Genetic population structure, queen supersedure and social polymorphism in a social Hymenoptera

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1 doi:0./j x Genetic population structure, queen supersedure and social polymorphism in a social Hymenoptera K. BARGUM,* H. HELANTERÄ* & L. SUNDSTRÖM* *Department of Biological and Environmental Sciences, University of Helsinki, Helsinki, Finland Laboratory of Apiculture and Social Insects, Department of Animal & Plant Sciences, University of Sheffield, Sheffield, UK Keywords: ants; behavioural flexibility; Formica fusca; kin selection; polygyny; queen turnover. Abstract In social insects, the emergence of multiple queening is linked to changes in a suite of traits such as the reproductive life span of queens, mating patterns and population structure. We investigated queen turnover, colony longevity, spatial distribution patterns and genetic differentiation in a population of the socially polymorphic ant Formica fusca. Genetic differentiation between the social forms was absent, and mating patterns were similar in the two forms. The spatial distribution of single- and multi-queen colonies indicated an absence of colony reproduction by budding in both colony types. However, the rate of queen supersedure was high in multi-queen colonies and absent in single-queen ones. The social structure of colonies remained stable across years, but colony mortality did not differ between the two social forms. These results imply that differences between social types may appear and persist also in sympatry, and that these differences may occur in some traits, but not others, despite the presence of homogenizing gene flow. Introduction A key challenge in evolutionary biology is to explain the emergence of cooperatively breeding groups, and identify the factors that influence the composition and function of such groups (Seger, 993; Cahan et al., 2002). Cooperation in social groups often entails a reduction in personal reproduction for some individuals, whereas others monopolize breeding. The decision to join a group or remain solitary is influenced both by inclusive fitness returns to potential helpers and by ecological factors that influence the success of a solitary vs. a social mode of life (Hamilton, 964; Keller & Reeve, 994). Sociality, in turn, impacts the genetic structure of populations by introducing an additional level of organization, the family unit (Chesser, 99; Chesser et al., 993; Sugg et al., 996; Ross, 200). In addition, the extent of gene flow among and within populations will affect the prospects for evolutionary divergence of social traits, which raises the question to what extent variation in Correspondence: Katja Bargum, Department of Biological and Environmental Sciences, PO Box 65, 0004 University of Helsinki, Helsinki, Finland. Tel.: ; fax: ; katja.bargum@helsinki.fi such traits can be maintained within populations of interbreeding groups (Chapuisat et al., 2004). Cooperative breeding, sensu West et al. (2007), occurs in a variety of taxa ranging from mammals and birds to several groups of arthropods (reviewed in e.g. Emlen, 99; Jennions & Macdonald, 994; Sherman et al., 995). In most species all individuals retain their full reproductive potential, with the exception of some social Hymenoptera (Crozier & Pamilo, 996), and some other arthropods (Crespi, 996) which have largely sterile castes. All lineages of social insects, however, also display extensive variation in the number of breeders per colony (Hölldobler & Wilson, 990; Keller, 995; Boomsma & Ratnieks, 996), as mature colonies may contain one or several breeding queens (polygyny), and queens may mate with one or several males (polyandry). Polygyny may be transient, as is the case in many species of semelparous or iteroparous wasps, bees and ants (Michener, 974; Ross & Matthews, 99), or permanent as in many species of ants, and some species of wasps and bees, all of which are iteroparous (Keller, 993b; Bourke & Franks, 995). In the latter species polygyny usually arises through the adoption of daughter queens (Keller, 993b; Bourke & Franks, 995; Keller, 995; but see e.g. Queller et al., 993; Heinze et al., 200; Johnson, 2004). ª 2007 THE AUTHORS 20 (2007)

2 352 K. BARGUM ET AL. Table Costs and benefits associated with the polygyny syndrome relative to monogyny, and predictions drawn from these. Predictions in bold are tested in this study. Costs Benefits Predictions for polygyne colonies Queens Queens Life history selection for short lifespan, leading to rapid queen turnover Loss in personal reproduction Avoiding risky dispersal and colony founding Adoption of related queens (colony daughters) Local mating Genetic structuring within populations Workers Queens and workers Isolation by distance between populations Indirect fitness loss Colony persistence Higher colony survival Colony productivity Higher colony productivity This paves the way for the evolution of special characteristics associated with the regular presence of multiple queens (the polygyny syndrome sensu Keller, 993a; Table ). The permanent presence of multiple breeders in a group dilutes intra-group relatedness and thus comes at a potential cost to the inclusive fitness of nonreproductive group members (Ross, 200). Moreover, the per capita reproductive output per female is usually also lower than in groups with a single breeding female reproducing to her full potential without competition from other females (Elmes, 973; Herbers, 984; Keller, 988; Sherman et al., 995; Sundström, 995a; Rosset & Chapuisat, 2006; but see Walin et al., 200). If polygyny is to be evolutionary stable it must therefore convey benefits that offset the reduced fitness returns for workers and costs of increased competition among breeders (Wilson, 97). Three main factors have been identified in this context: ecological constraints on independent colony founding (Emlen, 982; Pamilo & Rosengren, 984; Rosengren et al., 993; Hatchwell & Komdeur, 2000), enhanced group productivity (Kokko et al., 200; Shreeves & Field, 2002), and enhanced colony longevity (Nonacs, 988; Keller, 995; Kokko et al., 200). To date