Final Activity Report Summary - LEAFCUTTER IMMUNITY (Disease resistance, caste, and genetic diversity in leaf-cutting ants)
Advanced social organisation has evolved in ants, bees, wasps and termites, which all live in colonies characterised by kinship-based reproductive division of labour. Leaf-cutting ants are particularly advanced because they rear fungi for food; this obligate 'agricultural' mutualism allowed these ants to develop colonies of up to 5 million workers. These ant societies became so well organised that the major threats no longer come from predators or periods of food shortage but from continuously present and rapidly evolving diseases.
To survive for the last circa 50 million years leafcutters developed effective social defences against infections. These include:
1. unique actinomycete bacteria that the ants grow on their bodies to produce antibiotics targeted at a chronic disease of their gardens;
2. glandular secretions with general antimicrobial properties; and
3. sophisticated preventive allo- and self-grooming behaviours.
However, the objective of this proposal was to develop repeatable immune assays of the much older individual innate immune defences that ants shared with all non-social insects. The immune assays I adapted included:
1. spectrophotometric measurement of a key immune enzyme, namely phenoloxidase, and its precursor, prophenoloxidase;
2. an antibacterial activity assay; and
3. a blood cell count.
I also used the encapsulation response assay which had been previously used for leaf-cutting ants.
These assays allowed me to answer questions about immunity in an ecological context. For example, given that an organism was likely to have limited resources at its disposal, how would it decide to allocate these between the concurrent needs of immune defence and other life-history traits? I used the encapsulation response and haemocyte count assays to examine whether there was a trade-off between the immune system and reproductive traits in atta colombica. The queens of this species can live for up to a couple of decades and they must store all the sperm that they will need for the rest of their lives in one short reproductive bout, lasting one night, when they mate with several males, and then store on average 240 million sperm. After this they dig their own nest in parasite and pathogen rich soil, and have to rely on their own body resources for the next couple of months, an energetically costly time of their lives, until they raise their own workers.
Using microsatellite analysis on their stored sperm to determine the number of males they mated with and counting the number of stored sperm I examined the possible costs of these two reproductive traits in relation to immunity. Nine days after the queen mated, there was a trade-off whereby the more sperm she stored, the lower was her encapsulation response; thus, at least one component of her immune response was compromised after mating, which could be risky for her given the parasite rich environment in which she was living.
My yet unpublished results included an experiment on whether group living affected innate immunity and whether immune senescence occurred and effects of caste on immune defence. Furthermore, during field work I discovered a specialised entomopathogenic fungal parasite, in two leaf-cutting ant species, which enabled me and Dr David Hughes to examine the molecular ecology of the fungus and the possibility for experimenting with artificial host-shifts in the laboratory. I also examined potential behavioural defences whereby the ants actively covered their brood with their mutualistic fungal and I accumulated a large data set of colonies from 19 fungus growing ant species. The behaviour occurred in most species, but interestingly not in the evolutionarily most derived genera, atta, suggesting that this behaviour was secondarily lost. I also had collaborative work in progress examining the degree to which innate immune variation was accounted for by genetic and environmental effects.
To survive for the last circa 50 million years leafcutters developed effective social defences against infections. These include:
1. unique actinomycete bacteria that the ants grow on their bodies to produce antibiotics targeted at a chronic disease of their gardens;
2. glandular secretions with general antimicrobial properties; and
3. sophisticated preventive allo- and self-grooming behaviours.
However, the objective of this proposal was to develop repeatable immune assays of the much older individual innate immune defences that ants shared with all non-social insects. The immune assays I adapted included:
1. spectrophotometric measurement of a key immune enzyme, namely phenoloxidase, and its precursor, prophenoloxidase;
2. an antibacterial activity assay; and
3. a blood cell count.
I also used the encapsulation response assay which had been previously used for leaf-cutting ants.
These assays allowed me to answer questions about immunity in an ecological context. For example, given that an organism was likely to have limited resources at its disposal, how would it decide to allocate these between the concurrent needs of immune defence and other life-history traits? I used the encapsulation response and haemocyte count assays to examine whether there was a trade-off between the immune system and reproductive traits in atta colombica. The queens of this species can live for up to a couple of decades and they must store all the sperm that they will need for the rest of their lives in one short reproductive bout, lasting one night, when they mate with several males, and then store on average 240 million sperm. After this they dig their own nest in parasite and pathogen rich soil, and have to rely on their own body resources for the next couple of months, an energetically costly time of their lives, until they raise their own workers.
Using microsatellite analysis on their stored sperm to determine the number of males they mated with and counting the number of stored sperm I examined the possible costs of these two reproductive traits in relation to immunity. Nine days after the queen mated, there was a trade-off whereby the more sperm she stored, the lower was her encapsulation response; thus, at least one component of her immune response was compromised after mating, which could be risky for her given the parasite rich environment in which she was living.
My yet unpublished results included an experiment on whether group living affected innate immunity and whether immune senescence occurred and effects of caste on immune defence. Furthermore, during field work I discovered a specialised entomopathogenic fungal parasite, in two leaf-cutting ant species, which enabled me and Dr David Hughes to examine the molecular ecology of the fungus and the possibility for experimenting with artificial host-shifts in the laboratory. I also examined potential behavioural defences whereby the ants actively covered their brood with their mutualistic fungal and I accumulated a large data set of colonies from 19 fungus growing ant species. The behaviour occurred in most species, but interestingly not in the evolutionarily most derived genera, atta, suggesting that this behaviour was secondarily lost. I also had collaborative work in progress examining the degree to which innate immune variation was accounted for by genetic and environmental effects.