Shift in frequency-dependent selection across the life-cycle in obligately interbreeding harvester ant lineages

Inter-genomic sexual conflict drives antagonistic coevolution in harvester ants

The reproductive interests of males and females are not always aligned, leading to sexual conflict over parental investment, rate of reproduction and mate choice. Traits that increase the genetic interests of one sex often occur at the expense of the other, selecting for counter-adaptations leading to antagonistic coevolution. Reproductive conflict is not limited to intraspecific interactions; interspecific hybridization can produce pronounced sexual conflict between males and females of different species, but it is unclear whether such conflict can drive sexually antagonistic coevolution between reproductively isolated genomes. We tested for hybridization-driven sexually antagonistic adaptations in queens and males of the socially hybridogenetic 'J' lineages of Pogonomyrmex harvester ants, whose mating system promotes hybridization in queens but selects against it in males. We conducted no-choice mating assays to compare patterns of mating behaviour and sperm transfer between inter- and intra-lineage pairings. There was no evidence for mate discrimination on the basis of pair type, and the total quantity of sperm transferred did not differ between intra- and inter-lineage pairs; however, further dissection of the sperm transfer process into distinct mechanistic components revealed significant, and opposing, cryptic manipulation of copulatory investment by both sexes. Males of both lineages increased their rate of sperm transfer to high-fitness intra-lineage mates, with a stronger response in the rarer lineage for whom mating mistakes are the most likely. By contrast, the total duration of copulation for intra-lineage mating pairs was significantly shorter than for inter-lineage crosses, suggesting that queens respond to prevent excessive sperm loading by prematurely terminating copulation. These findings demonstrate that sexual conflict can lead to antagonistic coevolution in both intra-genomic and inter-genomic contexts. Indeed, the resolution of sexual conflict may be a key determinant of the long-term evolutionary potential of host-dependent reproductive strategies, counteracting the inherent instabilities arising from such systems.

Does an ecological advantage produce the asymmetric lineage ratio in a harvester ant population?

In dependent-lineage harvester ant populations, two lineages interbreed but are genetically distinct. The offspring of a male and queen of the same lineage are female reproductives; the offspring of a male and queen of different lineages are workers. Geographic surveys have shown asymmetries in the ratio of the two lineages in many harvester ant populations, which may be maintained by an ecological advantage to one of the lineages. Using census data from a long-term study of a dependent-lineage population of the red harvester ant, Pogonomyrmex barbatus, we identified the lineage of 130 colonies sampled in 1997–1999, ranging in age from 1 to 19 years when collected, and 268 colonies sampled in 2010, ranging in age from 1 to 28 years when collected. The ratio of lineages in the study population is similar across an 11-year interval, 0.59 J2 in 1999 and 0.66 J2 in 2010. The rare lineage, J1, had a slightly but significantly higher number of mates of the opposite lineage than the common lineage, J2, and, using data from previous work on reproductive output, higher male production. Mature colonies of the two lineages did not differ in nest mound size, foraging activity, or the propensity to relocate their nests. There were no strong differences in the relative recruitment or survivorship of the two lineages. Our results show no ecological advantage for either lineage, indicating that differences between the lineages in sex ratio allocation may be sufficient to maintain the current asymmetry of the lineage ratio in this population.

Intergenerational effect of juvenile hormone on offspring in Pogonomyrmex harvester ants

Parents can influence the phenotypes of their offspring via a number of mechanisms. In harvester ants, whether female progeny develop into workers or daughter queens is strongly influenced by the age and temperature conditions experienced by their mother, which is associated with variation in maternal ecdysteroid deposition in fertilized eggs. In many insects, juvenile hormone (JH) is antagonistic to ecdysteroid release, suggesting that seasonal and age-based variation in maternal JH titers may explain maternal effects on offspring size and reproductive caste. To test this hypothesis, we artificially increased maternal JH titers with methoprene, a JH analog, in laboratory colonies of two Pogonomyrmex populations exhibiting genetic caste determination. Increasing maternal JH resulted in a 50% increase in worker body size, as well as a sharp reduction in total number of progeny reared, but did not alter the genotype of progeny reared to adulthood. The intergenerational effect of JH manipulation was not mediated by a reduction in ecdysteroid deposition into eggs; instead, changes in egg size, trophic egg availability or brood/worker ratio may have altered the nutritional environment of developing larvae. Egg ecdysteroid content was significantly negatively correlated with natural variation in worker body size, however, suggesting that there are multiple independent routes by which queens can modify offspring phenotypes.

Dynamics of an ant-ant obligate mutualism: colony growth, density dependence and frequency dependence

In insect societies, worker vs. queen development (reproductive caste) is typically governed by environmental factors, but many Pogonomyrmex seed-harvester ants exhibit strict genetic caste determination, resulting in an obligate mutualism between two reproductively isolated lineages. Same-lineage matings produce fertile queens while alternate-lineage matings produce sterile workers. Because new virgin queens mate randomly with multiple males of each lineage type, and both worker and queen phenotypes are required for colony growth and future reproduction, fitness is influenced by the relative frequency of each lineage involved in the mutualistic breeding system. While models based solely on frequency-dependent selection predict the convergence of lineage frequencies towards equal (0.5/0.5), we surveyed the lineage ratios of 49 systems across the range of the mutualism and found that the global lineage frequency differed significantly from equal. Multiple regression analysis of our system survey data revealed that the density and relative frequency of one lineage decreases at lower elevations, while the frequency of the alternate lineage increases with total colony density. While the production of the first worker cohort is largely frequency dependent, relying on the random acquisition of worker-biased sperm stores, subsequent colony growth is independent of lineage frequency. We provide a simulation model showing that a net ecological advantage held by one lineage can lead to the maintenance of stable but asymmetric lineage frequencies. Collectively, these findings suggest that a combination of frequency-dependent and frequency-independent mechanisms can generate many different localized and independently evolving system equilibria.