Reproductive and worker castes in the primitively eusocial wasp Belonogaster petiolata (DeGeer) (Hymenoptera: Vespidae): evidence for pre-imaginal differentiation

Until recently, morphological differences between castes in independent-founding polistine wasps were considered absent. This paper investigates the extent of morphological and physiological differences between reproductive (foundress and gyne) and worker castes of Belonogaster petiolata, and tests the hypothesis that caste differentiation in this species occurs pre-imaginally.Foundresses were significantly larger than workers, to the extent that foundress/worker ratios were comparable with those between queens and workers in some swarm-founding Polistinae. Early emerging workers were small, but body size increased over the colony cycle such that late-season workers were similar in size to gynes. In proportion to body size, workers possessed broader heads while foundresses and gynes had broader thoraces and gasters. All queens, 98% of subordinate foundresses, and 95% of over-wintering gynes were inseminated. Workers were never inseminated and lacked mature ovaries in colonies with active queens. However, in the absence of the queen (and other foundresses), 11% of workers developed mature ovaries. Ovarian size and fat content of foundresses and gynes was significantly greater than that of workers. The differences in external morphology and reproductive physiology between castes support the hypothesis that differentiation occurs pre-imaginally. However, imaginal factors, in particular social dominance of the queen, maintain the reproductive subordinance of workers.     

Gyne development in the Argentine ant Iridomyrmex humilis: role of overwintering and queen control

Abstract A possible stimulatory effect of overwintering on gyne development in Iridomyrmex humilis (Mayr) was investigated. Although gyne-potent larvae are present in the nest throughout the year, small queenless colony fragments composed of freshly overwintered ants (collected in late winter or early spring) produced 3–8 times more gynes than comparable fragments made up of non-freshly overwintered ants (collected at other times of the year). Apparently, this stimulatory effect of overwintering acts on both the developmental potential of larvae and the tendency of workers to rear sexually competent larvae as gynes; queenless colony fragments in which either the brood or workers were freshly overwintered produced more gynes than fragments composed of non-freshly overwintered workers or brood but fewer than fragments containing both brood and workers freshly overwintered. This increased sexualization potential of larvae due to overwintering is enough to overcome weak pheromonal inhibition of queens associated with low queen number; a single queen in a large freshly overwintered colony fragment is insufficient to inhibit gyne development, whereas ten queens are totally inhibitory. In non-freshly overwintered colony fragments one queen is completely inhibitory. Variability in egg developmental potential according to queen age does not appear to play a major role in the seasonal production of gynes, because at least some eggs of very young queens (less than 3 weeks old) are capable of gyne development. In the field this stimulatory effect of overwintering is superimposed on a seasonal fluctuation in the combined strength of pheromonal queen control. In southern France, gynes are produced only in spring where they arise primarily from overwintered larvae just after a sharp drop in queen number, and presumably the total level of inhibitory queen pheromone, due to the massive execution of queens by workers.     

Host queen killing by a slave-maker ant queen: when is a host queen worth attacking?

A new colony of the slave-making ant Polyergus breviceps is initiated when a newly mated gyne invades a host nest and kills the resident queen. This process seems to result in chemical camouflage of the invading gyne and allows her to usurp the position of colony reproductive. Young, recently mated Formica gynes, however, are not attacked. To determine whether worker and/or immature presence is the basis for aggression, we placed eggs, larvae, pupae and workers from mature F. gnava queens with newly mated F. gnava queens and observed the responses of introducedP. breviceps queens. Because no newly mated gyne was attacked, we tested newly mated F. gnava queens (1) once they had produced eggs, (2) when the offspring reached the larval, pupal and callow stages of development, and (3) every 2 weeks until aggression ensued. Eventually all F. gnava queens were attacked but only 29 weeks after having mated. Thus, although offspring are the ultimate benefit from attacking an established F. gnava queen,P. breviceps queens detect mature queens using another time-dependent feature that is reliably indicative of reproductive status. The similarity of host queen hydrocarbon profiles, often correlated with reproductive status in other ant species, suggests that other compounds reflect queen fecundity and produce a kairomonal effect, or that another cue signals host queen and colony suitability. Our findings indicate P. breviceps gynes have evolved to respond aggressively to a host gyne cue that appears long after mating, preventing attacks on gynes without the workers necessary for colony founding.Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

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.     

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.     

