2 Explain the originevolution of viruses by these hypotheses

2.) Explain the origin/evolution of viruses by these hypotheses: reductive, additive, horizontal gene transfer, and common ancestor. Why are there so many different ideas? Is there any one right answer? Why?

Solution

Origin of virus has been hypothesized by three main views:

1. Reductive hypothesis: This states that viruses were initially small parasites of large cell. Just like parasites, they used to take nutrition from the host; and left the host when host\'s body became diseased and useless for the virus. This hypothesis is also known as degeneracy hypothesis. This hypothesis is not accepted much, especially in case of large viruses like pox virus. Moreover, no parasites on earth resemble viruses at all.

2. Additive hypothesis: Complex protein and nucleic acids combined together to form the infectious agents, i.e. viruses. This hypothesis is also known as virus-first hypothesis. In this hypothesis, the actual definition of virus is totally changed. Why couldn\'t the genetic material of replication machinery combine together during the course of evolution; even after being more fit than the rest of the genetic material?

3. Horizontal gene transfer: Small bits of RNA or DNA escaped from large organisms and then underwent homologous recombination; to form infectious agents known as virus. The process was random. This is also known as escape hypothesis. This hypothesis was not accepted much because it could not explain the formation of capsid around the genetic material.

So many different answers are there, because the genetic material of virus is of different types like ssRNA (+ and -), dsRNA (+ and -), ssDNA, dsDNA etc. This is not the case with cellular organisms like prokaryotes and eukaryotes. All of them have dsDNA as genetic material. Now, because of different genetic material, it is very difficult to trace the common ancestor of all viruses. Therefore, different researchers have proposed different hypothesis from time to time.

None of these hypotheses has been accepted. One right answer is Common ancestor hypothesis. This states that there must be some common ancestor from which all viruses originated.