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Home | Seminars and Symposia | Past seminars/symposia: Friday, February 1, 2008

Towards a molecular description of adaptive evolution


Gavin Sherlock
Stanford University

Friday, February 1, 2008
12:00 Lunch
12:15 Seminar

402 Walter Library

In the classical model of asexual evolution by Muller in 1932, microorganisms undergo periodic selection, and as a result, a beneficial mutant sweeps through the population. These series of expansions of beneficial mutants are termed adaptive sweeps. Recent evidence has emerged suggesting that adaptive sweeps may not be complete in evolving populations, but as of yet, no population-wide experimental studies have determined whether this is indeed true. To answer this question, we evolved fluorescently marked yeast strains under nutrient-limited conditions, and were able to track more precisely the expansion of beneficial mutants and determine whether these adaptive sweeps are indeed complete. The data revealed complex population dynamics and unequivocally disprove the traditional model of clonal replacement. Using FACS to sort out and count the subpopulations throughout the course of the evolution, we were able track the expansion of beneficial mutants, and obtain a more accurate estimate of the fitness coefficient of the adaptive mutants. Using these data, we can estimate the relative abundance of each adaptive mutant within each subpopulation. We are also determining whether the subsequent adaptive mutants carrying the same fluorescent marker are transitive (a latter mutant derived from an earlier one) or independent. Using tiling microarrays, we have been able to identify the exact nucleotide changes that are exist in the emerging adaptive clones, which allows us to clarify whether some later clones are derived from earlier ones, or whether they are independent. Transcriptome analysis, to determine the changes in transcriptional programming in the evolved strains reveals the consequences of these changes. Thus, we know the molecular events responsible for each adaptive mutant, and the phenotypic consequences at the level of the transcriptome and in relative fitness. These data help to reveal the fitness landscape of evolving populations and provide a comprehensive molecular description of evolutionary events.