Evolutionary ecology and natural history of fungus-growing ants : host-switching, divergence, and asexuality
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Host-switch associated divergence is an important generator of diversity among insects. Here, I investigate whether host-switching plays a role in fungus-growing ant divergence. There are over 210 known species of ants that cultivate fungus as their primary food source. This diversity of ants and their fungal cultivars offers a rich comparative system to investigate the complexities of host-switch associated divergence. This work complements previous studies utilizing the fungus-growing ant system to investigate coevolution, speciation, conflict and cooperation. In chapter one, I introduce the system and discuss features that make it ideal for studying evolutionary and ecological aspects of host-switch associated divergence. In chapter two, I examine whether switching to a new cultivar crop could trigger speciation events in two Central and South American species complexes of fungus-growing ants. Using behavior experiments and molecular phylogenetics, I investigate whether cultivar switches are associated with ant genetic divergence. It appears that, while some fungusgrowing ants specialize on a narrow group of fungal cultivars and do not switch, other fungus-growing ant species exchange fungal cultivars more frequently. Varying degrees of host-fidelity will have different consequences for coevolutionary dynamics in this symbiosis. In the course of investigating fungus-growing ant host-associated divergence, I discovered new facets of the system that are the subjects of chapters three and four. In chapter three, I investigate whether Mycocepurus smithii is the first completely asexual ant. Female mating anatomy studies, field and laboratory surveys, and DNA fingerprinting not only support complete asexuality in this widespread ant species, but also rule out bacteria and the fungal cultivars as causative agents inducing asexuality. In chapter four, I reconstruct the phylogenetic placement of a new species of fungusgrowing ant in the Cyphomyrmex longiscapus species group and discuss their unique nesting biology. In chapter five I detail the first population study of nest architecture and sex ratios in Mycetosoritis hartmanni, one of the few North American fungus-growing ants. As these studies attest, after over one hundred years of research on the fungusgrowing ant symbiosis, novel aspects of the system continue to emerge, providing a rich resource to test coevolutionary hypotheses within the context of this complex and ancient mutualism.