Identification of seedling resistance in wild oat relatives against oat crown rust

Faculty advisor: Melania FigueroaThis research was supported by the Undergraduate Research Opportunities Program (UROP)

Exceptional Diversity, Maintenance of Polymorphism, and Recent Directional Selection on the APL1 Malaria Resistance Genes of Anopheles gambiae

The three-gene APL1 locus encodes essential components of the mosquito immune defense against malaria parasites. APL1 was originally identified because it lies within a mapped QTL conferring the vector mosquito Anopheles gambiae natural resistance to the human malaria parasite, Plasmodium falciparum, and APL1 genes have subsequently been shown to be involved in defense against several species of Plasmodium. Here, we examine molecular population genetic variation at the APL1 gene cluster in spatially and temporally diverse West African collections of A. gambiae. The locus is extremely polymorphic, showing evidence of adaptive evolutionary maintenance of genetic variation. We hypothesize that this variability aids in defense against genetically diverse pathogens, including Plasmodium. Variation at APL1 is highly structured across geographic and temporal subpopulations. In particular, diversity is exceptionally high during the rainy season, when malaria transmission rates are at their peak. Much less allelic diversity is observed during the dry season when mosquito population sizes and malaria transmission rates are low. APL1 diversity is weakly stratified by the polymorphic 2La chromosomal inversion but is very strongly subdivided between the M and S “molecular forms.” We find evidence that a recent selective sweep has occurred at the APL1 locus in M form mosquitoes only. The independently reported observation of a similar M-form restricted sweep at the Tep1 locus, whose product physically interacts with APL1C, suggests that epistatic selection may act on these two loci causing them to sweep coordinately

Some Things I Have Written About Flies

Host-pathogen interactions shape the evolution of host immune defense. Genetic variation in genes of the immune system influences resistance to infection, but host immune defenses can additionally be influenced by environmental and physiological factors, including the presence of symbiotic microbes. In this thesis I examine both non-genetic and genetic factors involved in host-pathogen interactions. In the first chapter I ask whether the antibacterial immune response of D. melanogaster is influenced by the endosymbiotic bacterium Wolbachia pipientis, and in the second chapter I investigate whether immune system genes of the malaria mosquito Anopheles coluzzii show signatures of pathogen-driven coevolution. The endosymbiotic bacterium Wolbachia pipientis confers D. melanogaster and other insects with resistance to infection by RNA viruses. I investigated whether Wolbachia infection plays a role in protecting D. melanogaster against secondary bacterial infection, and in particular against pathogenic intracellular bacteria. I found no evidence that Wolbachia alters resistance to any of the bacterial pathogens tested, irrespective of how they colonize the host. The ability to resist and/or survive infection is a critical determinant of host fitness, so natural selection is predicted to drive the evolution of resistance mechanisms in response to novel or coevolving pathogens. I used molecular population genetic analyses to investigate how natural selection operates on the immune system of Anopheles coluzzii. I found evidence of rapid adaptive evolution in STAT-B, and long-term balancing selection in CTLMA2. In contrast to these Anopheles-specific immune genes, we found that genes encoding the Imd immune pathway, which are orthologously conserved across insects, exhibit patterns of genetic variation consistent with relaxed purifying selection. These results indicate that adaptive coevolution between A. coluzzii and its pathogens is more likely to involve novel or lineage-specific molecular mechanisms than the canonical humoral immune pathways

No effect of Wolbachia on resistance to intracellular infection by pathogenic bacteria in Drosophila melanogaster.