polygyny and its consequences has been most intensely studied in ants (e.g. Bourke & Franks, 995; Keller, 995). A set of traits that characterize permanently polygyne societies has been identified, the polygyny syndrome (Keller, 993a; Table ). These traits entail changes both in the life history of queens and colonies, and in the genetic structure of populations. With respect to life history traits, polygyny may have two consequences. First, although adoptee queens are likely to face a reduction in personal reproduction, they stand to gain by forgoing the risks of independent colony foundation and initiating sexual production earlier, without a compulsory phase of colony growth and maturation (Oster & Wilson, 978; Rosengren & Pamilo, 983; Nonacs, 988; Pamilo, 99a; Seger, 993; Keller, 995; Cahan et al., 2002). Life history theory holds that this may lead to a reduced life span of queens if investment in reproduction and reproductive competition comes at a cost to the queens (see e.g. Williams, 966; Charlesworth & Leon, 976). As a result, the rate of queen turnover between breeding seasons is expected to be higher in polygyne societies. Secondly, if colony lifespan is potentially greater than queen life-span (i.e. mortality due to queen senescence, intrinsic mortality, exceeds that due to environmental causes, extrinsic mortality), resident queens and workers both stand to gain if related queens inherit the colony and so extend offspring production beyond the life span of individual queens (Nonacs, 988). As a result colony mortality should decrease under polygyny. This begs the question whether queen turnover and colony life span differ between polygyne and monogyne colonies. To date also genetic differences between social forms has been most intensively studied in ants (e.g. Pamilo et al., 997; Ross & Shoemaker, 997; Sundström et al., 2005). The results associate monogyny with panmixis with genetically distinct colonies, but with negligible within- and between-population structuring (e.g. Pamilo et al., 997; Sundström et al., 2005, but see Sundström et al., 2003), whereas polygyny tends to be associated with local mating and genetic structuring both within and between populations (e.g. Ross, 200; Sundström et al., 2005). Most studies on facultatively polygyne ants have focused on cases where monogyne and polygyne colonies are confined to different populations (reviewed in e.g. Pamilo et al., 997; Sundström et al., 2005), with some notable exceptions (Chapuisat et al., 2004; DeHeer & Herbers, 2004; Fournier et al., 2004; Rosset & Chapuisat, 2006). The studies on populations with a mixture of two social forms suggest that differences typically seen between social forms in allopatry are absent when the two forms occur in sympatry (Chapuisat et al., 2004; DeHeer & Herbers, 2004; Fournier et al., 2004). However, a recent study showed that some life history differences may appear also under panmixis (Rosset & Chapuisat, 2006). Furthermore, we are not aware of any studies that have assessed the degree to which individual colonies exhibit flexibility in social type over time. This begs the questions whether the social form of a colony is maintained across breeding seasons, whether gene flow ª 2007 THE AUTHORS 20 (2007)

3 Social polymorphism in ants 353 prevails between the two forms when they occur in sympatry, and whether mating and dispersal patterns differ between the two forms. In this study, we analyse the genetic population structure and social traits in sympatric polygyne and monogyne colonies of the facultatively polygyne ant Formica fusca. We test whether the two sympatric social forms are genetically distinct, which would indicate that they originate from different source populations and/or that gene flow between the forms is restricted. We also investigate whether polygyne colonies show geographical and genetic clustering, which would suggest restricted dispersal and local mating. Finally, we measure colony survival and queen turnover in the two social forms. A difference in one or both of these traits between monogyne and polygyne colonies may indicate that the two types have diverged with respect to life history traits of queens and/or colonies. Our results show that the social forms are neither genetically nor geographically distinct, and that neither mating behaviour nor colony mortality differ between forms. Nevertheless, monogyne colonies remained as such throughout the study period, whereas the turnover of queens was significantly greater in the polygyne colonies. This suggests that differences in life history traits readily appear between social forms also in mixed populations, and testifies to the evolutionarily flexible, facultative nature of social behaviour. Materials and methods Study species and social structure Formica (subgenus Serviformica) fusca is a boreal, soildwelling ant species which builds nests of workers in semi-open habitat (peat bogs, recently logged areas etc.). The species is facultatively polygyne, with a median effective number of queens between 2 and 5, and observed queen numbers ranging from one to c. 0 (Hannonen et al., 2004). The average life span of queens is not known, but the maximal life span has been recorded as 6 years in the laboratory (Lubbock, 888). In nature, colonies of this species are, however, most likely short-lived owing to extensive parasitism by and competition with the mound-building Formica rufa group species (subgenus Formica s. str.) (Collingwood, 979; Savolainen & Vepsäläinen, 988; Czechowski et al., 2002). Thus it is likely that the average life span of queens is considerably shorter than that of the Formica s. str. species (Pamilo, 99b). A previous study found that on average 36% of the queens are multiply, usually doubly, inseminated (Hannonen et al., 2004). The study population is located on an island, Lilla Träskön, off the coast of southern Finland, and was established after 995; the island was dominated by a dense population of the ecologically dominant ant species F. truncorum until its extinction in The cause of extinction is VI w VII N IV II 0 VIII unknown, but was not caused