OAbstract--The presence of Leptopilina heterotoma or Leptopilina boulardi eggs in the hemocoel of a Drosophila melanogaster larva induces the differentiation of lamellocytes, the blood ceils that encapsulate foreign objects. L. boulardi eggs are encapsulated by the newly differentiated lamellocytes, but L. heterotoma eggs are not. The induced lamellocytes in host larvae with L. heterotoma eggs undergo the same destructive morphological changes as reported previously for lamellocytes present in melanotic tumor mutant larvae at the time of parasitization. Thus, the virus-like particles produced by the L. heterotoma female to protect its eggs from encapsulation do not block the differentiation of lamellocytes, but rather destroy lamellocytes whenever they are present in the hemocoel.

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 endoparasitic wasps developing within the hemocoel of another insect must avoid the potential harm posed by the cellular defense system of the host. Some parasitoid eggs have surface features or coating materials that provide passive protection against host blood cell reactions (1-6). The blood cells of other hosts are inhibited from encapsulating parasitoid eggs by secretions that female parasitoids deposit along with their eggs in the host hemocoel (7-10). Leptopilina heterotoma females utilize an accessory gland factor to disrupt the function of Drosophila melanogaster lamellocytes (11). Lamellocytes are discoidal blood cells that layer around a foreign object to form a tightly bound capsule that subsequently melanizes and permanently seals the intruder (12).Lamellocytes are abundant in the Drosophila larval hemocoel when capsules are being formed, such as in larvae of melanotic tumor (tu) strains; otherwise their frequency is low (13). They are sticky cells that adhere to each other, so clumps of lamellocytes are commonly found in blood cell samples from tu larvae. In tu larvae parasitized by L. heterotoma, lamellocytes lose their adhesivity and change their shape to an elongated, bipolar form (11,14). These modifications render lamellocytes incapable of forming cellular capsules around foreign bodies.The L. heterotoma factor that interferes with encapsulation was named lamellolysin because its destructive effects are restricted to lamellocytes. Drosophila plasmatocytes, which are phagocytic, and crystal cells that function in the melanization of cellular capsules in the hemocoel (15) remain undisturbed in parasitized larvae (11). Lamellolysin is stored in the reservoir of an accessory gland associated with the reproductive system of the female wasp. The fluid contents of this reservoir can be injected into Drosophila larvae or added to Drosophila blood cell samples in vitro to show the destructive effe...

Foreign objects that enter the hemocoel of Drosophila melanogaster larvae are encapsulated by one type of blood cell, the lamellocyte, yet eggs of the parasitoid wasp Leptopilina heterotoma remain unencapsulated in D. melano. gaster larval hosts that have many lamellocytes. Here we demonstrate that shortly after a female wasp oviposits in the hemocoel the lamellocytes undergo morphological changes and lose their adhesiveness. These affected blood cells are eventually destroyed as the parasitoid egg continues its development. The factor responsible for lamellocyte destruction, lamellolysin, is contained in an accessory gland of the female reproductive system and is injected along with the egg into the host hemocoel.Lamellolysin does not alter the morphology or the defense functions of the other types of blood cells in the host.Large foreign bodies entering the hemocoel of an insect are encapsulated by the circulating blood cells and permanently sealed in melanized cellular capsules. Thus, the successful development of a parasitoid wasp egg in the hemocoel of an insect host depends upon its ability to avoid encapsulation by its host's blood cells. Some parasitoid wasp eggs have special surface features so that they do not arouse a host defense response (1, 2). Other wasps actively interfere with host blood cell function when they oviposit. In the latter category are the ichneumonids that coinfect their hosts with viruses during oviposition so that the host's immune system as well as its growth are influenced (3-5). When melanotic tumor (symbol, tu) mutant larvae of Drosophila melanogaster are parasitized by some strains of the cynipid wasp, Leptopilina heterotoma (formerly, Pseudeucoila bochei), the encapsulation of host aberrant tissues to form the inert black masses known as melanotic tumors is blocked together with the inhibition of encapsulation of the wasp eggs (6, 7). The hemocytes of parasitized and unparasitized insect hosts have been compared (8-10), but how the host's immune system is suppressed by parasitoid wasps has not been elucidated.In this report we describe the loss of adhesiveness and eventual destruction of the type of blood cell that encapsulates foreign bodies. We also demonstrate that the factor responsible for suppression of encapsulation in the host is present in the reservoir associated with an accessory gland of the female wasp reproductive system. MATERIALS AND METHODS Insects. A sex-linked, temperature-sensitive melanotic tumor mutant of D. melanogaster, tu(J)Sz's, which develops melanotic masses in the posterior fat body when the larvae are grown at 26°C but not at 18°C (11), was used for the experiments. Larvae were raised at 26°C on cream of wheat/ molasses medium seeded with live yeast. Two autosomal recessive melanotic tumor mutations, tu(2)W and tu(2)bw (12, 13), were used to confirm that the effects of parasitization on lamellocytes are not strain specific. For exposure to female wasps, early third instar larvae were rinsed with distilled H20 and transferred to filter pa...

Black cells (Bc, 2-80.6±) mutant larvae ofDrosophila melanogaster have pigmented cells in the hemolymph and lymph glands. In this report we present evidence that these melanized cells are a mutant form of the crystal cells, a type of larval hemocyte with characteristic paracrystalline inclusions.Bc larvae lack crystal cells. Furthermore, the distribution pattern of black cells inBc larvae parallels that of experimentally-blackened crystal cells in normal larvae (phenocopy).InBc/Bc zygotes black cells appear during mid embryonic development but inBc /Bc zygotes pigmented cells are not found until late in the first larval instar.Crystal cells are present in the heterozygous larvae until this time, and paracrystalline inclusions can be seen in some of the cells undergoing melanization in these larvae.The rate of phenol oxidase activity inBc/Bc larval cell-free extracts is less than half that ofBc /Bcextracts whereas enzyme activity is undetectable inBc/Bc larvae. We propose that theBc gene product is required for maintaining the integrity of the paracrystalline inclusions; inBc/Bc larvae either the product is absent or nonfunctional so an effective contact between substrate and enzyme results in melanization of the cells.Phenol oxidase itself is either destroyed or consumed in the melanization process accounting for the absence of enzyme activity inBc/Bc larvae. These studies confirm that the crystal cells store phenolic substrates and are the source of the hemolymph phenol oxidase activity in the larva ofD. melanogaster.

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