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Studies in different animal model systems have revealed the impact of odors on immune cells; however, any understanding on why and how odors control cellular immunity remained unclear. We find that Drosophila employ an olfactory-immune cross-talk to tune a specific cell type, the lamellocytes, from hematopoietic-progenitor cells. We show that neuronally released GABA derived upon olfactory stimulation is utilized by blood-progenitor cells as a metabolite and through its catabolism, these cells stabilize Sima/HIFα protein. Sima capacitates blood-progenitor cells with the ability to initiate lamellocyte differentiation. This systemic axis becomes relevant for larvae dwelling in wasp-infested environments where chances of infection are high. By co-opting the olfactory route, the preconditioned animals elevate their systemic GABA levels leading to the upregulation of blood-progenitor cell Sima expression. This elevates their immune-potential and primes them to respond rapidly when infected with parasitic wasps. The present work highlights the importance of the olfaction in immunity and shows how odor detection during animal development is utilized to establish a long-range axis in the control of blood-progenitor competency and immune-priming., Competing Interests: SM, MS, AK, MG, MJ, PU, KG, JS, TM No competing interests declared, (© 2020, Madhwal et al.)

It is common to find abundant genetic variation in host resistance and parasite infectivity within populations, with the outcome of infection frequently depending on genotype-specific interactions. Underlying these effects are complex immune defenses that are under the control of both host and parasite genes. We have found extensive variation in Drosophila melanogaster's immune response against the parasitoid wasp Leptopilina boulardi. Some aspects of the immune response, such as phenoloxidase activity, are predominantly affected by the host genotype. Some, such as upregulation of the complement-like protein Tep1, are controlled by the parasite genotype. Others, like the differentiation of immune cells called lamellocytes, depend on the specific combination of host and parasite genotypes. These observations illustrate how the outcome of infection depends on independent genetic effects on different aspects of host immunity. As parasite-killing results from the concerted action of different components of the immune response, these observations provide a physiological mechanism to generate phenomena like epistasis and genotype-interactions that underlie models of coevolution., Competing Interests: The authors have declared that no competing interests exist.

Endoparasitoid wasps, which lay eggs inside the bodies of other insects, use various strategies to protect their offspring from the host immune response. The hymenopteran species of the genus Leptopilina , parasites of Drosophila , rely on the injection of a venom which contains proteins and peculiar vesicles (hereafter venosomes). We show here that the injection of purified L. boulardi venosomes is sufficient to impair the function of the Drosophila melanogaster lamellocytes, a hemocyte type specialized in the defense against wasp eggs, and thus the parasitic success of the wasp. These venosomes seem to have a unique extracellular biogenesis in the wasp venom apparatus where they acquire specific secreted proteins/virulence factors and act as a transport system to deliver these compounds into host lamellocytes. The level of venosomes entry into lamellocytes of different Drosophila species was correlated with the rate of parasitism success of the wasp, suggesting that this venosome-cell interaction may represent a new evolutionary level of host-parasitoid specificity.

Acute inflammation can cause serious tissue damage and disease in physiologically-challenged organisms. The precise mechanisms leading to these detrimental effects remain to be determined. In this study, we utilize a reproducible means to induce cellular immune activity in Drosophila larvae in response to mechanical stress. That is, forceps squeeze-administered stress induces lamellocytes, a defensive hemocyte type that normally appears in response to wasp infestation of larvae. The posterior signaling center (PSC) is a cellular microenvironment in the larval hematopoietic lymph gland that is vital for lamellocyte induction upon parasitoid attack. However, we found the PSC was not required for mechanical stress-induced lamellocyte production. In addition, we observed that mechanical injury caused a systemic expression of Unpaired3. This cytokine is both necessary and sufficient to activate the cellular immune response to the imposed stress. These findings provide new insights into the communication between injured tissues and immune system induction, using stress-challenged Drosophila larvae as a tractable model system., (Copyright © 2018. Published by Elsevier Inc.)