by changes in the habitat or human influence. The population consists of eight putative demes, based on spatially separated patches of suitable nesting habitat (Fig. ). A genetic study of the population confirmed a recent population expansion and showed that the population is not inbred (Helanterä, 2004). We collected adult workers from 56 colonies in 997. From 7 of these, worker pupae could also be collected. The locations of the colonies were mapped and the colonies marked in the field. In 999, all colonies still in existence were sampled again for workers and worker pupae. In F. fusca new workers are produced in clearly defined yearly cohorts and live for only year (K. Bargum and L. Sundström, personal observation). Because the samples were always collected before new workers had emerged in each season, the adult workers represent brood produced in 996 and 998 respectively, whereas the pupae represent brood produced in 997 and 999. This allowed us to measure queen turnover across 4 years. In addition, we sampled queens from 3 polygyne colonies in These queens were used to estimate the relatedness between queens and their mates, and the frequency of multiple mating in polygyne colonies. These data on mating frequency were augmented with data on maternal and paternal genotypes deduced from worker genotypes in the monogyne field colonies. We genotyped queens and the contents of their spermathecae, as well as eight to 24 workers and worker IX Fig. Map of the population showing the location of the demes I IX, and the location of colonies within demes. The numbers within each deme correspond to the genetic clustering from the Bayesian analysis. Boldface numbers represent monogyne and normal numbers polygyne colonies. Dots correspond to colonies that did not group with any other colony; filled dots represent polygyne and empty dots monogyne colonies I V ª 2007 THE AUTHORS 20 (2007)

4 354 K. BARGUM ET AL. pupae per colony at six polymorphic DNA-microsatellite loci (FL2, FL20: Chapuisat, 996; FE3, FE7, FE9, FE2: Gyllenstrand et al., 2002), following the protocol described in Hannonen et al. (2004). Based on the data from the first sampling occasion (workers 997), colonies were assigned as monogyne or polygyne based on inspection of the genotype distribution of the adult workers. Analysis of population structure We used hierarchical analysis of genetic variance to determine whether the population is structured according to either spatial patterns or variation in social structure. The analyses were based on the 56 colonies sampled for adult workers in 997. In the first analysis the hierarchy consisted of individuals nested within colonies, colonies nested within demes and demes nested within the population. In the second analysis the hierarchy consisted of individuals nested within colonies, colonies nested within social forms and social forms nested within the population. A significant F-statistic at the level of demes or social forms nested in the total population would indicate that either the demes or the social forms are genetically differentiated. We also tested for inbreeding separately for the two social forms. In these analyses the hierarchy comprised individuals nested within colonies, and colonies nested within the total population. Inbreeding was estimated as Fit, or gene correlation within individuals compared to the total population. Analyses were carried out using the population genetic software GDA. (Lewis & Zaykin, 200), and confidence intervals for the F-statistics were obtained by bootstrapping over loci 5000 times. We used the software BAPS 2.0 (Bayesian Analysis of Population Structure, Corander et al., 2003) to further analyse the genetic associations of colonies according to social type and geographical location. BAPS clusters genetically similar colonies to the most probable number of clusters and calculates the posterior probabilities for each of these groupings. The groupings obtained with BAPS were then compared both to the putative demes and the social structure of the clustered colonies. If demes indeed represent subpopulations with restricted gene flow, then the most probable groupings found by Bayesian analysis should correspond to the putative demes. Similarly, if social forms are genetically differentiated, they should be clustered with colonies of similar type. The genetic data were also used to estimate the average relatedness among nest mate workers for the entire population, and for monogyne and polygyne colonies separately using RELATEDNESS (Queller & Goodnight, 989). Average relatedness values were calculated by weighting nests equally and standard errors were obtained by jackknifing over colonies. Estimation of effective queen number, queen turnover and colony survival We genotyped 7 colonies for both workers and worker pupae in 997, and based on these we estimated queen turnover between the worker cohorts produced in 996 and 997. Of the 7 colonies, 2 also yielded worker and worker pupae samples in 999, and allowed estimation of queen turnover between the cohorts born in 996, 997, 998 and 999. We calculated the relatedness within and between yearly cohorts of workers using RELATEDNESS (Queller & Goodnight, 989). The background allele frequencies used were those of the whole population (both monogyne and polygyne colonies) for the two study years in question. We used the relatedness estimates to calculate the genetically effective number of queens, i.e. the number of queens required to explain the genetic diversity of the workers present. This was carried out using a formula that allows for multiple mating of queens and relatedness between queens and/or their mates (Ross, 993; Seppä, 994). The observed mating frequency of queens was used instead of the effective mating frequency, as we did not have access to offspring from the sampled queens to estimate paternity skew. Hence, we have a minimum estimate of effective queen number, but any bias in our estimates should be similar across years, and not affect our estimates of queen turnover. Because we did not have access to the offspring of the queens, we could not