Hybrid dysgenesis inDrosophila melanogaster: partial sterility associated with embryo lethality in theP-Msystem

Summary Variable frequencies of unhatched eggs were observed to be produced by a number of F 1 interstrain hybrids. This type of partial sterility resulting from F 2 embryo death was found to be associated with the P-M system of hybrid dysgenesis. Dysgenic hybrid progeny of crosses between M strain females and P strain males may therefore have reduced fertility due to the disruption of development at two different stages: early F 1 gonadal development and early F 2 embryo development. These disruptions result in the previously described F 1 gonadal dysgenesis ( GD sterility) and F 2 embryo lethality ( EL sterility) respectively. The two morphologically distinct types of P-M -associated sterility differ in their patterns of response to F 1 developmental temperature, and the temperature-sensitive period for EL sterility occurs considerably later in F 1 development than for GD sterility. EL sterility is similar to SF sterility, which is associated with the I–R system of hybrid dysgenesis in that both result from death during early F 2 embryogenesis. However, EL sterility differs from SF sterility in not being restricted to hybrids of the female sex and in showing different patterns of response to temperature and ageing in the F 1 generation. Some implications of the existence of EL sterility for methods of strain classification in the I–R system are explored.