Gene flow is maintained by polyandry and male dispersal in the army ant <Emphasis Type="Italic">Eciton burchellii</Emphasis>

The combination of haplodiploidy, complementary sex determination and eusociality constrains the effective population size (N _e) of social Hymenoptera far more than in any other insect group. Additional limitations on N _e occur in army ants since they have wingless queens and colony fission, both of which are factors causing restricted maternal gene flow and high population viscosity. Therefore, winged army ant males gain a particular significance to ensure dispersal, facilitate gene flow and avoid inbreeding. Based on population genetic analyses with microsatellite markers, we studied a population of the Neotropical army ant Eciton burchellii, finding a high level of heterozygosity, weak population differentiation and no evidence for inbreeding. Moreover, by using sibship reconstruction analyses, we quantified the actual number of male contributing colonies represented in a queen’s mate sample, demonstrating that, through extreme multiple mating, the queens are able to sample the genes of males from up to ten different colonies, usually located within an approximate radius of 1 km. We finally correlated the individual mating success of each male contributing colony with the relative siring success of individual males and found a significant colony-dependent male fitness component. Our results imply that the dispersal and mating system of these army ants seem to enhance gene flow and minimise the deleterious effects associated with small effective population size. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Reproductive conflicts in polyandrous and polygynous ant Formica sanguinea

The occurrence of multiple reproductives within an ant colony changes the balance between indirect fitness benefits and reproductive competition. We test whether the number of matings by an ant queen (polyandry) correlates negatively with the number of reproductive queens in the colony (polygyny), whether the patrilines and matrilines differ in their contribution to the sexual and worker progeny and whether there is an overall reproductive skew. For these aims, we genotyped both worker and sexual offspring from colonies of the ant Formica sanguinea in three populations. Most colonies were monogynous, but eight (11%) were polygynous with closely related queens. Most queens in the monogynous colonies (86%) had mated with multiple males. The effective paternity was lower than the actual number of mates, and the paternity skew was significant. Furthermore, in some monogynous colonies, the patrilines were differently represented in the worker pupae and sexual pupae produced at the same time. Likewise, the matrilines in polygynous colonies were differently present in worker pupae and male offspring. The effective number of matings by a queen was significantly lower in polygynous colonies (mean m_e = 1.68) than in monogynous colonies (means 2.06–2.61). The results give support to the hypotheses that polyandry and polygyny are alternative breeding strategies and that reproductive competition can lead to different representation of patrilines and matrilines among the sexual and worker broods.     

Colony genetic structure in the Australian jumper ant <Emphasis Type="Italic">Myrmecia pilosula</Emphasis>

Eusocial insects vary significantly in colony queen number and mating frequency, resulting in a wide range of social structures. Detailed studies of colony genetic structure are essential to elucidate how various factors affect the relatedness and the sociogenetic organization of colonies. In this study, we investigated the colony structure of the Australian jumper ant Myrmecia pilosula using polymorphic microsatellite markers. Nestmate queens within polygynous colonies, and queens and their mates, were generally unrelated. The number of queens per colony ranged from 1 to 4. Queens were estimated to mate with 1–9 inferred and 1.0–11.4 effective mates. This is the first time that the rare co-occurrence of polygyny and high polyandry has been found in the M. pilosula species group. Significant maternity and paternity skews were detected at the population level. We also found an isolation-by-distance pattern, and together with the occurrence of polygynous polydomy, this suggests the occurrence of dependent colony foundation in M. pilosula; however, independent colony foundation may co-occur since queens of this species have fully developed wings and can fly. There is no support for the predicted negative association between polygyny and polyandry in ants.     