calculate the relatedness between the multiple mates of the same queen. Therefore, we estimated effective queen numbers assuming that the male mates of individual queens are unrelated, which is a reasonable assumption as r m was shown to be zero in another population of the same age and species (Hannonen et al., 2004). Queen turnover leads to a decrease in relatedness among cohorts compared with that within cohorts (Pedersen & Boomsma, 999). To test for such a difference, we compared the relatedness within cohorts with that between cohorts using repeated measures ANOVA, with year as a repeated measure and type of relatedness measure (within or between cohorts) as a factor (Queller, 994). In the analysis, a significant difference between relatedness within cohorts and relatedness between cohorts implies queen turnover. A significant interaction between year and turnover reflects differences in the amount of turnover between years. We analysed polygyne and monogyne colonies separately to compare the rates of queen turnover in the two social forms. We estimated the magnitude of queen turnover as r w;w 0 t ¼ r w þ r w 0 r w;w 0 ðþ where r w is the estimated relatedness among workers of one cohort (w), r w the relatedness among workers of the next cohort and r w,w the relatedness between cohorts. ª 2007 THE AUTHORS 20 (2007)

5 Social polymorphism in ants 355 This estimate of queen turnover (Pedersen & Boomsma, 999) differs from that presented by Evans (996) in that it can accommodate for changes in queen number (and, hence, worker relatedness) between the years. This is especially important when working with facultatively polygyne species, where annual changes in queen number can occur (Elmes & Keller, 993; Pedersen & Boomsma, 999). We obtained confidence intervals for the turnover estimate by jackknifing over colonies. A signal indicating queen turnover will be obtained both when new queens are adopted while resident queens remain reproductive, and when reproductive queens are replaced between years. These two processes can be separated by analysing the change in offspring relatedness over time. In the first case, relatedness between offspring decreases across years, whereas in the second case, relatedness remains unchanged, or fluctuates without a trend. We tested for a decline in relatedness across years using a repeated measures ANOVA with the relatedness among workers across years as the repeated variable. Finally, we estimated colony survival and persistence of social structure of the two types of colony as the proportion of colonies present in 997 that were still in existence in 999. Results Social forms and population structure Of the 56 colonies sampled for workers in 997, 6 (29%) were monogyne with a singly mated queen and 38 (68%) were polygyne (Fig. 2). Two colonies (%) were consistent with monogyny allowing for double or triple mating of the queen, and were classified as such based on the observation that queens do mate multiply (Hannonen et al., 2004; this study). Nest mate workers Number of colonies Relatedness Polygyne Monogyne Fig. 2 The distribution of relatedness estimates in 56 colonies sampled for workers in 997. Black bars indicate colonies which by inspection of genotypes were classified as polygyne. Shaded bars indicate colonies classified as monogyne. were related (0.4 ± 0.046), more closely in monogyne than in polygyne colonies (0.66 ± and 0.26 ± respectively, Fig. 2). The estimated mating frequency for queens heading monogyne colonies was., assuming that the male mates of the queens are unrelated (Ross, 200). From the sample of 48 queens from 3 polygyne colonies collected in 2000, we estimated queen relatedness as 0.27 (±0.04). Sixteen per cent of these queens were doubly mated, yielding an estimated mating frequency of.7 assuming that the male mates of individual queens are unrelated (Ross, 200). The male mates of different queens from the same colony, i.e. all colony fathers, were unrelated (r ¼ 0.00 ± 0.). The estimated frequencies of multiple mating did not differ between monogyne and polygyne colonies (v 2 test, n ¼ 66, Fisher s exact P ¼ 0.45), and both correspond well to previous ones obtained for the species (Hannonen et al., 2004). These estimates were used to correct the estimate of effective queen number as reported below. No evidence of inbreeding was found when the social forms were analysed together (F individuals-total ¼ 0.023, 95% CI )0.06 to 0.070). When social forms were analysed separately, workers in monogynous colonies were slightly inbred (F individuals-total ¼ 0.068, 95% CI ) but no evidence of inbreeding was found for polygynous colonies (F individuals-total ¼ )0.0, 95% CI )0.072 to 0.06). We found a weak, but significant structuring among the putative demes (F demes - total ¼ 0.037, 95% CI ). All but two demes comprised both monogyne and polygyne colonies, and monogyne and polygyne colonies were not significantly unevenly distributed across demes (Binomial test for each deme, Fisher s combined P-value ¼ 0.09). Furthermore, we found no genetic differentiation between monogyne and polygyne colonies (F social form- total ¼ 0.020, 95% CI ) ). The most likely grouping of colonies obtained by the Bayesian analysis comprised 2 clusters (mean cluster size 2.4 colonies, SD ¼ 2.34), 3 of which encompassed a single colony. The clustering generated by BAPS did not follow the putative deme boundaries, as only one deme comprising two colonies corresponded to a genetic cluster (Fig. ). Hence, no clear geographical pattern of genetic differentiation could be detected. Finally, the social forms were evenly distributed among clusters (Binomial test separately for each cluster, Fisher s combined P-value ¼ 0.97). Effective queen number, queen turnover and colony survival Of the 7 colonies where we genotyped both worker pupae and adult workers, five were unambiguously assigned as monogyne and 2 as polygyne. In all but two cases, the social form was maintained across the four study years, so that monogyne colonies remained monogyne, and polygyne colonies remained polygyne. In two ª 2007 THE AUTHORS 20 (2007)

