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

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Abstract
Summary: Variable frequencies of unhatched eggs were observed to be produced by a number of F1interstrain hybrids. This type of partial sterility resulting from F2embryo death was found to be associated with theP-Msystem of hybrid dysgenesis. Dysgenic hybrid progeny of crosses betweenMstrain females andPstrain males may therefore have reduced fertility due to the disruption of development at two different stages: early F1gonadal development and early F2embryo development. These disruptions result in the previously described F1gonadal dysgenesis (GDsterility) and F2embryo lethality (ELsterility) respectively. The two morphologically distinct types ofP-M-associated sterility differ in their patterns of response to F1developmental temperature, and the temperature-sensitive period forELsterility occurs considerably later in F1development than forGDsterility.ELsterility is similar toSFsterility, which is associated with theI–Rsystem of hybrid dysgenesis in that both result from death during early F2embryogenesis. However,ELsterility differs fromSFsterility in not being restricted to hybrids of the female sex and in showing different patterns of response to temperature and ageing in the F1generation. Some implications of the existence ofELsterility for methods of strain classification in theI–Rsystem are explored.