Two systems of hybrid dysgenesis, the P–M system and the I–R system, are characterized by two different specific types of non-reciprocal hybrid sterility referred to, respectively, as GD and SF sterility. In order to determine the relationship between these two systems, strains representative of the four single-system interactive types were crossed in almost all possible combinations and tested for both types of sterility. The results suggest that the two systems are at least partially independent of one another. There are several examples of single strains that contribute the maternal component for interaction in the P–M system and contribute the paternal component in the I–R system. Using parents with the potential for the two types of interaction and. appropriate temperature manipulation, both GD and SF sterility can be manifested in the same hybrid females. In other crosses, a single type of sterility is observed, or none at all, according to the dual designation of the parental strains. The evidence from a number of additional crosses suggests that most strains of this species have the potential for both types of interaction to varying degrees when mated in appropriate combinations. Some theoretical and practical implications of these results are discussed.