Analysis of a lysozyme gene from the malaria vector mosquito, Anopheles gambiae

Sequence analysis of lysozyme C from the scorpion mesobuthus eupeus venom glands using semi-nested rt-PCR

BACKGROUND

Lysozyme is an antimicrobial protein widely distributed among eukaryotes and prokaryotes and take part in protecting microbial infection. Here, we amplified cDNA of MesoLys-C, a c-type lysozyme from the most common scorpion in Khuzestan Province, Southern Iran.

METHODS

Scorpions of Mesobuthus eupeus were collected from the Khuzestan Province. Using RNXTM solution, the total RNA was extracted from the twenty separated venom glands. cDNA was synthesized with extracted total RNA as template and modified oligo(dT) as primer. In order to amplify cDNA encoding a lysozyme C, semi-nested RT-PCR was done with the specific primers. Follow amplification, the fragment was sequenced.

RESULTS

Sequence determination of amplified fragment revealed that MesoLys-C cDNA had 438 bp, encoding for 144 aa residues peptide with molecular weight of 16.702 kDa and theoretical pI of 7.54. A putative 22-aminoacids signal peptide was identified. MesoLys-C protein was composed of one domain belonged to c-type lysosyme/ alphalactalbumin.

CONCLUSION

Multiple alignment of MesoLys-C protein with the related cDNA sequences from various organisms by ClustalW program revealed that some of the conserved residues of other c-type lysosymes were also seen in MesoLys-C. However, the comparison suggested that Mesobuthus eupeus of Khuzestan and east Mediterranean Mesobuthus eupeus belonged to different subspecies.

Characterization of expression, activity and role in antibacterial immunity of Anopheles gambiae lysozyme c-1

There are eight lysozyme genes in the Anopheles gambiae genome. Transcripts of one of these genes, LYSC-1, increased in Anopheles gambiae cell line 4a3B by 24 h after exposure to heat-killed Micrococcus luteus. Lysozyme activity was also identified in conditioned media from the cell line from which the protein was purified to homogeneity using ion exchange and gel filtration. Mass spectrometric analysis of the purified protein showed 100% identity to lysozyme c-1. Purified lysozyme c-1 was tested against non-mosquito-derived as well as culturable bacteria isolated from mosquito midguts. Lysozyme c-1 had negligible effects on the growth of most mosquito-derived bacteria in vitro but did inhibit the growth of M. luteus. Although Lys c-1 did not directly kill most bacteria, knockdown of LYSC-1 resulted in significant mortality in mosquitoes subjected to hemocoelic infections with Escherichia coli but not M. luteus thus suggesting that this protein plays an important role in antibacterial defense against selected bacteria.

Cloning and molecular characterization of two invertebrate-type lysozymes from Anopheles gambiae

We sequenced and characterized two novel invertebrate-type lysozymes from the mosquito Anopheles gambiae. Alignment and phylogenetic analysis of these and a number of related insect proteins identified through bioinformatics strategies showed a high degree of conservation of this protein family throughout the Class Insecta. Expression profiles were examined for the two mosquito genes through semiquantitative and real-time PCR analysis. Lys i-1 transcripts were found in adult females in the fat body and Malpighian tubules, whereas Lys i-2 was detected only in fat bodies. Blood-feeding resulted in significantly increased transcript abundance for both genes in the midguts. Neither gene was upregulated following bacterial challenge.

Differential gene expression in abdomens of the malaria vector mosquito, Anopheles gambiae, after sugar feeding, blood feeding and Plasmodium berghei infection

BACKGROUND

Large scale sequencing of cDNA libraries can provide profiles of genes expressed in an organism under defined biological and environmental circumstances. We have analyzed sequences of 4541 Expressed Sequence Tags (ESTs) from 3 different cDNA libraries created from abdomens from Plasmodium infection-susceptible adult female Anopheles gambiae. These libraries were made from sugar fed (S), rat blood fed (RB), and P. berghei-infected (IRB) mosquitoes at 30 hours after the blood meal, when most parasites would be transforming ookinetes or very early oocysts.

RESULTS

The S, RB and IRB libraries contained 1727, 1145 and 1669 high quality ESTs, respectively, averaging 455 nucleotides (nt) in length. They assembled into 1975 consensus sequences--567 contigs and 1408 singletons. Functional annotation was performed to annotate probable molecular functions of the gene products and the biological processes in which they function. Genes represented at high frequency in one or more of the libraries were subjected to digital Northern analysis and results on expression of 5 verified by qRT-PCR.