Male-biased dispersal promotes large scale gene flow in a subterranean army ant, Dorylus (Typhlopone) fulvus

Sex-biased dispersal is a widespread phenomenon in the animal kingdom, which strongly influences gene flow and population structure. Particularly army ants, important key-stone predators in tropical ecosystems, are prone to population fragmentation and isolation due to their extraordinary mating system: queens are permanently wingless, propagate via colony fission, and only the males disperse in mating flights. Here we report on sex-biased dispersal and the genetic population structure of an African subterranean army ant, Dorylus (Typhlopone) fulvus. Using maternally inherited mtDNA markers and bi-parentally inherited nuclear microsatellites we found strong geographical structuring of mtDNA haplotypes, whereas the nuclear genetic population structure was less pronounced. Strong mtDNA (Φ _ST = 0.85), but significantly lower nuclear (F _ST = 0.23) genetic differentiation translated into a more than an order of magnitude larger male migration rate compared to that of queens, reflecting the low motility of queens and strong, promiscuous dispersal by males. Thus, the well flying D. fulvus males appear to be the sex to promote large scale gene flow, and D. fulvus is indeed a species in which sex specific dispersal patterns and the mating system profoundly affect the population structure and phylogeography.     

Mating for convenience or genetic diversity? Mating patterns in the polygynous ant Plagiolepis pygmaea

Several genetic and nongenetic benefits have been proposed toexplain multiple mating (polyandry) in animals, to compensatefor costs associated with obtaining additional mates. The mostprominent hypotheses stress the benefits of increased geneticdiversity. In social insects, queens of most species mate onlyonce or have effective mating frequencies close to one. Yet,in a few species of ants, bees, and wasps, polyandry is therule. In these species, colonies are usually headed by a singlequeen, whereas multiple queening adds diversity in several ofthe remaining species, especially in ants. Here we investigatedmating frequency, inbreeding and relatedness between the queensand their mates in the polygynous ant Plagiolepis pygmaea, andthe effect of polyandry on the genetic diversity as a functionof the effective population size of individual colonies. Ourresults show that polyandry occurs frequently in the species.However, queens are frequently inseminated by close relatives,and additional sires add little genetic diversity among offspringof individual queens. In addition, the increase in diversityat the colony level is only marginal. Hence, contrary to establishednotions, polyandry in P. pygmaea seems not to be driven by substantialbenefits of genetic diversity. Nonetheless, very small or asyet unidentified genetic benefits to one party (males, workers,queens) in conjunction with low costs of mating may favor polyandry.Alternatively, nongenetic factors, such as convenience polyandry,may be more important than genetic factors in promoting polyandryin P. pygmaea.     

GENETIC VARIABILITY,QUEEN NUMBER,AND POLYANDRY IN SOCIAL HYMENOPTERA

Several hypotheses have been put forward to explain the adaptive significance of interspecific variation in mating frequencies by eusocial hymenopteran queens. Four of these hypotheses assert that polyandry is advantageous to queens because of the resultant increase in genetic variability within colonies (referred to as the “GV” hypotheses). Here we compare the frequency of polyandry between monogynous (single-queen) and polygynous (multiple-queen) ant species to test the hypotheses that (1) multiple mating functions primarily to increase intracolonial genetic variability, and (2) mating has costs (such as increased energetic losses or risk of predation or venereal disease). If one of the GV hypotheses is true and mating is costly, the frequency of polyandry should be lower in polygynous species (in which the presence of multiple queens results in low relatedness among workers) than in monogynous species. As predicted by the GV hypotheses, polyandry is less common among polygynous than among monogynous species. Furthermore, it seems that the causal relationship underlying this association is that the number of matings by queens depends on the number of queens present in the colony (rather than the number of queens being influenced by the number of matings), which also supports the GV hypotheses together with the assumption that mating has costs.     