Several signaling pathways, including the JAK/STAT and Toll pathways, are known to activate blood cells (hemocytes) in Drosophila melanogaster larvae. They are believed to regulate the immune response against infections by parasitoid wasps, such as Leptopilina boulardi, but how these pathways control the hemocytes is not well understood. Here, we discuss the recent discovery that both muscles and fat body take an active part in this response. Parasitoid wasp infection induces Upd2 and Upd3 secretion from hemocytes, leading to JAK/STAT activation mainly in hemocytes and in skeletal muscles. JAK/STAT activation in muscles, but not in hemocytes, is required for an efficient encapsulation of wasp eggs. This suggests that Upd2 and Upd3 are important cytokines, coordinating different tissues for the cellular immune response in Drosophila. In the fat body, Toll signaling initiates a systemic response in which hemocytes are mobilized and activated hemocytes (lamellocytes) are generated. However, the contribution of Toll signaling to the defense against wasps is limited, probably because the wasps inject inhibitors that prevent the activation of the Toll pathway. In conclusion, parasite infection induces a systemic response in Drosophila larvae involving major organ systems and probably the physiology of the entire organism.

The cellular immune response against parasitoid wasps in Drosophila involves the activation, mobilization, proliferation and differentiation of different blood cell types. Here, we have assessed the role of Edin (elevated during infection) in the immune response against the parasitoid wasp Leptopilina boulardi in Drosophila melanogaster larvae. The expression of edin was induced within hours after a wasp infection in larval fat bodies. Using tissue-specific RNAi, we show that Edin is an important determinant of the encapsulation response. Although edin expression in the fat body was required for the larvae to mount a normal encapsulation response, it was dispensable in hemocytes. Edin expression in the fat body was not required for lamellocyte differentiation, but it was needed for the increase in plasmatocyte numbers and for the release of sessile hemocytes into the hemolymph. We conclude that edin expression in the fat body affects the outcome of a wasp infection by regulating the increase of plasmatocyte numbers and the mobilization of sessile hemocytes in Drosophila larvae.

Type 3 copper proteins, including insect prophenoloxidase (PPO), contain two copper atoms in the active site pocket and can oxidize phenols. Insect PPO plays an important role in immunity. Insects and other invertebrates show limited recovery from pathogen invasion and wounds if phenoloxidase (PO) activity is low. In most insect PPOs, two disulfide bonds are present near the C-terminus. However, in Pimpla hypochondriaca (a parasitoid wasp), each PPO contains one disulfide bond. We thus questioned whether the formation of two sulfide bonds in insect PPOs improved protein stability and/or increased insect innate immunity over time. Using Drosophila melanogaster PPO1 as a model, one or two disulfide bonds were deleted to evaluate the importance of disulfide bonds in insect immunity. rPPO1 and mutants lacking disulfide bonds could be expressed and showed PO activity. However, the PO activities of mutants lacking one or two disulfide bonds significantly decreased. Deletion of disulfide bonds also reduced PPO thermostability. Furthermore, antibacterial activities against Escherichia coli and Bacillus subtilis significantly decreased when disulfide bonds were deleted. Therefore, the formation of two disulfide bond(s) in insect PPO enhances antibacterial activity by increasing PO activity and stability., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

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.

Drosophila larvae react against eggs from the endoparasitoid wasp Leptopilina boulardi by surrounding them in a multilayered cellular capsule. Once a wasp egg is recognized as foreign, circulating macrophage-like cells, known as plasmatocytes, adhere to the invader. After spreading around the wasp egg, plasmatocytes form cellular junctions between the cells, effectively separating the egg from the hemocoel. Next, a second sub-type of circulating immunosurveillance cell (hemocyte), known as lamellocytes, adhere to either the wasp egg or more likely the plasmatocytes surrounding the egg. From these events, it is obvious that adhesion and cell shape change are an essential part of Drosophila's cellular immune response against parasitoid wasp eggs. To date, very few genes have been described as being necessary for a proper anti-parasitization response in Drosophila. With this in mind, we performed a directed genetic miniscreen to discover new genes required for this response. Many of the genes with an encapsulation defect have mammalian homologues involved in cellular adhesion, wound healing, and thrombosis, including extracellular matrix proteins, cellular adhesion molecules, and small GTPases.

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.