CONCLUSION

13% of the 1965 ESTs showing identity to the A. gambiae genome sequence represent novel genes. These, together with untranslated regions (UTR) present on many of the ESTs, will inform further genome annotation. We have identified 23 genes encoding products likely to be involved in regulating the cellular oxidative environment and 25 insect immunity genes. We also identified 25 genes as being up or down regulated following blood feeding and/or feeding with P. berghei infected blood relative to their expression levels in sugar fed females.

Characterization, organization and expression of AmphiLysC, an acidic c-type lysozyme gene in amphioxus Branchiostoma belcheri tsingtauense

The study on lysozymes remains open in amphioxus, a cephalochordate. Here we show the existence of c-type lysozyme gene (AmphiLysC) in amphioxus, first such data in the basal chordates including urochordate and cephalochordate. This is in contrast to the absence of c-type lysozyme genes in urochordate. It is found that there exist two copies of c-type lysozyme genes in amphioxus genome, and their gene organization is similar to vertebrate c-type lysozyme genes with respect to the number and the size of both exons and introns. AmphiLysC possesses main features characteristic of the digestive c-type lysozyme such as lower number of basic amino acids (low pI values) and pH-optimum in acidic range. Moreover, AmphiLysC is predominantly expressed in the gut. These indicate that AmphiLysC is possibly a digestive c-type enzyme. However, the ubiquitous expression of AmphiLysC in non-digestive tissues such as ovaries, testes, notochord, gill and muscle suggests that it may also play a non-digestive role like antibacterial activity. It is highly likely that AmphiLysC is an enzyme with a combined function of digestion and bacteriolysis.

An updated catalogue of salivary gland transcripts in the adult female mosquito, Anopheles gambiae

Salivary glands of blood-sucking arthropods contain a variety of compounds that prevent platelet and clotting functions and modify inflammatory and immunological reactions in the vertebrate host. In mosquitoes, only the adult female takes blood meals, while both sexes take sugar meals. With the recent description of the Anopheles gambiae genome, and with a set of approximately 3000 expressed sequence tags from a salivary gland cDNA library from adult female mosquitoes, we attempted a comprehensive description of the salivary transcriptome of this most important vector of malaria transmission. In addition to many transcripts associated with housekeeping functions, we found an active transposable element, a set of Wolbachia-like proteins, several transcription factors, including Forkhead, Hairy and doublesex, extracellular matrix components and 71 genes coding for putative secreted proteins. Fourteen of these 71 proteins had matching Edman degradation sequences obtained from SDS-PAGE experiments. Overall, 33 transcripts are reported for the first time as coding for salivary proteins. The tissue and sex specificity of these protein-coding transcripts were analyzed by RT-PCR and microarray experiments for insight into their possible function. Notably, two gene products appeared to be differentially spliced in the adult female salivary glands, whereas 13 contigs matched predicted intronic regions and may include additional alternatively spliced transcripts. Most An. gambiae salivary proteins represent novel protein families of unknown function, potentially coding for pharmacologically or microbiologically active substances. Supplemental data to this work can be found at http://www.ncbi.nlm.nih.gov/projects/omes/index.html#Ag2.

Characterization of the c-type lysozyme gene family in Anopheles gambiae

Seven new c-type lysozyme genes were found using the Anopheles gambiae genome sequence, increasing to eight the total number of genes in this family identified in this species. The eight lysozymes in An. gambiae have considerable variation in gene structure and expression patterns. Lys c-6 has the most unusual primary amino acid structure as the predicted protein consists of five lysozyme-like domains. Transcript abundance of each c-type lysozyme was determined by semiquantitative RT-PCR. Lys c-1, c-6 and c-7 are expressed constitutively in all developmental stages from egg to adult. Lys c-2 and c-4 also are found in all stages, but with relatively much higher levels in adults. Conversely, Lys c-3 and c-8 transcripts are highest in larvae. Lys c-1, c-6 and c-7 transcripts are found in nearly all the adult tissue samples examined while Lys c-2 and Lys c-4 are more restricted in their expression. Lys c-1 and c-2 transcripts are clearly immune responsive and are increased significantly 6-12 h post challenge with bacteria. The functional adaptive changes that may have evolved during the expansion of this gene family are briefly discussed in terms of the expression patterns, gene and protein structures.

