Restricted gene flow between two social forms in the ant Formica truncorum

We studied genetic differentiation between two social forms (M-type: single queen, independent nest founding; P-type: multiple queens, dependent nest founding, building of colonial networks) of the ant Formica truncorum in a locality where the social types characterize two sympatric populations. The genetic results indicate restricted gene flow between the social forms. Female gene flow between the forms appears to be absent as they did not share mitochondrial haplotypes. Significant nuclear differentiation and the distribution of private alleles suggest that male gene flow between the forms is weak. However, the assignment analysis indicates some gene flow with P males mating with M females. The results have potentially important implications concerning social evolution within the forms but they need to be confirmed in other localities before they can be generalized. The colonies in the M-type population have earlier been shown to produce split sex ratios, depending on the mating frequency of the queens. The inferred gene flow from the P to the M type means that the split sex ratio is partly suboptimal, possibly because the P populations are not long-lived enough to influence the behavioural decisions in the M colonies.     

Contrasting population genetic structure for workers and queens in the putatively unicolonial ant Formica exsecta

The theory of inclusive fitness provides a powerful explanation for reproductive altruism in social insects, whereby workers gain inclusive fitness benefit by rearing the brood of related queens. Some ant species, however, have unicolonial population structures where multiple nests, each containing numerous queens, are interconnected and individuals move freely between nests. In such cases, nestmate relatedness values may often be indistinguishable from zero, which is problematic for inclusive fitness-based explanations of reproductive altruism. We conducted a detailed population genetic study in the polygynous ant Formica exsecta, which has been suggested to form unicolonial populations in its native habitat. Analyses based on adult workers indeed confirmed a genetic structuring consistent with a unicolonial population structure. However, at the population level the genetic structuring inferred from worker pupae was not consistent with a unicolonial population structure, but rather suggested a multicolonial population structure of extended family-based nests. These contrasting patterns suggest limited queen dispersal and free adult worker dispersal. That workers indeed disperse as adults was confirmed by mark-recapture measures showing consistent worker movement between nests. Together, these findings describe a new form of social organization, which possibly also characterizes other ant species forming unicolonial populations in their native habitats. Moreover, the genetic analyses also revealed that while worker nestmate relatedness was indistinguishable from zero at a small geographical scale, it was significantly positive at the population level. This highlights the need to consider the relevant geographical scale when investigating the role of inclusive fitness as a selective force maintaining reproductive altruism.     

Variable queen number in ant colonies: no impact on queen turnover, inbreeding, and population genetic differentiation in the ant Formica selysi

Variation in queen number alters the genetic structure of social insect colonies, which in turn affects patterns of kin-selected conflict and cooperation. Theory suggests that shifts from single- to multiple-queen colonies are often associated with other changes in the breeding system, such as higher queen turnover, more local mating, and restricted dispersal. These changes may restrict gene flow between the two types of colonies and it has been suggested that this might ultimately lead to sympatric speciation. We performed a detailed microsatellite analysis of a large population of the ant Formica selysi, which revealed extensive variation in social structure, with 71 colonies headed by a single queen and 41 by multiple queens. This polymorphism in social structure appeared stable over time, since little change in the number of queens per colony was detected over a five-year period. Apart from queen number, single- and multiple-queen colonies had very similar breeding systems. Queen turnover was absent or very low in both types of colonies. Single- and multiple-queen colonies exhibited very small but significant levels of inbreeding, which indicates a slight deviation from random mating at a local scale and suggests that a small proportion of queens mate with related males. For both types of colonies, there was very little genetic structuring above the level of the nest, with no sign of isolation by distance. These similarities in the breeding systems were associated with a complete lack of genetic differentiation between single- and multiple-queen colonies, which provides no support for the hypothesis that change in queen number leads to restricted gene flow between social forms. Overall, this study suggests that the higher rates of queen turnover, local mating, and population structuring that are often associated with multiple-queen colonies do not appear when single- and multiple-queen colonies still coexist within the same population, but build up over time in populations consisting mostly of multiple-queen colonies.     

