The determinants of queen size in a socially polymorphic ant

In social animals, body size can be shaped by multiple factors, such as direct genetic effects, maternal effects, or the social environment. In ants, the body size of queens correlates with the social structure of the colony: colonies headed by a single queen (monogyne) generally produce larger queens that are able to found colonies independently, whereas colonies headed by multiple queens (polygyne) tend to produce smaller queens that stay in their natal colony or disperse with workers. We performed a cross‐fostering experiment to investigate the proximate causes of queen size variation in the socially polymorphic ant Formica selysi. As expected if genetic or maternal effects influence queen size, eggs originating from monogyne colonies developed into larger queens than eggs collected from polygyne colonies, be they raised by monogyne or polygyne workers. In contrast, eggs sampled in monogyne colonies were smaller than eggs sampled in polygyne colonies. Hence, eggs from monogyne colonies are smaller but develop into larger queens than eggs from polygyne colonies, independently of the social structure of the workers caring for the brood. These results demonstrate that a genetic polymorphism or maternal effect transmitted to the eggs influences queen size, which probably affects the social structure of new colonies.     

Effect of mono- and polygyne social forms on transmission and spread of a microsporidium in fire ant populations

Thelohania solenopsae is a pathogen of the red imported fire ant, Solenopsis invicta, which debilitates queens and eventually causes the demise of colonies. Reductions of infected field populations signify its potential usefulness as a biological control agent. Thelohania solenopsae can be transmitted by introducing infected brood into a colony. The social forms of the fire ant, that is, monogyny (single queen per colony) or polygyny (multiple queens per colony), are associated with different behaviors, such as territoriality, that affect the degree of intercolony brood transfer. T. solenopsae was found exclusively in polygyne colonies in Florida. Non-synchronous infections of queens and transovarial transmission favor the persistence and probability of detecting infections in polygynous colonies. However, queens or alates with the monogyne genotype can be infected, and infections in monogyne field colonies have been reported from Louisiana and Argentina. Limited independent colony-founding capability and shorter dispersal of alate queens with the polygyne genotype relative to monogyne alates may facilitate the maintenance of infections in local polygynous populations. Demise of infected monogyne colonies can be twice as fast as in polygyne colonies and favors the pathogen's persistence in polygyne fire ant populations. The social form of the fire ant reflects different physiological and behavioral aspects of the queen and colony that will impact T. solenopsae spread and ultimate usefulness for biological control.     

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.     

A comparison of queen oviposition rates from monogyne and polygyne fire ant, Solenopsis invicta, colonies

Abstract. The oviposition rate of individual queens of Solenopsis invicta Buren (Hymenoptera: Formicidae) in relation to their weight and number of queens present in the colony was investigated by direct 2 h observations. There is a strong positive correlation between the weight of a queen and its oviposition rate in both monogyne and polygyne colonies. However, the number of eggs laid per mg queen is higher for moonogyne queens than for polygyne queens. This difference is more evident when the total weight of queens present in a colony is considered. The individual queen oviposition rate is negatively correlated with the number of queens in the colony. In addition, the weight loss per egg laid is significantly greater for polygyne than for monogyne queens, probably due to differences in egg size. These data suggest that oviposition is more efficient in monogyne than in polygyne queens at the individual level; however, at the colony level, polygyne colonies produce significantly more eggs. Comparison of colony level efficiency predicts that polygyne colonies must have at least nine queens to compete reproductively with a mature monogyne queen. Therefore, oligogyny does not appear to be a viable strategy for S.invicata.     

Sex investment ratios in monogyne and polygyne populations of the fire ant Solenopsis invicta

Both monogyne (single queen per colony) and polygyne (multiple queens per colony) populations of the fire ant Solenopsis invicta are good subjects for tests of kin selection theory because their genetic and reproductive attributes are well-characterized, permitting quantitative predictions about the degree to which sex investment ratios should be female-biased if workers and not queens control reproductive allocation. In the study populations, an investment ratio of 3 females: 1 male is predicted (a proportional investment in females of 0.75) in the monogyne form, whereas a proportional investment in females between 0.637 and 0.740 is expected in the polygyne form. To test these predictions, colonies from a single population of each social form were collected and censused during three different seasons. Consistent with their alternative modes of colony founding, monogyne colonies invested more in reproduction (sexual production) and less in growth/maintenance (worker production) than did the polygyne colonies. Overall, the sex investment ratios were female-biased in both forms, although there was considerable seasonal variation. After adjusting for sex-specific energetic costs, the proportional investment in females was 0.607 in the monogyne population, a value in between those expected under complete control by either the queen or the workers. However, when combined with data from four other previously studied monogyne populations in the U.S.A., the mean investment ratio did not differ significantly from the value predicted if workers have exclusive control. In the polygyne population, the proportional investment in females of 0.616 was consistent with the level of female bias expected under partial to complete worker control, although the potential influence of two confounding factors — possible contact with monogyne colonies and the preponderance of sterile diploid males — weakens this conclusion somewhat. Taken as a whole, the sex investment ratios of monogyne and polygyne populations of S. invicta are consistent with at least partial worker control. Of several ultimate and proximate explanations that have been proposed to explain inter-colonial variation in the sex investment ratio, only the effect of the primary sex ratio (female-determined eggs: male-determined eggs) laid by the queen appears to account for the observed variation among monogyne colonies. In the polygyne population, there is limited support for the hypothesis that greater resource abundance favors investment in females.     

