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Immunity & the butterfly: A functional genomic study of natural variation in immunity
Stockholm University, Faculty of Science, Department of Zoology.ORCID iD: 0000-0002-8782-3477
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Butterflies are ubiquitous and abundant, occurring in a wide variety of environments that contain diverse microbial communities with varied pathogenic pressures. These pathogens and parasites present a constant threat to organisms, and have led to the evolution of complex and intricate immune responses. Despite strong selection against immunological threats, organisms display great variation in their immune capabilities, both on the genetic and physiological level. Investigating this variation remains challenging, since differences in immune responses might arise from changes in the amount, size or performance of cells or organs. Disentangling these relative contributions is important, as the targets of selection are expected to differ, ranging from immune genes directly related to the phenotype to genes indirectly involved via cell proliferation. This thesis focuses on characterizing the immune system of the butterfly Pieris napi and investigating its remarkable variation across populations by using both phenotypic and genotypic measurements. By integrating RNA-seq with life history measurements, I found that the cost of infection and wounding in the final larval stage carries over the metamorphic boundary in P. napi (Paper II). Using population comparisons, I identified both the action and potential targets of natural selection in wild populations within their respective immune responses (Paper I, III & IV). The immune genes in P. napi show increased genetic variation compared to the rest of the genome, and microevolutionary selection dynamics act on these genes between and among populations (Paper I). I measured the cellular immune responses related to phagocytosis and melanization in common garden reared larvae originating from two allopatric populations (Spain, Sweden) (Paper III & IV). The two populations were found to differ in their blood cell composition, and overall phagocytic capability, driven by the increased phagocytic propensity of each cell type (Paper III). However, genome wide analysis of divergence between these populations found no excess genetic differentiation in genes annotated to phagocytic capacity, suggesting that our observed population differences might arise from genes affecting the activation or transdifferentiation of cells, which currently lack functional annotation. Interestingly, genes involved in glutamine metabolism, which have been linked to immune cell differentiation in mammals, did show divergence between the populations. In addition, the populations also differed in prophenoloxidase activity, a common method for quantifying immune related melanization in insects, along with the abundance of the cell-type (oenocytoids) related to this important immune function (Paper IV). Integrative analysis using both transcriptomic and genomic data revealed that the genes involved in this phenotype showed no significant differentiation between the populations. However, a gene involved with proper trafficking of melanogenic enzymes in vertebrates was found to be highly expressed and highly diverged between the two populations, providing an interesting candidate for future studies. This thesis demonstrates the advantages of integrating several genomic tools with lab experiments to quantify natural variation in the immune system of butterflies.

 

Place, publisher, year, edition, pages
Stockholm: Department of Zoology, Stockholm , 2019. , p. 31
Keywords [en]
eco-immunology, functional genomics, transcriptomics, innate immunity, selection dynamics, melanization, phagocytosis, population genetics, Pieris napi
National Category
Evolutionary Biology Genetics Zoology
Research subject
Population Genetics
Identifiers
URN: urn:nbn:se:su:diva-175465ISBN: 978-91-7797-825-1 (print)ISBN: 978-91-7797-826-8 (electronic)OAI: oai:DiVA.org:su-175465DiVA, id: diva2:1366284
Public defence
2019-12-13, Vivi Täckholmsalen (Q-salen), NPQ-huset, Svante Arrhenius väg 20, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2019-11-20 Created: 2019-10-28 Last updated: 2019-11-11Bibliographically approved
List of papers
1. Microevolutionary selection dynamics acting on immune genes of the green-veined white butterfly, Pieris napi
Open this publication in new window or tab >>Microevolutionary selection dynamics acting on immune genes of the green-veined white butterfly, Pieris napi
2018 (English)In: Molecular Ecology, ISSN 0962-1083, E-ISSN 1365-294X, Vol. 27, no 13, p. 2807-2822Article in journal (Refereed) Published
Abstract [en]

Insects rely on their innate immune system to successfully mediate complex interactions with their microbiota, as well as the microbes present in the environment. Previous work has shown that components of the canonical immune gene repertoire evolve rapidly and have evolutionary characteristics originating from interactions with fast-evolving microorganisms. Although these interactions are likely to vary among populations, there is a poor understanding of the microevolutionary dynamics of immune genes, especially in non-Dipteran insects. Here, we use the full set of canonical insect immune genes to investigate microevolutionary dynamics acting on these genes between and among populations by comparing three allopatric populations of the green-veined white butterfly, Pieris napi (Linne; Lepidoptera, Pieridae). Immune genes showed increased genetic diversity compared to genes from the rest of the genome and various functional categories exhibited different types of signatures of selection, at different evolutionary scales, presenting a complex pattern of selection dynamics. Signatures of balancing selection were identified in 10 genes, and 17 genes appear to be under positive selection. Genes involved with the cellular arm of the immune response as well as the Toll pathway appear to be enriched among our outlier loci, regardless of functional category. This suggests that the targets of selection might focus upon an entire pathway, rather than functional subsets across pathways. Our microevolutionary results are similar to previously observed macroevolutionary patterns from diverse taxa, suggesting that either the immune system is robust to dramatic differences in life history and microbial communities, or that diverse microbes exert similar selection pressures.

