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The evolution of immune defences in avian brood parasites and their hosts

Final Report Summary - HONEYGUIDEIMMUNITY (The evolution of immune defences in avian brood parasites and their hosts.)

All animals are exposed to disease and infection, with the immune system providing crucial defences to stop animals becoming sick. Yet immunological variation is widespread, with the costs of immunity and differences in disease exposure being two potential causes. This project investigates the importance of antigenic pressures during development in shaping immune function, by asking how variation in disease pressure during early life influences immunity, and what the consequences are for growth, a key developmental process against which investment in the immune system may be traded off. To achieve this, as model systems the project has used avian brood parasites called honeyguides (Indicator species) that exploit other species to raise their young. This exposes parasite nestlings to diverse antigens related to the range of hosts that might raise them. Honeyguides also destroy host eggs and kill host chicks, which remain rotting in the nest, further exposing parasite nestlings to high disease pressure. These behaviours make honeyguides an ideal system in which to test whether exposure to high levels of potential infectious antigens at an early age influences immune development. At a site in Zambia we monitored host nests containing honeyguide chicks, recording growth and taking small blood samples from chicks every three days throughout development. Half of the nests were exposed to an increased immunologic challenge, in order to boost the need for strong immune defences, while the remaining half were left unchanged as controls. We used the blood samples to profile the immune responses of the chicks as they grew, and compared between experimental and control groups to determine whether exposure to an increased immunologic challenge resulted in greater investment in immunity (stronger immune responses) at a cost to growth (slower growth rate). Chicks in the experimental group did appear to grow more slowly and had reduced fledging success. Lab work to determine several aspects of innate immunity (concentrations of acute phase proteins haptoglobin and ovotransferrin, and natural antibody mediated-complement activity) has been completed, but analyses of the resultant data is still to be completed.
In addition to chicks trading off investment in growth in favour of immune investment when disease exposure is high, mothers might preferentially deposit greater concentrations of immune defensive proteins when laying their eggs, in anticipation of challenging conditions when chicks hatch. We have collected a sample of eggs from honeyguide brood parasites and their hosts and will measure concentrations of defensive antimicrobial proteins (lysozyme and ovotransferrin) in these eggs to test this hypothesis. This work has recently been boosted by additional samples to increase the range of brood parasite species available for comparison, thereby greatly increasing the strength and applicability of any results we may find.
If early life conditions do influence immune investment, are there long-term consequences? We have used another brood parasite at our field site in Zambia, the cuckoo finch Anomalospiza imberbis to address this question. Cuckoo finches parasitize a range of small warblers, with different host species of different sizes. This leads to cuckoo finches raised by different hosts likely experiencing different rearing conditions such as how often and how much they are fed. Such differences during development could influence how much parasitic chicks are able to invest in immunity as they grow, leading to potential differences in immune function among individuals raised by different hosts. We wish to determine whether such differences might still be apparent in adult birds, long after they have left the nest. Cuckoo finches are unique in that adult birds can be linked to the particular host species that raised them, using just a small blood sample for genotyping. We captured adult cuckoo finches outside of the breeding season and used genotyping techniques developed by Prof. Mike Sorenson (University of Boston, USA) to determine which host species raised the individual birds that we sampled. The remainder of the same small blood sample was used to characterise the immune responses of each bird, using the same assays described above. We also injected half of the birds we captured with a small amount of dead bacterium to stimulate their immune systems, and collected a second blood sample afterwards to enable us to compare before and after the immune challenge. When analyses are complete, this study will be one of the first to determine whether early life conditions that could affect immune investment can still be detected in adult birds.
When complete, the combined results of these projects will provide greater insight into the causes and consequences of immunological variation, thereby contributing to our understanding of human and animal health.