Developmental plasticity in the queen-polymorphic ant Temnothorax longispinosus

Females of social Hymenoptera show developmental plasticity in response to varying social and environmental conditions, though some species have strong genetic influences on the form of the female reproductives. In ants, a queen polymorphism can occur in which large queens initiate new colonies on their own, while small queens enter established nests. Most queen polymorphisms studied to date originate due to genetic differences between individuals of differing form. Here, we report on the development of female form in response to social factors within the nest in the queen-polymorphic ant Temnothorax longispinosus. Three queen size morphs occur: a rare large queen with higher fat stores that can found new colonies independently, a large queen that has low fat stores and is behaviorally flexible, and a small queen that rejoins the natal nest. Both in nesting units collected from the field and those reared in the lab, queen presence during larval development led to fewer larvae developing as gynes (virgin, winged queens), and most of those gynes were the small morph. This queen effect is transferred to developing gyne larvae by close, physical interaction between queens and workers, and causes slower larval development. We conclude that gyne size, and therefore reproductive behavior, in T. longispinosus is developmentally plastic in response to queen presence. Plasticity in reproductive behavior may be an adaptive response to the nest sites utilized by this species. T. longispinosus nests predominantly in acorns and hickory nuts, which can vary dramatically from 1 year to the next. Since queens are more likely to be present in each nesting unit when fewer nest sites are available, the queen effect that results in more small gynes produced links the expression of colony-founding traits to ecological conditions across habitat patches.     

Colony social organization of <Emphasis Type="Italic">Lasioglossum malachurum</Emphasis> Kirby (Hymenoptera,Halictidae) in southern Greece

Summary We studied the nesting and social biology of two aggregations of the obligately eusocial halictine bee Lasioglossum malachurum at Agios Nikolaos Monemvasias (ANM) in southern Greece. Observations and nest excavations carried out from May to June 2000, revealed social and demographic variation between aggregations and years at ANM, as well as notable differences between these and other European populations. In southern Greece, the colony cycle includes multiple broods: the first two broods comprise only workers, whereas the third brood comprises workers, males, and gynes. Although pleometrosis is unknown in other populations, in the ANM region, as many as 10% of nests have more than one foundress. Newly emerged workers and gynes exhibit non-overlapping size distributions, but a few queens are worker-sized, indicating that workers occasionally overwinter and become foundresses. Although the vast majority of workers are unmated and most exhibit no ovarian development, an increase in worker ovarian development at the time of male production suggests that many males may develop from worker-laid eggs. Worker reproduction seems to be inhibited by the presence of queens, and annual variation in queen mortality may underlie annual variation in worker ovarian development. Across Europe, the major demographic and social differences among L. malachurum populations are in the number of worker broods and the extent of worker ovarian development. This contrasts with the results of a principal components analysis of social traits among 15 social L. (Evylaeus) populations, which shows that interspecific social variation is defined by the proportion of males in the early brood, the proportion of workers mated, queen-worker size dimorphism, gyne overwintering locale, and the proportion of workers with developed ovaries.Received 17 June 2002; revised 16 January 2002; accepted 27 January 2003.     

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.     

Effects of overwintering temperature on the survival of the black garden ant (Lasius niger)

The overwintering temperatures of ants might well be elevated due to climate change. We studied whether the overwintering temperature affects the survival of the queens and whole colonies of the black garden ant, Lasius niger (Linnaeus, 1758). In two consecutive years (2009, 2010) we collected mated, colony founding queens (n=280) from the urban area of Turku, Finland. Half of the queens overwintered in +7 to +8 °C and the other half in +2 °C. After the overwintering period, we determined their survival rate and measured the body fat content, body size and immune defence (encapsulation rate) of overwintering queens. Using the same setup, we studied the survival of 1-year-old L. niger colonies (queen & workers). Overwintering at a lower temperature (+2 °C) decreased the survival of workers. The survival of colony founding queens differed between years, but unlike with workers, the overwintering temperature did not affect their survival: neither in the colony experiment nor in the single queen experiment. All of the surviving queens managed to produce their worker offspring at the same rate. The relative amount of body fat of queens was higher for those who overwintered at a lower temperature, which is likely a result of lower energy consumption. We did not detect differences in the encapsulation rate between the temperature treatment groups. The ability of colony founding queens to tolerate wide overwintering temperature variations present in urban environments may explain the success of the colony in urban areas. As the colony grows, the overwintering chambers may extend more deeply into the ground. Thus, workers may not have to cope with such cold conditions as colony founding queens.     

