Paleovirology is the study of ancient viruses, typically over prehistoric or geological timescales. There is no physical ‘fossil record’ of viruses; virions persist for short time periods, and rapidly degrade leaving no direct trace of their existence. Many viruses can enter the genomes of their hosts—some, such as retroviruses, do so as an obligate step during their replication process, and others can occasionally do so, either by accident or as a latent part of their life cycle. When viral integrations occur in the germline of their host, they can be passed on to the next generation, potentially fixing in the host population. When this occurs, the integrated endogenous virus genomes evolve at host rates of mutation, and their sequence is relatively stably preserved. The study of this genomic ‘fossil record’ has led to the burgeoning field of paleovirology, which uses these endogenous viruses to disentangle the long-term evolutionary history of virus–host interactions.
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