Question

List three hypotheses for the evolution of viruses. Briefly describe the discovery and the nature of “Giant viruses” such as Mimivirus, describing how these viruses differ from most known viruses. How may the tree of life need to be reshaped to accommodate these viruses?

Answer 1

Viruses

These are acellular or non-cellular, non-cytoplasmic infectious agents formed of nucleic acid and protein. It is an obligate parasite & cannot complete it's life cycle without a host. Viruses are infectious agents. They are smaller than bacteria and can be seen only under Electron Microscope. Its genetic material is either DNA or RNA.

Evolution of VIruses

As regards the Origin or Evolution of Viruses, 3 theories or Hypothesis have been put forward. They are:

1. The Progressive Hypothesis: Viruses originated through a progressive process where genetic elements gained the ability of exiting one cell and entering another.

2.The Regressive Hypothesis: This theory or hypothesis opposes the Progressive Hypothesis. This hypothesis says that viruses may have originated via a reductive or regressive process. Existing viruses may have evolved from more complex free living organisms which lost genetic information as time progressed because they adapted parasitic approach to replication.

3. The Vius-First Hypothesis: Both the Progressive and Regressive Hypothesis believe that cells exixted before viruses. This theory says that viruses existed in the pre-cellular world as self-replicating units. As time progressed, they became more organised and more complex.Viruses may have existed before bacteria, archaea and eukaryotic cells.

Mimivirus is the largest and most complexes of the viruses known. It was first observed in 1992 as APMV or Acanthamoeba polyphaga mimivirus

Mimivirus has been named because of Mimicking Microbe. It also has gram staining properties.

Particles resembling bacteria (Gram positive) residing within amoeba was isolated from a water cooling tower. Since other bacterial species causing Pneumonia reside within amoeba, It was assumed to be a new Pneumonia causing bacterium which was later Classified as a large virus.

Features of Mimivirus:

Large size, Gram staining properties, fibres extend out from it's capsid. ( as compared to traditional viruses)

The giant viruses form the 4th branch of tree alongwith bacteria, archaea and eukarya( plants, animals and all other animals with nucleated cells).