Background: When the parasitoid wasp Leptopilina boulardi lays an egg in a Drosophila larva, phagocytic cells called plasmatocytes and specialized cells known as lamellocytes encapsulate the egg. The Drosophila β-integrin Myospheroid (Mys) is necessary for lamellocytes to adhere to the cellular capsule surrounding L. boulardi eggs. Integrins are heterodimeric adhesion receptors consisting of α and β subunits, and similar to other plasma membrane receptors undergo ligand-dependent endocytosis. In mammalian cells it is known that integrin binding to the extracellular matrix induces the activation of Rac GTPases, and we have previously shown that Rac1 and Rac2 are necessary for a proper encapsulation response in Drosophila larvae. We wanted to test the possibility that Myospheroid and Rac GTPases interact during the Drosophila anti-parasitoid immune response., Results: In the current study we demonstrate that Rac1 is required for the proper localization of Myospheroid to the cell periphery of haemocytes after parasitization. Interestingly, the mislocalization of Myospheroid in Rac1 mutants is rescued by hyperthermia, involving the heat shock protein Hsp83. From these results we conclude that Rac1 and Hsp83 are required for the proper localization of Mys after parasitization., Significance: We show for the first time that the small GTPase Rac1 is required for Mysopheroid localization. Interestingly, the necessity of Rac1 in Mys localization was negated by hyperthermia. This presents a problem, in Drosophila we quite often raise larvae at 29°C when using the GAL4/UAS misexpression system. If hyperthermia rescues receptor endosomal recycling defects, raising larvae in hyperthermic conditions may mask potentially interesting phenotypes.

Inactivation of host Rho GTPases is a widespread strategy employed by bacterial pathogens to manipulate mammalian cellular functions and avoid immune defenses. Some bacterial toxins mimic eukaryotic Rho GTPase-activating proteins (GAPs) to inactivate mammalian GTPases, probably as a result of evolutionary convergence. An intriguing question remains whether eukaryotic pathogens or parasites may use endogenous GAPs as immune-suppressive toxins to target the same key genes as bacterial pathogens. Interestingly, a RhoGAP domain-containing protein, LbGAP, was recently characterized from the parasitoid wasp Leptopilina boulardi, and shown to protect parasitoid eggs from the immune response of Drosophila host larvae. We demonstrate here that LbGAP has structural characteristics of eukaryotic RhoGAPs but that it acts similarly to bacterial RhoGAP toxins in mammals. First, we show by immunocytochemistry that LbGAP enters Drosophila immune cells, plasmatocytes and lamellocytes, and that morphological changes in lamellocytes are correlated with the quantity of LbGAP they contain. Demonstration that LbGAP displays a GAP activity and specifically interacts with the active, GTP-bound form of the two Drosophila Rho GTPases Rac1 and Rac2, both required for successful encapsulation of Leptopilina eggs, was then achieved using biochemical tests, yeast two-hybrid analysis, and GST pull-down assays. In addition, we show that the overall structure of LbGAP is similar to that of eukaryotic RhoGAP domains, and we identify distinct residues involved in its interaction with Rac GTPases. Altogether, these results show that eukaryotic parasites can use endogenous RhoGAPs as virulence factors and that despite their differences in sequence and structure, eukaryotic and bacterial RhoGAP toxins are similarly used to target the same immune pathways in insects and mammals.

Host larvae of Drosophila melanogaster injected with the eicosanoid biosynthesis inhibitor, dexamethasone, prior to parasitization by the wasp Leptopilina boulardi, exhibited significantly reduced rates of melanotic encapsulation in comparison with control and saline-injected larvae. The results of this investigation suggest that prostaglandins and other eicosanoids are involved as cell-signaling molecules in the hemocytic encapsulation reaction of D. melanogaster larvae.