6 356 K. BARGUM ET AL. cases the genotypes in an originally polygyne colony were consistent with both polygyny and monogyny with multiple mating (two and three times respectively) in one of the successive cohorts. However, none of the monogyne colonies became polygyne. The mean relatedness among workers in this subset of 7 colonies was 0.63 (±0.0) in monogyne and 0.27 (±0.0) in polygyne colonies. The mean effective number of queens across all years was.2 in monogyne and 3.09 in polygyne colonies assuming that the male mates of individual queens are unrelated, in good agreement with previous studies (Hannonen et al., 2004). In the polygyne colonies, the effective number of queens increased slightly across the study period from 2.73 to 3.2, but this increase was not statistically significant (repeatedmeasures ANOVA: F 3,5 ¼ 2.37, P ¼ 0.9). In monogyne colonies worker genotypes remained unchanged across years and queen turnover was not statistically significant (difference between within- vs. between-cohort relatedness analysed by repeated-measures ANOVA, F,3 ¼ 0.09, P ¼ 0.78, Table 2). Thus, the same queen was present and reproductively active in the monogyne colonies across all four study cohorts. In polygyne colonies, however, the mean queen turnover was 0.35, i.e. 35% of the queens in polygyne colonies were replaced each year, and statistically significant (repeated-measures ANOVA: F,7 ¼ 29.2, P < 0.00, Table 2). Frequent queen replacement was also evident in worker genotypes, with some alleles and/or genotypes present only in one or two cohorts. The rate of queen turnover did not differ between years, as indicated by the lack of a significant interaction between the study period and the difference between within- and between-cohort relatedness (F 2,6 ¼ 0.74, P ¼ 0.52). The estimates of queen turnover differed significantly between social forms, as evidenced by the lack of an overlap in the confidence intervals (Table 2). Ten (56%) of the monogyne colonies sampled in 997 were found again in 999, implying a yearly survival rate of 75%. Thirteen (34%) of the polygyne colonies were found again in 999, implying a yearly survival rate of 58% for polygyne colonies. Nevertheless, the two social forms did not differ significantly in colony survival (v 2 ¼ 0.9, d.f. ¼, P ¼ 0.34). Table 2 Queen turnover in colonies studied across four cohorts of workers. Monogyne Number of colonies Turnover C.I. Polygyne Number of colonies Turnover ()0.0 to 0.03) (0.34 to 0.36) ()0.04 to 0.04) (0.37 to 0.4) (0.04 to 0.06) (0.28 to 0.34) Average 0.02 (0.00 to 0.04) 0.35 (0.33 to 0.37) Discussion In this study, we compared genetic differentiation, colony life span and queen turnover in sympatric colonies of two social forms in the ant F. fusca (singleand multi-queen colonies). The two social forms were neither genetically differentiated nor significantly spatially segregated. In accordance with the predictions of the polygyny syndrome, the turnover rates of queens were significantly higher in polygyne than monogyne colonies. Indeed, none of the monogyne colonies accepted additional queens within the four years of our study. This implies that patterns of queen acceptance and/or queen mortality differ between social forms, as predicted (Nonacs, 988; Pamilo, 99a). Colony mortality did not differ significantly between the two social forms; if anything colony survival was higher in monogyne colonies. This suggests that limitations posed by queen life span (intrinsic mortality) play a negligible role in the emergence of polygyny in this species. Our result that colony survival over the study period did not differ significantly between the social forms holds two caveats. First, although colony mortality was quite high across years (25 40%) and thus provides a decent estimate of survival for the time frame of the study, differences in survival due to intrinsic mortality may become apparent only later because of the young age of the population. Secondly, we were not able to document whether some polygyne colonies had moved but remained alive. If polygyne nests move more easily than monogyne ones this would mask a difference in colony life span. If this is indeed the case, colony survival in monogyne and polygyne colonies converges, but a complete reversal of the pattern appears unlikely. Detailed data on the spatial structure of F. fusca colonies across several years are needed to resolve this issue. The fact that the two social forms are not genetically differentiated, with no evidence for heterozygote deficiency at the population level suggests that the population originates from the same ancestral population or that gene flow maintains genetic similarity between the social forms. In agreement with this polygyne and monogyne colonies also clustered together within the population. The only exception was deme number four, which encompassed exclusively polygyne colonies, most of which also clustered together in the BAPS-analysis (cluster ). However, both monogyne and polygyne colonies found outside this geographical deme were also included in the same BAPS-cluster. As geographical and genetic structuring did not correspond, we can conclude that even if polygyne colonies sometimes seem to disperse only short distances (perhaps by budding), it is by no means their main strategy of colony founding. When analysed separately, the Fit-value was higher for the monogyne than the polygyne colonies. However, the confidence limits for the estimates overlapped considerably, which suggests that mating behaviour does not ª 2007 THE AUTHORS 20 (2007)