Highly variable social organisation of colonies in the ant Formica cinerea

Social organisation of colonies was examined in the ant Formica cinerea by estimating the coefficient of genetic relatedness among worker nest mates. The estimates based on microsatellite genotypes at three loci ranged from values close to zero to 0.61 across the populations studied in Finland. These results showed that a fundamental feature of colonies, the number of reproductive queens, varied greatly among the populations. Colonies in some populations had a single queen, whereas the nests could have a high number number of queens in other populations. There was a weak but non-significant correlation between the genetic and metric distance of nests within two populations with intermediate level of relatedness. Differentiation among nearby populations (within the dispersal distance of individuals) in one locality indicated limited dispersal or founder effects. This could occur when females are philopatric and stay in the natal polygynous colony which expands by building a network of nest galleries within a single habitat patch.     

Nest structure and colony cycle of the Allegheny mound ant,Formica exsectoides Forel (Hymenoptera: Formicidae)

Summary The nest structure and colony cycle of a population of Allegheny mound ants,Formica exsectoides, were examined in central Michigan. The dispersion pattern of mounds was random. Nest structure and presence of brood were primarily determined by excavation of twenty-three nests over three intervals from June through September. Additional excavations of five nests in 1990 and ten nests in 1991 provided further details on nest structure and colony cycle. Most galleries occurred within the mound and upper 30 cm of soil, but some activity reached depths of 100 to 270 cm. Depth of nests showed little correlation with external measurements of height and diameter. Immature stages were recovered from two strata: the upper 20 cm of nest and mound and the lowest nest depths. Alate sexual forms were found in or near the mound in July, and numerous dealate queens were collected in September from peripheral galleries near the soil surface.     

Circadian rhythm of locomotor activity in individual workers of the wood ant Formica rufa

ABSTRACT. Individual worker ants isolated in an actograph exhibit circadian rhythms of locomotor activity. Entrainment occurs more readily in LD 18:6 h than in LD 12:12 h. The ants are either light-active or dark-active. Phase angle and duration of activity is influenced by photoperiod.     

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.     

中华红林蚁形态描述、行为观察及社群结构

调查显示,北京百花山上"蚂蚁山"的蚂蚁为我国林区特有的种类——中华红林蚁Formica sinensis。该蚂蚁分布海拔相对较高,通常在针叶林或针阔叶混交林中筑巢,进行社会性生活。成虫有4个品级:大型工蚁、小型工蚁、蚁后和雄蚁。每个成熟蚁巢中有2～4个蚁后,为多蚁后社群结构。夏季蚁巢中同时存在着卵、幼虫、蛹和成虫。目前还只发现工蚁和蚁后休眠越冬。对中华红林蚁的各个虫态和品级进行形态描述和测定,并记述工蚁的觅食、建巢及清巢、防御与进攻等行为习性。首次发现雄蚁具有明显的护巢行为。同时,还发现在工蚁的外出活动中,标记信息素的作用不是很明显。     

Deleterious Wolbachia in the ant Formica truncorum

Wolbachia is a maternally inherited bacterium that may manipulate the reproduction of its arthropod hosts. In insects, it is known to lead to inviable matings, cause asexual reproduction or kill male offspring, all to its own benefit, but to the detriment of its host. In social Hymenoptera, Wolbachia occurs widely, but little is known about its fitness effects. We report on a Wolbachia infection in the wood ant Formica truncorum, and evaluate whether it influences reproductive patterns. All 33 colonies of the study population were infected, suggesting that Wolbachia infection is at, or close to, fixation. Interestingly, in colonies with fewer infected workers, significantly more sexuals are produced, indicating that Wolbachia has deleterious effects in this species. In addition, adult workers are shown to have significantly lower infection rates (45%) than worker pupae (87%) or virgin queens (94%), suggesting that workers lose their infection over life. Clearance of Wolbachia infection has, to our knowledge, never been shown in any other natural system, but we argue that it may, in this case, represent an adaptive strategy to reduce colony load. The cause of fixation requires further study, but our data strongly suggest that Wolbachia has no influence on the sex ratio in this species.     

