Sze Huei Zoe:
Disease challenges and defences in leaf-cutting ants

Date: 15-09-2012    Supervisor: Jacobus J. Boomsma

The life history of social insects, often characterized by high density of related individuals, increases their susceptibility to parasitism. This thesis focuses on the Acromyrmex leaf-cutting ant, a social insect species that also engages in an obligate mutualism with fungal crops. The ants provide substrate and protections to their fungal crops and in return receive food and nutrition from the fungus. Leaf-cutting ants not only require efficient defence strategies to protect themselves, but also need defence mechanisms that keep their fungal crops free from parasites. This PhD thesis explores the complementary layers of defence, covering (1) innate immunity, (2) antimicrobial glandular secretions, (3) overview of specialized parasites, and (4) antibiotic-producing bacteria of the leafcutting ants.

Innate immunity is conserved across insects and vertebrates. Heightened parasite pressures from social living and fungiculture are expected to leave distinct signatures in the innate immune system. These signatures can be studied with differential gene expression analysis. Chapter 1 uses a genome-wide analysis of differential immune-related gene expressions, demonstrating for the first time how innate immunity responds differentially to fungal infections on ants, and to parasitism on the ants’ fungus garden. Fungal infections on ants induce up-regulations of all components of immune pathways (Toll and Imd), whereas parasitism on fungus garden did not, implying that ants utilize different genetic mechanisms for self protection and defences against parasitism on fungus garden.

One defence mechanisms unique to ants is the metapleural gland that produces antimicrobial secretions. This paired gland was a key innovation that allowed ants to diversify while nesting in microbe-ridden terrestrial environments. The leaf-cutting ants have well-developed metapleural glands, and the secretions are hypothesized to maintain ants’ hygiene. Previous studies on Atta leafcutting ants show that metapleural glands are not only important in ants’ protection but also important in protecting the fungus garden against parasitism. Chapter 2 demonstrates the ability of Acromyrmex leaf-cutting ants to regulate the glandular secretions in a context dependent manner, and that their glandular secretions are predominantly for ants’ protection, not fungus garden.

The generational propagation (i.e. vertical transmission) of ants’ fungus garden increases the fungal crops’ susceptibility to parasitism. Escovopsis, a specialized parasite of leaf-cutting ants have interacted and partly co-evolved with ant-fungus mutualism for ca. 50 million years. Chapter 3 reviews recent progress in understanding the patterns of specificity of this ant fungus gardenparasite association, using a modified version of Tinbergen’s four categories of evolutionary questions. The review identifies prospective areas for future studies, emphasizing use of next generation sequencing (NGS) technologies and genomic tools to answer some of the unresolved questions in ant fungus garden-parasite dynamics.

Leaf-cutting ants recruit antibiotic-producing bacteria into their medley of defences, to the extent of having specialized morphological adaptations on their cuticle to house these antibiotic-producing bacteria. The diversity of the ant hosts/bacteria associations on the ants has been subject of controversy, with one argument emphasizing long-term co-evolutions of ant hosts/bacteria associations, and another argument emphasizing dynamic recruitments of antibiotic-producing bacteria from surrounding environments. Chapter 4 used a cross-fostering approach to examine ant hosts/bacteria partner fidelity and their manifestations in prophylactic defences against Escovopsis parasitism. The study revealed no ant host/bacteria genotype-genotype differences in bacteria colonization on the ants, but showed differences in prophylactic defence action, implying that although non-native bacteria are able to colonize ants’ cuticle, native bacteria were more efficient in defences against Escovopsis parasitism.

The constant parasitism pressure from social living and fungiculture have shaped diverse and complementary defences in leaf-cutting ants. From genes to phenotypes, the four chapters of the thesis highlight several components of these defence strategies unique to leaf-cutting ants.