Ecoimmunology perspective of host-parasite interactions in Limosa limosa across its migratory flyway
Year: 2017
Type: dissertation
Abstract: For long-distance migrant birds, ecological changes along the flyway and strenuous work, can unbalance immune investments and thereby increasing the vulnerability to disease and reducing survival. Despite an essential self-maintenance component, immunity comes with costs as well as benefits. At an ecological context (i.e. limited energy), trade-offs are generated between immunity and other competing physiological components, leading to variations on immune response across time (annual-cycle) and space (different environments). Moreover, immune responses also vary between species and are optimized according to specific situations of an individual´s life, to maximize survival and fitness. Many ecoimmunology studies are focused on understanding the general patterns of immune responses in free-living birds, and what mechanisms mediate the changes in disease susceptibility, which in turn may affect population dynamics and survival. Information regarding these immunologic trade-offs remain largely unknown, and thus becoming of paramount importance when contextualized with climate change effects over migration ecology and the distribution of animal diseases. The work comprised in this thesis addressed the general question of how differences in environmental pathogen pressure shape the innate and adaptive immunity of a free-living bird species. The long-distance migrant Black-tailed godwit (Limosa limosa; hereafter godwit), was used as a study model, since the two subspecies, nominate (Limosa limosa limosa) and Icelandic (L. l. islandica), vary on migratory strategy and habitat-use, which consequently differ in pathogen pressure. Below, I summarize the main contributions of my work for the ecoimmunology research area. On chapter I, I tested whether the strength of immune response is indeed correlated with the environmental pathogen risk that differ with the migratory strategies and habitat use of godwit´s. The work focused innate baseline immunity parameters and I showed that it changes during the annual cycle of godwits in response to seasonal demands, availability of resources and physiological trade-offs. The investments in immune defence and the strategies deployed also varied in response to risk of getting infected. Trade-offs between innate immunity and other physiological components were more apparent during energetically demanding periods, such as the breeding season. For e.g. the Icelandic godwits downregulated innate responses when occupying parasite-poor breeding areas. Moreover, data indicates that immune adjustments and strategies are rather unique for each species. Nonetheless, migration seems to affect several bird species the same way, leading to an overall immunosuppression of the innate immune response and possibly to a higher vulnerability to disease. On chapter II, the MHC-I gene of the Icelandic godwit is characterized based on Sanger and ultradeep Illumina MiSeq sequencing. Characterization mostly focused the α2 domain (exon 3) of this gene, which is known to be highly polymorphic and for coding the proteins that recognize pathogens. I found 47 new alleles of MHC-I exon 3 and of a putatively classical nature (functional). Individuals had between one to four loci, with at least three being expressed. The gene organization was quite similar to their closest relative, the Red knots (Calidris canutus), but comparing to other Charadriiformes species, the alleles had a lower polymorphism and few sites subjected to positive selection. This pattern most likely reflects the lower pathogenic pressure that individuals experience along their migratory flyway. On chapter III, the role of pathogens as drivers of the diversifying selection of MHC genes is analysed. By using Illumina MiSeq sequencing we compared two godwit subspecies in terms of MHC-I exon 3 diversity and polymorphism. Both nominate and Icelandic godwits overlapped in terms of number of alleles (and loci) per individual, but nominate birds had significantly higher number of alleles and polymorphism. A population size effect could partly explain the differences seen between subspecies at the MHC-I diversity, but the number of positively selected sites for the nominate godwit was twice as high as those found for the Icelandic godwits. Positive selection differences suggest a stronger balancing selection for the nominate godwit and thus an adaptation to pathogen-rich habitats. Chapter IV is focus on sexual signals and whether they truly advertise the quality of mates and their capacity to fight-off pathogens. I tested whether investment on innate immunity could be reflected by the sexual characters of godwits. Results indicate that some plumage features of male and female godwits were linked to soluble parameters of innate immunity and were not cost-free. Moreover, the signal was honest for males but not for females, a difference that may be related to sex-specific energetic demands and roles undertaken in breeding. Results suggest that females select males that are better at combining moult and migration and more able to fight infections. Chapter V tackles a more pratical issue faced by researchers that work with free-living birds, and with no immediate access to laboratory facilities. Immunological assays have become a widespread tool for an integrate approach of immune variation, but it was unknown whether post-sampling repeated freeze-thawing (FT) cycles could affect the final outcome of the assays. An experimental approach testing post-sampling handling and methodological issues was implemented, and overall results showed that plasma (and serum) samples remain stable after repeated FT cycles, and thus the indices of immune function are mostly unaffected. Moreover, small methodological deviations of the assay´s protocols, caused no substantial variations on the final results.
Author: Sara Pardal
Language: English
Subfield: Parasitology
Field: Immunology and Microbiology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2808395396