Global gene expression analysis of Anopheles gambiae responses to microbial challenge

Anopheles gambiae transcript responses to experimental challenge with heat inactivated Salmonella typhimurium, Staphylococcus aureus and Beauveria bassiana have been analyzed with an Affymetrix GeneChip comprising the entire predicted mosquito transcriptome. Significant up- or down-regulation (greater than 2-fold) can be assayed for approximately 2% of the mosquito transcriptome and affected genes represent a variety of functional classes that include immunity, apoptosis, stress response, detoxification, metabolism, blood digestion, olfaction and others. Transcript responses to the 3 microbial elicitors exhibit an exceptionally high degree of specificity and only a few genes are significantly regulated by more than 1 of the tested elicitors. This study identifies several transcripts that have not been linked directly to immune response in A. gambiae previously; their infection responsiveness and sequence features do however suggest implication in defence reactions; examples are genes encoding leucine-rich repeat domain proteins, cuticle domain proteins and proteins containing immunoglobulin and fibronectin domains.

Innate immune defence in the human gastrointestinal tract

The mucosal surface of the gastrointestinal tract represents a major entry point and ecological niche for many microbes. It forms an important immune barrier, absorbing nutrients, whilst preventing invasion by organisms. Of the extra-ordinarily diverse species that comprise the microbial world, relatively few organisms are able to succeed in breaching this barrier in an otherwise healthy host. The production and secretion of antimicrobial peptides (AMPs) from surface epithelia and circulating immune cells are likely to play a key role in host protection and homeostasis. A number of these peptides are constitutively produced providing resident protection, whereas others are induced during infection and inflammation. In addition to directly eradicating microorganisms, it is becoming increasingly apparent that AMPs are multi-functional with diverse immuno-modulatory properties. This review focuses on three families of AMPs, defensins, cathelicidins, and lysozyme, and discusses their role in mucosal defence.

Molecular cloning and transcriptional activation of lysozyme-encoding cDNAs in the mosquito Aedes aegypti

Lysozymes are enzymes characterized by their ability to break down bacterial cell walls. In insects certain lysozymes are only found in the midgut, whereas others are only found in the haemolymph and fat body after immune challenge. We identified two lysozyme-encoding cDNAs from Aedes aegypti. Both deduced protein sequences are basic in nature, contain 148 amino acids including eight highly conserved cysteine residues, and their genomic sequences contain a single intron. Transcriptional profiles indicated that the predominant form is constitutively expressed and up-regulated upon immune challenge and blood feeding in adult mosquitoes. The second form is expressed during early developmental stages, larvae and pupae, and at low levels in adults after immune challenge. Lysozymes in Aedes aegypti play both roles, defined by the spatial and temporal regulation of their expression.

cDNA cloning of the lysozyme of the white shrimp Penaeus vannamei

Lysozyme, an antibacterial protein, has been implicated in innate immunity in invertebrates, but its activity in shrimp remained to be determined. We cloned the white shrimp lysozyme cDNA using a PCR strategy and detected its activity in haemocytes using a lytic-zone assay against Micrococcus luteus. The cloning was based on a reported EST (dbEST BE18831). The deduced amino acid sequence resulted in 150 amino with 46% identity to hen egg white lysozyme. RT-PCR was used to detect lysozyme mRNA in haemocytes. Analysis of the amino acid sequence of the shrimp lysozyme showed that it belongs to the C-type family of lysozymes. Furthermore, the lysozyme amino acid sequence contained extra residues at its C-terminus, which are characteristic of marine invertebrates. This information will be useful in future studies on the molecular mechanisms of immunity in marine invertebrates.

Isolation and characterization of a cDNA encoding for a lysozyme from the gut of the reduviid bug Triatoma infestans

We have isolated and characterised a Triatoma infestans cDNA encoding a lysozyme. A 174-bp fragment was amplified by PCR using degenerate oligodeoxyribonucleotide primers derived from the known amino acid sequences of lysozyme from other insects. This PCR fragment was used to screen a cDNA gut library of T. infestans. A clone containing the 3'-end of the lysozyme cDNA (219 bp) was isolated and sequenced. RACE was used to amplify the 5'-end of the lysozyme cDNA. After sequencing the complete lysozyme cDNA, the deduced 417 amino acid sequence showed high identity (40-50%) with other chicken-type lysozymes. The amino acid residues responsible for the catalytic activity and the binding of the substrate were essentially conserved. The expression pattern of the lysozyme gene in bugs at different molting and feeding states showed that this gene was upregulated in the digestive tract directly after the molt and after feeding. Additionally, this lysozyme gene was expressed differently in the different regions of the digestive tract, strongly in the cardia and stomach, the anterior regions of the midgut, and only traces of lysozyme mRNA could be detected in the small intestine, the posterior region of the midgut.

