Sex is generally thought of as meiosis, conjugation, and syngamy, with the primary function of sex believed to be genetic mixing. However, conjugation does not occur with complete automixis, whereas syngamy does not occur with restitutional automixis. Self sex in the forms of automixis and autogamy does not include genetic mixing. Yet sex, including self sex, is necessary for most eukaryotic lineages. What is the purpose of sex without genetic mixing? Obligate self sex is not an evolutionary dead end, but holds the key to understanding the evolutionary origin, function, maintenance, and ubiquity of sex. We extend the rejuvenescence hypothesis that sex provides a necessary developmental reset for multicellular eukaryotes and even many unicellular eukaryotes. Sex reduces additive genetic variance of epigenetic signals, especially cytosine methylation, and of ploidy levels. Furthermore, we argue that syngamy is a modified form of meiosis that maintains ploidy and resets epigenetic signals. Epigenetic resetting is consistent with sex being induced by starvation or desiccation. Diminution of additive genetic variance is consistent with the origin and maintenance of an adaptive trait, sex, that has been present for approximately two billion years.
Intersexual selection is often categorized as precopulatory or post-copulatory mate choice by individuals of one sex over showy individuals of the other sex. We extend the framework of post-copulatory choice to include post-plasmogamic pre-karyogamic sexual selection. That is, selection of haploid nuclei within the microcosm of a single fertilized egg cell after sperm has entered an egg cell but before fusion of their nuclear membranes, in which an egg nucleus chooses a sperm nucleus with which to fuse. The role of sexual selection at this nuclear level is probably small in monospermic out-crossers, but large in polyspermic outcrossers. Post-plasmogamic pre-karyogamic sexual selection may also explain how the sperm nucleus is rejected in gynogens, and how the egg nucleus is rejected in androgens. We propose that male and female nuclei behave independently within an egg cell, and that post-plasmogamic pre-karyogamic mate choice is largely based on either (1) a pre-existing sensory bias for certain characteristics of the sperm pronuclear membrane, or (2) detection of good genes through signal (mRNA) detection.
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