Stress induces the eukaryotic sexual cycle in protists

Abundant evidence indicates that facultative sexual protists tend to undergo sexual reproduction under stressful conditions. For instance, the budding yeast Saccharomyces cerevisiae reproduces mitotically (asexually) as diploid cells when nutrients are abundant, but switches to meiosis (sexual reproduction) under starvation conditions. The unicellular green alga, Chlamydomonas reinhardi grows as vegetative cells in nutrient rich growth medium, but depletion of a source of nitrogen in the medium leads to gamete fusion, zygote formation and meiosis. The fissioning yeast Schizosaccharomyces pombe, treated with H2O2 to cause oxidative stress, substantially increases the proportion of cells which undergo meiosis. The simple multicellular eukaryote Volvox carteri undergoes sex in response to oxidative stress or stress from heat shock. These examples, and others, indicate that, in protists and simple multicellular eukaryotes, meiosis is an adaptation to deal with stress.