Ployandry versus polygyny versus parasites

Although social insect colonies are most easily conceptualized as consisting of a single, once-mated queen and her worker progeny, the number of queens per colony and the number of times queens mate varies broadly in ants and other social insects. Various hypotheses have been suggested for the resulting range of breeding systems and social organizations, respectively; one set of hypotheses relating to both queen number and mate number at the same time is a need for genetic variation, especially in relation to disease resistance. We here carry out a comparative analysis using phylogenetic information and, contrary to one non-phylogenetic previous study, we find that polyandry and polygyny are not significantly associated. However, the level of relatedness within colonies, a quantity affected by both polyandry and polygyny, is significantly associated with parasite loads: species with colonies with low relatedness levels have lower parasite loads. Given that, under the variance-reduction principle, selection on queens for mating frequency ought to continue even in polygynous colonies, we suggest that while parasite loads indeed seem to correlate with intra-colony genetic variability, the relationship to polyandry and polygyny may be complex and requires considerably more experimental investigation.     

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.     

The costs and benefits of genetic heterogeneity in resistance against parasites in social insects

The occurrence of polygyny and polyandry in social insects has long puzzled evolutionary biologists. If cooperation requires genetic relatedness, how do we explain the occurrence and maintenance of mechanisms that reduce the degree of relatedness among colony members? A much-discussed hypothesis states that genetically diverse colonies are more resistant to parasitism than homogenous colonies because genetic diversity reduces the spread of a disease within a colony. However, as we will argue in this note, a necessary condition for the parasite hypothesis is that genetically heterogeneous colonies have a larger suite of parasites that are capable of infecting them. This implicit relationship is important because it implies that even if the cost per infection is reduced, this may not be sufficient to offset the increased rate of acquiring infections. The advantages of genetic heterogeneity as a defense against parasites thus may not be as big as commonly thought.     

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.     

No benefit in diversity? The effect of genetic variation on survival and disease resistance in a polygynous social insect

1. Multiple mating by queens has been shown to enhance disease resistance in insect societies, because higher genetic diversity among nestmates improves collective immune defences or offers a certain level of herd immunity. However, it has remained ambiguous whether polygynous societies with large numbers of queens also benefit from increased genetic diversity. 2. We used one of the very few ant species that can be reared across generations, the pharaoh ant, Monomorium pharaonis Linnaeus, to create experimental colonies with two types of enhanced genetic diversity: (i) mixed workers from three divergent inbred lineages representing the ‘polygyny‐equivalent' of multiple mating by queens (i.e. increased between‐worker variation); and (ii) uniform workers whose overall heterozygosity was increased by two subsequent generations of crossing between the same divergent inbred lineages (i.e. increased within‐worker variation). 3. We found significant differences in worker survival among the three inbred lineages, with exposure to conidiospores of the fungal pathogen Beauveria bassiana causing significant mortality to the workers independently of their diversity type. Increased diversity did not improve the resistance to Beauveria. 4. Enhanced heterozygosity colonies had worker survival rates similar to the most resistant inbred lineage, whereas colonies with mixed workers from the three inbred lineages had lower worker and larval survival. Workers did not show any infection‐avoidance behaviour. 5. Average larval survival appeared unaffected by the presence of conidiospores. It benefitted from increased heterozygosity but was reduced in mixed colonies independent of infection. This suggests that negative, but cryptic social interactions in mixed colonies may affect overall survival. 6. The present results do not provide evidence for or against a link between increased genetic variation and increased disease resistance in pharaoh ants, but show that colonies differ considerably in general survival. Thus, increasing the genetic diversity of pharaoh ant colonies may not provide survival advantages in the face of pathogen exposure, and polygyny and polyandry may not be directly comparable mechanisms for creating adaptive resistance towards pathogens.     

