mutation pathway
Beaumont HJE, Gallie J, Kost C, Ferguson GC & Rainey PB 2009 Experimental evolution of bet hedging. Nature 462:90-93.
- nine mutations separating 1B4 from the original ancestor were identified
- confirmed by Sanger sequencing
- ordered by inspection of the affected loci in the preceding genotypes
- with the exception of the final mutation, all mutations involved non-synonymous changes at loci previously demonstrated to be mutational targets in the evolution of wrinkly spreader types
- the final mutation was a single non-synonymous nucleotide change in carB (Arg674Cys, Fig. 3b)
- to examine the causal connection between the carB mutation and colony switching, we introduced this mutation in the immediate ancestor (1A4) by allelic replacement
- the engineered genotype displayed colony switching (Fig. 3g)
- reversion of the carB mutation in 1B4 to wild type abolished colony switching (Fig. 3h)
- the carB mutation is sufficient and necessary to cause stochastic colony morphology switching in 1B4
- colony switching was caused by a single point mutation but nonetheless took nine rounds of selection to evolve
- this led us to question the importance of the previously fixed mutations
- (Blount et al. 2008)
- one round of the evolutionary experiment was repeated from both 1A4 and the original ancestor (1A0)
- colony switching evolved from 1A4 (3 from 36 replicates)
- never from the ancestral genotype (0 from 138 replicates)
- indicating that these genotypes differed in their capacity to give rise to colony switching
- the reliance on previously fixed mutations might stem from epistatic interactions essential for the carB mutation to cause colony switching
- or to confer the requisite fitness benefit in static microcosms
- we distinguished between these hypotheses by introducing the carB mutation in 1A0
- in this background it did cause colony switching (Fig. 3f) but appeared not to confer a significant fitness increase (Fig. 4)
- the latter was confirmed by a direct statistical comparison with the effect of the carB mutation in 1A4
- which identified a significant epistatic interaction
- and indicated that the carB mutation is beneficial in 1A4 but deleterious in 1A0
- the evolutionary history of 1A4 'set the stage' for the evolution of stochastic colony morphology switching by altering the relative fitness effect of the carB mutation