by Merry Youle
Over 30,000 species of parasitoid wasps can't be wrong. Au contraire, their reproductive strategy has been highly successful. They deposit their eggs inside the larvae (or eggs) of other insects, typically lepidopterans (butterflies and moths). Although commonly called parasitic wasps, they qualify as "parasitoids" because parasitization is always deadly for the host. Normally, a caterpillar would recognize a foreign invader, such as a wasp egg, and activate its immune defenses to encapsulate the foreign object. The wasp prevents this by depositing venom and what looks to be viruses in the caterpillar along with her eggs. The venoms are chemically complex and act in a variety of ways to convert the caterpillar into a nursery for wasp larvae. In some species, the venom paralyzes the caterpillar and/or arrests its development prior to pupation. For the moment, we'll call those "viruses" virus-like particles (VLPs), a term used for particles that look like a virus but whose infectivity and components have not yet been characterized. The VLPs suppress the host immune response and are required for successful parasitization.
The two lineages of wasp parasitoids (the braconids and ichneumonids) independently evolved similar reproductive strategies – an elegant example of convergent evolution. More is known about the braconids, so we'll focus on their story, especially on the essential role of their VLPs. VLPs are produced only by females and only in a small number of specialized ovarian cells called calyx cells. VLP production begins when the level of molting hormones, called ecdysteroids, have peaked and initiated ovarian development early in pupation. VLPs are then produced throughout the female's adult life. The capsids of the VLPs are assembled in specialized regions of calyx cell nuclei. The VLPs are bounded by an envelope derived from the nuclear membrane and released into the oviduct by cell lysis. The capsids themselves are rod-shaped, 35–40 nm in diameter, varying between 30 and 100 nm in length – even in the same wasp. Structurally they resemble virions of the baculoviruses and their sister group, the nudiviruses. Both of those groups infect arthropods, particularly insects.
What are these particular VLPs? Well, viruses they’re not, at least not in the usual sense. Each particle contains no viral nucleic acid, but rather 10 to 30 circles of double-stranded DNA derived from the wasp genome, the number depending on the wasp species. Total packaged DNA ranges from ~150 to over 275 kb. When the VLPs are injected into a host larva along with the wasp eggs, the VLPs are seen to enter host cells very quickly – in as little as 2 hours, with the phagocytic hemocytes being most heavily infected. They enter the nucleus where their DNA is released and transcribed, but never replicated. The gene products play a vital role in the takeover of the host, including suppression of the immune response. (For a review of this topic, click here.)
This is a pretty unusual and highly sophisticated arrangement for the delivery of genes from a parasite to its host. So what genes are packaged and delivered to the caterpillar? Some answers are known for one of the braconid wasps, Cotesia congregata. You might be familiar with its host of choice, the caterpillar of the hummingbird or sphinx moth (Manduca sexta). These huge caterpillars, known as tobacco hornworms, often make their presence known in vegetable gardens where they noticeably defoliate prized tomato and pepper plants. The VLPs delivered with the eggs by this wasp contain 30 DNA circles. The DNA has been sequenced and 156 genes identified. The sequences of the DNA circles are found as linear genome segments dispersed throughout the wasp genome. In the ovarian calyx cells, those segments are selectively and intensively amplified, then excised and circularized for packaging in the VLPs.
None of the packaged sequences show similarity to any major virus gene. Two-thirds of them contain introns, a quarter of them are potential virulence factors. Strikingly, 66 genes are members of 9 gene families. The proteins encoded by 4 of these families contain domains found in toxins utilized by pathogenic bacteria or parasitic worms. The largest gene family (27 genes) encodes protein tyrosine phosphatases, key players in signal transduction cascades. The proteins encoded by the second largest family have akyrin repeat motifs that are typically found in eukaryotes where they participate in varied protein-protein interactions.
The abundance of gene families with members of a family distributed on several of the DNA circles suggests a history of gene duplication. Having multiple copies of these delivered genes may have been a key factor allowing the wasp to rapidly adapt to new hosts or to changes in host immune defenses. Indeed, although the VLP packaging mechanism is highly conserved among the braconid wasps, the contents vary greatly. Cross-infection experiments with three co-generic species demonstrated that the effectiveness of the VLPs is highly specific for particular host species.
If there is a capsid, then isn't there a virus lurking somewhere? Well, yes – long, long ago. Some of its genes remain in the wasp genome, to tell the tale. Recently, researchers identified 22 actively-transcribed virus-related genes in that same braconid wasp (C. congregata). These genes are most closely related to nudivirus genes. Some of them are in the core group that is shared by nudiviruses and baculoviruses. The researchers investigated a group of those genes further and found that they are integrated in the wasp genome, that they are expressed only in ovarian calyx cells that are producing VLPS, and that some of their protein products are present in the capsids. No viral genes are packaged in the VLPs. These viral-derived genes are highly conserved among the 17,000 species of this lineage of braconid wasps (up to 80% identity). In contrast, their similarity to the nudivirus genes is less than 60% — not too surprising given that this wasp-nudivirus association is estimated to have occurred 100 MYA ago.
Early on the VLPs were dubbed polydnaviruses. Knowing what we know now, would you call them viruses? Or parasitoid-wasp-gene-delivery devices?