therefore, as predicted, reproductive B. terrestris workers did not significantly reduce the production of new queens.     

Mate number,kin selection and social conflicts in stingless bees and honeybees

Microsatellite genotyping of workers from 13 species (ten genera) of stingless bees shows that genetic relatedness is very high. Workers are usually daughters of a single, singly mated queen. This observation, coupled with the multiple mating of honeybee queens, permits kin selection theory to account for many differences in the social biology of the two taxa. First, in contrast to honeybees, where workers are predicted to and do police each other''s male production, stingless bee workers are predicted to compete directly with the queen for rights to produce males. This leads to behavioural and reproductive conflict during oviposition. Second, the risk that a daughter queen will attack the mother queen is higher in honeybees, as is the cost of such an attack to workers. This explains why stingless bees commonly have virgin queens in the nest, but honeybees do not. It also explains why in honeybees the mother queen leaves to found a new nest, while in stingless bees it is the daughter queen who leaves.     

Queen-worker conflict over sex ratio: A comparison of primary and secondary sex ratios in the Argentine ant,Iridomyrmex humilis

We compare the primary sex ratio (proportion of haploid eggs laid by queens) and the secondary sex ratio (proportion of male pupae produced) in the Argentine ant Iridomyrmex humilis with the aim of investigating whether workers control the secondary sex ratio by selectively eliminating male brood. The proportion of haploid eggs produced by queens was close to 0.5 in late winter, decreased to less than 0.3 in spring and summer, and increased again to a value close to 0.5 in fall. Laboratory experiments indicate that temperture is a proximate factor influencing the primary sex ratio with a higher proportion of haploid eggs being laid at colder temperatures. Production of queen pupae ceased in mid-June, about three weeks before that of male pupae. After this time only worker pupae were produced. During the period of production of sexuals, the proportion of male pupae ranged from 0.30 to 0.38. Outside this period no males were reared although haploid eggs were produced all the year round by queens. Workers thus exert a control on the secondary sex ratio by eliminating a proportion of the male brood during the period of sexual production and eliminating all the males during the remainder of the cycle. These data are consistent with workers preferring a more female-biased sex ratio than queens. The evolutionary significance of the production of male eggs by queens all the year round is as yet unclear. It may be a mechanism allowing queen replacement in the case of the death of the queens in the colony.     

Worker policing in the common wasp Vespula vulgaris is not aimed at improving colony hygiene

In insect societies, eggs laid by workers are frequently killed by other workers – a behaviour known as “worker policing”. The traditional explanation of worker policing is that it is a mechanism to resolve intracolony conflict, and maintain the reproductive monopoly of the queen. Recently, Pirk et al. (2004) proposed that worker policing instead is aimed at removing unviable worker-laid eggs and is ultimately just another example of hygienic behaviour. Here we test this hypothesis for the common wasp Vespula vulgaris, a species with highly effective worker policing. We show that worker-laid eggs from queenless colonies have a lower hatch rate (68%) than queen-laid eggs (82%). Analysis of egg laying rates of queens and workers, however, shows that the difference is not big enough to explain the apparent absence of adult worker-derived males in this species. Received 30 January 2006; revised 2 May 2006; accepted 5 May 2006.     

REPRODUCTIVE CONFLICT IN BUMBLEBEES AND THE EVOLUTION OF WORKER POLICING

Worker policing (mutual repression of reproduction) in the eusocial Hymenoptera represents a leading example of how coercion can facilitate cooperation. The occurrence of worker policing in “primitively” eusocial species with low mating frequencies, which lack relatedness differences conducive to policing, suggests that separate factors may underlie the origin and maintenance of worker policing. We tested this hypothesis by investigating conflict over male parentage in the primitively eusocial, monandrous bumblebee, Bombus terrestris. Using observations, experiments, and microsatellite genotyping, we found that: (a) worker‐ but not queen‐laid male eggs are nearly all eaten (by queens, reproductive, and nonreproductive workers) soon after being laid, so accounting for low observed frequencies of larval and adult worker‐produced males; (b) queen‐ and worker‐laid male eggs have equal viabilities; (c) workers discriminate between queen‐ and worker‐laid eggs using cues on eggs and egg cells that almost certainly originate from queens. The cooccurrence in B. terrestris of these three key elements of “classical” worker policing as found in the highly eusocial, polyandrous honeybees provides novel support for the hypothesis that worker policing can originate in the absence of relatedness differences maintaining it. Worker policing in B. terrestris almost certainly arose via reproductive competition among workers, that is, as “selfish” policing.     

