One type of social interaction that we have mentioned in passing is sex. Sexual reproduction involves a cooperative interaction between organisms of different mating types, something unnecessary in asexual reproduction. While we are used to two distinct sexes (male and female), this is not universal: many unicellular eukaryotes are characterized by an number of distinct mating types. Typically, sexual reproduction involves the fusion of specialized cells, known as gametes, of different mating types (or sexes). Through mechanisms we will consider later, the outcome of sexual reproduction leads to increased diversity among offspring.
So what are the common hallmarks of sexual reproduction? Let us return to the slime mold Dictyostelium as an exemplar. We have already considered its asexual life cycle, but Dictyostelium also has a sexual life cycle. Under specific conditions, two amoeboid cells of different mating types will fuse together to form a single cell. The original cells are haploid, meaning that they have a single copy of their genome. When two haploid cells fuse, the resulting cell has two copies of the genetic material and is referred to as diploid. This diploid cell will then go through a series of events, known collectively as meiosis, that results in the production of four haploid cells. During meiosis, genetic material is shuffled, so that the genotypes of the haploid cells that emerge from the sexual process are different from those of the haploid cells that originally fused with one another.