Multiple mating and offspring quality in <Emphasis Type="Italic">Lasius</Emphasis> ants

Genetic diversity benefits for social insect colonies headed by polyandrous queens have received intense attention, whereas sexual selection remains little explored. Yet mates of the same queen may engage in sperm competition over the siring of offspring, and this could confer benefits on queens if the most successful sire in each colony (the majority sire) produces gynes (daughter queens) of higher quality. These benefits could be increased if high-quality sires make queens increase the percentage of eggs that they fertilize (unfertilized eggs develop into sons in social hymenopterans), or if daughters of better genetic quality are over-represented in the gyne versus worker class. Such effects would lead to female-biased sex ratios in colonies with high-quality majority gynes. I tested these ideas in field colonies of Lasius niger black garden ants, using body mass of gynes as a fitness trait as it is known to correlate with future fecundity. Also, I established the paternity of gynes through microsatellite DNA offspring analyses. Majority sires did not always produce heavier gynes in L. niger, but whenever they did do so colonies produced more females, numerically and in terms of the energetic investment in female versus male production. Better quality sires may be able to induce queens to fertilize more eggs or so-called caste shunting may occur wherever the daughters of better males are preferentially shunted to into the gyne caste. My study supports that integrating sexual selection and social evolutionary studies may bring a deeper understanding of mating system evolution in social insects.     

A mechanical signal biases caste development in a social wasp

Understanding the proximate mechanisms of caste development in eusocial taxa can reveal how social species evolved from solitary ancestors. In Polistes wasps, the current paradigm holds that differential amounts of nutrition during the larval stage cause the divergence of worker and gyne (potential queen) castes. But nutrition level alone cannot explain how the first few females to be produced in a colony develop rapidly yet have small body sizes and worker phenotypes. Here, we provide evidence that a mechanical signal biases caste toward a worker phenotype. In Polistes fuscatus, the signal takes the form of antennal drumming (AD), wherein a female trills her antennae synchronously on the rims of nest cells while feeding prey-liquid to larvae. The frequency of AD occurrence is high early in the colony cycle, when larvae destined to become workers are being reared, and low late in the cycle, when gynes are being reared. Subjecting gyne-destined brood to simulated AD-frequency vibrations caused them to emerge as adults with reduced fat stores, a worker trait. This suggests that AD influences the larval developmental trajectory by inhibiting a physiological element that is necessary to trigger diapause, a gyne trait.     

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.     

Brood development into sexual females depends on the presence of a queen but not on temperature in an ant dispersing by colony fission,Aphaenogaster senilis

Abstract. 1. In eusocial insects, colony fission is a mode of dispersal by which a young queen leaves her nest with some workers to found a new colony. In these species, adult females (workers and the queen) should allocate most resources to increasing their colony size, which constrains the possibility of fission. In contrast, developing diploid larvae should have a preference for becoming a queen and having their own offspring, rather than becoming workers and rearing the offspring of other females. 2. In the ant Aphaenogaster senilis, queens are produced in very small numbers, suggesting that adult females control larval development. We used a 6‐year series of data on more than 300 nests to determine the annual cycle of worker and queen production. Although both overlapped, the latter mostly occurred in the second half of the summer, after a major peak of worker emergence. Young queens were also often produced in nests whose reproductive queen had died, thus allowing her replacement. Overall, we estimate that only 0.07% of diploid larvae actually develop into gynes. 3. Laboratory experiments indicated that brood is bipotent until the second larval instar. Diploid larval development into queen was favoured by the removal of the mother queen, but was not affected by rearing temperature. 4. Our data suggest that most diploid broods are forced by the adults to develop into workers rather than into gynes. However, when the queen is not present due to death or after a fission event, a few larvae are allowed to develop into gynes. One way for workers to limit the development of larvae might be by controlling the amount of food they receive.     