Comparative analysis of DNA vectors at mediating RNAi in Anopheles mosquito cells and larvae

Heritable RNA interference (RNAi) mediated by transgenes exhibiting dyad symmetry represents an important tool to study the function of genes expressed at late developmental stages. In this study, we determined whether the transcriptional machinery of Anopheles mosquitoes is capable of directing suppression of gene expression from DNA constructs designed to transcribe double-stranded RNA (dsRNA) as extended hairpin-loop RNAs. A series of DNA vectors containing sense and antisense regions of the green fluorescent protein EGFP target gene was developed. The effect of these vectors on a transiently expressed or stably integrated EGFP gene was assessed in an Anopheles gambiae cell line and in Anopheles stephensi larvae. Our data indicate that dsRNA-mediated silencing of a target gene from plasmid DNA can be achieved at high levels in Anopheles cell lines and larvae. The region that links the sense and antisense sequences of the target gene plays a determining role in the degree of silencing observed. These results provide important information for the development of heritable RNAi in Anopheles.

Detection of lysozyme-like enzymatic activity secreted by an immune-responsive mosquito cell line

Using molecular approaches, we have recently shown that the C7-10 mosquito cell line from Aedes albopictus, and the Aag-2 line from Aedes aegypti, secrete a variety of immune peptides into the culture medium, including cecropins, defensins, transferrin, and lysozyme. The diversity of these peptides makes it difficult to quantify the relative activities of each molecule, because possible synergistic interactions may occur. Using a microtiter plate assay with live bacteria, we now show that C7-10 cells secrete an activity that is more potent against the Gram-positive bacterium, Micrococcus luteus, than against Gram-negative Escherichia coli. This lysozyme-like activity is accompanied by production of a lytic zone in an agarose plate assay containing commercially available, lyophilized M. luteus. Properties of the lysozyme-like activity from C7-10 cells included a broad pH optimum from 5.5 to 6.5, and heat-sensitivity above 42 degrees C. Amounts of secreted activity increased during the initial 24h of incubation with heat-killed bacteria. During this induction, lysozyme-like activity was found primarily in the cell culture supernatant.

Characterization and cDNA cloning of an immune-induced lysozyme from cultured Aedes albopictus mosquito cells

Protein chemistry and cDNA sequencing were used to identify an Aedes albopictus mosquito lysozyme secreted after treatment of cultured cells with heat-killed bacteria. On acid gels, the putative lysozyme activity ran just ahead of the cecropin band. Elution of this activity yielded a single band on SDS gels, with a mass of approximately 14 kDa. Mass spectral analysis of the silver-stained band uncovered five tryptic peptides with masses that matched peptides from an Aedes aegypti lysozyme, which we had previously characterized from the Aag-2 mosquito cell line. Based on this tentative identification, the Ae. albopictus lysozyme cDNA was cloned using PCR-based approaches. The full length cDNA sequence was used to deduce the sequences and masses of theoretical tryptic peptides that would be detected after matrix-assisted laser desorption ionization time of flight (MALDI-TOF) and tandem mass spectrometry (MS/MS). In aggregate, this analysis uncovered seven peptides that encoded 75 of the 125 amino acids in the mature Ae. albopictus lysozyme. In a phylogenetic analysis, the Aedes lysozymes were most closely related to the Anopheles lysozymes. As a group the mosquito lysozymes were more closely related to lysozymes from various Lepidopteran species than to those from higher Diptera such as Drosophila and Musca, which have evolved a digestive function.

Molecular cloning of lysozyme-encoding cDNAs expressed in the salivary gland of a wood-feeding termite, Reticulitermes speratus

Two kinds of PCR-product cDNAs that encode premature lysozyme peptides (Rs-Lys1 and Rs-Lys2) were cloned from workers of a Japanese damp-wood termite, Reticulitermes speratus. The Rs-Lys1 and Rs-Lys2 cDNAs encoded deduced sequences of 170 and 164 amino acids, respectively. Alignment of these sequences with those of other insect lysozymes showed that the cDNAs encode lysozyme homologues with putative signal peptides, insertions eight amino acids long, and a relatively long C-terminus (13-17 amino acids). A maximum likelihood tree, constructed using the cDNA sequences, indicated that the termite lysozymes are related to those of mosquitoes and lepidopterans. Southern-blotting analysis identified single copies of these lysozyme genes in the termite. Reverse transcript (RT)-PCR and in situ hybridization experiments showed that Rs-Lys1 and Rs-Lys2 are expressed in the salivary glands of worker termites. Here, we discuss the possible digestive function of these lysozymes.