Reproductive allocation within a polygyne, polydomous colony of the ant Myrmica rubra

1. Ant colonies commonly have multiple egg‐laying queens (secondary polygyny). Polygyny is frequently associated with polydomy (single colonies occupy multiple nest sites) and restricted dispersal of females. The production dynamics and reproductive allocation patterns within a population comprising one polygyne, polydomous colony of the red ant Myrmica rubra were studied. 2. Queen number per nest increased with nest density and the number of adult workers increased with the number of resident queens and with nest density. This suggests that nest site limitation promotes polygyny and that workers accumulate in nest units incapable of budding. 3. Nest productivity increased with the number of adult workers and production per queen was independent of queen number. Productivity increased with nest density, suggesting local resource enhancement. This shows that productivity can be a linear function of queen numbers and that the limiting factor is not the egg‐laying capacity of queens. 4. The total and per capita production of reproductives decreased towards the periphery of the colony, suggesting that the spatial location of nest units affects sexual production. Thus nests at the periphery of the colony invested more heavily in new workers. This is consistent with earlier observations in plants and could either represent investment in future budding or increased defence. 5. The colony produced only five new queens and 2071 males, hence the sex ratio was extremely male biased.     

Reproductive parameters vary with social and ecological factors in the polygynous ant Formica exsecta

Due to their haplo‐diploid sex determination system and the resulting conflict over optimal sex allocation between queens and workers, social Hymenoptera have become important model species to study variation in sex allocation. While many studies indeed reported sex allocation to be affected by social factors such as colony kin structure or queen number, others, however, found that sex allocation was impacted by ecological factors such as food availability. In this paper, we present one of the rare studies that simultaneously investigated the effects of social and ecological factors on social insect nest reproductive parameters (sex and reproductive allocation, nest productivity) across several years. We found that the sex ratio was extremely male biased in a polygynous (multiple queens per nest) population of the ant Formica exsecta. Nest‐level sex allocation followed the pattern predicted by the queen‐replenishment hypothesis, which holds that gynes (new queens) should only be produced and recruited in nests with low queen number (i.e. reduced local resource competition) to ensure nest survival. Accordingly, queen number (social factor) was the main determinant on whether a nest produced gynes or males. However, ecological factors had a large impact on nest productivity and therefore on a nest's resource pool, which determines the degree of local resource competition among co‐breeding queens and at what threshold in queen number nests should switch from male to gyne production. Additionally, our genetic data revealed that gynes are recruited back to their parental nests after mating. However, our genetic data are also consistent with some adult queens dispersing on foot from nests where they were produced to nests that never produced queens. As worker production is reduced in gyne‐producing nests, queen migration might be offset by workers moving in the other direction, leading to a nest network characterized by reproductive division of labour. Altogether our study shows that both, social and ecological factors can influence long‐term nest reproductive strategies in insect societies.     