<Emphasis Type="Italic">Polistes japonicus</Emphasis> (Hymenoptera,Vespidae) queens monopolize ovipositing but are not the most active aggressor in dominant-subordinate interactions

In order to elucidate the dominant–subordinate relationship between the foundress and workers, five colonies of the paper wasp Polistes japonicus were observed in a netted and covered cage located outdoors. The number of workers in each colony ranged from four to eight. Workers were divided into first and second broods. Abdominal wagging and ovipositing were performed almost exclusively by the foundress throughout colony development. However, an analysis of aggressive encounters indicated that although the foundress hardly received dominance behaviors (aggression) from workers, it lacked either partially or completely the following characteristics of the queen that are usually seen in paper-wasp colonies with independent-founding queens (except in one colony that produced no second brood): the queen being socially dominant over any worker (the queen had more wins than losses in one-on-one dominance contests with any worker), exhibiting the highest frequency of dominance behaviors, and directing dominance behaviors primarily toward the socially most-dominant worker. In particular, during the mixed-brood period (when all first- and second-brood workers were present on the nest) the foundress hardly exhibited dominance behaviors toward socially dominant workers (mainly second brood) but frequently directed dominance behaviors toward socially subordinate workers (mainly first brood). The foundress disappeared in two colonies before the reproductives emerged; in these colonies the socially most-dominant worker inherited the colony and laid many eggs. The frequency of abdominal wagging by these two foundresses decreased during colony development, while it did not in the other colonies. This suggests that abdominal wagging provides information about the vigor of the performer. The superseder was socially dominant over all other workers, but spent little time wagging its abdomen and allowed some workers to lay eggs.     

Facultative use of thelytokous parthenogenesis for queen production in the polyandrous ant Cataglyphis cursor

The evolutionary paradox of sex remains one of the major debates in evolutionary biology. The study of species capable of both sexual and asexual reproduction can elucidate factors important in the evolution of sex. One such species is the ant Cataglyphis cursor, where the queen maximizes the transmission of her genes by producing new queens (gynes) asexually while simultaneously maintaining a genetically diverse workforce via the sexual production of workers. We show that the queen can also produce gynes sexually and may do so to offset the costs of asexual reproduction. We genotyped 235 gynes from 18 colonies and found that half were sexually produced. A few colonies contained both sexually and asexually produced gynes. Although workers in this species can also use thelytoky, we found no evidence of worker production of gynes based on genotypes of 471 workers from the six colonies producing sexual gynes. Gynes are thus mainly, and potentially exclusively, produced by the queen. Simulations of gynes inbreeding level following one to ten generations of automictic thelytoky suggest that the queen switches between or combines thelytoky and sex, which may reduce the costs of inbreeding. This is supported by the relatively small size of inbred gynes in one colony, although we found no relationship between the level of inbreeding and immune parameters. Such facultative use of sex and thelytoky by individual queens contrasts with other known forms of parthenogenesis in ants, which are typically characterized by distinct lineages specializing in one strategy or the other.     

Sex allocation in fungus‐growing ants: worker or queen control without symbiont‐induced female bias

The fungal cultivars of fungus‐growing ants are vertically transmitted by queens but not males. Selection would therefore favor cultivars that bias the ants’ sex ratio towards gynes, beyond the gyne bias that is optimal for workers and queens. We measured sex allocation in 190 colonies of six sympatric fungus‐growing ant species. As predicted from relatedness, female bias was greater in four singly mated Sericomyrmex and Trachymyrmex species than in two multiply mated Acromyrmex species. Colonies tended to raise mainly a single sex, which could be partly explained by variation in queen number, colony fecundity, and fungal garden volume for Acromyrmex and Sericomyrmex, but not for Trachymyrmex. Year of collection, worker number and mating frequency of Acromyrmex queens did not affect the colony sex ratios. We used a novel sensitivity analysis to compare the population sex allocation ratios with the theoretical queen and worker optima for a range of values of k, the correction factor for sex differences in metabolic rate and fat content. The results were consistent with either worker or queen control, but never with fungal control for any realistic value of k. We conclude that the fungal symbiont does not distort the ants’ sex ratio in these species.     

