Phenotypic Effects of Somatic Mutations Accumulating during Vegetative Growth

The unique life form of plants promotes the accumulation of large numbers of somatic mutations that can be passed on to offspring in the next generation. However, rates of mutation accumulation remain similar in plants and animals. The solution to this paradox may be that plants have the potential to filter somatic mutations prior to seed dispersal via three forms of intra-generation selection. To provide an experimental test of this hypothesis, we used plants of Mimulus guttatus to compare the performance of offspring from self-pollinations made within the same flower (autogamy), to offspring from self-pollinations between stems on the same plant (geitonogamy). The fitness effects of somatic mutations are expected to differ between progeny from these crosses, as autogamy will result in homozygosity of a proportion of somatic mutations, but geitonogamous progeny will remain heterozygous. We developed a novel analytical approach to test whether the homozygous effects of somatic mutations were evident from differences in the means and variances of fitness between autogamous and geitonogamous progeny. Consistent with our predictions, the variance in progeny fitness among stems was significantly greater for autogamous compared to geitonogamous progeny from the same stem. Surprisingly, several autogamous progeny groups displayed increased fitness compared to geitonogamous controls, indicating that the effects of beneficial somatic mutations were prevalent in progeny. Phenotypic effects of somatic mutations were also evident from the positive association between mean fitness and the within-group variance in fitness for autogamous progeny. These results support the hypothesis that somatic mutations accumulate during vegetative growth, but they are filtered by the different forms of intra-generation selection, resulting in the culling of expressed deleterious mutations and the retention of beneficial mutations.