Mating system evolution and worker caste diversity in Pheidole ants

The efficiency of social groups is generally optimized by a division of labour, achieved through behavioural or morphological diversity of members. In social insects, colonies may increase the morphological diversity of workers by recruiting standing genetic variance for size and shape via multiply mated queens (polyandry) or multiple‐breeding queens (polygyny). However, greater worker diversity in multi‐lineage species may also have evolved due to mutual worker policing if there is worker reproduction. Such policing reduces the pressure on workers to maintain reproductive morphologies, allowing the evolution of greater developmental plasticity and the maintenance of more genetic variance for worker size and shape in populations. Pheidole ants vary greatly in the diversity of worker castes. Also, their workers lack ovaries and are thus invariably sterile regardless of the queen mating frequency and numbers of queens per colony. This allowed us to perform an across‐species study examining the genetic effects of recruiting more patrilines on the developmental diversity of workers in the absence of confounding effects from worker policing. Using highly variable microsatellite markers, we found that the effective mating frequency of the soldier‐polymorphic P. rhea (avg. me_N = 2.65) was significantly higher than that of the dimorphic P. spadonia (avg. me_N = 1.06), despite a significant paternity skew in P. rhea (avg. B = 0.10). Our findings support the idea that mating strategies of queens may co‐evolve with selection to increase the diversity of workers. We also detected patriline bias in the production of different worker sizes, which provides direct evidence for a genetic component to worker polymorphism.     

Multiple mating in the ant Cataglyphis cursor: testing the sperm limitation and the diploid male load hypotheses

Multiple mating (i.e., polyandry) by queens in social Hymenoptera is expected to weaken social cohesion since it lowers within-colony relatedness, and hence, indirect fitness benefits from kin selection. Yet, there are many species where queens mate multiply. Several hypotheses have been put forward to explain the evolution and maintenance of polyandry. Here,we investigated the ‘sperm limitation’ and the ‘diploid male load’ hypotheses in the ant Cataglyphis cursor. Genetic analyses of mother-offspring combinations showed that queens mate with up to 8 males, with an effective mating frequency of 3.79. Significant paternity skew (unequal contribution of the fathers) was detected in 1 out of 5 colonies. The amount of sperm stored in the spermatheca was not correlated with the queen mating frequency, and males carry on average enough sperm in their seminal vesicles to fill one queen’s spermatheca. Analyses of the nuclear DNA-content of males also revealed that all were haploid. These results suggest that the ‘sperm limitation’ and the ‘diploid male load’ hypotheses are unlikely to account for the queen mating frequency reported in this ant. In light of our results and the life-history traits of C. cursor, we discuss alternative hypotheses to account for the adaptive significance of multiple mating by queens in this species. Received 13 August 2008; revised 19 November 2008; accepted 21 November 2008.     

PATTERNS OF PATERNITY SKEW AMONG POLYANDROUS SOCIAL INSECTS: WHAT CAN THEY TELL US ABOUT THE POTENTIAL FOR SEXUAL SELECTION?

Monogamy results in high genetic relatedness among offspring and thus it is generally assumed to be favored by kin selection. Female multiple mating (polyandry) has nevertheless evolved several times in the social Hymenoptera (ants, bees, and wasps), and a substantial amount of work has been conducted to understand its costs and benefits. Relatedness and inclusive fitness benefits are, however, not only influenced by queen mating frequency but also by paternity skew, which is a quantitative measure of paternity biases among the offspring of polyandrous females. We performed a large‐scale phylogenetic analysis of paternity skew across polyandrous social Hymenoptera. We found a general and significant negative association between paternity frequency and paternity skew. High paternity skew, which increases relatedness among colony members and thus maximizes inclusive fitness gains, characterized species with low paternity frequency. However, species with highly polyandrous queens had low paternity skew, with paternity equalized among potential sires. Equal paternity shares among fathers are expected to maximize fitness benefits derived from genetic diversity among offspring. We discuss the potential for postcopulatory sexual selection to influence patterns of paternity in social insects, and suggest that sexual selection may have played a key, yet overlooked role in social evolution.     

Multiple paternity or multiple queens: two routes to greater intracolonial genetic diversity in the eusocial Hymenoptera

Understanding the evolution of multiple mating by females (polyandry) is an important question in behavioural ecology. Most leading explanations for polyandry by social insect queens are based upon a postulated fitness benefit from increased intracolonial genetic diversity, which also arises when colonies are headed by multiple queens (polygyny). An indirect test of the genetic diversity hypotheses is therefore provided by the relationship between polyandry and polygyny across species, which should be negative if the genetic diversity hypotheses are correct. Here, we conduct a powerful comparative investigation of the relationship between polyandry and polygyny for 241 species of eusocial Hymenoptera (ants, bees and wasps). We find a clear and significant negative relationship between polyandry and polygyny after controlling for phylogeny. These results strongly suggest that fitness benefits resulting from increased intracolonial genetic diversity have played an important role in the evolution of polyandry, and possibly polygyny, in social insects.     

