TESTING FOR EPISTASIS BETWEEN DELETERIOUS MUTATIONS

Determining the way in which deleterious mutations interact in their e ffects on fitness is crucial to numerous areas in population genetics and evolutionary biology. For example, if each additional mutation lea ds to a greater decrease in log fitness than the last (synergistic epi stasis), then the evolution of sex and recombination may be favored to facilitate the elimination of deleterious mutations. However, there i s a severe shortage of relevant data. Three relatively simple experime ntal methods to test for epistasis between deleterious mutations in ha ploid species have recently been proposed. These methods involve cross ing individuals and examining the mean and/or skew in log fitness of t he offspring and parents. The main aim of this paper is to formalize t hese methods, and determine the most effective way in which tests for epistasis could be carried out. We show that only one of these methods is likely to give useful results: crossing individuals that have very different numbers of deleterious mutations, and comparing the mean lo g fitness of the parents with that of their offspring. We also reconsi der experimental data collected on Chlamydomonas moewussi using two of the three methods. Finally, we suggest how the test could be applied to diploid species.