Keywords
genomics, innate immunity, Lepidoptera, microevolutionary dynamics, selection dynamics
National Category
Biological Sciences
Research subject
Population Genetics
Identifiers
urn:nbn:se:su:diva-158376 (URN)10.1111/mec.14722 (DOI)000436113300003 ()29772089 (PubMedID)
Available from: 2018-08-10 Created: 2018-08-10 Last updated: 2019-11-01Bibliographically approved
2. The consequences of surviving infection across the metamorphic boundary: tradeoff insights from RNAseq and life history measures
Open this publication in new window or tab >>The consequences of surviving infection across the metamorphic boundary: tradeoff insights from RNAseq and life history measures
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The broad diversity of insect life has been shaped, in part, by pathogen pressure, yet the influence of injury and infection during critical periods of development is understudied. During development, insects undergo metamorphosis, wherein the organism experiences a dramatic shift in their overall morphology, and physiology. In temperate zones, metamorphosis is often directly followed by a developmental arrest called diapause, for which the insect needs to acquire enough energy reserves before the onset of winter. We investigated the long-term effects of injury and infection using two bacteria in the butterfly Pieris napi, revealing that the negative consequences of bacterial infection carry across the metamorphic boundary. Initial direct effects of infection were weight loss and slower development, as well as an increased mortality at higher infection levels. The detrimental effects were stronger in the gram-positive Micrococcus luteus compared to gram-negative Escherichia coli. Transcriptome-wide differences between the two bacteria were already observed in the gene expression profile of the first 24 hours after infection. Larvae infected with M. luteus showed a strong suppression of all non-immunity related processes, with several types of immune responses being activated. The impact of these transcriptomic changes, a tradeoff between homeostasis and immune response, were visible in the life history data, wherein individuals infected with M. luteus had the highest mortality rate, along with the lowest pupal weight, developmental rate and adult weight of all the treatments. Overall, we find that the cost of infection and wounding in the final larval instar carries over the metamorphic boundary, and is expected to negatively affect their lifetime fitness.

National Category
Evolutionary Biology
Research subject
Population Genetics
Identifiers
urn:nbn:se:su:diva-175461 (URN)10.1101/792176 (DOI)
Available from: 2019-10-28 Created: 2019-10-28 Last updated: 2019-11-01
3. Geographic variation in hemocyte diversity and phagocytic propensity shows a diffuse genomic signature in the green veined white butterfly
Open this publication in new window or tab >>Geographic variation in hemocyte diversity and phagocytic propensity shows a diffuse genomic signature in the green veined white butterfly
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Insects rely on their innate immune system to successfully mediate complex interactions with their internal microbiota, as well as the microbes present in the environment. Given the variation in microbes across habitats, the challenges to respond to them is likely to result in local adaptation in the immune system. Here we focus upon phagocytosis, a mechanism by which pathogens and foreign particles are engulfed in order to be contained, killed and processed for antigen presentation. We investigated the phenotypic and genetic variation related to phagocytosis, in two allopatric populations of the butterfly Pieris napi. We found that the populations differ in their hemocyte composition, and overall phagocytic capability, driven by the increased phagocytic propensity of each cell type. However, no evidence for divergence in phagocytosis-related genes was observed, though an enrichment of genes involved in glutamine metabolism was found, which have recently been linked to immune cell differentiation in mammals.

National Category
Evolutionary Biology
Research subject
Population Genetics
Identifiers
urn:nbn:se:su:diva-175463 (URN)10.1101/790782 (DOI)
Available from: 2019-10-28 Created: 2019-10-28 Last updated: 2019-11-01Bibliographically approved
4. Integrating immune cell expression with signatures of selection suggests novel targets in melanin trafficking for geographic differences in melanin-related immune performance
Open this publication in new window or tab >>Integrating immune cell expression with signatures of selection suggests novel targets in melanin trafficking for geographic differences in melanin-related immune performance
(English)Manuscript (preprint) (Other academic)
National Category
Evolutionary Biology
Research subject
Population Genetics
Identifiers
urn:nbn:se:su:diva-175464 (URN)
Available from: 2019-10-28 Created: 2019-10-28 Last updated: 2019-11-01Bibliographically approved

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