Worker reproduction in the ant Formica fusca

We studied the kin conflict over male parentage in the ant Formica fusca. The conflict arises because each worker and queen is most related to her own sons and is thus predicted to lay eggs. Microsatellite analysis of eggs revealed that workers laid eggs in more than half the queenright experimental nests. Nevertheless, almost exclusively diploid offspring were reared in the presence of a queen. This also occurred when worker-laid haploid male eggs were experimentally introduced in to the nests. Because our experimental setup allowed us to exclude the possibility of queen policing, we conclude that worker laid eggs are removed by other workers, either as a response to their parentage or gender. Our results suggest that worker reproduction in F. fusca is ultimately an interplay of conflicts over male parentage and sex allocation and that both worker and self policing have roles as proximate mechanisms of resolution.     

Role of Early Experience in Mixed-colony Odor Recognition in the Ants Manica rubida and Formica selysi

An experimental model of mixed societies of ants was used to test the effect of early conditioning to a mixed colony odor on interspecific recognition after various periods of separation, with Manica rubida (Myrmicinae) and Formica selysi (Formicinae). Recognition was evaluated by recording the behaviour of individuals interacting after they had been grouped in mixed societies. The observed frequency and nature of these interactions depended on the way the societies were made up (whether with the same or different species, and with or without previous exposure to that species). The recognition displayed by the workers was much more altered in interspecific than in intraspecific groupings, which is evidence of the existence of some species recognition. In addition, the adult is capable of recognizing the olfactory characteristics of its group, the group odor would be a mixture of individual odors.     

Polyandry and colony genetic structure in the primitive ant Nothomyrmecia macrops

The Australian endemic ant Nothomyrmecia macrops is considered one of the most ‘primitive’ among living ants. We investigated the genetic structure of colonies to determine queen mating frequencies and nestmate relatedness. An average of 18.8 individuals from each of 32 colonies, and sperm extracted from 34 foraging queens, were genotyped using five highly variable microsatellite markers. Queens were typically singly (65%) or doubly mated (30%), but triple mating (5%) also occurred. The mean effective number of male mates for queens was 1.37. No relationship between colony size and queen mate number was found. Nestmate workers were related by b=0.61 ± 0.03, significantly above the threshold under Hamilton’s rule over which, all else being equal, altruistic behaviour persists, but queens and their mates were unrelated. In 25% of the colonies we detected a few workers that could not have been produced by the resident queen, although there was no evidence for worker reproduction. Polyandry is for the first time recorded in a species with very small mature colonies, which is inconsistent with the sperm‐limitation hypothesis for the mediation of polyandry levels. Facultative polyandry is therefore not confined to the highly advanced ant genera, but may have arisen at an early stage in ant social evolution.     

Blochmannia endosymbionts improve colony growth and immune defence in the ant Camponotus fellah

Background  

Microorganisms are a large and diverse form of life. Many of them live in association with large multicellular organisms, developing symbiotic relations with the host and some have even evolved to form obligate endosymbiosis [1]. All Carpenter ants (genus Camponotus) studied hitherto harbour primary endosymbiotic bacteria of the Blochmannia genus. The role of these bacteria in ant nutrition has been demonstrated [2] but the omnivorous diet of these ants lead us to hypothesize that the bacteria might provide additional advantages to their host. In this study, we establish links between Blochmannia, growth of starting new colonies and the host immune response.     

Nestmate recognition in the unicolonial ant Formica paralugubris

In unicolonial populations of ants, individuals can mix freelywithin large networks of nests that contain many queens. Ithas been proposed that the absence of aggression in unicolonialpopulations stems from a loss of nest mate recognition, butfew studies have tested this hypothesis. We investigated patternsof aggression and nest mate recognition in the unicolonial woodant, Formica paralugubris. Little aggression occurred, evenbetween workers from nests separated by up to 5 km. However,when aggression took place, it was directed toward non–nestmates rather than nest mates. Trophallaxis (exchange of liquidfood) occurred very frequently, and surprisingly, workers performedsignificantly more trophallaxis with non–nest mates thanwith nest mates (bias 2.4:1). Hence, workers are able to discriminatenest mates from non–nest mates. Higher rates of trophallaxisbetween non–nest mates may serve to homogenize the colonyodor or may be an appeasement mechanism. Trophallaxis rate andaggression level were not correlated with geographical distanceand did not differ within and between two populations separatedby several kilometers. Hence, these populations do not representdifferentiated supercolonies with clear-cut behavioral boundaries.Overall, the data demonstrate that unicoloniality can evolvedespite well-developed nest mate recognition. Reduced levelsof aggression might have been favored by the low rate of interactionswith foreign workers, high cost of erroneously rejecting nestmates, and low cost of accepting foreign workers.     

