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