Kin-selected conflict in the bumble-bee Bombus terrestris (Hymenoptera: Apidae)

Kin selection theory predicts conflict in social Hymenoptera between the queen and workers over male parentage because each party is more closely related to its own male offspring. Some aspects of the reproductive biology of the bumble-bee Bombus terrestris support kin selection theory but others arguably do not. We present a novel hypothesis for how conflict over male parentage should unfold in B. terrestris colonies. We propose that workers delay laying eggs until they possess information showing that egg laying suits their kin-selected interests. In colonies where queens start to lay haploid eggs early, we hypothesize that this occurs when workers detect the presence of queen-produced male brood in the brood's larval stage. In colonies where queens start to lay haploid eggs late, we hypothesize that it occurs when workers detect a signal from the queen to female larvae to commence development as queens. Our hypothesis accounts for previously unexplained aspects of the timing of reproductive events in B. terrestris, provides ultimate explanations for the results of a recent study of mechanisms underlying queen-worker conflict and helps explain this species' characteristic bimodal (split) sex ratios. Therefore, kin selection theory potentially provides a good explanation for reproductive patterns in B. terrestris.     

Sex ratio determination by queens and workers in the ant Pheidole desertorum

Because workers in colonies of eusocial Hymenoptera are more closely related to sisters than to brothers, theory predicts workers should bias investment in reproductive broods to favour reproductive females over males. However, conflict between queens and workers is predicted. Queens are equally related to daughters and sons, and should act to prevent workers from biasing investment. Previous study of the ant Pheidole desertorum showed that workers are nearly three times more closely related to reproductive females than males; however, the investment sex ratio is very near equal, consistent with substantial queen control of workers. Near-equal investment is produced by an equal frequency of colonies whose reproductive broods consist of only females (female specialists) and colonies whose reproductive broods consist of only males or whose sex ratios are extremely male biased (male specialists). Because natural selection should act on P. desertorum workers to bias investment in favour of reproductive females, why do workers in male-specialist colonies rear only (or mostly) males? We tested the hypothesis that queens prevent workers from rearing reproductive females by experimentally providing workers with immature reproductive broods of both sexes. Workers reared available reproductive females, while failing to rear available males. Worker preference for rearing reproductive females is consistent with queens preventing their occurrence in colonies of male specialists. These results provide evidence that queens and workers will act in opposition to determine the sex ratio, a fundamental prediction of queen-worker conflict theory. Copyright 2000 The Association for the Study of Animal Behaviour.     

Worker interests and male production in Polistes gallicus, a Mediterranean social wasp

The resolution of social conflict in colonies may accord with the interests of the most numerous party. In social insect colonies with single once-mated queens, workers are more closely related to the workers' sons than they are to the queens' sons. Therefore, they should prefer workers to produce males, against the queen's interests. Workers are capable of producing males as they arise from unfertilized eggs. We found Polistes gallicus to have colonies of single, once-mated queens, as determined by microsatellite genotyping of the workers, so worker interests predict worker male production. In colonies lacking queens, workers produced the males, but not in colonies with original queens. Thus worker interests were expressed only when the queen was gone. The high fraction of missing queens and early end to the colony cycle relative to climate so early in the season is surprising and may indicate a forceful elimination of the queen.     

The honeybee queen influences the regulation of colony drone production

Social insect colonies invest in reproduction and growth, buthow colonies achieve an adaptive allocation to these life-historycharacters remains an open question in social insect biology.Attempts to understand how a colony's investment in reproductionis shaped by the queen and the workers have proved complicatedbecause of the potential for queen–worker conflict overthe colony's investment in males versus females. Honeybees,in which this conflict is expected to be minimal or absent,provide an opportunity to more clearly study how the actionsand interactions of individuals influence the colony's productionand regulation of males (drones). We examined whether honeybeequeens can influence drone regulation by either allowing orpreventing them from laying drone eggs for a period of timeand then examining their subsequent tendency to lay drone andworker eggs. Queens who initially laid drone eggs subsequentlylaid fewer drone eggs than the queens who were initially preventedfrom producing drone eggs. This indicates that a colony's regulationof drones may be achieved not only by the workers, who buildwax cells for drones and feed the larvae, but also by the queen,who can modify her production of drone eggs. In order to betterunderstand how the queen and workers contribute to social insectcolony decisions, future work should attempt to distinguishbetween actions that reflect conflict over sex allocation andthose that reflect cooperation and shared control over the colony'sinvestment in reproduction.     

