Accelerated evolution as a consequence of transitions to mutualism.

Differential rates of nucleotide substitutions among taxa are a common observation in molecular phylogenetic studies, yet links between rates of DNA evolution and traits or behaviors of organisms have proved elusive. Likelihood ratio testing is used here for the first time to evaluate specific hypotheses that account for the induction of shifts in rates of DNA evolution. A molecular phylogenetic investigation of mutualist (lichen-forming fungi and fungi associated with liverworts) and nonmutualist fungi revealed four independent transitions to mutualism. We demonstrate a highly significant association between mutualism and increased rates of nucleotide substitutions in nuclear ribosomal DNA, and we demonstrate that a transition to mutualism preceded the rate acceleration of nuclear ribosomal DNA in these lineages. Our results suggest that the increased rate of evolution after the adoption of a mutualist lifestyle is generalized across the genome of these mutualist fungi.