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Perfect (hermaphrodite) flowers can, assuming no other constraints, self-pollinate, and fertilize their own ovules. This guaranteed sexual reproduction gives self-pollinating plants the ability to colonize new habitats, and it is a common trait in weedy species. However, self-fertilization does carry a disadvantage relative to outcrossing, which is that the genetic variability produced, although greater than in an asexual population, is considerably less than that seen in an outbreeding population. The balance between the relative importance of assured reproduction and genetic variability differs in different species, largely as a result of their habitats, lifecycles, and the niches that they occupy. This chapter considers the ways in which self-fertilization can be reduced or prevented through dichogamy, herkogamy, monoecy, dioecy, and biochemical self-incompatibility.

Most flowers are ‘perfect’ (hermaphrodite), containing both male and female reproductive structures, and producing both male and female gametes. Hermaphroditism carries both advantages and disadvantages. Perfect flowers can, assuming no other constraints, self-pollinate and fertilize their own ovules, which carries certain advantages such as guaranteed reproduction. It is perhaps not surprising, then, that around 20% of angiosperm species are predominantly self-pollinating. This chapter discusses the evolution of traits that promote self-pollination as a breeding system. However, self-fertilization does carry a disadvantage relative to outcrossing, which is that the genetic variability produced is considerably less than that seen in an outbreeding population. The reduction in viability of inbred progeny compared to outcrossed progeny is known as inbreeding depression, and is recognized as the primary selective pressure resulting in strategies to avoid self-fertilization. This chapter considers the developmental and biochemical ways in which self-fertilization can be reduced or prevented. Developmental mechanisms involve spatial or temporal separation of male and female reproductive organs, within a flower, between flowers of an individual or between individuals. Bu the majority of angiosperm species employ biochemical self incompatibility systems to recognize and reject self pollen.

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.

This chapter first describes the overall structure of sex-determination cascades and the function of the main upstream and downstream actors (Section 3.1). Given the fundamental bipotentiality of genomes, the mechanisms of sex determination must insure proper development towards one or the other sex, and not towards intermediate phenotypes of reduced fertility. This is achieved via antagonisms that inhibit alternative pathways, and feedback auto-regulatory loops, acting as memory devices that maintain sexual identity throughout life. Transitions in sex-determination pathways may occur at any step along the cascade, through neo- or sub-functionalization following gene duplication, changes in the hierarchical position or timing of gene expression, alterations of gene function, or changes in transcriptional and translational regulation of genes. Section 3.2 delineates the structure and functioning of MAT loci, and describes specific pathways of haploid mating-type determination in a few model systems, as well as the molecular mechanisms of mating-type switching and homothallism. Section 3.3 focuses on diploid sex determination in two contrasted systems. In angiosperms, where floral meristems are organized by MADS-box transcription factors, different genders evolve via the selective abortion of male or female organs, as exemplified in a few model systems. In contrast, genders in animals evolve via the development of undifferentiated gonads into either testes or ovaries. The crucial roles of DM-domain genes in all animals, Sox genes in vertebrates, and transformer genes in insects, are developed. Finally, Section 3.4 outlines the molecular mechanisms of self-incompatibility and induction types in some diplontic or sub-diplontic lineages.