The mechanisms by which an organism becomes immune competent during its development are largely unknown. When infected by eggs of parasitic wasps, Drosophila larvae mount a complex cellular immune reaction in which specialized host blood cells, lamellocytes and crystal cells, are activated and recruited to build a capsule around the parasite egg to block its development. Here, we report that parasitization by the wasp Leptopilina boulardi leads to a dramatic increase in the number of both lamellocytes and crystal cells in the Drosophila larval lymph gland. Furthermore, a limited burst of mitosis follows shortly after infection, suggesting that both cell division and differentiation of lymph gland hemocytes are required for encapsulation. These changes, observed in the lymph glands of third-instar, but never of second-instar hosts, are almost always accompanied by dispersal of the anterior lobes themselves. To confirm a link between host development and immune competence, we infected mutant hosts in which development is blocked during larval or late larval stages. We found that, in genetic backgrounds where ecdysone levels are low (ecdysoneless) or ecdysone signaling is blocked (nonpupariating allele of the transcription factor broad), the encapsulation response is severely compromised. In the third-instar ecdysoneless hosts, postinfection mitotic amplification in the lymph glands is absent and there is a reduction in crystal cell maturation and postinfection circulating lamellocyte concentration. These results suggest that an ecdysone-activated pathway potentiates precursors of effector cell types to respond to parasitization by proliferation and differentiation. We propose that, by affecting a specific pool of hematopoietic precursors, this pathway thus confers immune capacity to third-instar larvae., ((C)2002 Elsevier Science (USA).)

Host-parasite relationships represent integrating adaptations of considerable complexity involving the host's immune capacity to both recognize and destroy the parasite, and the latter's ability to successfully invade the host and to circumvent its immune response. Compatibility in Drosophila-parasitic wasp (parasitoid) associations has been shown to have a genetic basis, and to be both species and strain specific. Studies using resistant and susceptible strains of Drosophila melanogaster infected with virulent and avirulent strains of the wasp Leptopilina boulardi demonstrate that the success of the host cellular immune response depends on the genetic status of both host and parasitoid. Immunological, physiological, biochemical, and genetic data form the bases of a two-component model proposed here to account for the observed specificity and complexity of two coevolved adaptations, host nonself recognition and parasitoid virulence.

Upon infection with the wasp parasitoid Leptopilina boulardi, the blood cells or hemocytes of Drosophila melanogaster larvae become activated and manifest a type of communal phagocytosis wherein eggs of the parasitoid are enveloped by multicellular, melanotic capsules. Hemocytes engaged in this collaborative response generate reactive oxygen intermediates (ROI). These molecules, together with melanogenic intermediates, are believed to destroy intrahemocoelic parasites. Cellular uptake of 2',7'-dichlorofluorescin diacetate (DCF-DA) and the oxidation of its deacetylated form (DCF) to yield the fluorescent product dichlorofluorescein (DC) was used as an intracellular probe for oxidant generation. The selective uptake of the fluorescent probe only by activated plasmatocytes from immune-reactive larvae identified these hemocytes as the source of ROI. Inhibition of DCF oxidation by catalase established hydrogen peroxide (H2O2) as 1 of the principal oxidants generated during melanotic encapsulation. A sensitive spectrometric assay for assessing iron oxidation and complex formation with xylenol orange (FOX assay) also was used to document in vitro-enhanced H2O2-mediated oxidations by hemolymph from immune-competent larvae. Cumulative evidence now establishes both superoxide anion (O2-*) and its dismutation product H2O2 in the cellular encapsulation response of D. melanogaster.

The role of dietary yeast in Drosophila melanogaster cellular immunity was investigated. Host larvae deprived of yeast immediately after parasitization by the cynipid wasp Leptopilina boulardi encapsulated a significantly lower percentage of the parasitoid's eggs than hosts transferred to a medium with yeast. When the transfers of hosts were made 24 hr after exposure to the parasite, diet had no effect on the immune response that had commenced prior to the transfers. This study demonstrates for the first time the effect of a specific dietary component on the immune responsiveness of Drosophila against a larval parasitoid.

Larvae of Drosophila melanogaster produce a haemocytic reaction against eggs of the parasitoid, Asobara tabida, which leads to the formation of a multicellular capsule surrounding the foreign object. The same phenomenon was observed with the parasitoid, Leptopilina boulardi. Concerning the resistance of D. melanogaster to L. boulardi, a single major segregating locus with the resistant allele dominant to the susceptible one was found. The host strain susceptible to this parasitoid species was found to be highly immune reactive against the eggs of A. tabida. The inheritance of the capacity to encapsulate A. tabida was analysed by comparing reciprocal crosses made using inbred resistant and susceptible parental strains. We conclude that differences in the encapsulation capacity are inherited autosomally, with the reactive phenotype showing complete dominance over the non-reactive one. These data suggest the existence of two independent gene systems, each being concerned with the recognition of one species only.

