Mutation profile under salt stress in C. reinhardtii

The genetic basis of adaptations is driven not only by selection, but also by the spectrum of spontaneous mutations. Given that the rate of mutation is not uniformly distributed across the genome and varies depending on the environment, understanding the signatures of selection across the genome is aided by first establishing what the expectations of genetic change are from mutation. To determine the interaction between salt stress, selection, and mutation across the genome, We compared the rates and patterns of mutation observed in a selection experiment for salt tolerance in \textit{Chlamydomonas reinhardtii} to those observed in mutation accumulation experiments with and without salt exposure. We found that salt stress led to an increased rate of indel mutations, but that these mutations were removed under selection. Finally, lines adapted to salt also showed spatial and network-based clustering of mutations, suggesting a role for multi-nucleotide mutations in the evolution of salt tolerance. The differences we observed indicate a complex interaction between the selective environment, fitness, and mutation.