We have investigated the blood cell types present in Drosophila at postembryonic stages and have analysed their modifications during development and under immune conditions. The anterior lobes of the larval hematopoietic organ or lymph gland contain numerous active secretory cells, plasmatocytes, few crystal cells, and a number of undifferentiated prohemocytes. The posterior lobes contain essentially prohemocytes. The blood cell population in larval hemolymph differs and consists mainly of plasmatocytes which are phagocytes, and of a low percentage of crystal cells which reportedly play a role in humoral melanisation. We show that the cells in the lymph gland can differentiate into a given blood cell lineage when solicited. Under normal nonimmune conditions, we observe a massive differentiation into active macrophages at the onset of metamorphosis in all lobes. Simultaneously, circulating plasmatocytes modify their adhesion and phagocytic properties to become pupal macrophages. All phagocytic cells participate in metamorphosis by ingesting doomed larval tissues. The most dramatic effect on larval hematopoiesis was observed following infestation by a parasitoid wasp. Cells within all lymph gland lobes, including prohemocytes from posterior lobes, massively differentiate into a new cell type specifically devoted to encapsulation, the lamellocyte.
Efficient wound healing including clotting and subsequent reepithelization is essential for animals ranging from insects to mammals to recover from epithelial injury. It is likely that genes involved in wound healing are conserved through the phylogeny and therefore, Drosophila may be an useful in vivo model system to identify genes necessary during this process. Furthermore, epithelial movement during specific developmental processes, such as dorsal closure, ressembles of those seen in mammalian wound healing. As puckered (puc) gene is a target of the JUN N-terminal kinase signaling pathway during dorsal closure, we investigated puc gene expression during wound healing in Drosophila. We showed that puc gene expression is induced at the edge of the wound in epithelial cells and Jun kinase is phosphorylated in wounded epidermal tissues, suggesting that the JUN N-terminal kinase signaling pathway is activated by a signal produced by an epidermal wound. In the absence of the Drosophila c-Fos homologue, puc gene expression is no longer induced. Finally, impaired epithelial repair in JUN N-terminal kinase deficient flies demonstrates that the JUN N-terminal kinase signaling is required to initiate the cell shape change at the onset of the epithelial wound healing. We conclude that the embryonic JUN N-terminal kinase gene cassette is induced at the edge of the wound. In addition, Drosophila appears as a good in vivo model to study morphogenetic processes requiring epithelial regeneration such as wound healing in vertebrates.
Insects belonging to the recent orders of the endopterygote clade (Lepidoptera, Diptera, Hymenoptera and Coleoptera) respond to bacterial challenge by the rapid and transient synthesis of a battery of potent antibacterial peptides which are secreted into their haemolymph. Here we present the first report on inducible antibacterial molecules in the sap-sucking bug Pyrrhocoris apterus, a representative species of the Hemiptera, which predated the Endoptergotes by at least 50 million years in evolution. We have isolated and characterized from immune blood of this species three novel peptides or polypeptides: (i) a 43-residue cysteine-rich anti-(Gram-positive bacteria) peptide which is a new member of the family of insect defensins; (ii) a 20-residue proline-rich peptide carrying an O-glycosylated substitution (N-acetylgalactosamine), active against Gram-negative bacteria; (iii) a 133-residue glycine-rich polypeptide also active against Gram-negative bacteria. The proline-rich peptide shows high sequence similarities with drosocin, an O-glycosylated antibacterial peptide from Drosophila, and also with the N-terminal domain of diptericin, an inducible 9 kDa antibacterial peptide from members of the order Diptera, whereas the glycine-rich peptide has similarities with the glycine-rich domain of diptericin. We discuss the evolutionary aspects of these findings.
Insect defensins are a family of 4‐kDa, cationic, inducible antibacterial peptides which bear six cysteine residues engaged in three intramolecular disulfide bridges. They owe their name to certain sequence similarities with defensins from mammalian neutrophiles and macrophages. We report the characterization of a novel defensin isoform from Drosophila and the cloning of the gene encoding a preprodefensin. The gene, which is intronless and present in a single copy/haploid genome, maps at position 46CD on the right arm of the second chromosome. The analysis of the upstream region of the gene reveals the presence of multiple putative cis‐regulatory sequences similar to mammalian regulatory motifs of acute‐phase‐response genes. Transcriptional profiles indicate that the Drosophila defensin gene is induced by bacterial challenge with acute‐phase kinetics. It is also expressed in the absence of immune challenge during metamorphosis. These and other data on the Drosophila defensin gene lead us to suggest that insect and mammalian defensins have evolved independently.
