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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 reviews the influences of environmental factors on sex determination, sex ratios, and reproductive behavior in the Crustacea, focusing in particular on amphipod and isopod examples. A range of abiotic and biotic environmental factors influence reproduction in Crustacea, including temperature, day length, pollutants, and parasites. Individual crustaceans may benefit from these environmental influences, but in other cases, reproductive biology responses to biotic and abiotic environments may be detrimental to individual fitness. Environmental Sex Determination (ESD) falls into the former category. ESD is an adaptive mechanism of sex determination that is rare, but has evolved in diverse taxa. Evidence from gammarid amphipods is used to explore the evolution of ESD in response to a patchy environment. While ESD is an adaptive mechanism of sex determination, the impact of other environmental factors can be very costly. Parasitic castrators can lead to a reduction or total cessation of reproduction in crustacean hosts, driving population declines. In contrast, parasitic feminizers convert male hosts into females, enhancing maternal parasite transmission but also leading to sex ratio distortion in the host population. The chapter discusses parasite-host coevolutionary conflict and reviews evidence that selection on the host in response to parasitic sex ratio distortion has led to altered mate choice in amphipods, and to the evolution of a novel system of sex determination in isopods. Human-induced environmental influences can also be seen in Crustacea, and the chapter discusses how parasites, ESD, and endocrine-disrupting chemicals can each affect sex determination and lead to abnormal intersex phenotypes. It ends by highlighting areas for future research on the diverse world of crustacean reproduction.

Standard examples in biological game theory are introduced. The degree of cooperation at evolutionary stability is analysed in models that deal with situations such as the Prisoner’s Dilemma, the Tragedy of the Commons and the conflict of interest between parents over care of their common young. Several models of aggressive interactions are treated in this book. In this chapter the Hawk–Dove game, which is the simplest of these models, is analysed. Further models in the chapter deal with the situation in which individuals vary in their fighting ability and the situation in which information about the opponent is available before an individual decides whether to be aggressive. The problem of the allocation of resources to sons versus daughters has played a central role in biological game theory. This chapter introduces the basic theory, as well as a model in which the environmental temperature affects the development of the sexes differentially, so that at evolutionary stability the sex of offspring is determined by this temperature. Coordination games, alternative mating tactics, dispersal to avoid kin competition, and the idea that signals can evolve from cues are also introduced.