7 Social polymorphism in ants 357 differ significantly between the social forms. In fact, polygyny in combination with high queen turnover leads to a rapid decline in within-colony relatedness, so intranidal mating in polygyne colonies (as opposed to monogyne ones) would not necessarily involve close relatives and therefore not result in a measurable increase in homozygosity. By contrast, also limited intranidal mating would produce a measurable increase in homozygosity among the monogyne colonies. Some degree of local mating in the population at large may therefore be the case. In the polygyne colonies a third of the queens are replaced each year, an estimate that agrees with previous ones of turnover in polygyne colonies of Myrmica ants (Evans, 996; Pedersen & Boomsma, 999). The observed high queen turnover was not due to accidental sampling of new colonies, because all colonies displayed positive relatedness values across sampling occasions and turnover was not markedly higher between sampling occasions ( ) than between cohorts sampled simultaneously ( and ; Table 2). In fact, the actual rate of turnover is probably higher than 35%, because colony queens are related, and this masks some events of queen replacements. As a result the detected rate of turnover underestimates the true rate (Pedersen & Boomsma, 999). Furthermore, the observed changes in queen composition in polygyne colonies can be attributed to queen replacement, rather than the addition of queens, as the effective number of queens remained unchanged across years. The lack of queen recruitment in monogyne colonies and the high and constant rates of queen turnover observed in F. fusca stand in contrast to polygynous, perennial wasps, where replacement queens are produced and accepted only when the number of resident queens is low (Queller et al., 993, Henshaw et al., 2000, 2004; Tsuchida et al., 2000). As a result high relatedness between queens and colony members is maintained in these wasps (Queller et al., 993; Henshaw et al., 2000, 2004; Tsuchida et al., 2000), whereas relatedness can decline to near-zero values in ants (e.g. Nonacs, 988; Pamilo, 99a; Ross & Shoemaker, 997; Sundström et al., 2005). A higher rate of queen turnover in polygyne colonies may arise in two ways. First, queens may have shorter life span in polygyne colonies (Keller & Genoud, 997), owing to inherent differences, e.g. in size or condition, between queens of different social types (Keller & Ross, 995; Sundström, 995b; Rosset & Chapuisat, 2006). Indeed, we have found that queen condition declines with the number of resident queens also in F. fusca (K. Bargum, unpublished data). Such differences in condition may be a result of a life history trade-off if reproductive competition in polygyne colonies comes at cost to queen longevity. Alternatively queen fertility may fluctuate over time, so that queen turnover does not reflect queen mortality but reproductive shifts among the same set of queens across years. In F. fusca there is strong reproductive skew among queens which mainly arises from differences in queen fecundity (Hannonen et al., 2002; Hannonen & Sundström, 2002). Hence, at any given time a colony will comprise queens at their reproductive peak, and queens which are past their reproductive peak, are yet to reach it, or are between two peaks, creating a situation akin to social queuing (Kokko & Johnstone, 999; Ragsdale, 999). The combined facts that queen turnover is high in polygyne colonies, but colony longevity equals that of monogyne colonies nevertheless imply that the total reproductive life span of a queen in a polygyne colony is shorter than in a monogyne colony. The wider implication of this is that instantaneous measures of reproductive skew may not reflect the total reproductive success of individuals accurately. Given the young age of our study population the observed pattern with two social forms and an absence of genetic structuring may not represent a stable state. If the population persists for a long enough time one form may come to dominate or the two forms may continue to coexist. In the latter case the social forms may diverge genetically and socially owing to reduced gene flow between the forms (Ross & Keller, 995; Gyllenstrand et al., 2005), or gene flow mediated by males may maintain genetic homogeneity between social forms (Gyllenstrand & Seppä, 2003; Sundström et al., 2003; Seppä et al., 2004; Sundström et al., 2005). Alternatively, the transient character of populations of pioneer species such as F. fusca (Savolainen & Vepsäläinen, 988), may prevent the build-up of genetic differentiation between social forms, or prevent the eventual domination of one form altogether. Indeed, the populations of F. fusca studied to date all encompass a mixture of monogyne and polygyne colonies, and queen number does not increase with population age (Hannonen et al., 2004; Helanterä, 2004). The polygyny syndrome entails a suite of characteristics related to dispersal, mating system, life history and genetic population structure. Our results, in conjunction with previous studies (Chapuisat et al., 2004; DeHeer & Herbers, 2004; Fournier et al., 2004; Rosset & Chapuisat, 2006), imply that these characteristics may appear independently from one another, even when the social forms occur in sympatry. Our results also highlight the importance of measuring individual fitness over the lifespan of the individual (Kokko & Johnstone, 999; Ragsdale, 999; Heinze & Keller, 2000). The question is then to what extent our insights may be applicable to social insects other than ants. Although social wasps and bees have been studied extensively, and encompass many polygyne species (Michener, 974; Ross & Matthews, 99), factors related to the polygyny syndrome have not been studied comprehensively (for a few notable exceptions see Itô, 993; Queller et al., 993; Henshaw et al., 2000, 2004; Tsuchida et al., 2000; Henshaw & Crozier, 2004). For example, approximately ª 2007 THE AUTHORS 20 (2007)

