For most people, the most obvious thought or image that sexual reproduction brings to mind is that of sexual intercourse, a mating between two individuals of opposite sexes, which will result in the birth of their common offspring. While biparental reproduction is certainly the most common mode of sexual reproduction among all eukaryotes, it is not the only one, and the way it is carried out can depart substantially, in many different ways, from the ‘canonical’ description above. What is common to all these modes is that two distinct sexually compatible individuals (parents) undertake a sexual exchange that leads to the generation of new individuals with a genetic constitution obtained from the association and/or the reassortment of those parents’ genomes. The key event in this mode of reproduction, technically called amphigony, is the fusion of two gametes or two nuclei functioning as gametes (syngamy), each produced by one parent, to form a zygote. While in species with anisogamy (i.e. with distinct male and female gametes; Chapter 4), only gametes of opposite sex are compatible, the two individuals that produce them are not necessarily a male and a female.