The evolution of multiple mating in army ants

The evolution of mating systems in eusocial Hymenoptera is constrained because females mate only during a brief period early in life, whereas inseminated queens and their stored sperm may live for decades. Considerable research effort during recent years has firmly established that obligate multiple mating has evolved only a few times: in Apis honeybees, Vespula wasps, Pogonomyrmex harvester ants, Atta and Acromyrmex leaf-cutting ants, the ant Cataglyphis cursor, and in at least some army ants. Here we provide estimates of queen-mating frequency for New World Neivamyrmex and Old World Aenictus species, which, compared to other army ants, have relatively small colonies and little size polymorphism among workers. To provide the first overall comparative analysis of the evolution of army ant mating systems, we combine these new results with previous estimates for African Dorylus and New World Eciton army ants, which have very large colonies and considerable worker polymorphism. We show that queens of Neivamyrmex and Aenictus mate with the same high numbers of males (usually ca. 10-20) as do queens of army ant species with very large colony sizes. We infer that multiple queen mating is ancestral in army ants and has evolved over 100 million years ago as part of the army ant adaptive syndrome. A comparison of army ants and honeybees suggests that mating systems in these two distantly related groups may have been convergently shaped by strikingly similar selective pressures.     

Intraspecific support for the polygyny-vs.-polyandry hypothesis in the bulldog ant Myrmecia brevinoda

The number of queens per colony and the number of matings per queen are the most important determinants of the genetic structure of ant colonies, and understanding their interrelationship is essential to the study of social evolution. The polygyny-vs.-polyandry hypothesis argues that polygyny and polyandry should be negatively associated because both can result in increased intracolonial genetic variability and have costs. However, evidence for this long-debated hypothesis has been lacking at the intraspecific level. Here, we investigated the colony genetic structure in the Australian bulldog ant Myrmecia brevinoda. The numbers of queens per colony varied from 1 to 6. Nestmate queens within polygynous colonies were on average related (r(qq) = 0.171 ± 0.019), but the overall relatedness between queens and their mates was indistinguishable from zero (r(qm) = 0.037 ± 0.030). Queens were inferred to mate with 1-10 males. A lack of genetic isolation by distance among nests indicated the prevalence of independent colony foundation. In accordance with the polygyny-vs.-polyandry hypothesis, the number of queens per colony was significantly negatively associated with the estimated number of matings (Spearman rank correlation R = -0.490, P = 0.028). This study thus provides the rare intraspecific evidence for the polygyny-vs.-polyandry hypothesis. We suggest that the high costs of multiple matings and the strong effect of multiple mating on intracolonial genetic diversity may be essential to the negative association between polygyny and polyandry and that any attempt to empirically test this hypothesis should place emphasis upon these two key underlying aspects.     

Honey Bee Colonies Headed by Hyperpolyandrous Queens Have Improved Brood Rearing Efficiency and Lower Infestation Rates of Parasitic Varroa Mites

A honey bee queen mates on wing with an average of 12 males and stores their sperm to produce progeny of mixed paternity. The degree of a queen’s polyandry is positively associated with measures of her colony’s fitness, and observed distributions of mating number are evolutionary optima balancing risks of mating flights against benefits to the colony. Effective mating numbers as high as 40 have been documented, begging the question of the upper bounds of this behavior that can be expected to confer colony benefit. In this study we used instrumental insemination to create three classes of queens with exaggerated range of polyandry– 15, 30, or 60 drones. Colonies headed by queens inseminated with 30 or 60 drones produced more brood per bee and had a lower proportion of samples positive for Varroa destructor mites than colonies whose queens were inseminated with 15 drones, suggesting benefits of polyandry at rates higher than those normally obtaining in nature. Our results are consistent with two hypotheses that posit conditions that reward such high expressions of polyandry: (1) a queen may mate with many males in order to promote beneficial non-additive genetic interactions among subfamilies, and (2) a queen may mate with many males in order to capture a large number of rare alleles that regulate resistance to pathogens and parasites in a breeding population. Our results are unique for identifying the highest levels of polyandry yet detected that confer colony-level benefit and for showing a benefit of polyandry in particular toward the parasitic mite V. destructor.     

