Shift in frequency-dependent selection across the life-cycle in obligately interbreeding harvester ant lineages
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The long-term persistence of hybridogenetic systems, in which one genetic lineage is dependent on another as a sperm host, is paradoxical because they require continual coexistence between ecological competitors. Several species of ants display a social form of hybridogenesis, in which two distinct genetic lineages obligately interbreed to produce sterile workers, while intra-lineage progeny become the reproductives. A recent model suggests that persistence of such a system depends on the relative strengths of negative frequency-dependent selection acting during colony growth and positive frequency-dependent selection during reproduction. We used path analysis to investigate the effect of lineage frequency on reproductive output and colony sex ratio over a single reproductive season in a natural population of the H1/H2 lineage pair in the genus Pogonomyrmex. Results suggest that lineage frequency does impact reproduction via two opposing routes: the more common lineage procures more same-lineage mates, resulting in a higher proportion of same-lineage mates increases reproduction through higher queen-egg availability, but the rarer lineage procures more alternate-lineage mates, yielding a more genetically diverse worker caste that increases productivity. As a result, lineage frequency has only a weakly positive overall effect on colony reproduction, and overall frequency-dependence in this system across the entire life cycle is likely to be negative as predicted by the persistence model.
KeywordsGenetic caste determination Hybridogenesis Frequency-dependent selection Pogonomyrmex Reproduction Sex ratio
We are grateful to the 2003 Earthwatch Institute Student Challenge Awards Program team of high school students for their hard work and diligence in constructing traps and conducting alate collections. J. Gadau was gracious enough to share his unpublished microsatellite primers with us, for which we are very appreciative. A number of volunteers from the research volunteer program at the Southwestern Research Station also provided much-needed field assistance. C. Bernasconi conducted the majority of the genotyping for this project. B. Cole provided constructive comments on an earlier version of the manuscript. This work was partially funded by a generous research grant from the Earthwatch Institute.
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