Adaptive Evolution of a Novel Drosophila Lectin Induced by Parasitic Wasp Attack

Abstract Drosophila melanogaster has long been used as a model for the molecular genetics of innate immunity. Such work has uncovered several immune receptors that recognize bacterial and fungal pathogens by binding unique components of their cell walls and membranes. Drosophila also act as hosts to metazoan pathogens such as parasitic wasps, which can infect a majority of individuals in natural populations, but many aspects of their immune responses against these more closely related pathogens are poorly understood. Here, we present data describing the transcriptional induction and molecular evolution of a candidate Drosophila anti-wasp immunity gene, lectin-24A. Lectin-24A has a secretion signal sequence and its lectin domain suggests a function in sugar group binding. Transcript levels of lectin-24A were induced significantly stronger and faster following wasp attack than following wounding or bacterial infection, demonstrating lectin-24A is not a general stress response or defense response gene but is instead part of a specific response against wasps. The major site of lectin-24A transcript production is the fat body, the main humoral immune tissue of flies. Interestingly, lectin-24A is a new gene of the D. melanogaster/Drosophila simulans clade, displaying very little homology to any other Drosophila lectins. Population genetic analyses of lectin-24A DNA sequence data from African and North American populations of D. melanogaster and D. simulans revealed gene length polymorphisms segregating at high frequencies as well as strong evidence of repeated and recent selective sweeps. Thus, lectin-24A is a rapidly evolving new gene that has seemingly developed functional importance for fly resistance against infection by parasitic wasps

Evolutionary Responses of Drosophila melanogaster Under Chronic Malnutrition

Drosophila species have successfully spread and adapted to diverse climates across the globe. For Drosophila melanogaster, rotting vegetative matter provides the primary substrate for mating and oviposition, and also acts as a nutritional resource for developing larvae and adult flies. The transitory nature of decaying vegetation exposes D. melanogaster to rapidly changing nutrient availability. As evidenced by their successful global spread, flies are capable of dealing with fluctuating nutritional reserves within their respective ecological niches. Therefore, D. melanogaster populations might contain standing genetic variation to support survival during periods of nutrient scarcity. The natural history and genetic tractability of D. melanogaster make the fly an ideal model for studies on the genetic basis of resistance to nutritional stress. We review artificial selection studies on nutritionally-deprived D. melanogaster and summarize the phenotypic outcomes of selected animals. Many of the reported evolved traits phenocopy mutants of the nutrient-sensing PI3K/Akt pathway. Given that the PI3K/Akt pathway is also responsive to acute nutritional stress, the PI3K/Akt pathway might underlie traits evolved under chronic nutritional deprivation. Future studies that directly test for the genetic mechanisms driving evolutionary responses to nutritional stress will take advantage of the ease in manipulating fly nutrient availability in the laboratory

The Nutritional Environment Influences the Impact of Microbes on Drosophila melanogaster Life Span

D. melanogaster ingests microorganisms growing within its rotting vegetation diet. Some of these microbes form associations with flies, while others pass through the gut with meals. Fly-microbe-diet interactions are dynamic, and changes to the fly culture medium can influence microbial growth in the overall environment. In turn, these alterations in microbial growth may not only impact the nutritional value of fly meals but also modulate behavior and health, at least in part due to direct contributions to fly nutrition. The interactive ecology between flies, microbes, and their environment can cause a specific microbe to be either beneficial or detrimental to fly life span, indicating that the environment should be considered a key influential factor in host-microbe interactions.Microbes can extend Drosophila melanogaster life span by contributing to the nutritional value of malnourishing fly culture medium. The beneficial effect of microbes during malnutrition is dependent on their individual ability to proliferate in the fly environment and is mimicked by lifelong supplementation of equivalent levels of heat-killed microbes or dietary protein, suggesting that microbes can serve directly as a protein-rich food source. Here, we use nutritionally rich fly culture medium to demonstrate how changes in dietary composition influence monocolonized fly life span; microbes that extend fly life span on malnourishing diets can shorten life on rich diets. The mechanisms employed by microbes to affect host health likely differ on low- or high-nutrient diets. Our results demonstrate how Drosophila-associated microbes can positively or negatively influence fly life span depending on the nutritional environment. Although controlled laboratory environments allow focused investigations on the interaction between fly microbiota and nutrition, the relevance of these studies is not straightforward, because it is difficult to mimic the nutritional ecology of natural Drosophila-microbe interactions. As such, caution is needed in designing and interpreting fly-microbe experiments and before categorizing microbes into specific symbiotic roles based on results obtained from experiments testing limited conditions

Mgat1-dependent N-glycosylation of membrane components primes Drosophila melanogaster blood cells for the cellular encapsulation response.

