We have identified 242 Anopheles gambiae genes from 18 gene families implicated in innate immunity and have detected marked diversification relative to Drosophila melanogaster. Immune-related gene families involved in recognition, signal modulation, and effector systems show a marked deficit of orthologs and excessive gene expansions, possibly reflecting selection pressures from different pathogens encountered in these insects' very different life-styles. In contrast, the multifunctional Toll signal transduction pathway is substantially conserved, presumably because of counterselection for developmental stability. Representative expression profiles confirm that sequence diversification is accompanied by specific responses to different immune challenges. Alternative RNA splicing may also contribute to expansion of the immune repertoire.

The response of drosophila to bacterial and fungal infections involves two signaling pathways, Toll and Imd, which both activate members of the transcription factor NF-kappaB family. Here we have studied the global transcriptional response of flies to infection with drosophila C virus. Viral infection induced a set of genes distinct from those regulated by the Toll or Imd pathways and triggered a signal transducer and activator of transcription (STAT) DNA-binding activity. Genetic experiments showed that the Jak kinase Hopscotch was involved in the control of the viral load in infected flies and was required but not sufficient for the induction of some virus-regulated genes. Our results indicate that in addition to Toll and Imd, a third, evolutionary conserved innate immunity pathway functions in drosophila and counters viral infection.

Oligonucleotide DNA microarrays were used for a genome-wide analysis of immune-challenged Drosophila infected with Grampositive or Gram-negative bacteria, or with fungi. Aside from the expression of an established set of immune defense genes, a significant number of previously unseen immune-induced genes were found. Genes of particular interest include corin-and Stubblelike genes, both of which have a type II transmembrane domain; easter-and snake-like genes, which may fulfil the roles of easter and snake in the Toll pathway; and a masquerade-like gene, potentially involved in enzyme regulation. The microarray data has also helped to greatly reduce the number of target genes in large gene groups, such as the proteases, helping to direct the choices for future mutant studies. Many of the up-regulated genes fit into the current conceptual framework of host defense, whereas others, including the substantial number of genes with unknown functions, offer new avenues for research.

The fruit fly Drosophila melanogaster is a good model to unravel the molecular mechanisms of innate immunity and has led to some important discoveries about the sensing and signaling of microbial infections. The response of Drosophila to virus infections remains poorly characterized and appears to involve two facets. On the one hand, RNA interference involves the recognition and processing of dsRNA into small interfering RNAs by the host RNase Dicer-2 (Dcr-2), whereas, on the other hand, an inducible response controlled by the evolutionarily conserved JAK-STAT pathway contributes to the antiviral host defense. To clarify the contribution of the small interfering RNA and JAK-STAT pathways to the control of viral infections, we have compared the resistance of flies wild-type and mutant for Dcr-2 or the JAK kinase Hopscotch to infections by seven RNA or DNA viruses belonging to different families. Our results reveal a unique susceptibility of hop mutant flies to infection by Drosophila C virus and cricket paralysis virus, two members of the Dicistroviridae family, which contrasts with the susceptibility of Dcr-2 mutant flies to many viruses, including the DNA virus invertebrate iridescent virus 6. Genome-wide microarray analysis confirmed that different sets of genes were induced following infection by Drosophila C virus or by two unrelated RNA viruses, Flock House virus and Sindbis virus. Overall, our data reveal that RNA interference is an efficient antiviral mechanism, operating against a large range of viruses, including a DNA virus. By contrast, the antiviral contribution of the JAK-STAT pathway appears to be virus specific.