Читать книгу Basic Virology - Martinez J. Hewlett - Страница 22

There is ample genetic evidence that the interaction between viruses and their hosts has a measurable impact on evolution of the host. Viruses provide environmental stresses to which organisms evolve responses. Also, it is possible that the ability of viruses to acquire and move genes between organisms provides a mechanism of gene transfer between lineages.

Development of the immune system, the cellular‐based antiviral interferon (IFN) response, and many of the inflammatory and other responses that multicellular organisms can mount to ward off infection is the result of successful genetic adaptation to infection. In addition, virus infection may provide an important (and as yet underappreciated) basic mechanism to affect the evolutionary process in a direct way.

There is good circumstantial evidence that the specific origin of placental mammals is the result of an ancestral species being infected with an immunosuppressive proto‐retrovirus. It is suggested that this immunosuppression permitted an immunological accommodation in the mother to the development of a genetically distinct individual in the placenta during a prolonged period of gestation!

Two current examples provide very strong evidence for the continued role of viruses in the evolution of animals and plants. Certain parasitic wasps lay their eggs in the caterpillars of other insects. As the wasp larvae develop, they devour the host, leaving the vital parts for last to ensure that the food supply stays fresh! Naturally, the host does not appreciate this attack and mounts an immune defense against the invader – especially at the earliest stages of the wasp's embryonic development. The wasps uninfected with a polydnavirus do not have a high success rate for their parasitism, and their larvae are often destroyed. The case is different when the same species of wasp is infected with a polydnavirus that is then maintained as a persistent genetic passenger in the ovaries and egg cells of the wasps. The polydnavirus inserted into the caterpillar along with the wasp egg induces a systemic, immunosuppressive infection so that the caterpillar cannot eliminate the embryonic tissue at an early stage of development! The virus maintains itself by persisting in the ovaries of the developing female wasps.

A further example of a virus's role in development of a symbiotic relationship between its host and another organism can be seen in replication of the Chlorella viruses. These viruses are found at concentrations as high as 4 × 10⁴ infectious units/ml in freshwater throughout the United States, China, and probably elsewhere in the world. Such levels demonstrate that the virus is a very successful pathogen. Despite this success, the viruses can only infect free algae; they cannot infect the same algae when the algae exist semi‐symbiotically with a species of paramecium. Thus, the algae cells that remain within their symbiotes are protected from infection, and it is a good guess that existence of the virus is a strong selective pressure toward establishing or stabilizing the symbiotic relationship.