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In order to
reproduce, the wasp, Cotesia congregata, lays her eggs in the
tobacco sphinx caterpillar (Manduca sexta). The caterpillar
thus serves as a protective storehouse for the larva of the wasp, and
dies when the larva attain a sufficient stage of maturity to burst out
of the bloodless body of their host where they continue their
transformation, metamorphosing into pupas, and then flying adults.
More than half the Hymenoptera reproduce in this way. Of course the insect host does not let itself be taken advantage of easily. Its main defense mechanism consists in surrounding the egg of the parasite in a sort of capsule of cells which liberate toxic substances to destroy the invader. Ever since the adoption of a parasitic mode of life by the Hymenoptera in the Jurassic era, the host species has constantly evolved, because the genes which permit an escape from parasitism are advantageous for the wasp. As a response to this, new means of avoiding the immune defenses of the caterpillar have been selected in Hymenoptera.
The two organisms have thus evolved in parallel such that the parasite
always finds something to eat. It turns out that there is a virus
which permits the wasp to manipulate the physiology of the caterpillar
so that the wasp larva can develop.
Virus particles, which have the form of cylindrical rods in a protein matrix, are introduced at the same time as the parasite eggs, when they are laid in the body of the caterpillar. There, the genes of the virus are expressed, leading to modifications in the physiology of the host.
These induced modifications may differ depending on the biological system, but they are always advantageous for the parasite. They can be divided into categories: inactivation of host defense mechanisms, and perturbation of its development. Generally,the parasitized caterpillar does not go through metamorphosis and remains blocked in a prepupal stage, which allows the parasite to continue its own development.
To understand the role of these viruses, more precisely polydnaviruses, in the success of wasps as parasites, it is important to know not only the proteins which are expressed in the host, but also the manner in which their production is controlled by the wasp, as well as their precise characteristics.
This is the goal of the genetics group of the IRBI
(Institute for Research on the Biology of Insects). One of the goals, at the end of the project, will be to develop new methods of control of harmful insects based on a better understanding of the molecular bases for these mechanisms of circumventing the defenses of the host. The results published in Science (8 October 2004; 306:286-289) removed some of the obscurity surrounding the organization of the viral genome, its evolutionary origin and its characteristics.
The first surprise is that the complete sequence of the DNA (about 568
000 base pairs) of the virus particle introduced into the caterpillar
reveals a complex genomic organization that resembles a genomic region
of a eukaryote more than that of a virus. In contrast to other known
viruses, the DNA of the polydnavirus is very gene-dense. It contains a
total of 156 coding regions, of which 42% have no homology with known
genes. Furthermore, this genome does not contain any groups of genes
which can be linked to a known viral family, and no gene which is
similar to a major virus gene.
Another unusual characteristic is the abundance of gene families: 66 genes are organized into 9 families. Another interesting fact: the proteins produced from 4 of these gene families contain domains previously described in toxins utilized by pathogenic bacteria (Pseudomonas, Yersinia, Salmonella,...) or parasitic worms, i.e. by bacteria and eukaryotes.
The current hypothesis is that these virulence genes may have come from the wasp genome during the evolution of lineages. These polydnaviruses may therefore not be descended from a large viral genome, but were constructed from the genome of the wasp using a system of production of circular DNA - because the genome present in particles produced in the ovary of the wasp is composed of 30 DNA circles.
On the other hand, the proteins of each of the capsids containing this genome would have a viral origin, probably acquired in the course of an infection. The genes implicated in the replication of this original virus were probably not transferred into the genome of the wasp.
“Nature,” estimates CNRS scientist Jean-Michel Drezen, "seems to have developed here a technique for gene transfer which is similar to the medical approaches currently being used to produce pseudoviruses used to transfer genes with a therapeutic goal, and to allow their expression."
|The study of the molecular bases of host-parasite interactions in the insect could lead to
the identification of new natural insecticides and the development of products which are
not dangerous for the environment, and also to the creation of innovative technologies for
The molecules produced by the parasites and/or their associated viruses, which are toxic for the host, should be more selective because of the high specificity of the mechanisms of regulation of the host by the parasite.