Colony kin structure and breeding system in the ant genus Plagiolepis

Relatedness is a central parameter in the evolution of sociality, because kin selection theory assumes that individuals involved in altruistic interactions are related. At least three reproductive characteristics are known to profoundly affect colony kin structure in social insects: the number of reproductive queens per colony, the relatedness among breeding queens and queen mating frequency. Both the occurrence of multiple queens (polygyny) and multiple mating (polyandry) decrease within-colony relatedness, while mating among sibs increases relatedness between the workers and the brood they rear. Using DNA microsatellites, we performed a detailed genetic analysis of the colony kin structure and breeding system in three ant species belonging to the genus Plagiolepis: P. schmitzii, P. taurica and P. maura. Our data show that queens of the three species mate multiply: queens of P. maura mate with 1-2 males, queens of P. taurica with 3-11 males and queens of P. schmitzii may have 1-14 different mates. Moreover, colonies are headed by multiple queens: P. taurica and P. maura are facultatively polygynous, while P. schmitzii is obligately polygynous. Despite polyandry and polygyny, relatedness within colonies remains high because all species are characterized by sib-mating, with a fixation index F(it) = 0.25 in P. taurica, 0.24 in P. schmitzii and 0.26 in P. maura, and because the male mates of a queen are on average closely related.     

Genetic diversity and disease resistance in leaf-cutting ant societies

Multiple mating by females (polyandry) remains hard to explain because, while it has substantial costs, clear benefits have remained elusive. The problem is acute in the social insects because polyandry is probably particularly costly for females and most material benefits of the behavior are unlikely to apply. It has been suggested that a fitness benefit may arise from the more genetically diverse worker force that a polyandrous queen will produce. One leading hypothesis is that the increased genetic diversity of workers will improve a colony's resistance to disease. We investigated this hypothesis using a polyandrous leaf-cutting ant and a virulent fungal parasite as our model system. At high doses of the parasite most patrilines within colonies were similarly susceptible, but a few showed greater resistance. At a low dose of the parasite there was more variation between patrilines in their resistance to the parasite. Such genetic variation is a key prerequisite for polyandry to result in increased disease resistance of colonies. The relatedness of two hosts did not appear to affect the transmission of the parasite between them, but this was most likely because the parasite tested was a virulent generalist that is adapted to transmit between distantly related hosts. The resistance to the parasite was compared between small groups of ants of either high or low genetic diversity. No difference was found at high doses of the parasite, but a significant improvement in resistance in high genetic diversity groups was found at a low dose of the parasite. That there is genetic variation for disease resistance means that there is the potential for polyandry to produce more disease-resistant colonies. That this genetic variation can improve the resistance of groups even under the limited conditions tested suggests that polyandry may indeed produce colonies with improved resistance to disease.     

Polygyny in the nest-site limited acacia-ant Crematogaster mimosae

Polygyny is common in social insects despite inevitable decreases in nestmate relatedness and reductions to the inclusive fitness returns for cooperating non-reproductive individuals. We studied the prevalence and mode of polygyny in the African acacia-ant Crematogaster mimosae. These ants compete intensively with neighboring colonies of conspecifics and with three sympatric ant species for resources associated with the whistling-thorn acacias in which they all obligately nest. We used the genotypes of alate males at ten microsatellite loci to reconstruct queen genotypes and found that C. mimosae colonies are frequently secondarily polygynous, in that they include multiple closely related (and sometimes full-sib) queens, and (more rarely) unrelated queens. We also found that individual queens in both monogynous and polygynous colonies had mated with multiple males, making C. mimosae an interesting example of simultaneous polygyny and polyandry. The presence of polygyny in C. mimosae and the intense competition for nest-sites between C. mimosae and its conspecifics support the association between nest-site limitation and polygyny. Polygyny may allow for increased worker populations and a competitive advantage, as inter-colony conflicts are typically won by the colony with the larger number of workers.     

