Recently we have shown that genetic factors on chromosome 3 of Drosophila melanogaster control some processes involved in x-ray induced chromosome loss [Racine et al, 19791 . These genetic factors lead to differential maternal effects which can be studied by introducing identically irradiated ring-X-bearing sperm into oocytes from genotypically different females. We observed female-genotype dependent differences of ring-X chromosome loss frequencies. These differences are due to genetic factors present in the maternal genome which control the processing of premutational lesions [Graf et al, 19791 . Since the chromosome loss experiments alone did not allow us to decide whether or not DNA repair is involved, we tested the chromosome substitution stocks, which showed different maternal effects, for MMS (methyl methanesulfonate) sensitivity.In Drosophila several MMS-sensitive mutants are also defective in either excision or postreplication repair (for a review see Baker et al, 1976). A correlation between MMS sensitivity and maternal effects would therefore strongly suggest that in our strains the genetic factors on chromosome 3 are involved in DNA repair processes.The stocks used are given in Table I. For the MMS treatment of larvae a modification of the method described by Mollet and Weilenmann [1977] was used. Egg batches were collected from 1.5-day-old females for 4-hr collection periods [Buchi and Burki, 19751. To get comparable population densities in the tubes, depending on the spontaneous lethality of the stocks, 100 to 150 eggs were put into individual plastic tubes. In addition we had to take into account the increased MMS sensitivity of the slow-growing portion of the larval populations in some stocks [Racine et al, 19781. We intended to