In nature, larvae of the fruitfly Drosophila melanogaster are commonly infected by parasitoid wasps, and so have evolved a robust immune response to counter wasp infection. In this response, fly immune cells form a multilayered capsule surrounding the wasp egg, leading to death of the parasite. Many of the molecular mechanisms underlying this encapsulation response are conserved with human immune responses. Our findings suggest that protein N-glycosylation, a common protein post-translational modification of human immune proteins, may be one such conserved mechanism. We found that membrane proteins on Drosophila immune cells are N-glycosylated in a temporally specific manner following wasp infection. Furthermore we have identified mutations in eight genes encoding enzymes of the N-glycosylation pathway that decrease fly resistance to wasp infection. More specifically, loss of protein N-glycosylation in immune cells following wasp infection led to the formation of defective capsules, which disintegrated over time and were thereby unsuccessful at preventing wasp development. Interestingly, we also found that one species of Drosophila parasitoid wasp, Leptopilina victoriae, targets protein N-glycosylation as part of its virulence mechanism, and that overexpression of an N-glycosylation enzyme could confer resistance against this wasp species to otherwise susceptible flies. Taken together, these findings demonstrate that protein N-glycosylation is a key player in Drosophila cellular encapsulation and suggest that this response may provide a novel model to study conserved roles of protein glycosylation in immunity

Microbial Quantity Impacts Drosophila Nutrition, Development, and Lifespan

Summary: In Drosophila, microbial association can promote development or extend life. We tested the impact of microbial association during malnutrition and show that microbial quantity is a predictor of fly longevity. Although all tested microbes, when abundantly provided, can rescue lifespan on low-protein diet, the effect of a single inoculation seems linked to the ability of that microbial strain to thrive under experimental conditions. Microbes, dead or alive, phenocopy dietary protein, and the calculated dependence on microbial protein content is similar to the protein requirements determined from fly feeding studies, suggesting that microbes enhance host protein nutrition by serving as protein-rich food. Microbes that enhance larval growth are also associated with the ability to better thrive on fly culture medium. Our results suggest an unanticipated range of microbial species that promote fly development and longevity and highlight microbial quantity as an important determinant of effects on physiology and lifespan during undernutrition. : Nutrition in Life Cycle; Microbiology; Microbiome Subject Areas: Nutrition in Life Cycle, Microbiology, Microbiom

<i>Mgat1</i> is required in hemocytes, but loss of <i>Mgat1</i> does not result in reduced hemocyte numbers.

<p>(A) Proportion of wasp eggs encapsulated in the indicated genotypes crossed to hemocyte- (<i>He-Gal4</i>, blue bars) and fat body- (<i>C833</i>, green bars) specific Gal4 drivers. RNAi-mediated knockdown of <i>Mgat1</i> specifically in hemocytes led to a reduced encapsulation rate. (B) Circulating hemocyte counts in <i>L. clavipes</i> attacked <i>w¹¹¹⁸</i> (blue bars) and <i>Mgat1¹/+</i> (green bars) larvae. <i>Mgat1¹/+</i> larvae showed comparable counts with control for both total hemocyte count (THC) and lamellocyte differentiation, suggesting that hematopoiesis occurs normally in these mutants. Error bars indicate standard error of the mean; * indicates p<0.01 relative to control.</p

N-glycosylation of hemocytes is temporally specific following wasp attack.

<p>Paired brightfield and FITC images of hemocytes stained with FITC-WGA at the indicated time after wasp attack reveals that WGA staining of hemocytes is dynamic following wasp attack. (A,B) Plasmatocytes from unattacked <i>w¹¹¹⁸</i> larvae, age matched with the 24–48 hour post attack (PA) time point, did not stain with WGA. (C,D) At 0–24 hours PA, lamellocytes from <i>L. clavipes</i> attacked <i>w¹¹¹⁸</i> larvae began to show WGA staining. (E–H) A strong, speckled, WGA staining pattern was seen in lamellocytes (E,F), and to a lesser extent, plasmatocytes (G,H) from attacked <i>w¹¹¹⁸</i> larvae at 24–48 hours PA. (I,J) By 48–72 hours PA, lamellocytes from attacked <i>w¹¹¹⁸</i> larvae no longer stained with WGA. (K,L) Lamellocytes from attacked <i>Mgat1¹</i> mutant larvae did not stain with WGA at 24–48 hours PA, demonstrating that the WGA signal is dependent on N-glycosylation pathway function. Scale bar in all panels indicate 10 µm.</p

Data from: Fruit flies diversify their offspring in response to parasite infection

The evolution of sexual reproduction is often explained by Red Queen dynamics: Organisms must continually evolve to maintain fitness relative to interacting organisms, such as parasites. Recombination accompanies sexual reproduction and helps diversify an organism’s offspring, so that parasites cannot exploit static host genotypes. Here we show that Drosophila melanogaster plastically increases the production of recombinant offspring after infection. The response is consistent across genetic backgrounds, developmental stages, and parasite types but is not induced after sterile wounding. Furthermore, the response appears to be driven by transmission distortion rather than increased recombination. Our study extends the Red Queen model to include the increased production of recombinant offspring and uncovers a remarkable ability of hosts to actively distort their recombination fraction in rapid response to environmental cues