8 358 K. BARGUM ET AL. 25% of polistine wasps are swarm-founding and polygyne (Jeanne, 99), and many are also iteroparous. Thus, selection regimes similar to those in ants may well prevail and should result in traits defined by the polygyny syndrome. Diversity in queen number and colony life cycles in polistine wasps provides an excellent opportunity to test the generality of this concept, and study the evolution of queen number in the light of ecological and genetic idiosyncrasies of wasps and bees versus ants (Henshaw & Crozier, 2004). Acknowledgments This paper is dedicated to the memory of Rainer Rosengren. We also wish to thank M. Hannonen and P. Seppä for their help, S. Kupiainen and M. Rehn for lab assistance, and the Helsinki Team::Antzz and Laboratory of Ecological and Evolutionary Dynamics for stimulating discussions. KB was supported by the Finnish School in Wildlife Biology, Conservation and Management and by grants from Societas pro Fauna et Flora Fennica and the Waldemar von Frenckell and Otto A. Malm foundations. HH was supported by the Evolutionary Ecology Graduate School and the Academy of Finland (2382). LS was supported by research grants from the Academy of Finland (42725 and ) and the Finnish Society for Sciences and Letters. References Boomsma, J.J. & Ratnieks, F.L.W Paternity in eusocial Hymenoptera. Phil. Trans. R. Soc. Lond. B 35: Bourke, A.F.G. & Franks, N.R Social Evolution in Ants. Princeton University Press, Princeton. Cahan, S.H., Blumstein, D.T., Sundström, L., Liebig, J. & Griffin, A Social trajectories and the evolution of social behavior. Oikos 96: Chapuisat, M Characterization of microsatellite loci in Formica lugubris B and their variability in other ant species. Mol. Ecol. 5: Chapuisat, M., Bocherens, S. & Rosset, H Variable queen number in ant colonies: no impact on queen turnover, inbreeding, and population genetic differentiation in the ant Formica selysi. Evolution 58: Charlesworth, B. & Leon, J.A Relation of reproductive effort to age. Am. Nat. 0: Chesser, R.K. 99. Influence of gene flow and breeding tactics on gene diversity within populations. Genetics 29: Chesser, R.K., Rhodes, O.E., Sugg, D.W. & Schnabel, A Effective sizes for subdivided populations. Genetics 35: Collingwood, C The Formicidae (Hymenoptera) of Fennoscandia and Denmark. Fauna Entomol. Scand. Vol. 8. Scandinavian Science Press Ltd, Klampenborg, Denmark. Corander, J., Waldmann, P. & Sillanpää, M.J Bayesian analysis of genetic differentiation between populations. Genetics 63: Crespi, B.J Comparative analysis of the origins and losses of eusociality: causal mosaics and historical uniqueness. In: Phylogenies and the Comparative Method in Animal Behaviour (E. Martins, ed.), pp Oxford University Press, Oxford. Crozier, R.H. & Pamilo, P Evolution of Social Insect Colonies. Oxford University Press, Oxford. Czechowski, W., Radchenko, A. & Czechowska, W Ants of Poland. Warszawa, Warsaw. DeHeer, C.J. & Herbers, J.M Population genetics of the socially polymorphic ant Formica podzolica. Insectes Soc. 5: Elmes, G Observations on the density of queens in natural colonies of Myrmica rubra (Hymenoptera: Formicidae). J. Anim. Ecol. 42: Elmes, G. & Keller, L Distribution and ecology of queen number in ants of the genus Myrmica. In: Queen Number and Sociality in Insects (L. Keller, ed.), pp Oxford University Press, Oxford. Emlen, S.T The evolution of helping. I. An ecological constraints model. Am. Nat. 9: Emlen, S.T. 99. Evolution of cooperative breeding in birds and mammals. In: Behavioral Ecology, 2nd edn (J. R. Krebs & N. B. Davies, eds), pp Blackwell Publications, Oxford. Evans, J.D Queen longevity, queen adoption, and posthumous indirect fitness in the facultatively polygyne ant Myrmica tahoensis. Behav. Ecol. Sociobiol. 39: Fournier, D., Aron, S. & Keller, L Significant reproductive skew in the facultatively polygyne ant Pheidole pallidula. Mol. Ecol. 3: Gyllenstrand, N. & Seppä, P Conservation genetics of the wood ant, Formica lugubris, in a fragmented landscape. Mol. Ecol. 2: Gyllenstrand, N., Gertsch, P.J. & Pamilo, P Polymorphic microsatellite DNA markers in the ant Formica exsecta. Mol. Ecol. Notes 2: Gyllenstrand, N., Seppä, P. & Pamilo, P Restricted gene flow between two social forms in the ant Formica truncorum. J. Evol. Biol. 8: Hamilton, W.D The genetical evolution of social behaviour, I, II. J. Theor. Biol. 7: 52. Hannonen, M. & Sundström, L Proximate determinants of reproductive skew in polygyne colonies of the ant Formica fusca. Ethology 08: Hannonen, M., Sledge, M.F., Turillazzi, S. & Sundström, L Queen reproduction, chemical signalling and worker behaviour in polygyne colonies of the ant Formica fusca. Anim. Behav. 64: Hannonen, M., Helanterä, H. & Sundström, L Habitat age, breeding system and kinship in the ant Formica fusca. Mol. Ecol. 3: Hatchwell, B.J. & Komdeur, J Ecological constraints, life history traits and the evolution of cooperative breeding. Anim. Behav. 59: Heinze, J. & Keller, L Alternative reproductive strategies: a queen perspective in ants. Trends Ecol. Evol. 5: Heinze, J., Trunzer, B., Hölldobler, B. & Delabie, J.H.C Reproductive skew and queen relatedness in an ant with primary polygyny. Insectes Soc. 48: Helanterä, H Kinship and Conflicts over Male Production in Formica Ants. PhD thesis, University of Helsinki, Helsinki. Henshaw, M.T. & Crozier, R.H Mating system and population structure of the primitively eusocial wasp Ropalidia ª 2007 THE AUTHORS 20 (2007)

9 Social polymorphism in ants 359 revolutionalis: a model system for the evolution of complex societies. Mol. Ecol. 3: Henshaw, M.T., Strassmann, J.E. & Queller, D.C The independent origin of a queen number bottleneck that promotes cooperation in the African swarm-founding wasp, Polybioides tabidus. Behav. Ecol. Sociobiol. 48: Henshaw, M.T., Robson, S.K.A. & Crozier, R.H Queen number, queen cycling and queen loss: the evolution of complex multiple queen societies in the social wasp genus Ropalidia. Behav. Ecol. Sociobiol. 55: Herbers, J.M Queen-worker conflict and eusocial evolution in a polygyne ant species. Evolution 38: Hölldobler, B. & Wilson, E.O The Ants. Belknap Press, Cambridge, MA, USA. Itô, Y The evolution of polygyny in primitively eusocial polistine wasps with special reference to the genus Ropalidia. In: Queen Number and Sociality in Insects (L. Keller, ed.), pp Oxford University Press, Oxford. Jeanne, R.L. 99. The swarm-founding Polistinae. In: The Social Biology of Wasps (K. G. Ross & R. W. Matthews, eds), pp Cornell University Press, New York. Jennions, M.D. & Macdonald, D.W Cooperative breeding in mammals. Trends Ecol. Evol. 9: Johnson, R.A Colony founding by pleometrosis in the semiclaustral seed-harvester ant Pogonomyrmex californicus (Hymenoptera: Formicidae). Anim. Behav. 