Does genetic diversity hinder parasite evolution in social insect colonies?

Polyandry is often difficult to explain because benefits of the behaviour have proved elusive. In social insects, polyandry increases the genetic diversity of workers within a colony and this has been suggested to improve the resistance of the colony to disease. Here we examine the possible impact of host genetic diversity on parasite evolution by carrying out serial passages of a virulent fungal pathogen through leaf-cutting ant workers of known genotypes. Parasite virulence increased over the nine-generation span of the experiment while spore production decreased. The effect of host relatedness upon virulence appeared limited. However, parasites cycled through more genetically diverse hosts were more likely to go extinct during the experiment and parasites cycled through more genetically similar hosts had greater spore production. These results indicate that host genetic diversity may indeed hinder the ability of parasites to adapt while cycling within social insect colonies.     

Increased genetic diversity as a defence against parasites is undermined by social parasites: Microdon mutabilis hoverflies infesting Formica lemani ant colonies

Genetic diversity can benefit social insects by providing variability in immune defences against parasites and pathogens. However, social parasites of ants infest colonies and not individuals, and for them a different relationship between genetic diversity and resistance may exist. Here, we investigate the genetic variation, assessed using up to 12 microsatellite loci, of workers in 91 Formica lemani colonies in relation to their infestation by the specialist social parasite Microdon mutabilis. At the main study site, workers in infested colonies exhibited lower relatedness and higher estimated queen numbers, on average, than uninfested ones. Additionally, estimated queen numbers were negatively correlated with estimated average numbers of mates per queen within infested colonies. At another site, infested colonies also exhibited significantly lower worker relatedness, and estimated queen numbers were comparable in trend. In contrast, in two populations of F. lemani where M. mutabilis was absent, relatedness within colonies was high (40 and 90% with R>0.6). While high genetic variation can benefit social insects by increasing their resistance to pathogens, there may be a cost in the increased likelihood of infiltration by social parasites owing to greater variation in nestmate recognition cues. This study provides the first empirical test of this hypothesis.     

Fitness in invasive social wasps: the role of variation in viral load,immune response and paternity in predicting nest size and reproductive output

Within any one habitat, the relative fitness of organisms in a population can vary substantially. Social insects like the common wasp are among the most successful invasive animals, but show enormous variation in nest size and other fitness‐related traits. Some of this variation may be caused by pathogens such as viruses that can have serious consequences in social insects, which range from reduced productivity to colony death. Both individual immune responses and colony‐level traits such as genetic diversity are likely to influence effects of pathogen infections on colony fitness. Here we investigate how infections with Kashmir Bee Virus (KBV), immune response and intracolony genetic diversity (due to queen polyandry) affect nest size in the invasive common wasp Vespula vulgaris. We show that KBV is highly prevalent in wasps and expression of antiviral immune genes is significantly increased with higher viral loads across individuals. Patriline membership within a nest did not influence KBV susceptibility or immune response. A permutational MANCOVA revealed that polyandry, viral load and expression of the immune gene Dicer were significant predictors of variation in nest size. High intracolony genetic diversity due to polyandry has previously been hypothesized to improve colony‐level resistance to parasites and pathogens. Consistent with this hypothesis, we observed genetically diverse colonies to be significantly larger and to produce more queens, although this effect was not driven by the pathogen we investigated. Invasive wasps clearly suffer from pathogens and expend resources, as indicated here by elevated immune gene expression, toward reducing pathogen‐impact on colony fitness.     

Unrelated queens coexist in colonies of the termite Macrotermes michaelseni

Relatedness increases the likelihood of cooperation within colonies of social insects. Polygyny, the coexistence of numerous reproductive females (queens) in a colony, is common in mature colonies of the termite Macrotermes michaelseni. In this species, polygyny results from pleometrosis and from several female alates that jointly found a new colony. To explain this phenomenon, it was suggested that only related females cooperate and survive during maturation of colonies. Using multilocus fingerprints as well as microsatellites, we showed that nestmate queens in mature colonies are unrelated. Furthermore, we found that all nestmate queens contributed to the production of steriles. Even in mature colonies, several matrilines of steriles coexist within a colony. Although genetic diversity within colonies may increase the likelihood of conflicts, high genetic diversity may be important for foraging, colony growth, and resistance to disease and parasites.     

