Let's jump right in, shall we?
The hottest topic in virology these days is the sudden reemergence of the age old "are viruses alive?" debate. The basic properties used to describe something as "living" are:
- Does it have the ability to grow?
- Does it have the ability to reproduce?
- Can it maintain an internal homeostasis?
- Can it respond to stimuli, and carry out various metabolic processes?
- Has it, and can it evolve over time?
There are three theories for the origin of viruses that have been respectfully agreed-upon: (1) the progressive hypothesis, (2) the regressive hypothesis, and (3) the virus-first hypothesis. Let's discuss, shall we?
The progressive hypothesis:
The progressive hypothesis states that mobile genetic elements found the ability to leave one cell and enter another, progressively picking up additions as they traveled, forming infectious agents. The progressive hypothesis is supported by the existence and they behaviors of retrotransposons. Retrotransposons are the essence of the progressive hypothesis, as they are genes that move along the genome with the aid of RNA intermediates. While transitive within a single cell's genome, retrotransposons are currently limited to the confines of that cell. We've seen retrotransposons in many eukaryotic genomes, and attribute them to assisting with evolution.
The progressive hypothesis, which leans so heavily on retrotransposons for support, also has a viral example: retroviruses (which as you know, are some of my favorites). The single stranded RNA genome of a retrovirus utilizes a number of enzymes to fuel reverse transcription and integration into the host's genome. But, while retrotransposons are limited to one cell, retroviruses are able to bud from the host cell and move to another.
The regressive hypothesis:
The regressive hypothesis states that the formation of viruses occurred due to reductive evolution. Regressive evolution is not as common, but is supported. On a singular cell level, the best example of regressive evolution is that of the mitochondria. Mitochondria are organelles in eukaryotic cells that are responsible for the generation of most of the cell's energy. Mitochondria are comprised of multiple layers and membranes, and have their own set of DNA that is handed down through maternal ties. Due to their complex structure and their individual set of DNA, researchers believe that mitochondria were once bacteria that devolved to become an organelle.
The virus-first hypothesis:
The virus-first hypothesis takes a completely different approach to the origin of viruses, as it suggests that viruses originated before other cellular life. This hypothesis is really hard for some people to grasp, since we currently identify viruses by their dependency on a host for continued replication and existence. But what if viruses existed first? The simplistic design of a virus and the high rate of mutations that occur within viral replication are both promising reasons for the virus-first hypothesis.
Why am I talking about this?
A study published in Nature seems to have illustrated an adaptive quality in bacteriophages, wherein they begin to express somewhat of an "immune response" to the host's innate immune response. Researchers have shown that specific short palindromic repeats and associated proteins (CRISPR/Cas, for those who are interested) protect vital regions of the bacteriophage genome when exposed to potential threats, like invading amino acids or other bacteriophages.
Now, this article on Discovery News jumps the gun a little bit in the headline alone. "Viruses Pass Major Test to Enter Ranks of Living"? Now just hold on a gosh darned second.
Well, actually, I think I might let you decide. What do you think? Does this mean viruses are living things? Additionally, how do you think people will react to hearing that viruses may be living?
Or do you even care?