The effects of pathogen infection and mutation on life history characteristics in Arabidopsis thaliana

The nature of the interaction among deleterious mutations is important to m odels in many areas:of evolutionary biology. In addition, interactions betw een genetic and environmental factors may affect the predictions of such mo dels. Individuals of unknown genotypes of Arabidopsis thaliana, ecotype Mar burg, were-exposed to five levels of chemical (EMS) mutagenesis and three l evels of Pseudomonas syringae infection. Survival, growth and flowering cha racteristics of each individual were measured. The logarithm of fitness is expected to be a linear function of mutation number if mutations act indepe ndently. Furthermore, the expected number of mutations should be approximat ely a linear function of time of exposure to mutagen. Therefore, nonlinear effects of mutagen exposure on the logarithm of fitness characters would su ggest epistasis between mutations. Similarly, if pathogen infection and mut ation act independently of each other, their effects should be additive on a log scale. Statistical interactions between these factors would suggest t hey do not act independently; particularly, if highly mutated individuals s uffer more when infected than do less mutated individuals, this, suggests t hat pathogens and mutations act synergistically. Pseudomonas-infected indiv iduals were shown to have an increased probability of flowering under-condi tions of short day length, but to ultimately produce fewer flowers than uni nfected individuals. This suggests a plastic response to stress and, despit e that response, an ultimately deleterious effect of infection on fitness. Leaf rosette growth was negatively and linearly related to the expected num ber of mutations, and the effects of mutation on different life-cycle stage s appeared to be uncorrelated. No significant interactions between pathogen and mutation main effects were found. These results suggest that mutations act multiplicatively with each other and with pathogen infection in determ ining individual fitness.