Inbred and outbred honey bees (Apis mellifera) have similar innate immune responses

Social insect colonies provide ideal conditions for the spread of pathogens. It has been proposed that the extreme polyandry and genetic diversity seen in the colonies of some eusocial insect species is central to a colony’s defence against disease. Indeed, empirically, colonies headed by polyandrous queens have lower incidence of pathogens than genetically uniform monoandrous colonies. The mechanisms of improved resistance in genetically diverse colonies could arise from the genetic diversity among worker genotypes or from increased innate immunity arising from heterozygosity at immune gene loci within individual workers. Here, we investigate the effects of heterozygosity on two components of the honey bee (Apis mellifera) innate immune system: encapsulation and phenoloxidase (PO) activity. No significant effect of heterozygosity on immune system activity was evident for either encapsulation or PO activity. Thus, we conclude that while encapsulation and PO activity are important components of the immune response, it seems that they do not underlie the positive effects of genetic diversity on parasite and pathogen resistance in honey bees.     

Investigation of the population genetic structure and mating system in the ant Pheidole pallidula

The origin of eusociality in haplo-diploid organisms such as Hymenoptera has been mostly explained by kin selection. However, several studies have uncovered decreased relatedness values within colonies, resulting primarily from multiple queen matings (polyandry) and/or from the presence of more than one functional queen (polygyny). Here, we report on the use of microsatellite data for the investigation of sociogenetic parameters, such as relatedness, and levels of polygyny and polyandry, in the ant Pheidole pallidula. We demonstrate, through analysis of mother-offspring combinations and the use of direct sperm typing, that each queen is inseminated by a single male. The inbreeding coefficient within colonies and the levels of relatedness between the queens and their mate are not significantly different from zero, indicating that matings occur between unrelated individuals. Analyses of worker genotypes demonstrate that 38% of the colonies are polygynous with 2-4 functional queens, and suggest the existence of reproductive skew, i.e. unequal respective contribution of queens to reproduction. Finally, our analyses indicate that colonies are genetically differentiated and form a population exhibiting significant isolation-by-distance, suggesting that some colonies originate through budding.     

Cheating honeybee workers produce royal offspring

The Cape bee (Apis mellifera capensis) is unique among honeybees in that workers can lay eggs that instead of developing into males develop into females via thelytokous parthenogenesis. We show that this ability allows workers to compete directly with the queen over the production of new queens. Genetic analyses using microsatellites revealed that 23 out of 39 new queens produced by seven colonies were offspring of workers and not the resident queen. Of these, eight were laid by resident workers, but the majority were offspring of parasitic workers from other colonies. The parasites were derived from several clonal lineages that entered the colonies and successfully targeted queen cells for parasitism. Hence, these parasitic workers had the potential to become genetically reincarnated as queens. Of the daughter queens laid by the resident queen, three were produced asexually, suggesting that queens can 'choose' to produce daughter queens clonally and thus have the potential for genetic immortality.     

The significance of multiple mating in the social wasp Vespula maculifrons

The evolution of the complex societies displayed by social insects depended partly on high relatedness among interacting group members. Therefore, behaviors that depress group relatedness, such as multiple mating by reproductive females (polyandry), are unexpected in social insects. Nevertheless, the queens of several social insect species mate multiply, suggesting that polyandry provides some benefits that counteract the costs. However, few studies have obtained evidence for links between rates of polyandry and fitness in naturally occurring social insect populations. We investigated if polyandry was beneficial in the social wasp Vespula maculifrons. We used genetic markers to estimate queen mate number in V. maculifrons colonies and assessed colony fitness by counting the number of cells that colonies produced. Our results indicated that queen mate number was directly, strongly, and significantly correlated with the number of queen cells produced by colonies. Because V. maculifrons queens are necessarily reared in queen cells, our results demonstrate that high levels of polyandry are associated with colonies capable of producing many new queens. These data are consistent with the explanation that polyandry is adaptive in V. maculifrons because it provides a fitness advantage to queens. Our research may provide a rare example of an association between polyandry and fitness in a natural social insect population and help explain why queens in this taxon mate multiply.     

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.     

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.     

