Extremely High Mutation Rate of a Hammerhead Viroid by Gago et al

This Science paper confirms the MGD hypothesis. The author stated: "Such error-prone replication can only be tolerated by extremely simple genomes such as those of viroids." The simpler the organism, the more mutations it can tolerate.

Science 6 March 2009:
Vol. 323. no. 5919, p. 1308
DOI: 10.1126/science.1169202 Prev | Table of Contents | Next
BREVIA
Extremely High Mutation Rate of a Hammerhead Viroid
Selma Gago,1 Santiago F. Elena,1 Ricardo Flores,1 Rafael Sanjuán1,2*
The mutation rates of viroids, plant pathogens with minimal non-protein-coding RNA genomes, are unknown. Their replication is mediated by host RNA polymerases and, in some cases, by hammerhead ribozymes, small self-cleaving motifs embedded in the viroid. By using the principle that the population frequency of nonviable genotypes equals the mutation rate, we screened for changes that inactivated the hammerheads of Chrysanthemum chlorotic mottle viroid. We obtained a mutation rate of 1/400 per site, the highest reported for any biological entity. Such error-prone replication can only be tolerated by extremely simple genomes such as those of viroids and, presumably, the primitive replicons of the RNA world. Our results suggest that the emergence of replication fidelity was critical for the evolution of complexity in the early history of life.

Also see this Science comment: "Fast-Mutating Viroids Hold Clues to Early Life" by Carl Zimmer
http://blogs.sciencemag.org/origins/2009/03/fast-mutating-viroids-hold-clu.html

"What's intriguing about this pattern is the size of the genomes involved: The higher the mutation rate, the smaller the genome."