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.     

The socially parasitic ant Polyergus mexicanus has host‐associated genetic population structure and related neighbouring colonies

The genetic structure of populations can be both a cause and a consequence of ecological interactions. For parasites, genetic structure may be a consequence of preferences for host species or of mating behaviour. Conversely, genetic structure can influence where conspecific interactions among parasites lay on a spectrum from cooperation to conflict. We used microsatellite loci to characterize the genetic structure of a population of the socially parasitic dulotic (aka “slave‐making”) ant (Polyergus mexicanus), which is known for its host‐specificity and conspecific aggression. First, we assessed whether the pattern of host species use by the parasite has influenced parasite population structure. We found that host species use was correlated with subpopulation structure, but this correlation was imperfect: some subpopulations used one host species nearly exclusively, while others used several. Second, we examined the viscosity of the parasite population by measuring the relatedness of pairs of neighbouring parasitic ant colonies at varying distances from each other. Although natural history observations of local dispersal by queens suggested the potential for viscosity, there was no strong correlation between relatedness and distance between colonies. However, 35% of colonies had a closely related neighbouring colony, indicating that kinship could potentially affect the nature of some interactions between colonies of this social parasite. Our findings confirm that ecological forces like host species selection can shape the genetic structure of parasite populations, and that such genetic structure has the potential to influence parasite‐parasite interactions in social parasites via inclusive fitness.     

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 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.     

Experimentally increased group diversity improves disease resistance in an ant species

A leading hypothesis linking parasites to social evolution is that more genetically diverse social groups better resist parasites . Moreover, group diversity can encompass factors other than genetic variation that may also influence disease resistance. Here, we tested whether group diversity improved disease resistance in an ant species with natural variation in colony queen number. We formed experimental groups of workers and challenged them with the fungal parasite Metarhizium anisopliae . Workers originating from monogynous colonies (headed by a single queen and with low genetic diversity) had higher survival than workers originating from polygynous ones, both in uninfected groups and in groups challenged with M. anisopliae . However, an experimental increase of group diversity by mixing workers originating from monogynous colonies strongly increased the survival of workers challenged with M. anisopliae , whereas it tended to decrease their survival in absence of infection. This experiment suggests that group diversity, be it genetic or environmental, improves the mean resistance of group members to the fungal infection, probably through the sharing of physiological or behavioural defences.     

Evidence for a quiet revolution: seasonal variation in colonies of the specialist tansy aphid, Macrosiphoniella tanacetaria (Kaltenbach) (Hemiptera: Aphididae) studied using microsatellite markers

In cyclical parthenogens, clonal diversity is expected to decrease due to selection and drift during the asexual phase per number of asexual generations. The decrease in diversity may be counteracted by immigration of new genotypes. We analysed temporal variation in clonal diversity in colonies of the monophagous tansy aphid, Macrosiphoniella tanacetaria (Kaltenbach), sampled four times over the course of a growing season. In a related field study, we recorded aphid colony sizes and the occurrence of winged dispersers throughout the season. The number of colonies increased from April, when asexual stem mothers hatched from the sexually produced eggs, to the end of June. The proportion of colonies with winged individuals also increased over this period. After a severe reduction in colony sizes in late summer, a second expansion phase occurred in October when sexuals were produced. At the season's end, the only winged forms were males. A linked genetic study showed that the number of microsatellite multilocus genotypes and genetic variability assessed at three polymorphic loci per colony decreased from June to October. Overall, the relatedness of wingless to winged individuals within colonies was lower than average relatedness among wingless individuals, suggesting that winged forms mainly originated in different colonies. The results demonstrate that patterns of genetic diversity within colonies can be explained by the antagonistic forces of clonal selection, migration and genetic drift (largely due to midsummer population bottlenecks). We further suggest that the males emigrate over comparatively longer distances than winged asexual females.     

