Relatedness among queens in polygynous nests of the ant Leptothorax acervorum

Social life: the paradox of multiple-queen colonies

The evolution of animal societies in which some individuals forego their own reproductive opportunities to help others to reproduce poses an evolutionary paradox that can be traced to Darwin. Altruism may evolve through kin selection when the donor and recipient of altruistic acts are related to each other, as generally is the case in social birds and mammals. Similarly, social insect workers are highly related to the brood they rear when colonies are headed by a single queen. However, recent studies have shown that insect colonies frequently contain several queens, with the effect of decreasing relatedness among colony members. How can one account for the origin and maintenance of such colonies? This evolutionary enigma presents many of the same theoretical challenges as does the evolution of cooperative breeding and eusociality.

Polymorphic social organization in an ant

Identifying species exhibiting variation in social organization is an important step towards explaining the genetic and environmental factors underlying social evolution. In most studied populations of the ant Leptothorax acervorum, reproduction is shared among queens in multiple queen colonies (polygyny). By contrast, reports from other populations, but based on weaker evidence, suggest a single queen may monopolize all reproduction in multiple queen colonies (functional monogyny). Here we identify a marked polymorphism in social organization in this species, by conclusively showing that functional monogyny is exhibited in a Spanish population, showing that the social organization is stable and not purely a consequence of daughter queens overwintering, that daughter queen re-adoption is frequent and queen turnover is low. Importantly, we show that polygynous and functionally monogynous populations are not genetically distinct from one another based on mtDNA and nDNA. This suggests a recent evolutionary divergence between social phenotypes. Finally, when functionally monogynous and polygynous colonies were kept under identical laboratory conditions, social organization did not change, suggesting a genetic basis for the polymorphism. We discuss the implications of these findings to the study of reproductive skew.

The coevolutionary dynamics of obligate ant social parasite systems--between prudence and antagonism

In this synthesis we apply coevolutionary models to the interactions between socially parasitic ants and their hosts. Obligate social parasite systems are ideal models for coevolution, because the close phylogenetic relationship between these parasites and their hosts results in similar evolutionary potentials, thus making mutual adaptations in a stepwise fashion especially likely to occur. The evolutionary dynamics of host-parasite interactions are influenced by a number of parameters, for example the parasite's transmission mode and rate, the genetic structure of host and parasite populations, the antagonists' migration rates, and the degree of mutual specialisation. For the three types of obligate ant social parasites, queen-tolerant and queen-intolerant inquilines and slavemakers, several of these parameters, and thus the evolutionary trajectory, are likely to differ. Because of the fundamental differences in lifestyle between these social parasite systems, coevolution should further select for different traits in the parasites and their hosts. Queen-tolerant inquilines are true parasites that exert a low selection pressure on their host, because of their rarity and the fact that they do not conduct slave raids to replenish their labour force. Due to their high degree of specialisation and the potential for vertical transmission, coevolutionary theory would predict interactions between these workerless parasites and their hosts to become even more benign over time. Queen-intolerant inquilines that kill the host queen during colony take-over are best described as parasitoids, and their reproductive success is limited by the existing worker force of the invaded host nest. These parasites should therefore evolve strategies to best exploit this fixed resource. Slavemaking ants, by contrast, act as parasites only during colony foundation, while their frequent slave raids follow a predator prey dynamic. They often exploit a number of host species at a given site, and theory predicts that their associations are best described in terms of a highly antagonistic coevolutionary arms race.

