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.