Alternative genetic foundations for a key social polymorphism in fire ants

Little is known about the genetic foundations of colony social organization. One rare example in which a single major gene is implicated in the expression of alternative social organizations involves the presumed odorant-binding protein gene Gp-9 in fire ants. Specific amino acid substitutions in this gene invariably are associated with the expression of monogyny (single queen per colony) or polygyny (multiple queens per colony) in fire ant species of the Solenopsis richteri clade. These substitutions are hypothesized to alter the abilities of workers to recognize queens and thereby regulate their numbers in a colony. We examined whether these same substitutions underlie the monogyny/polygyny social polymorphism in the distantly related fire ant S. geminata. We found that Gp-9 coding region sequences are identical in the polygyne and monogyne forms of this species, disproving our hypothesis that one or a few specific amino acid replacements in the protein are necessary to induce transitions in social organization in fire ants. On the other hand, polygyne S. geminata differs genetically from the monogyne form in ways not mirrored in the two forms of S. invicta, a well-studied member of the S. richteri clade, supporting the conclusion that polygyny did not evolve via analogous routes in the two lineages. Specifically, polygyne S. geminata has lower genetic diversity and different gene frequencies than the monogyne form, suggesting that the polygyne form originated via a founder event from a local monogyne population. These comparative data suggest an alternative route to polygyny in S. geminata in which loss of allelic variation at genes encoding recognition cues has led to a breakdown in discrimination abilities and the consequent acceptance of multiple queens in colonies.     

Differences in nestmate recognition for drones and workers in the honeybee, Apis mellifera (L.)

Nestmate recognition is the basic mechanism for rejecting foreign individuals and is essential for maintaining colony integrity in insect societies. However, in honeybees, Apis mellifera, both workers and males occasionally gain access to foreign colonies in spite of nest guards (=drifting). Instead of conducting direct behavioural observations, we inferred nestmate recognition for males and workers from the genotypes of naturally drifting individuals in honeybee colonies. We evaluated the degree of polyandry of the resident queens, because nestmate recognition theory predicts that the genotypic composition of insect colonies may affect the recognition precision of guards. Workers (N=1346) and drones (N=407) from 38 colonies were genotyped using four DNA microsatellite loci. Foreign bees were identified by maternity testing. The proportion of foreign individuals in a host colony was defined as immigration. Putative mother queens were identified if a queen's genotype corresponded with the genotype of a drifted individual. The proportion of a colony's individuals in the total number of drifted individuals was defined as emigration. Drones immigrated significantly more frequently than workers. The impact of polyandry was significantly different between drones and workers. Whereas drones immigrated more readily into less polyandrous colonies, worker immigration was not correlated with the degree of polyandry of the host colony. Furthermore, colonies with high levels of emigrated drones did not show high levels of emigration for workers, and colonies that adopted many workers did not adopt many foreign drones. Our data indicate that genetically derived odour cues are important for honeybee nestmate recognition in drones and show that different nestmate recognition mechanisms are used to identify drones and workers.     

Prediction of social structure and genetic relatedness in colonies of the facultative polygynous stingless bee Melipona bicolor (Hymenoptera, Apidae)

Stingless bee colonies typically consist of one single-mated mother queen and her worker offspring. The stingless bee Melipona bicolor (Hymenoptera: Apidae) shows facultative polygyny, which makes this species particularly suitable for testing theoretical expectations concerning social behavior. In this study, we investigated the social structure and genetic relatedness among workers from eight natural and six manipulated colonies of M. bicolor over a period of one year. The populations of M. bicolor contained monogynous and polygynous colonies. The estimated genetic relatedness among workers from monogynous and polygynous colonies was 0.75 ± 0.12 and 0.53 ± 0.16 (mean ± SEM), respectively. Although the parental genotypes had significant effects on genetic relatedness in monogynous and polygynous colonies, polygyny markedly decreased the relatedness among nestmate workers. Our findings also demonstrate that polygyny in M. bicolor may arise from the adoption of related or unrelated queens.     