Two novel antimicrobial peptides, which we propose to name termicin and spinigerin, have been isolated from the fungus-growing termite Pseudacanthotermes spiniger (heterometabole insect, Isoptera). Termicin is a 36-amino acid residue antifungal peptide, with six cysteines arranged in a disulfide array similar to that of insect defensins. In contrast to most insect defensins, termicin is C-terminally amidated. Spinigerin consists of 25 amino acids and is devoid of cysteines. It is active against bacteria and fungi. Termicin and spinigerin show no obvious sequence similarities with other peptides. Termicin is constitutively present in hemocyte granules and in salivary glands. The presence of termicin and spinigerin in unchallenged termites contrasts with observations in evolutionary recent insects or insects undergoing complete metamorphosis, in which antimicrobial peptides are induced in the fat body and released into the hemolymph after septic injury.The termite species Pseudacanthotermes spiniger, like the other members of the Macrotermitinae, depends for its nutrition on a symbiotic fungus of the basidiomycete genus Termitomyces (1). This fungus grows inside the nests of the termites on piles of fecal pellets; it predigests the lignocellulosic substances and is responsible for food supply in cellulases. Macrotermitinae also live in symbiosis with anaerobic bacteria present in their posterior gut, which are in part responsible for the complete digestion of cellulose. In this environment inhabited with microorganisms, how do termites protect themselves?The first line of defense of insects against pathogens is the cuticle. Once this barrier has been breached, their defense reactions rely both on cellular and humoral mechanisms. The cellular aspects include phagocytosis and encapsulation of invading microorganisms (for a review, see Ref.2). The humoral facet involves the activation of proteolytic cascades leading to melanization and coagulation. In the evolutionary recent insect orders, the best characterized aspect of the humoral immune response is the rapid synthesis of antimicrobial peptides/ polypeptides by the fat body and certain blood cells and release of these factors into the hemolymph after bacterial challenge. Since the first report of an inducible antibacterial peptide from an insect, cecropin from the moth Hyalophora cecropia (3), more than 200 antimicrobial peptides/polypeptides have been characterized in insects. On the basis of their structural features, the peptides are classified into three classes: (i) linear peptides, devoid of cysteines and forming ␣-helices (the prototype of this family are the insect cecropins (4)), (ii) peptides with an overrepresentation in proline and/or glycine residues, and (iii) open-ended cyclic peptides containing cysteine residues (5). Cecropins, proline-rich, and glycine-rich peptides are essentially active against Gram-negative cells, but their activity spectrum sometimes includes Gram-positive bacteria as targets (6). Two recent reports have also indicated that c...
Wheat seedlings germinating in the presence of the systemic fungicide fenpropimorph accumulate 9.8,19-cyclopropylsterols (95% of total sterols) in place of A5-sterols, which are normally produced in these plants. Adult females of the phytophagous insect Locusta migratoria show a dramatic decrease in their cholesterol content when reared on fenpropimorph-treated wheat. These females lay eggs with the ecdysteroid concentration reduced by up to 80% as compared to controls. Injection of fenpropimorph to the insects or feeding them on wheat coated with the fungicide (normal sterol composition) does not affect their sterol or ecdysteroid profiles; addition of cholesterol to fenpropimorph-treated wheat prior to feeding restores normal ecdysteroid titers in the insects. The severe reduction of the ecdysteroid content in eggs laid by females reared on fenpropimorph-treated wheat is associated with a series of developmental arrests and/or abnormalities. The results show that the dietary 9,3,19-cyclopropylsterols cannot be used by Locusta in place of A5-sterols for ecdysteroid biosynthesis. They suggest that the selective inhibition of specific enzymes in the sterol biosynthetic pathway of the plants can be used as a strategy to control insect development.Insects are unable to synthesize de novo the steroid nucleus and depend on an exogenous source for their steroid economy. In particular, the biosynthesis of the molting hormone family, the ecdysteroids, which are polyhydroxylated steroids, is dependent on the uptake of sterols from the diet. The phytophagous grasshopper Locusta migratoria uses phytosterols such as sitosterol, stigmasterol, and campesterol, which it dealkylates to cholesterol, the most common precursor for ecdysone biosynthesis ( Fig. 1) (1, 2). We have recently shown that long-term treatment of maize seedlings with low concentrations of the systemic fungicides tridemorph or fenpropimorph leads to an almost complete replacement in these plants of the A5-sterols by the 9P,19-cyclopropylsterols [>95% of total sterols (3-5)]. This replacement could be traced back to a selective inhibition by the fungicide of two enzymes in the biosynthetic pathway of plant sterols: cycloeucalenolobtusifoliol isomerase and A8 A7-sterol isomerase (6).We have investigated the effect of abnormal sterol diet on the sterol economy of Locusta and have focused on possible interferences with ecdysteroid titers. This study was prompted by previous reports showing that lanosterol could not meet the sterol requirements of Dermestes vulpinus (7) and that some species of Drosophila were unable to complete development when raised on yeast mutants lacking A8 A7-sterol isomerase (8). MATERIALS AND METHODSChemicals. Fenpropimorph {4-[3-(4-t-butylphenyl)-2-methyl]propyl-2,6-dimethylmorpholine} was a gift of H. Pommer (BASF, Limburgerhof, F.R.G.). The cyclopropyl sterols used as standards were extracted from maize seedlings treated with tridemorph (3).Plant Material and Insects. Wheat (Triticum sativum) caryopses were purchased locally. ...
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