The Eucoilid parasitoid Leptopilina boulardi is able to suppress its host Drosophila melanogaster immune reaction. Some strains, however, are non-immune suppressive to that host. Virus-like particles (VLPs) responsible for the immune suppressive ability were investigated in different strains of L. boulardi with histochemical and ultrastructural techniques. Membrane-bound particles containing vesicles were observed in the reservoir of the long gland and also in the egg canal of the ovipositor. These particles are homologous with the immune suppressive VLPs already described in the reservoir of L. heterotoma. Similarities were also observed with the L2 particles described previously around the chorion of the parasitoid egg after infestation. A weak positive DNA specific histochemical reaction was observed inside the reservoir and at the ultrastructural level. Feulgen-derived techniques demonstrated that the reaction was localized inside the particles. The morphology of the particles as well as the immune suppressive ability varied between strains. Two morphotypes of VLPs are described; the 'Is' morphotype (always observed in immune suppressive or Is strains) and the 'NIs' morphotype (observed in the non-immune suppressive or NIs strain). The hybrids between Is and NIs strains produce a third type of particle, the 'HIs' morphotype with half-immune suppressive ability and intermediate morphology. The origin of the particles' immune suppressive activity against D. melanogaster is discussed within the scope of host specificity.

Eggs of an immune suppressive strain (= virulent) of the parasitoid Leptopilina boulardi are encapsulated neither in resistant nor in susceptible strains of Drosophila melanogaster but are encapsulated in Drosophila yakuba. Eggs of a nonimmune suppressive strain (= avirulent) of the same parasitoid are encapsulated in a resistant strain of D. melanogaster and in D. yakuba but are not encapsulated in a susceptible strain of D. melanogaster. Egg chorion in the 2 parasitoid strains showed the same morphology and the same modifications after egg laying whatever the host strain. When a capsule is built, a small dotted dense layer was first spread on the chorion, followed by accumulation layers of cells (plasmatocytes and lamellocytes) and lastly necrosis of the inner haemocytes. The encapsulated eggs darken only at the time of necrosis of haemocytes. In susceptible hosts, neither the tiny dense layer nor haemocyte accumulation occurred. We concluded that (1) this tiny dense layer was present before the deposition of the first haemocytes, (2) inhibition of deposition of this dense layer was the initial event of the induced immunosuppression, (3) haemocytes other than lamellocytes were engaged in capsule formation, (4) the immunosuppressive factors did not target only the lamellocytes but also the plasmatocytes, (5) darkening of the encapsulated eggs was due to cell necrosis rather than to extracellular melanin deposition.

L. heterotoma females inject VLPs along with their eggs into the hemocoel of the Drosophila larva. The VLPs destroy lamellocytes that are potentially harmful for the parasite eggs, but the other types of host hemocytes retain their functions. The parasitized third instar Drosophila larvae continue to grow and pupariate, showing no outward adverse effects. Minimum disruption to the growth and development of the host is advantageous to the endoparasite egg, since survival of the endoparasite depends on the health of the host to the developmental stage at which the endoparasite begins feeding on host tissues. The target of the VLPs must be microtubule components, because VLP entry into the lamellocyte induces depolymerization and repolymerization of microtubules. Microtubule rearrangement changes the discoidal lamellocyte to a bipolar cell. Concomitant with the modification in cell morphology, lamellocytes lose their surface adhesivity so they cannot adhere to a foreign object or to each other to form a capsule around endoparasite eggs. VLP-affected lamellocytes continue to elongate, lose cytoplasmic contents at the poles as blebs full of microvesicles, and are eventually destroyed. The molecular nature of the L. heterotoma VLP and the mechanisms underlying its specificity for lamellocyte cytoplasm are not known. An interesting consideration is the evolution of a particle whose selectivity for a host hemocyte protects the eggs of the parasitic insect. On the basis of information available at this time, it is clear that the L. heterotoma VLP is a useful tool for studying molecular aspects of microtubules and cytoskeleton.