Dispersal polymorphism and physiological condition of males and females in the ant, Formica truncorum

This study deals with dispersal behavior of sexuals and intraspecificvariation in queen numbers. The specific questions are: (1)Is there an association between male and female dispersal behaviorand the number of queens in a colony? (2) Is there an associationbetween individual behavior and physiological condition? (3)Do males and females from monogyne (one queen per colony) andpolygyne (several functional queens per colony) colonies differwith respect to size, weight, and physiological condition? Theresults show that both males and females are more prone to dispersein monogyne than in polygyne colonies. Moreover, males and femalesof both monogyne and polygyne colonies show dispersal polymorphism,suggesting that an increased tendency of reproductive femalesto stay in the maternal colony may cause monogyne colonies toswitch to polygyny. The propensity to disperse is associatedwith the physiological condition of individuals. Larger andheavier females containing more fat and glycogen preferentiallydisperse, whereas smaller ones with less fat and glycogen moreeasily dealate and mate without a previous nuptial flight. Maledispersal correlates positively to larger size and higher levelsof glycogen; fat contents do not increase during maturation.The females produced in monogyne colonies are larger, heavier,and contain more fat and glycogen than those produced in polygynecolonies. The males produced in monogyne colonies have relativelylonger wings and are heavier than those produced in polygynecolonies. However, there are no differences in size and fatcontents between males from monogyne and polygyne colonies.     

Selective male mortality in the red imported fire ant, Solenopsis invicta

Males in polygyne populations of Solenopsis invicta are primarily sterile diploids and thought to not express the Gp-9 gene coding for a pheromone-binding protein affecting complex social behavior. We examined an aspect of the breeding system hitherto not considered--male Gp-9 genotypes in relation to sperm stored in queens. Four sites with varying frequencies of sympatric monogyne and polygyne colonies were sampled, including sexuals, workers, and broods from four colonies. Most queens were heterozygotes storing B sperm. Although predicted to be common, only 14 of 504 males were B or BB genotypes, suggesting strong selection. Increased frequency of polygyne colonies at each site paralleled increases in queens with b sperm (1.9-32.8%) and of noninseminated queens. The presence of both B and b sperm in 1.9-18.9% of queens, genotype profiles of colonies, and genotypes of offspring from individual queens suggest some frequency of multiple mating. The bb genotype, rather than an obligate, developmental lethal, was present in some queens and common in alates, workers, and brood. Selective mortality of sexuals may affect multiple aspects of the breeding system, including female-mediated dispersal, mating success, and gene flow.     

Characterization of vitellogenin in the red imported fire ant, Solenopsis invicta (Hymenoptera: Apocrita: Formicidae)

Vitellin (VN) and vitellogenin (VG) profiles were analyzed in monogyne and polygyne colonies of the red imported fire ant, Solenopsis invicta. Non-denaturing and SDS-polyacrylamide gel electrophoresis (PAGE) analyses indicated that the native VN was likely 350 kDa and comprised of two subunits in the molecular size range of 170-185 kDa. SDS-PAGE of hemolymph showed that the relative mobilities and subunit patterns of VG and VN were similar. VG was present in the hemolymph of reproductive queens; alate, virgin queens; and workers, but not in males. Anti-VN, prepared from polygyne egg homogenates, reacted with egg homogenates and with hemolymph VG from reproductive, monogyne and polygyne queens and alate, virgin polygyne queens. Analysis of circulating VG and ovarian development in alate, virgin queens showed that low levels of VG appeared by five days following adult eclosion, but egg development was not observed until seven weeks. VG was evident in newly inseminated queens, and increased steadily for the first three weeks following dealation. VG levels declined slightly near eclosion of the first workers (= nanitics) and dropped sharply after nanitic emergence at five weeks following dealation. Oocyte maturation peaked at days 15-25 following dealation, but otherwise remained low but steady. These studies provide the basis for future investigations into endocrine regulations of vitellogenesis in S. invicta queens.     