Gambicin: a novel immune responsive antimicrobial peptide from the malaria vector Anopheles gambiae

A novel mosquito antimicrobial peptide, gambicin, and the corresponding gene were isolated in parallel through differential display-PCR, an expressed sequence tag (EST) project, and characterization of an antimicrobial activity in a mosquito cell line by reverse-phase chromatography. The 616-bp gambicin ORF encodes an 81-residue protein that is processed and secreted as a 61-aa mature peptide containing eight cysteines engaged in four disulfide bridges. Gambicin lacks sequence homology with other known proteins. Like other Anopheles gambiae antimicrobial peptide genes, gambicin is induced by natural or experimental infection in the midgut, fatbody, and hemocyte-like cell lines. Within the midgut, gambicin is predominantly expressed in the anterior part. Both local and systemic gambicin expression is induced during early and late stages of natural malaria infection. In vitro experiments showed that the 6.8-kDa mature peptide can kill both Gram-positive and Gram-negative bacteria, has a morphogenic effect on a filamentous fungus, and is marginally lethal to Plasmodium berghei ookinetes. An oxidized form of gambicin isolated from the cell line medium was more active against bacteria than the nonoxidized form from the same medium.

The gene of chlamysin, a marine invertebrate-type lysozyme, is organized similar to vertebrate but different from invertebrate chicken-type lysozyme genes

In a recent publication we reported the protein purification, characterization, and the gene isolation of a cDNA encoding the antibacterial cold-active lysozyme-like protein chlamysin from the marine bivalve Chlamys islandica. A 4.2 kb genomic chlamysin gene has now been amplified and sequence-analyzed. By comparison to the cDNA sequence and its translation product, the coding region was found separated in four exons of 38-252 bp. The introns range in size from 0.8 to 1.5 kb, and have traditional spliceosomal intron 5'-GT donor and 3'-AG acceptor sites for splicing. Two of the introns contain multiple copies of three sequence motifs not found repeated in other published genes. The over-all gene organization of chlamysin resembles chicken-type (c-type) lysozyme genes in vertebrates, but is different from the three-exon structure in invertebrate c-type lysozyme genes. A phylogenetic analysis of invertebrate-type (i-type) and c-type lysozyme proteins demonstrated a large evolutionary distance between the i-type and the c-type enzyme classes. Exons of the i-type genes are not equally organized according to their homolog protein domains.

Exploration of mosquito immunity using cells in culture

The propagation of immune-responsive cells in vitro has provided the basis for substantial contributions to our understanding of many aspects of the mammalian immune response. In contrast, the potential for exploring the innate immune response of insects using cultured cells is only beginning to be developed, particularly with various mosquito cell lines from the genera Aedes and Anopheles. Immune-reactive mosquito cell lines express various defensive factors, including transferrin, lysozyme, cecropin, defensin, and prophenoloxidase activities. In this review, we discuss insect immunity in the context of key concepts that have emerged in the study of the mammalian immune system, with emphasis on the properties of the cells that participate in the immune response. The nature of established cell lines and their contributions to our understanding of immune functions in humans and insects is described, with emphasis on our own work with the C7-10 and Aag-2 mosquito cell lines from Aedes albopictus and Aedes aegypti, respectively. Finally, we offer some speculation on further advances in insect immunology that may be facilitated by work with cells in culture.

Immune activation upregulates lysozyme gene expression in Aedes aegypti mosquito cell culture

After stimulation with heat-killed bacteria, cultured cells from the mosquito Aedes aegypti (Aag-2 cells) secreted an induced protein with a mass of approximately 16 kDa that cross-reacted with antibody to chicken egg lysozyme. To investigate whether lysozyme messenger RNA is induced in bacteria-treated cells, we used polymerase chain reaction-based approaches to obtain the complete lysozyme cDNA from Aag-2 cells. The deduced protein contained 148 amino acids, including a 23 amino acid signal sequence. The calculated mass of the precursor protein is 16 965 Da, which is processed to yield a mature lysozyme of 14 471 Da with a calculated pI of 10.1. The lysozyme from Ae. aegypti shared 50% amino acid identity with lysozymes from Anopheles gambiae and Anopheles darlingi, which in turn shared 70% identity between each other. Northern analysis with the lysozyme cDNA probe showed induction of a 1.3 kb messenger RNA during the first 3 h after treatment of Aag-2 cells with heat-killed bacteria, followed by maximal expression 12-36 h after treatment. Southern analysis suggested that the gene likely occurs as a single copy in the genome of Aag-2 cells.

