The Determination of Mating Types in Paramecium
The mating types of the P. aurelia complex of species1 originally were designated in P. primaurelia mating types I and II, P. biaurelia mating types III and IV, P. triaurelia mating types V and VI, and so on. Each of the fourteen species of the P. aurelia complex of species has two mating types, one called O (odd) and the other E (even). Moreover, there is no exception to the rule that the O mating types are all homologous to each other, as are the E mating types. The homologies depend on several factors: the time it takes for a new genotype to be expressed (phenomic lag), response to temperature, and cross-reactions. Cross-reactions occur only between the O of some species and the E of another (see Table 2.2).