Multiple studies have shown that infection with the endosymbiotic bacterium Wolbachia pipientis confers Drosophila melanogaster and other insects with resistance to infection by RNA viruses. Studies investigating whether Wolbachia infection induces the immune system or confers protection against secondary bacterial infection have not shown any effect. These studies, however, have emphasized resistance against extracellular pathogens. Since Wolbachia lives inside the host cell, we hypothesized that Wolbachia might confer resistance to pathogens that establish infection by invading host cells. We therefore tested whether Wolbachia-infected D. melanogaster are protected against infection by the intracellular pathogenic bacteria Listeria monocytogenes and Salmonella typhimurium, as well as the extracellular pathogenic bacterium Providencia rettgeri. We evaluated the ability of flies infected with Wolbachia to suppress secondary infection by pathogenic bacteria relative to genetically matched controls that had been cured of Wolbachia by treatment with tetracycline. We found no evidence that Wolbachia alters host ability to suppress proliferation of any of the three pathogenic bacteria. Our results indicate that Wolbachia-induced antiviral protection does not result from a generalized response to intracellular pathogens

Systemic bacterial load is not influenced by <i>Wolbachia</i> infection.

<p>Least squares means for bacterial load (±1SE) of five <i>Wolbachia</i>-infected lines [WOLB(+)TET(-)] and genetically matched lines that have been cured of <i>Wolbachia</i> [WOLB(+)TET(+)], as well as five <i>Wolbachia</i>-uninfected lines [WOLB(-)TET(-)] and genetically paired tetracycline treated lines[WOLB(-)TET(+)]. Note that the WOLB category on the x-axis refers to initial <i>Wolbachia</i>-infection status prior to antibiotic treatment, rather than infection status at the time of experimental infections. Bacterial load was measured 24 hours after infection with the pathogenic bacteria (A) <i>P. rettgeri</i> (B) <i>L. monocytogenes</i> and (C) <i>S. typhimurium</i>. Assays were performed 2, 4, and 6 generations after ending tetracycline treatment, with three replicates in each generation. For each replicate, bacterial load was measured in 3 pools of 5 flies from every line.</p

GWAS for immune response on high glucose diet

Columns are: SNP, factor, denominator DF, numerator DF, F, P, chromosom

Data from: The complex contributions of genetics and nutrition to immunity in Drosophila melanogaster

Both malnutrition and undernutrition can lead to compromised immune defense in a diversity of animals, and “nutritional immunology” has been suggested as a means of understanding immunity and determining strategies for fighting infection. The genetic basis for the effects of diet on immunity, however, has been largely unknown. In the present study, we have conducted genome-wide association mapping in Drosophila melanogaster to identify the genetic basis for individual variation in resistance, and for variation in immunological sensitivity to diet (genotype-by-environment interaction, or GxE). D. melanogaster were reared for several generations on either high-glucose or low-glucose diets and then infected with Providencia rettgeri, a natural bacterial pathogen of D. melanogaster. Systemic pathogen load was measured at the peak of infection intensity, and several indicators of nutritional status were taken from uninfected flies reared on each diet. We find that dietary glucose level significantly alters the quality of immune defense, with elevated dietary glucose resulting in higher pathogen loads. The quality of immune defense is genetically variable within the sampled population, and we find genetic variation for immunological sensitivity to dietary glucose (genotype-by-diet interaction). Immune defense was genetically correlated with indicators of metabolic status in flies reared on the high-glucose diet, and we identified multiple genes that explain variation in immune defense, including several that have not been previously implicated in immune response but which are confirmed to alter pathogen load after RNAi knockdown. Our findings emphasize the importance of dietary composition to immune defense and reveal genes outside the conventional “immune system” that can be important in determining susceptibility to infection. Functional variation in these genes is segregating in a natural population, providing the substrate for evolutionary response to pathogen pressure in the context of nutritional environment

Description of Factors Tested in Analyses of Variance.

<p>Description of Factors Tested in Analyses of Variance.</p

Analyses of variance for fixed effects relating genotype, <i>Wolbachia</i> status, tetracycline treatment, and generation to bacterial load.

<p>Analyses of variance for fixed effects relating genotype, <i>Wolbachia</i> status, tetracycline treatment, and generation to bacterial load.</p

Line means for all phenotypes from study

Columns are self-explanatory with all values for nutritional indices being mg/fly, bacterial load is natural log load per fly, weight is in mg. Data from linear models as described in manuscript