Symbionts often exhibit significant reductions in genome complexity while pathogens often exhibit increased complexity through acquisition and diversification of virulence determinants. A few organisms have evolved complex life cycles in which they interact as symbionts with one host and pathogens with another. How the predicted and opposing influences of symbiosis and pathogenesis affect genome evolution in such instances, however, is unclear. The Polydnaviridae is a family of double-stranded (ds) DNA viruses associated with parasitoid wasps that parasitize other insects. Polydnaviruses (PDVs) only replicate in wasps but infect and cause severe disease in parasitized hosts. This disease is essential for survival of the parasitoid's offspring. Thus, a true mutualism exists between PDVs and wasps as viral transmission depends on parasitoid survival and parasitoid survival depends on viral infection of the wasp's host. To investigate how life cycle and ancestry affect PDVs, we compared the genomes of Campoletis sonorensis ichnovirus (CsIV) and Microplitis demolitor bracovirus (MdBV). CsIV and MdBV have no direct common ancestor, yet their encapsidated genomes share several features including segmentation, diversification of virulence genes into families, and the absence of genes required for replication. In contrast, CsIV and MdBV share few genes expressed in parasitized hosts. We conclude that the similar organizational features of PDV genomes reflect their shared life cycle but that PDVs associated with ichneumonid and braconid wasps have likely evolved different strategies to cause disease in the wasp's host and promote parasitoid survival.

Pathogens often suppress the melanization response of host insects, but the underlying molecular mechanisms are largely unknown. Here we report that Microplitis demolitor bracovirus (MdBV) carried by the wasp M. demolitor produces a protein, Egf1.0, which inhibits the phenoloxidase (PO) cascade. Egf1.0 belongs to a larger gene family that shares a cysteine-rich motif with similarities to the trypsin inhibitor-like (TIL) domains of small serine proteinase inhibitors (smapins). Gain-of-function and RNAi experiments indicated that the Egf genes are the only MdBVencoded factors responsible for disabling the insect melanization response. Known smapins bind target proteinases in a substratelike fashion and are cleaved at a single reactive site bond. The P1-P1 position for Egf1.0 has the sequence Arg-Phe, which suggested that its target proteinase is a prophenoloxidase-activating proteinase (PAP). Wild-type Egf1.0 inhibited PAP-3 from Manduca sexta, whereas Egf1.0 R51A , whose reactive-site arginine was replaced with an alanine, had no PAP-3 inhibitory activity. Other experiments using wild-type and mutant constructs indicated that Egf1.0 blocks activation of the PO cascade via PAP inhibition. Overall, our results identify a novel inhibitor of the PO cascade and indicate that suppression of the host melanization response is functionally important for both the virus and its associated wasp. melanization ͉ virulence ͉ immunity ͉ parasite ͉ antiviral

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Most facultative intracellular bacteria replicate in specialized phagosomes after being taken up by mammalian cells. Relatively few intracellular bacteria escape the phagosomal compartment with the help of cytolytic (pore-forming) proteins and replicate in the host cell cytosol. Without such toxins, intracellular bacteria cannot reach this cellular compartment. To circumvent the requirement of an ''escape'' step, we developed a procedure allowing the efficient direct injection of bacteria into the cytosol of mammalian cells. With this technique, we show that most bacteria, including extracellular bacteria and intracellular pathogens that normally reside in a vacuole, are unable to replicate in the cytosol of the mammalian cells. In contrast, microorganisms that replicate in the cytosol, such as Listeria monocytogenes, Shigella flexneri, and, to some extent, enteroinvasive Escherichia coli, are able to multiply in this cellular compartment after microinjection. Further L. monocytogenes with deletion in its PrfAregulated hpt gene was found to be impaired in replication when injected into the cytosol. Complementation of the hpt mutation with a plasmid carrying the wild-type hpt gene restored the replication ability in the cytosol. These data indicate that cytosolic intracellular pathogens have evolved specific mechanisms to grow in this compartment of mammalian cells. M any pathogenic bacteria are able to trigger their uptake by mammalian cells, which is followed by efficient multiplication of the internalized bacteria inside of the host cells. Internalization of these bacteria involves normal phagocytosis when the host cells are professional phagocytes, e.g., macrophages, or triggered phagocytosis in the case of nonprofessional phagocytic host cells, such as epithelial cells, hepatocytes, fibroblasts, and endothelial cells (1, 2). After internalization, most intracellular bacteria reside and replicate inside membrane-bound vacuoles that are specifically modified by the different bacteria (3, 4). Salmonella enterica, Legionella pneumophila, members of the Mycobacterium tuberculosis complex, Mycobacterium leprae, Brucella spp., Chlamydia, Rhodococcus equi, and several others belong to this group of intracellular bacteria. A smaller group of intracellular bacteria, including Shigella spp., the closely related enteroinvasive Escherichia coli (EIEC), Listeria monocytogenes, Listeria ivanovii, and Ricksettia spp., can escape from the primary phagosome into the host cell cytosol where the bacteria proficiently replicate. These latter bacteria synthesize specific proteins that disrupt the phagosomal membrane, thus allowing bacterial entry into the cytosol. In L. monocytogenes, the required proteins are best characterized and comprise the pore-forming lysteriolysin (LLO) and two phospholipases C, PlcA and PlcB (5, 6).It has been reported that the introduction and expression of the listerial hly gene (encoding LLO) in Bacillus subtilis leads to the release of these avirulent bacteria into the cytosol of mammalian cells wher...