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.     

Host–parasite genotypic interactions in the honey bee: the dynamics of diversity

Parasites are thought to be a major driving force shaping genetic variation in their host, and are suggested to be a significant reason for the maintenance of sexual reproduction. A leading hypothesis for the occurrence of multiple mating (polyandry) in social insects is that the genetic diversity generated within‐colonies through this behavior promotes disease resistance. This benefit is likely to be particularly significant when colonies are exposed to multiple species and strains of parasites, but host–parasite genotypic interactions in social insects are little known. We investigated this using honey bees, which are naturally polyandrous and consequently produce genetically diverse colonies containing multiple genotypes (patrilines), and which are also known to host multiple strains of various parasite species. We found that host genotypes differed significantly in their resistance to different strains of the obligate fungal parasite that causes chalkbrood disease, while genotypic variation in resistance to the facultative fungal parasite that causes stonebrood disease was less pronounced. Our results show that genetic variation in disease resistance depends in part on the parasite genotype, as well as species, with the latter most likely relating to differences in parasite life history and host–parasite coevolution. Our results suggest that the selection pressure from genetically diverse parasites might be an important driving force in the evolution of polyandry, a mechanism that generates significant genetic diversity in social insects.     

Does cooperation mean kinship between spatially discrete ant nests?

Eusociality is one of the most complex forms of social organization, characterized by cooperative and reproductive units termed colonies. Altruistic behavior of workers within colonies is explained by inclusive fitness, with indirect fitness benefits accrued by helping kin. Members of a social insect colony are expected to be more closely related to one another than they are to other conspecifics. In many social insects, the colony can extend to multiple socially connected but spatially separate nests (polydomy). Social connections, such as trails between nests, promote cooperation and resource exchange, and we predict that workers from socially connected nests will have higher internest relatedness than those from socially unconnected, and noncooperating, nests. We measure social connections, resource exchange, and internest genetic relatedness in the polydomous wood ant Formica lugubris to test whether (1) socially connected but spatially separate nests cooperate, and (2) high internest relatedness is the underlying driver of this cooperation. Our results show that socially connected nests exhibit movement of workers and resources, which suggests they do cooperate, whereas unconnected nests do not. However, we find no difference in internest genetic relatedness between socially connected and unconnected nest pairs, both show high kinship. Our results suggest that neighboring pairs of connected nests show a social and cooperative distinction, but no genetic distinction. We hypothesize that the loss of a social connection may initiate ecological divergence within colonies. Genetic divergence between neighboring nests may build up only later, as a consequence rather than a cause of colony separation.     

Effective paternity in natural colonies of Japanese native bumble bees

Kin selection theory predicts a high coefficient of genetic relatedness among nestmates, explaining the frequent evolution of eusociality in a social hymenopteran colony. The bumble bee is a primitively eusocial hymenoptera whose colonies are founded by a single queen. In such a monogynous colony, mating frequency of the queen is the sole factor that affects genetic relatedness among nestmates. Although the queens of most bumble bee species are known to be monandrous, there are some species that are known to be polyandrous. Here, we estimated the effective paternity in the native colonies of Japanese bumble bees, Bombus ardens, B. diversus, and B. honshuensis, using genetic polymorphic data of microsatellites. We found no evidence for polyandry in any investigated colonies, which suggests that within-colony genetic relatedness is very high for all of these colonies.     

