Search Results

Sex and genders occupy a central place in the human mind and have a long history of alternative interpretations throughout human cultures. Although intimately linked to reproduction in animals and other multicellular eukaryotes, sex is fundamentally not a reproductive process. This chapter first discusses what sex is and why it evolved, arguing that it is best defined by the presence of meiosis, that it might have originated as a DNA-repair mechanism, and is likely maintained by indirect benefits stemming from recombination.We then present the several kinds of mating categories (mating types, sexes, genders, self-incompatibility systems), discussing their similarities and differences, and the reasons why they exist. We introduce the concepts of sex-ratio selection and sexual selection, which are arguably the main evolutionary forces underlying the dynamics of sex-determination systems. Finally, this chapter we argues against the traditional distinction between sex determination and primary sex differentiation, defining sex determination as the whole process that leads, from undifferentiated gonads or meristems, to the development of differentiated reproductive organs. This broad definition widens the scope of our book, to also include, for example, sex differentiation in simultaneous and sequential hermaphrodites.

The sexual cycles of eukaryotes vary immensely in terms of the relative importance of the haploid and diploid phases, the differentiation between gametes, and the timing and mode of sex determination. The chapter discusses the evolutionary advantages of haploid and diploid phases, the conditions for the maintenance of haplo-diplontic cycles, and the role of disruptive selection in the evolution from isogamy to anisogamy and oogamy. The chapter proposes a typology for sexual cycles based on the relative importance of haploid and diploid phase, whether sex is determined at the haploid or diploid stage, and whether the initial trigger is genetic or epigenetic. The chapter develops the concepts of heterothallism versus homothallism, haplo- versus diplo-genotypic sex determination, dioicy versus dioecy, monoicy versus monoecy, self-incompatibility systems and secondary mating types. The chapter considers the diversity of epigenetic sex-determination systems (mating-type switching, simultaneous and sequential hermaphroditism, as well as environmental, social, maternal, or parasite control of sex determination) and discusses the ultimate and proximate causes favouring their evolution, as well as their likely role in transitions from haplo- to diplo-genotypic sex determination.The electronic addendum of this chapter (Section 2.2) describes in more detail the diversity and phylogenetic distribution of sex-determination types among extant eukaryotes.

Well-studied cases of programmed DNA rearrangements, e.g., somatic recombination in the emergence of specific antibodies, suggest a rubric for specially evolved mutation systems: they amplify the rates of specific types of mutations (by orders of magnitude), subject to specific modulation, using dedicated parts, with the favored types of mutations being used repeatedly. Chapter 5 focuses on six types of systems that generate mutational diversity in a focused manner, often in an ecological context that makes sense of such a specialized feature, e.g., immune evasion or phage-host coevolution: cassette shuffling, phase variation (switching), CRISPR-Cas defenses, inversion shufflons, diversity-generating retro-elements, and mating-type switching. The emergence and influence of these systems relates to the concept of evolvability, here expressed in terms of three types of claims: evolvability as fact (E1), evolvability as explanans (E2), and evolvability as explanandum (E3).