Efficiency of I-factors in offspring of isogenic lines of Drosophila melanogaster.

In order to obtain information about mechanisms to control the strength of I-effects in Drosophila melanogaster, a genome from a spontaneously arising I-type line was introduced into an isogenic R-type w-K line with cytoplasm of strong R-reactivity. From the homozygous w-K offspring, five families, each with four sublines, were established by single pair-mating for five generations and then mass-mating. In a similar way, another 24 sub-lines were derived from six selected sublines in F9. The strength of the I-effect for the various sublines and sub-sublines was tested at the 8th, 17th, and 18th generations. The results indicated that there was great disparity between and within some families and the trends of family difference remained in successive generations. Such a result supports the hypothesis that the position effect is the main determinant of the I-effect.