Читать книгу Parasitology - Alan Gunn - Страница 25

Some endoparasitic wasps belonging to the families Ichneumonidae and Braconidae have a fascinating relationship with polydnaviruses. The polydnaviruses from these two wasp families are morphologically distinct, and they probably arose from the ‘domestication’ of two different viruses. However, through convergent evolution they exhibit many biological similarities (Drezen et al. 2017; Strand and Burke 2019).

The viruses replicate within the calyx cells of the wasps’ ovaries and are then secreted into the oviducts. Therefore, when a wasp injects her eggs into a suitable host, usually a lepidopteran caterpillar, she also injects the virus. The viruses cannot replicate within the caterpillar, but they do invade several of its cell types. Within these cells, the viruses integrate into the caterpillar’s genome and cause the expression of substances that facilitate the establishment of the parasitoid. For example, one of the main immune responses that insects express in response to an invader is encapsulation. Encapsulation depends upon recognition of the invader and then a co‐ordinated physiological response: amoeboid‐like cells present in the haemolymph surround the invader and then either kill it through the production of toxic chemicals and/or lack of oxygen or physically isolate it and thereby prevent it damaging the host.

If one implants wasp eggs without the virus into a host, then these are rapidly encapsulated and killed. The protective effect of the virus probably results from it causing the caterpillar to express protein tyrosine phosphatase enzymes and thereby interfering with the encapsulation process. Protein tyrosine phosphatases dephosphorylate the tyrosine residues of several regulatory proteins and are therefore closely involved in the regulation of signal transduction. Altering the levels of regulatory proteins makes it impossible for the host to express an effective immune response and therefore the parasitoid egg develops unmolested. The viruses also have other effects on the caterpillar including preventing its further development once it reaches the stage at which the parasitoid is to emerge. Consequently, the polydnaviruses have a mutualist‐like relationship with the parasitoid within which they replicate. They are transmitted vertically as an endogenous provirus that integrates into the wasp genome but have a pathogenic relationship with the parasitoid’s host, within which it cannot replicate.

It is probable that there are many other examples of symbiotic virus‐parasitoid/parasite relationships awaiting discovery. However, not all wasp parasitoids have relationships with viruses and these inject toxins that cause similar disruptions to the host immune response and host development.