Mechanisms of social regulation change across colony development in an ant

Background  

Mutual policing is an important mechanism for reducing conflict in cooperative groups. In societies of ants, bees, and wasps, mutual policing of worker reproduction can evolve when workers are more closely related to the queen's sons than to the sons of workers or when the costs of worker reproduction lower the inclusive fitness of workers. During colony growth, relatedness within the colony remains the same, but the costs of worker reproduction may change. The costs of worker reproduction are predicted to be greatest in incipient colonies. If the costs associated with worker reproduction outweigh the individual direct benefits to workers, policing mechanisms as found in larger colonies may be absent in incipient colonies.     

Colony success of the submissive ant Formica fusca within territories of the dominant Formica polyctena

Abstract. 1. Twenty-three nests of the submissive ant Formica fusca L. were sampled in two adjacent territories of the dominant wood ant Formica polyctena Först. The nests were dug up at different distances from the wood-ant mounds. Distance is assumed to be inversely related to the extent of disturbance of F. fusca by F. polyctena.
2. Colony success of F. fusca was assayed by counting the numbers of workers (colony size), and worker and sexual offspring in a nest. For individual size, the head width and dry weight of fifty workers per nest were measured.
3. Colony size correlated positively with the numbers of worker and sexual offspring in one territory, but only with worker offspring in the other.
4. Distance from the wood-ant mound correlated positively with colony size and numbers of worker and female offspring in one territory. In the other territory distance correlated positively with number of females, but negatively with number of worker offspring. No significant correlations between distance and the size measurements were obtained.
5. Because of its nonaggressive behaviour F. fusca may nest fairly close to a wood-ant mound but is likely to show reduced abundances in terms of nest density and forager number, and, ultimately, lowered fitness.     

Reproductive sharing among queens in the ant Formica fusca

Reproductive sharing among cobreeders, in which reproductiveshares may vary from equal contribution (low reproductive skew)to reproductive dominance by one individual (high reproductiveskew), is a fundamental feature of animal societies. Recenttheoretical work, the reproductive skew models, has focusedon factors affecting the degree to which reproduction is skewedwithin a society. We used the parameters provided by skew modelsas a guideline to study determinants of reproductive sharingin polygyne ants. As a model system we used two-queen laboratorycolonies of the ant Formica fusca in which the reproductiveshares of each queen was assessed from offspring by using allozymesand DNA microsatellites. We tested how the different variablesincluded in reproductive skew models (queen-queen relatedness,potential fighting ability, productivity, and worker relatednessreflected by queen number in the colony of origin) affect reproductivesharing among queens. The results showed that the relatednessamong queens explained 26% of the variation in reproductiveskew. The size difference between queens (reflecting potentialfighting ability), colony productivity, and worker relatednessdid not have an effect on reproductive partitioning among cobreeders.To our knowledge, this is the first study to test for the effectsof various determinants of skew in an experimental setting.     

Individual variation in immune function in the ant Formica exsecta; effects of the nest, body size and sex

It has been generally thought that sex differences in the immune system are the result of the differing life history strategies of the sexes, although the available data are not entirely consistent with the hypothesis. In this study, we studied the variation in the immune function in the mound-building wood ant Formica exsecta. F. exsecta has two forms of males, distinguished by size: the small males (micraners) and the large males (macraners), which die after the mating period, whereas females live tens of years laying their eggs. We found that in general males have a lower encapsulation response against nylon monofilament (i.e. lower immune function) than queens. Among males, the micraners had a lower encapsulation rate than the macraners. However, in queens, there was no correlation between size and encapsulation rate. The origin nest had an effect on the encapsulation rate of males: males from the large nests had a stronger encapsulation rate than males from small nests. However, in queens, nest size did not have any effect on encapsulation response. The observed variation between sexes and individuals in the encapsulation rate is discussed in the context of reproductive strategies and parasite-mediated sexual selection.