Larvae of Drosophila melanogaster produce a haemocytic reaction against eggs of the parasitoid Leptopilina boulardi, which leads to the formation of a multicellular capsule surrounding the foreign object. Melanization resulting from the conversion of phenol to o-quinones frequently accompanies the cellular reaction. Although various cytological and biochemical aspects of this reaction have been investigated, very little is known about genetic determinism of the insect immune response. The heredity of the capacity to encapsulate was analysed by comparing 16 reciprocal crosses made using inbred resistant and susceptible parental strains. We conclude that differences in the encapsulation capacity of D. melanogaster are inherited autosomally, with the reactive phenotype showing complete dominance over the non-reactive one. There were neither sex-linked nor maternal effects. The results of all crosses suggest a single major segregating locus with two alleles and complete dominance of the resistant allele, with cytoplasmic factors and minor modifying genes acting on the major locus.

Parasitoid wasps must avoid the destructive effects of the host's cellular defense system in order to exploit the host hemocoel as a suitable environment for their survival. To protect their eggs from encapsulation by Drosophila melanogaster blood cells, Leptopilina heterotoma females inject a factor that selectively destroys lamellocytes, the type of Drosophila blood cell involved in recognition and encapsulation of large foreign objects. Other types of host blood cells, including the phagocytic plasmatocytes, remain functional. This report demonstrates that the destructive factor for lamellocytes is a virus-like particle (VLP) stored in the reservoir of an accessory gland associated with the female wasp reproductive system. We show that VLPs enter Drosophila blood cells in vitro. VLPs are found free in the cytoplasm of lamellocytes but are confined to phagocytic vesicles of plasmatocytes. As lamellocytes are susceptible to the VLP infection and plasmatocytes are not, we conclude that the mode of VLP entry and its disposition in the cytoplasm determine the fate of the infected host blood cell.

Eggs of three strains of the cynipid parasitoid Leptopilina heterotoma and a Tunisian strain (G317) of L. boulardi are not encapsulated by hemocytes of Drosophila melanogaster hosts, but the eggs of a Congolese strain (L104) of L. boulardi are encapsulated. To determine the reason for the difference in host response against the parasitoid eggs, lamellocytes (hemocytes that encapsulate foreign objects and form capsules around endogenous tissues in melanotic tumor mutants) were examined in host larvae parasitized by the five Leptopilina strains. Parasitization by the three L. heterotoma strains affected the morphology of host lamellocytes and suppressed endogenous melanotic capsule formation in melanotic tumor hosts. L104 did not alter the morphology of host lamellocytes nor block tumor formation in melanotic tumor mutant hosts. The morphology of some lamellocytes was affected by G317 parasitization but host lamellocytes were still capable of forming melanotic tumors and encapsulating dead supernumerary parasitoid larvae. Therefore, the eggs of strains affecting lamellocyte morphology are protected from encapsulation by the host's blood cells. L. heterotoma eggs float freely in the host hemocoel but L. boulardi eggs are attached to host tissue surfaces. Lamellocytes cannot infiltrate the attachment site so the capsule around the L104 egg remains incomplete. The wasp larva uses this gap in the capsule as an escape hatch for emergence.

Insects are able to effectively recognize parasitoid eggs or larvae and to eliminate them by formation of a hemocytic capsule. Although the cellular process is now well documented, the genetic aspects of recognition of foreignness and the encapsulation process are still poorly understood. Experiments using the isofemale-strain method showed that the encapsulating ability of Drosophila melanogaster exercised against a parasitic wasp varies within a given population and that this variability is under partial genetic control.

In Drosophila melanogaster a temperature-induced change in immune competence accompanies cell surface alterations that cause its blood cells to adhere and to encapsulate a parasite. At 29 degrees C the blood cells of the tumorous-lethal (Tuml) mutant show a high degree of immune competence and encapsulate the eggs of the parasitic wasp Leptopilina heterotoma. At 21 degrees C the blood cells are essentially immune incompetent. High percentages of lectin binding cells were found under conditions which potentiated cellular encapsulation responses. Some immune reactive blood cells did not bind lectin. The low percentages of lectin binding cells in susceptible hosts suggest that developing parasites alter the cell surface of the blood cells of immune reactive hosts.