Mating frequency and mating system of the polygynous ant, Leptothorax acervorum

Multiple mating by queens (polyandry) and the occurrence of multiple queens in the same colony (polygyny) alter patterns of relatedness within societies of eusocial insects. This is predicted to influence kin-selected conflicts over reproduction. We investigated the mating system of a facultatively polygynous UK population of the ant Leptothorax acervorum using up to six microsatellite loci. We estimated mating frequency by genotyping 79 dealate (colony) queens and the contents of their sperm receptacles and by detailed genetic analysis of 11 monogynous (single-queen) and nine polygynous colonies. Results indicated that 95% of queens were singly mated and 5% of queens were doubly mated. The corrected population mean mating frequency was 1.06. Parentage analysis of adults and brood in 17 colonies (10 monogynous, 7 polygynous) showed that female offspring attributable to each of 31 queens were full sisters, confirming that queens typically mate once. Inbreeding coefficients, queen-mate relatedness of zero and the low incidence of diploid males provided evidence that L. acervorum sexuals mate entirely or almost entirely at random. Males mated to queens in the same polygynous colony were not related to one another. Our data also confirmed that polygynous colonies contain queens that are related on average and that their workers had a mixed maternity. We conclude that the mating system of L. acervorum involves queens that mate near nests with unrelated males and then seek readoption by those nests, and queens that mate in mating aggregations away from nests, also with unrelated males.

The ecology of communal breeding: the case of multiple-queen leptothoracine ants

Multiple-queen societies of ants are key subjects in the study of communal breeding. Societies of leptothoracine ants may be obligately monogynous (contain a single reproductive queen), functionally monogynous (only one of several mated queens lays eggs), or facultatively polygynous (some colonies contain more than one egg-laying, mated queen). This paper presents a framework for understanding these diverse social systems as a function of leptothoracine ecology. The framework is derived from a synthesis of empirical information - in particular, a link between the social system and the degree of habitat patchiness - with three bodies of theory. These are ecological constraints theory, ESS (evolutionarily stable strategy) models of dispersal, and kin selection models predicting the stable reproductive skew (allocation of reproduction). In contrast to several previous ecological hypotheses, multiplequeening in leptothoracines almost certainly results from high costs to single queens of dispersal and colony foundation (high ecological constraints), which select for queens to seek adoption in their natal colony. Factors raising these costs include nest-site limitation, cold climate, and habitat patchiness. ESS models suggest that high dispersal costs lead to a larger stable fraction of non-dispersers and hence to higher relatedness between queens. Skew models predict that high ecological constraints and high relatedness promote high skew (one or a few individuals dominate reproduction) and high within-colony aggression. Therefore, (i) extensive habitats with moderate costs of solitary colony foundation should promote multiple-queening with high dispersal levels, moderate queen relatedness, low reproductive skew, and low queen aggression. By contrast, (ii) patchy habitats should induce multiple-queening with less dispersal, higher queen relatedness, higher skew, and higher aggression. In addition, (iii) habitats with small or widely spaced nest-sites, or with low costs of founding colonies alone, should lead to universal dispersal without multiple-queening. These associations of traits occur in facultatively polygynous, functionally monogynous and obligately monogynous leptothoracines respectively. Therefore, the framework in this paper explains a substantial amount of the social and ecological diversity of leptothoracine ants.

Parentage analyses in ant colonies using simple sequence repeat loci

In ants of genus Myrmica, female progeny may be the offspring of one to several different queens. In addition, both workers and queens are capable of producing haploid male offspring. Even in such complex colonies, parentage can be assigned on the basis of genotypic variation at highly polymorphic simple sequence repeat loci. Methods are described for isolating and screening dinucleotide repeat loci in ants. Three independent loci, Myrt2, Myrt3 and Myrt4, show expected heterozygosities of 0.94, 0.92 and 0.95, respectively. When used in parallel these loci should be sufficient to establish parentage in the vast majority of screened colonies. An initial screening indicates that males are produced by workers in the subalpine ant Myrmica 'near tahoensis'.

Intrapopulation nestclusters of maternal mtDNA lineages in the polygynous ant Leptothorax acervorum (Hymenoptera: Formicidae)

Analysis of restriction fragment length polymorphism (RFLP) in mitochondrial DNA (mtDNA) in a population of Leptothorax acervorum demonstrates substantial population substructuring. Digestion with four restriction endonucleases, HaeIII, MboI, MspI and RsaI, gave six, four, three and two different patterns, respectively. Seven composite haplotypes were obtained from the observed cleavage patterns. Uniform aggregations of nests from the same maternal lineage (i.e. with the same haplotype) were found which suggests that nest-founding by a process of budding-off is common. Groups of nests seem to actively exclude other haplotypes from establishing nests within their territory. F-statistics suggests that individuals mate at random within the population.