Mosquito immune responses and malaria transmission: lessons from insect model systems and implications for vertebrate innate immunity and vaccine development

The introduction of novel biochemical, genetic, molecular and cell biology tools to the study of insect immunity has generated an information explosion in recent years. Due to the biodiversity of insects, complementary model systems have been developed. The conceptual framework built based on these systems is used to discuss our current understanding of mosquito immune responses and their implications for malaria transmission. The areas of insect and vertebrate innate immunity are merging as new information confirms the remarkable extent of the evolutionary conservation, at a molecular level, in the signaling pathways mediating these responses in such distant species. Our current understanding of the molecular language that allows the vertebrate innate immune system to identify parasites, such as malaria, and direct the acquired immune system to mount a protective immune response is very limited. Insect vectors of parasitic diseases, such as mosquitoes, could represent excellent models to understand the molecular responses of epithelial cells to parasite invasion. This information could broaden our understanding of vertebrate responses to parasitic infection and could have extensive implications for anti-malarial vaccine development.

Induction of mosquito hemolymph proteins in response to immune challenge and wounding

The rapid induction of proteins in the hemolymph of the mosquito, Anopheles gambiae, was examined after wounding or injection of immune elicitors (Escherichia coli, lipopolysaccharide, laminarin, zymosan). One-dimensional gel electrophoresis revealed at least six hemolymph polypeptides >25 kDa that consistently appeared after any breech of the cuticle. All of these polypeptides appeared in the hemolymph within 30 min and reached a maximum concentration after approx. 6 h. No proteins were specifically induced by bacteria or bacterial or fungal cell wall products, however two constitutively expressed proteins were repressed by these injections. Patterns of hemolymph proteins were further analyzed by two-dimensional electrophoresis. Seven spots were enhanced or induced 2 h after injection in four replicate experiments. An additional two spots demonstrated some variability between replicates, but were generally responsive to injection. These rapidly induced polypeptides are candidates for regulating and initiating the mosquito's responses to pathogens and wounding.

Plasmodium activates the innate immune response of Anopheles gambiae mosquitoes

Innate immune-related gene expression in the major disease vector mosquito Anopheles gambiae has been analyzed following infection by the malaria parasite, Plasmodium berghei. Substantially increased levels of mRNAs encoding the antibacterial peptide defensin and a putative Gram-negative bacteria-binding protein (GNBP) are observed 20-30 h after ingestion of an infected blood-meal, at a time which indicates that this induction is a response to parasite invasion of the midgut epithelium. The induction is dependent upon the ingestion of infective, sexual-stage parasites, and is not due to opportunistic co-penetration of resident gut micro-organisms into the hemocoel. The response is activated following infection both locally (in the midgut) and systemically (in remaining tissues, presumably fat body and/or hemocytes). The observation that Plasmodium can trigger a molecularly defined immune response in the vector constitutes an important advance in our understanding of parasite-vector interactions that are potentially involved in malaria transmission, and extends knowledge of the innate immune system of insects to encompass responses to protozoan parasites.

Insect lysozymes

Lysozymes, related to the chicken-type lysozymes in vertebrates, are ubiquitous components in the bacteriolytic armamentarium of insects. The enzyme is normally present in the blood, and together with other bactericidal factors lysozyme is often strongly induced when the insect is infected. This response is regulated by mechanisms that are related to those that activate inflammatory, acute-phase and immune responses in mammals, and the induction of lysozyme and other factors is now being investigated as a model for innate immune reactions in general. A special adaptation is seen in flies like Musca and Drosophila. These animals live on the microorganisms in decompositing matter, and they have developed a specialized set of lysozymes that are expressed in the alimentary tract. In Drosophila, at least seven different lysozyme genes are clustered in a small region on the third chromosome. The different genes are expressed in different parts of the digestive tract, and at different time points during development, and they are highly divergent in sequence. The major lysozymes in the fly gut have acidic isoelectric points and/or pH optima, and their evolution provides an interesting parallel to the ruminants.