Escuela de Lenguas UNLP/Academic Writing/Biology/Virology

 The origin of viruses 

Although there is no general consensus on the origin of viruses, most of the proposed models can be framed into three hypotheses, known as the pre-cellular, endogenous and reductive theories. Two of them (endogenous and reductive theories) usually postulate an "ancestral virus" contemporary with the "last universal common ancestor" (LUCA) or the first organisms of the three domains of life. Most viral proteins do not have cell homologues; even the replication mechanisms and the nature of the genomes differ (totally or partially) between viruses and cells. This low phylogeny contrasts with the similarities between viruses that infect the three domains of life, and suggests that the origin of viruses was temporally close to or even previous to LUCA. -The pre-cellular theory During the appearance of the first cell line, it is postulated that the genetic elements that were unable to generate machinery for their own replication could never have developed mechanisms to infect ancestral cells and use host machinery to ensure their propagation. All the genetic elements shared the same primitive broth. At first these primitive genomes would have shared a space with genetic elements that had replicative strategies, so its perpetuation occurred incidentally. The appearance of membranes that protected the replicative genomes would have been the evolutionary pressure responsible for the development of modes of entry by non-replicating genetic elements to the hosts, and their constitution as pre-cellular parasites. The latter would involve the production of proteins in the primitive broth, probably by ribozymes, due to the need for replications by non-replicative RNAs. -The endogenous theory In a scenario prior to LUCA, genomic fragments of RNA cells may have escaped from their cellular environment, giving autonomous and infectious elements. In the old RNA cells the molecular mechanisms related to the genome could have been simpler and less integrative than those of modern cells. It has been claimed that the genomes of these ancestral lines were segmented, probably semi-autonomous elements, which replicated independently and had the ability to transfer randomly between cells. Some of these segments may have coded for hedging proteins that aided the transfer, acquiring infective capacity (protovirus). -The reductive theory The possibility for the viruses to have derived from parasitic cellular organisms has been postulated, and it is believed that those cells had lost in the course of evolution many of the "dispensable" structures to replicate in a host. However, there is not an intermediary between cells and viruses, and the fact that parasites of the three domains of life (such as Mycoplasma in Bacteria, Microsporidia in Eucaria, and Nanoarchaea in Archaea) retain their cellular characteristics, makes the possibility of reversal of a modern cell unlikely. It is expected that these forced endosymbiont parasites have lost their own machines of synthesis and energy production, against the most effective metabolic routes of their guests in the course of evolution. An alternative hypothesis It has been postulated that viruses could be responsible for the onset of DNA genomes. According to this theoretical trend, the first cells had an RNA genome, just like the ancestral cellular parasites. It is believed that the development of mechanisms of protection against foreign RNA by these cells could have constituted an enough selection pressure for the viruses to develop a DNA genome in order to bypass the host defences. Over time, the cells might have acquired foreign DNA and incorporated it as their own. Due to the fact that DNA possesses greater chemical stability and a lower mutation rate, "transformed" cells would have evolutionary advantages, and would have displaced primitive RNA cells.