Sandra Breum Andersen:
Dynamics of ant-microbial interactions. Coevolution along the parasitism-mutualism continuum

Date: 15-12-2011    Supervisor: David P. Hughes & Jacobus J. Boomsma

The life history of social insects, with division of labour, cooperative brood care and overlapping generations, affects the strategies of the associated symbionts. A high density of related individuals could be an open invitation to recurrent disease epidemics, but the complementary layers of social and individual immunity efficiently protect the society. In this thesis the interaction between ants and three different microbial symbionts are dealt with, covering the spectrum from parasitism to mutualism and something in between.

Two chapters focus on the leaf‐cutting ants native to South and Central America that are (in)famous for their ability to defoliate vegetation surrounding their colonies, making them the dominant herbivores of the region. The leaf‐cutter ants are the most advanced of the fungus‐growing ants and this is what makes them capable of living on leaves: in underground chambers the ants farm fungus that degrade the substrate the ants bring it, in return letting the ants feed on the fungus. The association between the ants and the fungus are the corner stones in an intriguing multi‐trophic interaction involving also other fungi and bacteria.

One of these bacterial partners called Pseudonocardia grow on the cuticle of some leaf‐cutter ants, visible to the naked eye as a white patch on the ants’ chest, and used by the ants as antibiotic factories employed against a parasite of the fungus garden. The diversity of the bacteria on the ants has been the subject of some controversy. We found a low diversity with only one strain of bacteria dominating on the ants. By comparing ants collected in the field and in the lab from the same colonies we show that this association is highly stable, even after 10 years in the lab and exposure to many other bacteria.

Another bacterial partner is of the genus Wolbachia and these are live inside the ants’ tissues. Wolbachia are found associated with a wide range of insects and typically as a reproductive parasite, yet what they may do in the leafcutter ants is not well understood. Our work show that the ants are found in great numbers in sterile workers and surprisingly also extracellularly in the gut, suggesting a new potential role in the ant’s nutritional system.

In contrast to these likely helpful bacteria carpenter ants in tropical regions of the world are attacked by parasitic fungi of the genus Ophiocordyceps. When infected, the ants are manipulated into leaving their colony and die in ‘graveyards’, biting under leaves. The last two chapters of the thesis deals with the disease pressure of this parasite experienced by the ant colonies, which is found to be surprisingly low, and how the social structure of the host apparently has shaped the life‐strategy of the parasite into iteroparity. In addition, differences in the manipulation was found between species in Thailand and Brazil, likely reflecting variation in host behaviour and environmental parameters.

Together, the four chapters highlights different ways in which the symbionts of ants have adapted to the social structure of the host.