Prevalence of Thelohania solenopsae infected Solenopsis invicta newly mated queens within areas of differing social form distributions

Newly mated queens (NMQs) originating from monogyne red imported fire ant (Solenopsis invicta) colonies and following a mating flight, initiate new colonies by sealing themselves in a nuptial chamber and relying solely on their own fat and crop reserves, as well as no longer needed wing muscles to rear their first workers (claustral colony foundation). This method of colony-founding is rarely successful for polygyne-derived NMQs, whose low weight critically limits the number of first workers they are able to produce. However, this observation may be confounded by the parasitic microsporidium, Thelohania solenopsae, thus far found to persist only in association with polygyne colonies. Infections of this microsporidium reduce the weight of female alates and may explain why polygyne NMQs are unlikely to successfully found colonies claustrally. NMQs collected following mating flights in Gainesville and Ocala, Florida were sorted by weight, checked for insemination and T. solenopsae infection. Insemination levels were greater than 90% for all weight classes at both collection sites and were not related to infection. Infection levels were lower in Gainesville than Ocala, averaging 1.67% and 14.14%, respectively. Polygyne-derived NMQs collected in Ocala, defined here as weighing 12mg (social form correctly assigned in 85% of samples examined by PCR), had the highest infection levels, 25.37% (17/67) in 2003 and 21.43% (6/28) in 2004. We conclude that infection by T. solenopsae cannot be completely responsible for the inability of polygyne NMQs to claustrally establish colonies.     

QUEEN NUMBER IN COLONIES OF SOCIAL HYMENOPTERA AS A KIN-SELECTED ADAPTATION

Using a series of kin-selection models, I examine factors that favor multiple egg-laying queens (polygyny) in eusocial Hymenoptera colonies. One result is that there is a theoretical conflict of interest between the founding queens and their daughter workers over how many and which individuals should be the extra reproductives. Both castes should prefer their full sisters. Therefore, primary polygyny (multiple related foundresses) may favor queens while secondary polygyny (related queens added to mature colonies) may favor workers. Polygyny, itself, was found to be favored by high colony survivorship and low probability of queens contributing eggs to successive broods. Polygyne colonies, however, did not need to produce more offspring per brood to be selectively favored; they could be half as productive per brood as monogyne ones and still have higher lifetime fitness under some conditions. For reproductive data from eight ant species with both monogyne and polygyne colonies, the model generates results that are consistent with a kin-selection explanation of polygyny in all of them. It is proposed that queen number is an ecologically flexible trait that is influenced by a broad set of factors but is not necessarily linked to specific habitat types. Furthermore, neither polygyny nor monogyny may be reliably considered as the primitive or ancestral Hymenopteran social system. The optimal queen number within a species may evolutionarily increase or decrease, depending on the direction of environmental change.     

Experimental Conversion of Colony Social Organization in Fire Ants (<Emphasis Type="Italic">Solenopsis invicta</Emphasis>): Worker Genotype Manipulation in the Absence of Queen Effects

Colony social organization in the fire ant Solenopsis invicta appears to be under strong genetic control. In the invasive USA range, polygyny (multiple queens per colony) is marked by the presence of the Gp-9 ^b allele in most of a colony’s workers, whereas monogyny (single queen per colony) is associated with the exclusive occurrence of the Gp-9 ^B allele. Ross and Keller, Behav Ecol Sociobiol 51:287–295 (2002) experimentally manipulated social organization by cross-fostering queens into colonies of the alternate form, thereby changing adult worker Gp-9 genotype frequencies over time. Although these authors showed that social behavior switched predictably when the frequency of b-bearing adult workers crossed a threshold of 5–10%, the possibility that queen effects caused the conversions could not be excluded entirely. We addressed this problem by fostering polygyne brood into queenright monogyne colonies. All such treatment colonies switched social organization to become polygyne, coincident with their proportions of b-bearing workers exceeding 12%. Our results support the conclusion that polygyny in S. invicta is induced by a minimum frequency of colony workers carrying the b allele, and further confirm that its expression is independent of queen genotype or history, worker genotypes at genes not linked to Gp-9, and colony genetic diversity.     