Complex sociogenetic organization and reproductive skew in a primitively eusocial sweat bee,Lasioglossum malachurum,as revealed by microsatellites

The sweat bees (Family Halictidae) are a socially diverse taxon in which eusociality has arisen independently numerous times. The obligate, primitively eusocial Lasioglossum malachurum, distributed widely throughout Europe, has been considered the zenith of sociality within halictids. A single queen heads a colony of smaller daughter workers which, by mid-summer, produce new sexuals (males and gynes), of which only the mated gynes overwinter to found new colonies the following spring. We excavated successfully 18 nests during the worker- and gyne-producing phases of the colony cycle and analysed each nest's queen and either all workers or all gynes using highly variable microsatellite loci developed specifically for this species. Three important points arise from our analyses. First, queens are facultatively polyandrous (queen effective mating frequency: range 1-3, harmonic mean 1.13). Second, queens may head colonies containing unrelated individuals (n = 6 of 18 nests), most probably a consequence of colony usurpation during the early phase of the colony cycle before worker emergence. Third, nonqueen's workers may, but the queen's own workers do not, lay fertilized eggs in the presence of the queen that successfully develop into gynes, in agreement with so-called 'concession' models of reproductive skew.     

Influence of queen weight and colony origin on worker response in Solenopsis geminata

Abstract. The influence of weight and colony origin of the queen of Solenopsis geminata (F.) (Hymenoptera: Formicidae) on worker attraction is studied under laboratory conditions. In the first experiment, worker response to individual queens of different weight from the same colony is evaluated. Heavier queens are more attractive than smaller queens to their own workers. In subsequent experiments, the colony origin effect is investigated and worker response to a pair of queens of the same weight from the same or different colonies is compared. When queens are from the same colony, workers do not show a significant preference between queens. However, when queens are from a different colony, workers are significantly more attracted to their own queen than to the foreign queen. Finally, the response of workers to queens of different weight from the same or different colonies is investigated. In both cases, workers are significantly more attracted to a heavier queen than a lighter queen, even if the lighter queen is their own queen. A putative pheromonal component (E)‐6‐(1‐pentenyl)‐2H‐2‐pyranone, is not positively correlated with queen weight.     

Multiple breeders, breeder shifts and inclusive fitness returns in an ant

In social insects, colonies may contain multiple reproductively active queens. This leads to potential conflicts over the apportionment of brood maternity, especially with respect to the production of reproductive offspring. We investigated reproductive partitioning in offspring females (gynes) and workers in the ant Formica fusca, and combined this information with data on the genetic returns gained by workers. Our results provide the first evidence that differential reproductive partitioning among breeders can enhance the inclusive fitness returns for sterile individuals that tend non-descendant offspring. Two aspects of reproductive partitioning contribute to this outcome. First, significantly fewer mother queens contribute to gyne (new reproductive females) than to worker brood, such that relatedness increases from worker to gyne brood. Second, and more importantly, adult workers were significantly more related to the reproductive brood raised by the colony, than to the contemporary worker brood. Thus, the observed breeder shift leads to genetic benefits for the adult workers that tend the brood. Our results also have repercussions for genetic population analyses. Given the observed pattern of reproductive partitioning, estimates of effective population size based on worker and gyne samples are not interchangeable.     

Sex-ratio dependent execution of queens in polygynous colonies of the ant Formica exsecta

Formica exsecta has become an important model system for studying intraspecific variation in sex ratios. Patterns of sex allocation in polygynous (multiple queen per nest) populations of F. exsecta are generally consistent with the queen-replenishment hypothesis. This hypothesis states that colonies produce gynes (reproductive females) in order to increase queen number and enhance colony survival and/or productivity when the number of resident queens is low. However, the small proportion of colonies that raise gynes produce more than necessary for simple queen replenishment. It has been hypothesized that excess production of gynes may occur to reduce the frequency of accepting foreign unrelated gynes into the colony when workers cannot distinguish nestmate from non-nestmate queens. This explanation for excess gynes requires weak or no aggression between non-nestmates and is expected to lead to the selective execution of new queens by colonies that do not invest in the production of gynes. Experimental studies where gynes were introduced into natal and foreign colonies indeed suggested that polygynous populations of F. exsecta have a poor nestmate recognition system. Although gynes were significantly more likely to be accepted in their parental colony compared to another foreign female-producing colony, the difference was small. Moreover, encounters between workers from different colonies within the population showed very little aggression and were no more aggressive than encounters between nestmates, again suggesting a weak capacity for nestmate recognition. Our experiment also showed that colonies that produced only males executed most of the gynes that were experimentally introduced into the colony, whereas female-producing colonies accepted most gynes. This is consistent with ants using a simple rule of thumb to decrease parasitism by unrelated queens, whereby colonies selectively destroy gynes whenever gynes are not produced in the colonies.