Abstract
Chlamydomonas (Chlorophyta) can grow as a heterotroph on medium supplemented with acetate in the dark. A long-term experiment to investigate adaptation to dark conditions was set up with hundreds of replicate lines. Growth was initially slow, and most lines became extinct when transferred every few weeks. Some lines survived through the expansion of lineages derived from cells with extreme phenotypes and exhibited a U-shaped curve of collapse and recovery. Two short-term experiments were set up to evaluate the effect of sex on the frequency of 'evolutionary rescue' by deriving replicate lines from ancestral populations with contrasting sexual histories that had been cultured in the light for hundreds of generations. When transferred to dark conditions of growth, lines derived from obligately sexual populations survived more often than lines derived from facultatively sexual or asexual populations. This reflected the higher initial frequency of cells able to grow in the dark, due to greater genetic diversity supported by sexual fusion and recombination. The greater probability of evolutionary rescue suggests a general reason for the prevalence of sexual species.