Academic journal article Genetics

Survival Probability of Beneficial Mutations in Bacterial Batch Culture

Academic journal article Genetics

Survival Probability of Beneficial Mutations in Bacterial Batch Culture

Article excerpt

ABSTRACT The survival of rare beneficial mutations can be extremely sensitive to the organism's life history and the trait affected by the mutation. Given the tremendous impact of bacteria in batch culture as a model system for the study of adaptation, it is important to understand the survival probability of beneficial mutations in these populations. Here we develop a life-history model for bacterial populations in batch culture and predict the survival of mutations that increase fitness through their effects on specific traits: lag time, fission time, viability, and the timing of stationary phase. We find that if beneficial mutations are present in the founding population at the beginning of culture growth, mutations that reduce the mortality of daughter cells are the most likely to survive drift. In contrast, of mutations that occur de novo during growth, those that delay the onset of stationary phase are the most likely to survive. Our model predicts that approximately fivefold population growth between bottlenecks will optimize the occurrence and survival of beneficial mutations of all four types. This prediction is relatively insensitive to other model parameters, such as the lag time, fission time, or mortality rate of the population. We further estimate that bottlenecks that are more severe than this optimal prediction substantially reduce the occurrence and survival of adaptive mutations.

KEYWORDS fixation probability; serial passaging; adaptation; experimental evolution; life history

(ProQuest: ... denotes formulae omitted.)

IT is well understood that most de novo mutations, even if they confer a substantial fitness advantage to the organism, do not survive the vicissitudes of genetic drift when initially rare (Fisher 1922; Haldane 1927; Wright 1929; Kimura 1964). The influences of population size, population structure, and environmental fluctuations on the fate of beneficial mutations have all been well studied, including cyclic (Otto and Whitlock 1997; Pollak 2000) or dynamically changing population sizes (Lambert 2006; Parsons and uince 2007a,b), population subdivision (Barton 1993; Cherry 2003; Whitlock 2003) and migration (Lundy and Possingham 1998; Shpak and Proulx 2007), fluctuating selection (Haccou and Iwasa 1996; Lande 2007), or several of these factors in combination (Uecker and Hermisson 2011; Waxman 2011).

A phenomenon that is perhaps less well appreciated is the sensitivity of a beneficial mutation's fate-survival or extinction-to the details of the organism's life history and the trait affected by the mutation. Studies of the extinction process in a population of changing size have demonstrated that extinction probabilities sensitively depend on whether mutations increase birth rates or reduce death rates (Parsons and Quince 2007a,b) or if mutations affect other life-history traits (Lambert 2006). Previous work has also compared the fate of mutations that reduce generation times with those that increase offspring survival (Wahl and Dehaan 2004). From these studies a clear picture is emerging: beneficial mutations that confer the same selective advantage, once established, can have quite different probabilities of surviving drift when rare.

While these effects have been well studied in viruses (Alexander and Wahl 2008; Patwa and Wahl 2008), the survival of de novo mutations in bacterial populations has received relatively little attention. Using a model specificto bacterial fission, Johnson and Gerrish (2002) demonstrated that if single-strand DNA damage is incorrectly repaired immediately prior to replication, the fixation probability of the resulting mutation is ^ 4s,wheres is the selective advantage; in contrast, the familiar 2s was recovered for a mutation occurring through a copy error during DNA replication. The impact of the severe population bottlenecks inherent in bacterial serial passaging has also been examined in some detail (Wahl and Gerrish 2001; Wahl et al. …

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