Transfer of intracolonial genetic variability through gametes in Acropora hyacinthus corals

In recent years, the new phenomenon of intracolonial genetic variability within a single coral colony has been described. This connotes that coral colonies do not necessarily consist of only a single genotype, but may contain several distinct genotypes. Harboring more than one genotype could improve survival under stressful environmental conditions, e.g., climate change. However, so far it remained unclear whether the intracolonial genetic variability of the adult coral is also present in the gametes. We investigated the occurrence of intracolonial genetic variability in 14 mature colonies of the coral Acropora hyacinthus using eight microsatellite loci. A grid was placed over each colony before spawning, and the emerging egg/sperm bundles were collected separately in each grid. The underlying tissues as well as the egg/sperm bundles were genotyped to determine whether different genotypes were present. Within the 14 mature colonies, we detected 10 colonies with more than one genotype (intracolonial genetic variability). Four out of these 10 mature colonies showed a transfer of different genotypes via the eggs to the next generation. In two out of these four cases, we found additional alleles, and in the two other cases, we found only a subset of alleles in the unfertilized eggs. Our results suggest that during reproduction of A. hyacinthus, more than one genotype per colony is able to reproduce. We discuss the occurrence of different genotypes within a single coral colony and the ability for those to release eggs which are genetically distinct.     

Inbreeding and kinship in the ant Plagiolepis pygmaea

In ants the presence of multiple reproductive queens (polygyny) decreases the relatedness among workers and the brood they rear, and subsequently dilutes their inclusive fitness benefits from helping. However, adoption of colony daughters, low male dispersal in conjunction with intranidal (within nest) mating and colony reproduction by budding may preserve local genetic differences, and slow down the erosion of relatedness. Reduced dispersal and intranidal mating may, however, also lead to detrimental effects owing to competition and inbreeding. We studied mating and dispersal patterns, and colony kinship in three populations of the polygynous ant Plagiolepis pygmaea using microsatellite markers. We found that the populations were genetically differentiated, but also a considerable degree of genetic structuring within populations. The genetic viscosity within populations can be attributed to few genetically homogeneous colony networks, which presumably have arisen through colony reproduction by budding. Hence, selection may act at different levels, the individuals, the colonies and colony networks. All populations were also significantly inbred (F=0.265) suggesting high frequencies of intranidal mating and low male dispersal. Consequently the mean regression relatedness among workers was significantly higher (r = 0.529-0.546) than would be expected under the typically reported number (5-35) of queens in nests of the species. Furthermore, new queens were mainly recruited from their natal or a neighbouring related colony. Finally, the effective number of queens coincided with that found upon excavation, suggesting low reproductive skew.     

Lack of patriline-specific differences in chemical composition of the metapleural gland secretion in <Emphasis Type="Italic">Acromyrmex octospinosus</Emphasis>

Summary. Multiple queen-mating (polyandry) in social insects increases the genetic variability among worker offspring, which may enhance colony survival, social productivity and defence against parasites. The unique and complex symbiosis of leaf-cutting ants with a clonal mutualistic fungus makes this social system particularly vulnerable to contamination by pathogenic and unwanted saprophytic fungi and bacteria. Proper defence against such threats requires effective and flexible chemical defence mechanisms. A prime candidate for providing such defences is the metapleural gland secretion, which is known to have broad antibiotic properties. Here we use the leaf-cutting ant Acromyrmex octospinosus to specifically test the hypothesis that genetically more diverse worker-offspring produce a more variable spectrum of metapleural gland compounds. We used DNA microsatellite markers to assign workers from two colonies to the six most common patrilines in each colony, and have analysed the degree to which the observed variance in the quantitative chemical composition of the metapleural gland secretion can be explained by genetic differences among patrilines. We found a marginally significant patriline-effect on the overall variability of metapleural gland compounds in one colony, but could not detect such effect in the other colony. We discuss a number of possible reasons why the genetic variance component for quantitative variation in metapleural gland secretion may be low.     