Multiple paternity,relatedness and genetic diversity in Acromyrmex leaf-cutter ants

Multiple queen-mating occurs in many social insects, but high degrees of multiple paternity have only been found in honeybees and some yellowjacket wasps. Here we report the first case of an ant species where multiple mating reduces relatedness among female offspring to values significantly lower than 0.5. Genetic analysis of a Panamanian population of the leaf-cutter ant Acromyrmex octospinosus showed that queens mate with at least 4 to 10 males. The detected (minimum) genetically effective paternity of nestmate females was 3.9 and estimates of mean relatedness among nestmate females were ca. 0.33. This implies that multiple queen-mating in Acromyrmex octospinosus reduces relatedness to 44% of the value in full-sib colonies (0.75), realizing 84% of the maximum reduction (to 0.25) that would be obtained with an infinite number of matings. Queens of Panamanian Acromyrmex octospinosus mate with more males than sympatric queens of Atta colombica, which is contrary to the positive relationship between queen-mating frequency and colony size found across more distantly related ant species. Possible selective forces that maintain high queen-mating frequencies in leaf-cutter ants are discussed.     

Population- and sociogenetic structure of the leaf-cutter ant <Emphasis Type="Italic">Atta colombica</Emphasis> (Formicidae,Myrmicinae)

Relatedness and genetic variability in colonies of social insects are strongly influenced by the number of queens present and the number of matings per queen, but also by the genetic variability in the population. Thus, multiple paternity will enhance within-colony genetic variability more strongly when the males a queen mates with are unrelated. To study the kin-structure within colonies of the leaf-cutter ant Atta colombica and the population structure of this species around Barro Colorado Island, Panama, we developed five polymorphic microsatellite loci with a range of three to 17 alleles in At. colombica, all of which cross-amplify in other higher attines as well. The average effective mating frequency calculated from four-locus microsatellite genotypes was 1.89 ± 0.12 (harmonic mean ± SE) and thus slightly lower than the average observed mating frequency of 2.50 ± 0.11 (arithmetic mean ± SE) over the 55 colonies studied, confirming former studies that utilized fewer loci. The discrepancy between observed mating frequency and effective mating frequency is most probably due to paternity skew within colonies. The study population proved to be genetically diverse and in Hardy-Weinberg equilibrium, suggesting random mating within the study area. No population substructure was observed, neither considering nuclear (global F _ST = 0.011 ± 0.003 SE) nor mitochondrial markers (mean Φ_ST = 0.008). Consequently, gene flow is obviously promoted by both sexes across the range investigated here. Thus, multiple mating and long-distance dispersal appear to be two interconnected behavioural mechanisms to create and maintain genetic diversity in At. colombica. The advantages of this system are partly offset by paternity skew and the non-zero relatedness among colony fathers found in the study population. Received 18 March 2008; revised 14 July 2008; accepted 18 July 2008.     

Worker caste determination in the army ant Eciton burchellii

Elaborate division of labour has contributed significantly to the ecological success of social insects. Division of labour is achieved either by behavioural task specialization or by morphological specialization of colony members. In physical caste systems, the diet and rearing environment of developing larvae is known to determine the phenotype of adult individuals, but recent studies have shown that genetic components also contribute to the determination of worker caste. One of the most extreme cases of worker caste differentiation occurs in the army ant genus Eciton, where queens mate with many males and colonies are therefore composed of numerous full-sister subfamilies. This high intracolonial genetic diversity, in combination with the extreme caste polymorphism, provides an excellent test system for studying the extent to which caste determination is genetically controlled. Here we show that genetic effects contribute significantly to worker caste fate in Eciton burchellii. We conclude that the combination of polyandry and genetic variation for caste determination may have facilitated the evolution of worker caste diversity in some lineages of social insects.     

