Reconciling Luria-Delbrück fluctuation test with phenotypic delay by effective polyploidy.

<p>(A) The original Luria-Delbrück mutation model disregards polyploidy. For instance, a phenotypic delay of two generations results in four mutants appearing at once. (B) The observation of many one-mutant (“singleton”) populations was interpreted as evidence against the existence of a delay [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004644#pbio.2004644.ref001" target="_blank">1</a>,<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004644#pbio.2004644.ref003" target="_blank">3</a>]. (C) With polyploidy considered, cells with four genome copies require two divisions to generate a homozygous mutant that expresses a selectable recessive phenotype. Therefore, a delay of two generations can generate just one mutant. (D) Heterozygous cells containing recessive mutations will not survive selection, leading to an underestimation of mutational events.