Stress, Sex, Cysts and Spores: Selection for syngamy, meiosis and dormancy under resource limitation in eukaryotic microbes

In facultatively sexual species, which reproduce both sexually and clonally, sexual reproduction is often triggered by stress. There exists an extensive theoretical literature to explain this pattern, broadly relying on the fact that the benefits of sexual reproduction are highest when an organism is maladapted to its environment. However, in unicellular eukaryotes a single round of sexual reproduction is often followed by a dormant phase, with clonal reproduction resuming under the restoration of resource-rich conditions to which the organism appears already adapted. In this paper we review the typical life cycles of four model eukaryotic organisms; the green alga Chlamydomonas reinhardtii, the fission yeast Schizosaccharomyces pombe, the budding yeast Saccharomyces cerevisiae and the amoebozoa Dictyostelium discoideum. We also review existing theory as it pertains to these life histories, paying particular attention to the conception of gametes, which are here often morphologically indistinguishable from vegetative cells. In this context we use mathematical models to illustrate how dormancy, syngamy and meiosis could each independently be selected for in response to a particular form of stress, namely resource limitation, under which the opportunity costs of sexual reproduction are minimized (rather than the benefits being maximized). We conclude by addressing open questions.