Genetic diversity within honeybee colonies prevents severe infections and promotes colony growth

Multiple mating by social insect queens increases the genetic diversity among colony members, thereby reducing intracolony relatedness and lowering the potential inclusive fitness gains of altruistic workers. Increased genetic diversity may be adaptive, however, by reducing the prevalence of disease within a nest. Honeybees, whose queens have the highest levels of multiple mating among social insects, were investigated to determine whether genetic variation helps to prevent chronic infections. I instrumentally inseminated honeybee queens with semen that was either genetically similar (from one male) or genetically diverse (from multiple males), and then inoculated their colonies with spores of Ascosphaera apis, a fungal pathogen that kills developing brood. I show that genetically diverse colonies had a lower variance in disease prevalence than genetically similar colonies, which suggests that genetic diversity may benefit colonies by preventing severe infections.     

Colony social structure in native and invasive populations of the social wasp Vespula pensylvanica

Social insects rank among the most invasive of terrestrial species. The success of invasive social insects stems, in part, from the flexibility derived from their social behaviors. We used genetic markers to investigate if the social system of the invasive wasp, Vespula pensylvanica, differed in its introduced and native habitats in order to better understand variation in social phenotype in invasive social species. We found that (1) nestmate workers showed lower levels of relatedness in introduced populations than native populations, (2) introduced colonies contained workers produced by multiple queens whereas native colonies contained workers produced by only a single queen, (3) queen mate number did not differ significantly between introduced and native colonies, and (4) workers from introduced colonies were frequently produced by queens that originated from foreign nests. Thus, overall, native and introduced colonies differed substantially in social phenotype because introduced colonies more frequently contained workers produced by multiple, foreign queens. In addition, the similarity in levels of genetic variation in introduced and native habitats, as well as observed variation in colony social phenotype in native populations, suggest that colony structure in invasive populations may be partially associated with social plasticity. Overall, the differences in social structure observed in invasive V. pensylvanica parallel those in other, distantly related invasive social insects, suggesting that insect societies often develop similar social phenotypes upon introduction into new habitats.     

Sympatric speciation and radiative evolution of socially parasitic ants - Heretic hypotheses and their factual background

According to current hypotheses the main types of social parasitism among ants, namely slavery, temporary parasitism, and inquilinism, arose from such features as predation on other ants, or territorial behavior, both presumed precursors of slavemaking, and polygyny, a presumed precursor of temporary parasitism and inquilinism. The latter is believed also to represent a final instar in several evolutionary pathways leading from slavery, temporary parasitism, and xenobiosis to this permanently parasitic, workerless condition. Speciation, the origin of parasitic species from their usually closely related host species, is suggested to occur due to temporary geographic isolation and subsequent transition of one of the newly formed daughter species to parasitism in the nests of the other. Evidence is presented suggesting that the main types of social parasitism originated independently of each other. 15 ant genera are parasitized exclusively by inquilines, Eve other genera exclusively by temporary parasites. Only four groups of non-parasitic ant species (Formica, Tet-ramorium, Leptothorax subgenera Leptothorax and Myrafant) have parasites of several types each. Within these roups, however, there is little evidence of evolutionary transitions from one type to another. The few exceptions, mainly workerless species of the genera Epimyrma and Chalepoxenus, represent parasites which clearly derive from slave-making congeners, but differ from ordinary inquilines in that they eliminate the host colony queens like their actively dulotic ancestors. The new hypothesis suggests that all forms of interspecific true social parasitism (excluding xenobiosis) orginated from a common “preparasitic” stage, a subpopulation of reproductives in polygynous colonies and species, with diverging sexual behavior (near-nest mating vs. swarming) and caste ratios (production of more sexuals vs. workers). Arguments for sympatric speciation are compiled. Various features of the ancestral, and then host species (colony sizes, population density and structure, transition from polygyny to monoyny, etc.), and of the “preparasite” (production of few, or no workers, etc.) may shape the developing parasite to become a slave-maker, inquiline, or temporary parasite. These features usually leave open only one, or in a few genera, several options. The different types of parasitism within one host species group thus may have developed in a radiative manner from the common, preparasitic stage, which explains that independent colony foundation is a common feature of all true social parasites among ants.