A stress-induced polyphenic switch from apomeiosis to meiosis occurs in Boechera (Brassicaceae) that is cytologically and molecularly comparable to those of other kingdoms

In many eukaryotic apomicts, stress activates an epigenetic switch from apomixis (unreduced gamete formation and parthenogenesis) to sex (meiotic gamete formation and syngamy). For this to occur, a third polyphenic gender, an apomictic female, is often observed. She is well adapted to favorable conditions, is adept at producing clonal progeny, and, though genetically identical, often differs phenologically and morphologically from sexual females. Stress-activated epigenome reprogramming occurs in her progeny during their development such that sexually-functional males and females form. The genetically-identical siblings often mate to produce fertilized spores or eggs, which tolerate the stresses that induced their formation, e.g., starvation, cold, heat, desiccation, etc. When favorable conditions return, apomictic females form from the fertilized spores or eggs. This cyclical-apomixis pattern occurs in animals, fungi, protists, diatoms, and brown and green algae. In apomictic angiosperms, polyphenic genders are not generally recognized. Instead, apomixis is usually considered to be facultative, with seeds often forming sexually and apomictically on the same plant. Herein, we document for Boechera (Brassicaceae) stress-induced switching from mostly apomeiotic to mostly meiotic ovule development, and we show that this switching includes global epigenetics-based changes in gene expression that produce gene ontology patterns in ovaries typical of those observed during reproduction in obligate sexual plants. Our findings unify angiosperms with other eukaryotes in terms of i) tendencies for stress-induced polyphenic reversions from apomixis to sex and ii) physiological adaptations in apomicts that maximize fecundity thus facilitating geographic parthenogenesis. A primitive origins hypothesis for sex-apomixis polyphenisms, with functionality more-or-less conserved among eukaryotes, is supported.