Multiple virus infections occur in individual polygyne and monogyne Solenopsis invicta ants

Concurrent infections of Solenopsis invicta colonies with S. invicta virus 1 (SINV-1), SINV-2, and SINV-3 has been reported. However, whether individual ants were capable of supporting multiple virus infections simultaneously was not known, nor whether the social form of the colony (polygyne or monogyne) had an influence on the occurrence of multiple infection rates in individual ants. S. invicta field populations were sampled sequentially to establish whether multiple virus infections co-occurred in individual worker ants. In addition, the intra-colony virus infection rates were compared in monogyne and polygyne field colonies to determine whether social form played a role in the viral infection prevalence. All combinations of virus infection (SINV-1, SINV-2, or SINV-3 alone, SINV-1 & SINV-2, SINV-1 & SINV-3, SINV-2 & SINV-3, and SINV-1, SINV-2 & SINV-3) were detected in individual worker ants as well as queens in the field. Thus, individual S. invicta ants can be infected simultaneously with all combinations of the S. invicta viruses. Colony social form did have an influence on the intra-colony prevalence of multiple S. invicta virus infections. Polygyne colonies exhibited significantly greater intra- and inter-colony single and multiple virus infections compared with monogyne colonies.     

Unexpected,well-developed nestmate recognition in laboratory colonies of polygyne imported fire ants (Hymenoptera: Formicidae)

Aggression bioassays were used to investigate nestmate recognition in polygyne laboratory colonies of the imported fire ant, Solenopsis invictaBuren. Unlike workers from polygyne field colonies, laboratory-maintained (>10 weeks) workers exhibited well-developed nestmate recognition. As in monogyne colonies of this species, both heritable and environmentally acquired (diet) odors provided recognition cues and were roughly additive in their effect. Within diet treatments, polygyne colonies responded in a graded fashion to polygyne conspecifics, monogyne conspecifics, and heterospecifics (S. richteri Forel),thus suggesting incipient genetic divergence between the two S. invictasocial forms. Hypotheses to account for the acute intraspecific discrimination observed in the laboratory are presented. Empirical testing of these hypotheses will illuminate ecological constraints and proximate mechanisms underlying the reduced intercolony discrimination associated with natural polygyne colonies of this and other ant species.     

A formal assessment of gene flow and selection in the fire ant Solenopsis invicta

Abstract.— Recent studies of the introduced fire ant Solenopsis invicta suggest that introduced polygyne (with multiple queens per nest) populations are strongly influenced by male-mediated gene flow from neighboring monogyne (single queen per nest) populations and selection acting on a single locus, general protein-9 (Gp-9) . This investigation formally tests this hypothesis and determines if these processes can account for the genotypic structure of polygyne S. invicta . To increase the statistical power of this test, we considered the genotypes of polygyne queens and workers at both Gp-9 and the closely linked, selectively neutral locus Pgm-3 . We then constructed and analyzed a novel mathematical model to delimit the effects of monogyne male gene flow and selection on the joint genotypes at the Pgm-3/Gp-9 superlocus. Using this framework, a hierarchical maximum-likelihood method was developed to estimate the best-fitting gene flow and selection parameters based on the fit of our model to data from both the current study and an earlier one of the same population. In each case, selection on polygyne queens and workers alone, with no monogyne male gene flow, provides the most parsimonious explanation for the observed genotype frequencies. The apparent discrepancy between this result and the empirical evidence for monogyne male gene flow indicates that undocumented factors, such as other forms of selection in polygyne males or workers, are operating in introduced polygyne S. invicta .     

Chemical communication of queen supergene status in an ant

Traits of interest to evolutionary biologists often have complex genetic architectures, the nature of which can confound traditional experimental study at single levels of analysis. In the fire ant Solenopsis invicta, the presence of a Mendelian ‘supergene’ is both necessary and sufficient to induce a shift in a fundamental property of social organization, from single‐queen (monogyne) to multiple‐queen (polygyne) colonies. This selfish genetic element, termed the Social b (Sb) supergene, contains > 600 genes that collectively promote its fitness by inducing the characteristic polygyne syndrome, in part by causing polygyne workers to accept only queens bearing the Sb element (a behaviour termed ‘worker Sb discrimination’). Here, we employ a newly developed behavioural assay to reveal that polygyne workers, many of which bear the Sb element, employ chemical cues on the cuticle of queens to achieve worker Sb discrimination, but we found no evidence for such pheromonally mediated worker Sb discrimination in monogyne workers, which universally lack the Sb element. This polygyne worker Sb discrimination was then verified through a ‘green beard’ effect previously described in this system. We thus have demonstrated that the Sb element is required both for production of relevant chemical cues of queens and for expression of the behaviours of workers that collectively result in worker Sb discrimination. This information fills a critical gap in the map between genotype and complex phenotype in S. invicta by restricting the search for candidate genes and molecules involved in producing this complex social trait to factors associated with the Sb element itself.     

