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

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

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

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

GWAS results for immune defense when data from both diets pooled

Columns are: SNP position, factor, denominator DF, numerator DF, F, P, Chromosom

The Complex Contributions of Genetics and Nutrition to Immunity in <i>Drosophila melanogaster</i>

<div><p>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 <i>Drosophila melanogaster</i> to identify the genetic basis for individual variation in resistance, and for variation in immunological sensitivity to diet (genotype-by-environment interaction, or GxE). <i>D</i>. <i>melanogaster</i> were reared for several generations on either high-glucose or low-glucose diets and then infected with <i>Providencia rettgeri</i>, a natural bacterial pathogen of <i>D</i>. <i>melanogaster</i>. 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.</p></div

GWAS for immune response on low glucose diet

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