Queen Dispersal Research Articles

GENETIC ANALYSIS OF DISPERSAL DYNAMICS IN AN INVADING POPULATION OF ARGENTINE ANTS

The ecology and dispersal dynamics of unicolonial ant invaders are poorly understood. In this study, we investigated the genetic structure of a well-documented, invading population of Argentine ants in the Jasper Ridge Biological Preserve in northern California to examine the dispersal distances of reproductives, the direction and mode of population expansion, and changes in the genetic differentiation among nests over time. Using microsatellite data, we measure both traditional FST statistics and multi-locus ge- notype assignment distances to determine the patterns of genetic structure at three spatial scales: population-wide gene flow, population substructure, and mixing between neigh- boring nests. At the population level, there was little viscosity across the Jasper Ridge population, suggesting recent rapid expansion and/or considerable long-distance gene flow, presumably mediated by winged males. The pattern of genetic structure across distance indicates that the scale of queen dispersal was limited to less than 100 m. At the level of population substructure, hierarchical F statistics measures were low across subpopulations, locations within subpopulations, and nests. However, multi-locus genotype assignment tests revealed significant structure between subpopulations and between locations. Genetic dis- tances between nests were lower within locations than between locations, indicating that nests are most closely related to neighboring nests and that the expansion of subpopulations is primarily due to the local budding of new nests from existing nests at the invasion front. At the level of nest connectivity, the genetic differentiation among neighboring nests was associated with the time since invasion. As the invasion proceeds, nearby nests tend to be less closely related, indicating that extensive local mixing does not occur among more established nests. Our results show that the genetic structure of nests was not homogenous across unicolonial populations. Instead, the patterns of genetic structure reflect the limi- tations of and barriers to the dispersal of Argentine ant reproductives, the demographic history of the Jasper Ridge invasion, and changes in the genetic and ecological environment during the course of the invasion.

Restricted effective queen dispersal at a microgeographic scale in polygynous populations of the ant Formica exsecta.

Ecological constraints on effective dispersal have been suggested to be a key factor influencing social evolution in animal societies as well as the shift from single queen colonies (monogyny) to multiple queen colonies (polygyny) in ants. However, little is known about the effective dispersal patterns of ant queens. Here we investigate the microgeographic genetic structure of mitochondrial haplotypes in polygynous populations of the ant Formica exsecta, both between pastures and among nests within pastures. An analysis of molecular variance revealed a very high genetic differentiation (phiST = 0.72) between pastures, indicating that queens rarely disperse successfully between pastures, despite the fact that pastures were sometimes as close as 1 km. Most of the pastures contained only a single haplotype, and haplotypes were frequently distinct between nearby pastures and even between groups of nests within the same pasture. In the three pastures that contained several haplotypes, haplotypes were not randomly distributed, the genetic differentiation between nests being phiST = 0.17, 0.52, and 0.69. This indicates that most queens are recruited within their parental colonies. However, a large proportion of nests contained more than one haplotype, demonstrating that colonies will sometimes accept foreign queens. The relatedness of mitochondrial genes among nestmates varied between 0.62 and 0.75 when relatedness was measured within each pasture and ranged between 0.72 and 1.0 when relatedness was assessed with all pastures as a reference population. Neighboring nests were more genetically similar than distant ones, and there was significant isolation by distance. This pattern may be due to new nests being formed by budding or by limited effective queen dispersal, probably on foot between neighboring nests. These results show that effective queen dispersal is extremely restricted even at a small geographical scale, a pattern consistent with the idea that ecological constraints are an important selective force leading to the evolution and maintenance of polygyny.

Positive association of queen number and queen-mating frequency Myrmica ants: a challenge to the genetic-variability hypotheses

Detailed knowledge of the mating system in specific social insect populations is essential for testing general evolutionary hypotheses of multiple paternity in eusocial Hymenoptera. We have studied the mating frequency of queens in a polygynous population of the red ant Myrmica sulcinodis. Genetic mother-offspring analysis showed that double mating occurred at a considerable frequency, but that the effective number of queen-mates remained close to one. After quantifying the effects of multiple maternity (polygyny) and multiple paternity (polyandry) on the genetic diversity of workers, we conclude that multiple paternity in M. sulcinodis did not evolve as an adaptation to increase genetic variation within colonies. Contrary to the predictions from `genetic variability' hypotheses, we found a positive correlation between colony-specific queen number and the average number of mates per queen. Such positive association of queen number and frequency of multiple mating was also found after analysing comparative data across six species of Myrmica ants. These results suggest that resticted dispersal of young queens may be a common factor promoting both polygyny and polyandry at the same time, and that moderate degrees of multiple mating may be an unselected consequence of (1) mating at low cost when mating occurs close to the nest and (2) mating in swarms with a highly male biased operational sex ratio. Future comparative tests of genetic-variability hypotheses should therefore not include species with such evolutionary derived mating system characteristics.

Facultative polygyny and habitat succession in boreal ants

High dispersal risks of ant queens make staying in the natal patch more attractive than long range dispersal. These alternative strategies and the mode of colony founding determine the average number of queens in the population. Increasing competition and queen predation make independent colony founding increasingly difficult and the only option for new queens to reproduce in the habitat patch may be to enter an existing colony. The effect of nest-site availability to the number of queens was studied in successional spruce-dominated taiga forests in facultatively polygynous ants Myrmica ruginodis, M. sulcinodis, Leptothorax acervorum, Formica sanguinea and F. truncorum. Decreasing relatedness among worker nestmates supports an association between increasing habitat age and polygyny to some extent. M. sulcinodis and L. acervorum persist in this type of taiga only for a relatively short period. Relatedness varied only slightly among populations, but lower relatedness estimated in other studies suggested higher levels of polygyny in older populations. In M. ruginodis there was more variation in relatedness and it was possibly connected to the relative proportions of the two social forms of the species. In F. sanguinea and F. truncorum the decrease in relatedness with increasing age of the habitat was clearest. Other factors favouring limited dispersal and acceptance of new queens in the colonies are, however, hard to separate.

Sex Ratio Strategy as Related to Queen Number, Dispersal Behaviour and Habitat Quality in Formica Ants (Hymenoptera: Formicidae)

Sex Ratio Strategy as Related to Queen Number, Dispersal Behaviour and Habitat Quality in Formica Ants (Hymenoptera: Formicidae)

An equilibrium theory of queen production in honeybee colonies preparing to swarm

An equilibrium model is developed which seeks to explain the regulation of queen rearing in honeybee colonies preparing to swarm. The model postulates that there is a balance between nurse bees becoming inhibited from queen rearing and nurses losing their inhibition, and that whether a colony does or does not rear queens reflects the equilibrium percentage of inhibited nurses. This model leads to a quantitative prediction about the size of a conoly's nurse population at which queen rearing should start. Comparing the model's predictions with empirical observations pinpoints data needed for a more complete explanation of control of queen rearing. In particular, the model suggests a central regulatory role for density-dependent changes in the behaviors involved in queen substance dispersal.

THE BIOLOGY OF THE SOCIAL WASPS (HYMENOPTERA, VESPIDAE)

THE BIOLOGY OF THE SOCIAL WASPS (HYMENOPTERA, VESPIDAE)