68: Keller, L Evolutionary implications of polygyny in the Argentine ant, Iridomyrmex humilis (Mayr) (Hymenoptera: Formicidae): an experimental study. Anim. Behav. 36: Keller, L. 993a. The assessment of reproductive success of queens in ants and other social insects. Oikos 67: Keller, L. ed. 993b. Queen Number and Sociality in Insects. Oxford University Press, New York. Keller, L Social life: the paradox of multiple-queen colonies. Trends Ecol. Evol. 0: Keller, L. & Genoud, M Extraordinary lifespans in ants: a test of evolutionary theories of ageing. Nature 389: Keller, L. & Reeve, H.K Partitioning of reproduction in animal societies. Trends Ecol Evol 9: Keller, L. & Ross, K.G Gene by environment interaction: effects of a single gene and social environment on reproductive phenotypes of Fire Ant queens. Funct. Ecol. 9: Kokko, H. & Johnstone, R.A Social queuing in animal societies: a dynamic model of reproductive skew. Proc. R. Soc. Lond. B 266: Kokko, H., Johnstone, R.A. & Clutton-Brock, T.H The evolution of cooperative breeding through group augmentation. Proc. R. Soc. Lond. B 268: Lewis, P.O. & Zaykin, D Genetic Data Analysis: Computer Program for the Analysis of Allelic Data, version.0 (d6c). Lubbock, J Observations on ants, bees, and wasps. Part XI. J. Linn. Soc. Lond. Zool. 20: Michener, C.D The Social behavior of the Bees. Harvard University Press, MA, USA. Nonacs, P Queen number in colonies of social Hymenoptera as a kin-selected adaptation. Evolution 42: Oster, G. & Wilson, E Caste and Ecology in the Social Insects. Princeton University Press, Princeton. Pamilo, P. 99a. Evolution of colony characteristics in social insects. II. Number of reproductive individuals. Am. Nat. 38: Pamilo, P. 99b. Life-span of queens of the ant Formica exsecta. Insectes Soc 38: 9. Pamilo, P. & Rosengren, R Evolution of nesting strategies in ants genetic evidence from different population types. Biol. J. Linn. Soc. 2: Pamilo, P., Gertsch, P. Thorén, P. & Seppä, P Molecular population genetics of social insects. Annu. Rev. Ecol. Syst. 28: 25. Pedersen, J.S. & Boomsma, J.J Effect of habitat saturation on the number and turnover of queens in the polygyne ant, Myrmica sulcinodis. J. Evol. Biol. 2: Queller, D.C A method for detecting kin discrimination within natural colonies of social insects. Anim. Behav. 47: Queller, D.C. & Goodnight, K.F Estimation of genetic relatedness using allozyme data. Evolution 43: Queller, D.C., Strassmann, J.E., Solis, C.R., Hughes, C.R. & Deloach, D.M A selfish strategy of social insect workers that promotes social cohesion. Nature 365: Ragsdale, J.E Reproductive skew theory extended: The effect of resource inheritance on social organization. Evol. Ecol. Res. : Rosengren, R. & Pamilo, P The evolution of polygyny and polydomy in mound-building Formica ants. Acta Entomologica Fennica 42: Rosengren, R., Sundström, L. & Fortelius, W. 993: Monogyny and polygyny in Formica ants: the result of alternative dispersal tactics. In: Queen Number and Sociality in Insects (L. Keller, ed.), pp Oxford University Press, Oxford. Ross, K.G The breeding system of the fire ant Solenopsisinvicta effects on colony genetic-structure. Am. Nat. 4: Ross, K.G Molecular ecology of social behaviour: analyses of breeding systems and genetic structure. Mol. Ecol. 0: Ross, K.G. & Keller, L Ecology and evolution of social organization: insights from fire ants and other highly eusocial insects. Annu. Rev. Ecol. Syst. 26: Ross, K.G. & Matthews, R.W. (ed). 99. The Social Biology of Wasps. Cornell University Press, New York. Ross, K.G. & Shoemaker, D.D Nuclear and mitochondrial genetic structure in two social forms of the fire ant Solenopsis invicta: insights into transitions to an alternate social organization. Heredity 78: Rosset, H. & Chapuisat, M Alternative life-histories in a socially polymorphic ant. Evolutionary Ecology, doi: 0.007/ s Savolainen, R. & Vepsäläinen, K A competition hierarchy among boreal ants: impact on resource partitioning and community structure. Oikos 5: Seger, J Opportunities and pitfalls in co-operative reproduction. In: Queen Number and Sociality in Insects (L. Keller, ed.), pp. 6. Oxford University Press, Oxford. Seppä, P Sociogenetic organization of the ants Myrmica ruginodis and Myrmica lobicornis number, relatedness and longevity of reproducing individuals. J. Evol. Biol. 7: Seppä, P., Gyllenstrand, N., Corander, J. & Pamilo, P Coexistence of the social types: genetic population structure in the ant Formica exsecta. Evolution 58: Sherman, P.W., Lacey, E.A., Reeve, H.K. & Keller, L The eusociality continuum. Behav. Ecol. 6: ª 2007 THE AUTHORS 20 (2007)

10 360 K. BARGUM ET AL. Shreeves, G. & Field, J Group size and direct fitness in social queues. Am. Nat. 59: Sugg, D.W., Chesser, R.K., Dobson, F.S. & Hoogland, J.L Population genetics meets behavioral ecology. Trends Ecol. Evol. : Sundström, L. 995a. Sex allocation and colony maintenance in monogyne and polygyne colonies of Formica truncorum (Hymenoptera; Formicidae): the impact of kinship and mating structure. Am. Nat. 46: Sundström, L. 995b. Dispersal polymorphism and physiological condition of males and females in the ant, Formica truncorum. Behav. Ecol. 6: Sundström, L., Keller, L. & Chapuisat, M Inbreeding and sex-biased gene flow in the ant Formica exsecta. Evolution 57: Sundström, L., Seppä, P. & Pamilo, P Genetic population structure and dispersal patterns in Formica ants a review. Ann. Zool. Fenn. 42: Tsuchida, K., Ito, Y., Katada, S. & Kojima, J Genetical and morphological colony structure of the Australian swarmfounding polistine wasp, Ropalidia romandi (Hymenoptera, Vespidae). Insectes Soc. 47: 3 6. Walin, L., Seppä, P. & Sundström, L Reproductive allocation within a polygyne, polydomous colony of the ant Myrmica rubra. Ecol. Ent. 26: West, S.A., Griffin, A.S. & Gardner, A Social semantics: altruism, cooperation, mutualism, strong reciprocity and group selection. J. Evol. Biol. 20: Williams, G.C Natural selection costs of reproduction and a refinement of Lack s principle. Am. Nat. 00: Wilson, E.O. 97. The Insect Societies. Belknap Press, Cambridge, MA, USA. Received 8 February 2007; accepted 20 February 2007 ª 2007 THE AUTHORS 20 (2007)