Morphology of reproductive and trophic eggs and their controlled release by workers in Trigona (Tetragonisca) angustula llliger (Apidae, Meliponinae)

Abstract. To study the reproductive potential of workers in the stingless bee Trigona (Tetragonisca) angustula Illiger (Apidae, Meliponinae), we examined the morphological quality of their eggs. Worker-eggs were all placed on the inside of the upper wall of brood cells. Normally, such eggs are consumed by the queen and are therefore referred to as trophic. The provisioning and oviposition processes in queenright colonies are characterized by the occurrence of circular aggregations of workers, 'rosettes', around the broodcell opening immediately after the release of a worker-egg. The ovaries of 35% of these rosette workers contained mature, chorionated eggs. In a single worker ovary, always only one mature egg was found. Some of the eggs, dissected from the ovarioles, showed a reticulate chorion pattern. Worker-eggs which lacked this pattern were significantly bigger than patterned eggs. Scanning electron micrographs revealed that the patterned worker-eggs are similar in appearance to queen-eggs.
After a worker-egg had been removed experimentally from a broodcell, the same cell could be oviposited by a worker again. Light microscopic analysis revealed that all these worker-eggs lacked the reticulate chorion pattern and were very similar in their morphology. In a colony without a laying queen workers laid eggs which had a reticulate pattern on the chorion. Since these eggs developed into males, we assume that the pattern on the chorion is characteristic for reproductive eggs. We also assume that the queen prevents the release of reproductive eggs by the workers. However, she does not inhibit the development of this type of egg.     

Significant reproductive skew in the facultatively polygynous ant Pheidole pallidula

Reproductive skew - the extent to which reproduction is unevenly shared between individuals in a social group - varies greatly between and within animal species. In this study, we investigated how queens share parentage in polygynous (multiple queen) colonies of the Mediterranean ant Pheidole pallidula. We used highly polymorphic microsatellites markers to determine parentage of gynes (new queens), males and workers in P. pallidula field colonies. The comparison of the genotypes of young and adult workers revealed a very low queen turnover (less than 2%). The first main finding of the study of reproductive skew in these colonies was that there was a significant departure from equal contribution of queens to gyne, male and worker production. Reproductive skew was greater for male production than for queen and worker production. There was no relationship between the magnitude of the reproductive skew and the number of reproductive queens per colony, their relatedness and the overall colony productivity, some of the factors predicted to influence the extent of reproductive skew. Finally, our study revealed for the first time a trade-off in the relative contribution of nestmate queens to gyne and worker production. The queens contributing more to gyne production contributed significantly less to worker production.     

Mating structure and male production in Vespa analis and Vespa simillima (Hymenoptera: Vespidae)

We estimated queen mating frequency, genetic relatedness between workers and worker reproduction in the hornets Vespa analis and Vespa simillima using microsatellite DNA genotyping. The 20 V. analis colonies studied each contained a queen inseminated by a single male. Of the 15 V. simillima colonies studied, nine had a queen inseminated by a single male, four had a queen inseminated by two males, and two had a queen inseminated by three males. The estimated effective number of matings was 1.33 ± 0.74 (mean ± SD), with 75–85% of the offspring of the six multiply mated queens sired by single males. The values for genetic relatedness between the workers of V. analis and V. simillima were 0.739 ± 0.004 and 0.698 ± 0.013 (mean ± SD), respectively. We conclude that V. analis and V. simillima colonies are genetically monogynous and monandrous. When high relatedness between the workers occurs within colonies, kin selection theory predicts a potential conflict between queens and workers over male production. To determine whether males were derived from queens or workers, males from V. analis and V. simillima colonies were genotyped at four microsatellite loci and the level of ovary activation in workers was determined. None of the 787 V. analis workers and only 15 of 3520 V. simillima workers had developed ovaries. Furthermore, the genotyping identified no worker‐produced males in any colony. The presence of reproductive workers correlated positively with the number of workers within the colony. These results suggest that eusocial colonies with an annual life cycle tend to break down socially when they become large and are close to dying.