Abstract
Crosses between various strains of Drosophila melanogaster may give rise to a female sterility of non-Mendelian determination. Reduced fertility is observed in females, known as SF females, bred from crosses between females of "reactive" strains and males of "inducer" strains. The reduced fertility of the SF females is the result of an interaction between an extrachromosomal property varies considerably in its ability to reduce fertility. The fertility reduction of the SF females corresponds to what is known as the reactivity level of their reactive mothers. Two nongenetic factors can modify the level of reactivity: aging and temperature. The action of aging is cumulative. When the flies of a reactive strain are submitted at each generation to the action of this factor, the level of reactivity of this strain is gradually modified. The modifications induced are reversible. Indeed, when such a modified strain is returned to standard breeding conditions, the reactivity returns progressively to its initial level. The effect of thermic treatments also seems to be cumulative and reversible.