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.     

The influence of social structure on brood survival and development in a socially polymorphic ant: insights from a cross‐fostering experiment

Animal societies vary in the number of breeders per group, which affects many socially and ecologically relevant traits. In several social insect species, including our study species Formica selysi, the presence of either one or multiple reproducing females per colony is generally associated with differences in a suite of traits such as the body size of individuals. However, the proximate mechanisms and ontogenetic processes generating such differences between social structures are poorly known. Here, we cross‐fostered eggs originating from single‐queen (= monogynous) or multiple‐queen (= polygynous) colonies into experimental groups of workers from each social structure to investigate whether differences in offspring survival, development time and body size are shaped by the genotype and/or prefoster maternal effects present in the eggs, or by the social origin of the rearing workers. Eggs produced by polygynous queens were more likely to survive to adulthood than eggs from monogynous queens, regardless of the social origin of the rearing workers. However, brood from monogynous queens grew faster than brood from polygynous queens. The social origin of the rearing workers influenced the probability of brood survival, with workers from monogynous colonies rearing more brood to adulthood than workers from polygynous colonies. The social origin of eggs or rearing workers had no significant effect on the head size of the resulting workers in our standardized laboratory conditions. Overall, the social backgrounds of the parents and of the rearing workers appear to shape distinct survival and developmental traits of ant brood.     

On the relationship between queen number and fecundity in polygyne colonies of the fire ant Solenopsis invicta

ABSTRACT. The reproductive status of individual queens in relation to the number of queens in polygyne colonies of Solenopsis invicta Buren was investigated under both field and laboratory conditions. In field colonies, the weight of individual queens is inversely related to the number of resident queens present. The weight of a queen is positively correlated with fecundity, as measured by the number of eggs laid in 5 h in isolation. Thus increasing queen number has a strong negative effect on the fecundity of individual queens in these colonies. A similar relationship was established in standardized laboratory colonies using queens from a single experimental population. Colony size by itself had no significant effect on queen weight in either the field or laboratory colonies, suggesting that the inverse relationship between queen number and fecundity results from mutual inhibition among queens, possibly involving pheromones, rather than reduced nutrition through lower worker/queen ratios. While uninseminated queens can comprise up to 50% of the functional (i.e. egglaying) queens in polygyne colonies of this ant, they tend to be less fecund than their inseminated nestmates; uninseminated queens within a given colony generally weigh 5–25% less.     

Benefits and costs of secondary polygyny in the dampwood termite Zootermopsis angusticollis

Newly molted female neotenic reproductives of the dampwood termite Zootermopsis angusticollis Hagen were allowed to mature in the presence of a neotenic male, a fixed number of larval helpers, and varying numbers of sibling neotenic queens to assess the impact of secondary polygyny to the individual and colony. Under monogyne conditions, neotenics developed more ovarioles per ovary and had higher individual fecundities after 60 d compared with females under polygyne conditions. Queens in groups of three females were able to gain more body mass than those in groups of five. Although the division of resources provided by helpers reduced individual female development and fecundity under polygyne conditions, it resulted in an overall increase in colony fecundity. In addition, neotenic females in polygynous colonies did not differ significantly in reproductive competence. There was no evidence that neotenics were attacked or injured by other reproductives or larval helpers, suggesting little if any reproductive competition among sibling queens. The physiological responses of neotenics to the increasing queen/worker ratio may have the benefit of enhancing the colony growth at the cost of the fecundity of individual queens.     

Unusual Behavior of Polygyne Fire Ant Queens on Nuptial Flights

This study reports previously undescribed behavior of fire ant queens (Solenopsis invicta) on their nuptial flights. We captured large numbers of alate (winged) queens flying at low altitudes in dense swarms that were virtually devoid of males. We assayed the genotypes of these alate queens at the locus Gp-9, which exhibits strong genotype frequency differences between monogyne (single-queen) and polygyne (multiple-queen) populations, and found that almost all of these low-flying queens originated from polygyne colonies. Comparisons of mtDNA haplotype distributions of these queens to those of alates leaving polygyne nests suggest that the flying queens had not dispersed more than a few hundred meters. Moreover, the proportion of flying queens that were mated did not differ significantly from the proportion of reproductive queens that were mated within the same sites. Thus the flight behavior appears to occur subsequent to mating. We suggest that the flying queens are sampling the local environment in order to select a suitable landing site. Such a site would contain established polygyne nests into which the queens may be adopted as new reproductives.