Microbiology

An Overview of the Quasispecies Model of Sequence Evolution



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The quasispecies model of sequence evolution describes a group or cloud of related genotypes with a great deal of mutations. In comparison to a species in which genotypes tend to remain stable from one generation to the next, the offspring in a quasispecies have different genotypes from the parents because of mutations. There are often seen in self-replicating DNA and RNA, as well as bacteria and viruses. Manfred Eigen put forth the original research, as did Peter Schuster.

High reproductive fidelity explains a situation in which a species of one genotype tends to reproduce more of the same genotype. In a quasispecies, however, the fidelity is low as the mutations occur rapidly and in many different offspring. They may come and go, and the individuals of the group may only be one mutation away from each other. Thus they are clearly still related. Because they are so close, they can go back and forth. For instance, the offspring of one can contain a mutation making it different from the parent, but then the subsequent offspring of that can go back to the grandparent.

A cloud has been used to represent the concept of quasispecies, such that the members of the group are all within the figurative cloud, and offspring jump to different points within it. For a quasispecies to exist requires a great amount of individuals and many mutations. 

Even in species, mutations obviously exist, and thus works the theory of evolution. Genotypes compete, and the strongest takes over by producing an organism with the greatest chance of reproducing. Even a genotype in small numbers may become dominant if its fitness is much more powerful.

In a quasispecies, these mechanisms do not work the same way because of the rapid mutations. Even when a change in a genotype produces greater reproduction, its children will feature more mutations, thus the line of expanded fitness will not continue. In a quasispecies, researchers instead focus on how connected the cloud is. They consider how viable the related genotypes are of the group. If most are viable and can reproduce, then the fitness is maintained as many descendents will be produced. If a high rate of the sequences are not viable, then many will not reproduce (yet they will still be produced as offspring as the mutations will continue to occur despite their lack of reproductive ability). In this case, they would have a low fitness. 

The quasispecies model contains four important assumptions. The first is that the individuals are made of a small amount of building blocks. One example would be RNA with its bases adenine, guanine, cytosine, and uracil. The second is that a copy process creates new sequences into the system. This process occurs from sequences that are in existence. The third assumption is that there are always enough materials for replication to constantly continue. Finally, sequences can decay. New or old have the same probability of this decay, which leads back to the building blocks.

The mutations that occur in a quasispecies do so through errors as existing sequences are copies. Some sequences will replicate faster, however even those that replicate slowly will always still appear due to replenishing.

The quasispecies model may or may not have practical use when applying them to organisms.  Scientists have different viewpoints on this theory, however it is of importance when studying viruses that mutate in great numbers such as HIV. Research continues, and as science advances new information should become available about this fascinating evolutionary view. 

Resources:

http://www.cdc.gov/ncidod/eid/vol4no4/domingo.htm

http://arxiv.org/PS_cache/cond-mat/pdf/0202/0202054v1.pdf

http://www.ncbi.nlm.nih.gov/pubmed/16386760

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