Somatic Mutations in Organisms with Complex Life Histories

Abstract

A theoretical model is developed of the fate of mutations for organisms with such life-history characteristics as indeterminate growth and clonal reproduction. It focuses on how the fate of a particular mutant depends on whether it arises during mitotic cell division (somatic mutation) or during meiotic cell division (meiotic mutation). At gamete production, individuals carrying somatic mutations will produce some proportion of gametes reflecting the original, zygotic genotype and some proportion reflecting genotypes carrying the somatic mutation. Focusing on allele frequencies at gamete production allows the effects of growth and clonal reproduction to be summarized. The relative strengths of somatic and meiotic mutation can be determined, as well as the conditions under which the change in allele frequency due to one is greater than that due to the other. Examples from a published demographic study of clonal corals are used to compare somatic and meiotic mutation. When there is no selection acting on either type of mutation, only a few cell divisions per time unit on average are needed for the change in allele frequency due to somatic mutation to be greater, given empirically based mutation rates. When somatic selection is added, the most dramatic effect is seen with fairly strong negative selection acting against the somatic mutation within individuals. In this case, selection within organisms can effectively counteract the effects of somatic mutation, and the change in allele frequency due to somatic mutations will not be greater than that due to meiotic mutations for reasonable numbers of within-generation cell divisions. The majority of the mutation load, which would have been due to somatic mutation, is purged by selection within the individual organism.