Sex ratios and the distribution of elaiosomes in colonies of the ant, Aphaenogaster rudis

Summary: Genetic theory predicts that workers in monogynous ant colonies with singly-mated queens should capitalize on higher relatedness with sisters than with brothers by altering the sex investment ratio of a colony in favor of females. Sex investment ratios, however, may also be influenced by the amount of resources available to colonies, in part because more mating opportunities might be obtained by investing scarce resources in males, which are much smaller than queens. Female larvae that reach a critical size by a particular point in development become queens while underfed larvae develop into workers, so workers could potentially influence the sex investment ratio of a colony by selectively feeding female larvae. In a previous experiment on the ant, Aphaenogaster rudis, colonies increased female sex investment after their diet was supplemented with elaiosomes, a lipid-rich food gained from a seed dispersal mutualism. In order to investigate the mechanisms producing this shift, we radio-labeled Sanguinaria canadensis elaiosomes with fatty acids and compared uptake among castes within a colony. The experiment was performed in both the laboratory and field. Lab colonies produced female-biased sex investment ratios, while field colonies mainly invested in males. We hypothesize that this discrepancy is related to differing levels of background food availability in the lab and field. The results of the elaiosome distribution experiment do not support a hypothesis that elaiosomes play a qualitative role in queen determination, because all individuals in a colony receive this nutrient. There is, however, support for the hypothesis that elaiosomes have a quantitative effect on larval development because larvae that accumulated more radio-label from elaiosomes tended to develop into gynes (virgin queens), while other female larvae developed into workers.     

Conflict over male parentage in social insects

Mutual policing is an important mechanism that maintains social harmony in group-living organisms by suppressing the selfish behavior of individuals. In social insects, workers police one another (worker-policing) by preventing individual workers from laying eggs that would otherwise develop into males. Within the framework of Hamilton's rule there are two explanations for worker-policing behavior. First, if worker reproduction is cost-free, worker-policing should occur only where workers are more closely related to queen- than to worker-produced male eggs (relatedness hypothesis). Second, if there are substantial costs to unchecked worker reproduction, worker-policing may occur to counteract these costs and increase colony efficiency (efficiency hypothesis). The first explanation predicts that patterns of the parentage of males (male parentage) are associated with relatedness, whereas the latter does not. We have investigated how male parentage varies with colony kin structure and colony size in 50 species of ants, bees, and wasps in a phylogenetically controlled comparative analysis. Our survey revealed that queens produced the majority of males in most of the species and that workers produced more than half of the males in less than 10% of species. Moreover, we show that male parentage does not vary with relatedness as predicted by the relatedness hypothesis. This indicates that intra- and interspecific variation in male parentage cannot be accounted for by the relatedness hypothesis alone and that increased colony efficiency is an important factor responsible for the evolution of worker-policing. Our study reveals greater harmony and more complex regulation of reproduction in social insect colonies than that expected from simple theoretical expectations based on relatedness only.     

The role of swarming sites for maintaining gene flow in the brown long-eared bat (Plecotus auritus)

Bat-swarming sites where thousands of individuals meet in late summer were recently proposed as 'hot spots' for gene flow among populations. If, due to female philopatry, nursery colonies are genetically differentiated, and if males and females of different colonies meet at swarming sites, then we would expect lower differentiation of maternally inherited genetic markers among swarming sites and higher genetic diversity within. To test these predictions, we compared genetic variance from three swarming sites to 14 nursery colonies. We analysed biparentally (five nuclear and one sex-linked microsatellite loci) and maternally (mitochondrial D-loop, 550 bp) inherited molecular markers. Three mtDNA D-loop haplolineages that were strictly separated at nursery colonies were mixed at swarming sites. As predicted by the 'extra colony-mating hypothesis', genetic variance among swarming sites (V(ST)) for the D-loop drastically decreased compared to the nursery population genetic variance (V(PT)) (31 and 60%, respectively), and genetic diversity increased at swarming sites. Relatedness was significant at nursery colonies but not at swarming sites, and colony relatedness of juveniles to females was positive but not so to males. This suggests a breakdown of colony borders at swarming sites. Although there is behavioural and physiological evidence for sexual interaction at swarming sites, this does not explain why mating continues throughout the winter. We therefore propose that autumn roaming bats meet at swarming sites across colonies to start mating and, in addition, to renew information about suitable hibernacula.