Host-parasite interactions: bird immune genes, blood parasites and climate change implications

Zamora-Vilchis, Itzel (2013) Host-parasite interactions: bird immune genes, blood parasites and climate change implications. PhD thesis, James Cook University.

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It is predicted that rising temperatures, and changes in other climatic variables as a consequence of global warming, will increase the global distribution of infectious diseases. In contrast, many potential host species will experience reductions in geographical range size and abundance and changes in their distribution to higher altitudes or latitudes as a result of climate change. The potential interactions between climate change and disease dynamics is of significant importance to both natural biodiversity and human health. There is a vital need to evaluate the potential effect of climate change on host-parasite interactions. Understanding how environmental factors, especially temperature, affect parasite distribution and how these two variables directly or indirectly affect host immunity in comparative studies is one approach to fill this gap in current knowledge. The aim of this thesis was to study a model system including temperature, vector-borne diseases (VBD) and host Major Histocompatibility Complex (MHC) genes along altitudinal gradients, and predict the effects of climate change on these host-parasite interactions. The avian community of the Australian Wet Tropics (AWT) was analyzed as a model case system, in which I investigated blood parasite pressure in relation to temperature and host MHC genes along altitudinal gradients. MHC genes were used because they show extreme polymorphism within populations and this gene diversity is thought to arise via interactions of host MHC gene products and parasites. I used PCR screening of cytochrome b to investigate the prevalence and lineage diversity of four of the main genera of blood parasites (Haemoproteus spp., Plasmodium spp., Leucocytozoon spp. (Haemosporida) and Trypanosoma spp. (Kinetoplastida)) in birds of the Australian Wet Tropics. I found that parasite prevalence and lineage richness were positively and strongly associated with temperature. The phylogenetic relationships among parasite lineages were analyzed to determine the host specificity of each parasite genus. I found that Plasmodium spp. and Trypanosoma spp. displayed low specificity, whereas Haemoproteus spp. seemed to display specificity at host family level. I also amplified a 173 bp fragment of the second exon of the MHC class II β gene of fifteen species from two bird families (Acanthizidae and Meliphagidae) in order to analyze their allele diversity and reveal evidence for selection (average d(N)/d(S) ratio and number of positive selected sites; NCBS). MHC diversity and selection were positively correlated with prevalence of blood parasites. The results suggest that the stronger the parasite pressure the higher the MHC allele diversity average d(N)/d(S) ratio and NCBS. It appears that higher parasite prevalence imposed stronger selective pressure in the host immune system, therefore the higher MHC allele diversity and selection allowed them to tolerate higher parasite prevalence. These results suggest an interaction between temperature, parasite prevalence and lineage richness, and bird MHC diversity and selection on MHC genes. Higher temperatures in lowland areas promote the development of parasites. This strong parasite pressure on the host immune system promotes higher diversity and selection of MHC genes. As elevation increases both temperature and parasite prevalence decreases. The lower temperature of highland areas inhibits development of parasites, creating a low-parasite environment and hence lower MHC diversity and selection in birds. To understand the possible effects of climate change on these host-parasite interactions, I used the regression of overall parasite prevalence and temperature documented to estimate increases in prevalence of parasites with temperature rise. This relationship predicts an increase of about 10% in the prevalence of parasites, for each 1°C increment in temperature. The shifts of host distribution along the elevation gradient that would be required to hold parasite prevalence to current values were determined using parasite prevalence data from this study. For each 1°C increase in temperature, bird distributions would need to ascend 200 m in elevation. Given a 4°C temperature increase, only birds that currently live at 400 m or below would be able to offset increases in parasite prevalence by shifting their distributions upwards; for birds currently living above 400 m, some increase in parasite prevalence would be unavoidable. It was shown that upland birds have lower MHC diversity, and rapid adaptation of their immunity could be unlikely due to the long life cycles of birds. My results also predict that lineage richness will increase with temperature, and that Plasmodium spp. and Trypanosoma spp. may have greater opportunities for host-switching than other more host-specific genera like Haemoproteus spp. Increased parasite pressure are expected to have negative effects on the bird populations of the region, particularly those inhabiting the upland areas and populations unable to shift upwards. The predicted increase of parasite prevalence and lineage richness could interact with, and further exacerbate, the projected impacts of climate change on this bird community, leading to an increased risk of extinction for many bird species. In conclusion, temperature is one of the main variables driving patters of distribution of avian haematozoa in this avian community. Plasmodium spp. and Trypanosoma spp. showed low specificity and as such higher host-switching potential than Haemoproteus spp. Blood parasites are driving selection and diversity of bird MHC genes. Increasing parasite pressure was predicted with rising temperature as a consequence of climate change. Shifts upwards of bird distributions along the elevation gradient can help to reduce the impact of increment of parasite pressure in this community, but upland bird communities and populations unable to shift upwards will be susceptible to the consequences of increased parasite pressure.

Item ID: 40682
Item Type: Thesis (PhD)
Keywords: birds; climate change adaption; climate change; climatic; community ecology; ecosystem adaption; ecosystem management; genetic polymorphisms; high temperatures; host-parasite interactions; immune genes; immunological; leucocytozoon; parasites; plasmodium; protozoa; protozoan diseases; Queensland ; Wet Tropics of Queensland; Wet Tropics
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Zamora-Vilchis, Itzel, Williams, Stephen E., and Johnson, Christopher N. (2012) Environmental temperature affects prevalence of blood parasites of birds on an elevation gradient: implications for disease in a warming climate. PLoS ONE, 7 (6). pp. 1-8.

Date Deposited: 01 Oct 2015 04:30
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060202 Community Ecology (excl Invasive Species Ecology) @ 33%
06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060307 Host-Parasite Interactions @ 34%
05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050101 Ecological Impacts of Climate Change @ 33%
SEO Codes: 96 ENVIRONMENT > 9603 Climate and Climate Change > 960305 Ecosystem Adaptation to Climate Change @ 33%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960501 Ecosystem Assessment and Management at Regional or Larger Scales @ 33%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960805 Flora, Fauna and Biodiversity at Regional or Larger Scales @ 34%
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