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.     

Habitat age, breeding system and kinship in the ant Formica fusca

In polygyne ants (multiple queens per colony) factors that affect the distribution and survival of queens may play a key role in shaping the population-wide mating system and colony kin structure. The aim of this paper was to study the breeding system in two populations of different age in the facultatively polygyne ant Formica fusca. Both the observed numbers of queens, and the relatedness patterns among queens, workers and colony fathers were compared in two adjacent populations (ages 17 years and > 100 years) in Southern Finland. The results showed that both the mating system and colony kin structure differed between the study populations. In the old population the relatedness among workers, queens and colony fathers was high. The queens were also related to their mates, resulting in significant inbreeding in workers, but not in queens. Finally, the number of queens per colony fluctuated between years, suggesting queen turnover, and nest-mate queens shared their reproduction unequally (reproductive skew). In the younger population relatedness among queens and workers was lower than in the old population, and the colony fathers were unrelated. Furthermore, inbreeding was absent, and no conclusive evidence was found for reproductive skew among nest-mate queens. Finally, the number of queens per colony appeared more stable between years, although queen turnover occurred also in this population. The observed differences in dispersal and mating behaviour are discussed in the light of a potential connection between population age and habitat saturation.     

Sib‐mating in the ant Plagiolepis pygmaea: adaptative inbreeding?

Multiple functional queens in a colony (polygyny) and multiple mating by queens (polyandry) in social insects challenge kin selection, because they dilute inclusive fitness benefits from helping. Colonies of the ant Plagiolepis pygmaea brash contain several hundreds of multiply mated queens. Yet, within‐colony relatedness remains unexpectedly high. This stems from low male dispersal, extensive mating among relatives and adoption of young queens in the natal colony. We investigated whether inbreeding results from workers expelling foreign males, and/or from preferential mating between related partners. Our data show that workers actively repel unrelated males entering their colony, and that queens preferentially mate with related males. These results are consistent with inclusive fitness being a driving force for inbreeding: by preventing outbreeding, workers reduce erosion of relatedness within colonies due to polygyny and polyandry. That virgin queens mate preferentially with related males could result from a long history of inbreeding, which is expected to reduce depression in species with regular sibmating.     

Genetic evidence for landscape effects on dispersal in the army ant Eciton burchellii

Inhibited dispersal, leading to reduced gene flow, threatens populations with inbreeding depression and local extinction. Fragmentation may be especially detrimental to social insects because inhibited gene flow has important consequences for cooperation and competition within and among colonies. Army ants have winged males and permanently wingless queens; these traits imply male‐biased dispersal. However, army ant colonies are obligately nomadic and have the potential to traverse landscapes. Eciton burchellii, the most regularly nomadic army ant, is a forest interior species: colony raiding activities are limited in the absence of forest cover. To examine whether nomadism and landscape (forest clearing and elevation) affect population genetic structure in a montane E. burchellii population, we reconstructed queen and male genotypes from 25 colonies at seven polymorphic microsatellite loci. Pairwise genetic distances among individuals were compared to pairwise geographical and resistance distances using regressions with permutations, partial Mantel tests and random forests analyses. Although there was no significant spatial genetic structure in queens or males in montane forest, dispersal may be male‐biased. We found significant isolation by landscape resistance for queens based on land cover (forest clearing), but not on elevation. Summed colony emigrations over the lifetime of the queen may contribute to gene flow in this species and forest clearing impedes these movements and subsequent gene dispersal. Further forest cover removal may increasingly inhibit Eciton burchellii colony dispersal. We recommend maintaining habitat connectivity in tropical forests to promote population persistence for this keystone species.