Quasispecies-like behavior observed in catalytic RNA populations evolving in a test tube.

Carolina Diaz Arenas,Niles Lehman

doi:10.1186/1471-2148-10-80

Abstract

BackgroundDuring the RNA World, molecular populations were probably very small and highly susceptible to the force of strong random drift. In conjunction with Muller's Ratchet, this would have imposed difficulties for the preservation of the genetic information and the survival of the populations. Mechanisms that allowed these nascent populations to overcome this problem must have been advantageous.ResultsUsing continuous in vitro evolution experimentation with an increased mutation rate imposed by MnCl2, it was found that clonal 100-molecule populations of ribozymes clearly exhibit certain characteristics of a quasispecies. This is the first time this has been seen with a catalytic RNA. Extensive genotypic sampling from two replicate lineages was gathered and phylogenetic networks were constructed to elucidate the structure of the evolving RNA populations. A common distribution was found in which a mutant sequence was present at high frequency, surrounded by a cloud of mutant with lower frequencies. This is a typical distribution of quasispecies. Most of the mutants in these clouds were connected by short Hamming distance values, indicating their close relatedness.ConclusionsThe quasispecies nature of mutant RNA clouds facilitates the recovery of genotypes under pressure of being removed from the population by random drift. The empirical populations therefore evolved a genotypic resiliency despite a high mutation rate by adopting the characteristics of quasispecies, implying that primordial RNA pools could have used this strategy to avoid extinction.

Highlights

During the RNA World, molecular populations were probably very small and highly susceptible to the force of strong random drift
The continuous evolution (CE) protocol is a means to induce the rapid evolution of ligase ribozymes using Moloney Murine Leukaemia Virus Reverse Transcriptase (MMLV-RT) and T7 RNA polymerase to sustain RNA populations through sequential serial transfers [20,23,24]
All four lineages were carried out for 50 bursts without a sign of population decay via Muller’s Ratchet, and a mutational meltdown was never observed [Additional file 1: Supplemental Figure S1]. These results were unlike those of previous data obtained in the absence of MnCl2, in which a meltdown was observed at an average of 24.3 bursts [22]

Summary

Introduction

During the RNA World, molecular populations were probably very small and highly susceptible to the force of strong random drift. A quasispecies is basically a steady-state dynamic of mutant molecules distributed around a parental genotype, the so-called master sequence, which occupies a central position in the genotypic network space. This dynamic occurs after a sufficient amount of time at high mutational rates, such that the progeny of an individual genotype (the mutant cloud) can be rapidly produced. Different genotypes can form mutant clouds of various sizes that can compete for survivorship during the evolution of the population This can generate a fluctuating equilibrium dynamic as clouds of mutants are replaced by other ones at the interplay of selection and random drift [6]

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