Figure 2.

Properties of the s element ([Het-s]) during the sexual cycle. s and S strains are incompatible at the vegetative stage. In a s x S sexual cross (1 and 2), one recovers the expected 50% S progeny but no s progeny are recovered instead a novel phenotype designated s^S appears, s^S strains are compatible both with s and S. Even when the s parent is used as maternal parent—contributing most of the cytoplasm to the zygote—there are very few s progeny; Janine Beisson reports that in the analysis of 727 asci, only one contained s progeny. In s x s^S crosses (3 and 4), all progeny have the phenotype of the maternal parent, which illustrates maternal inheritance of the s element. In a s x S cross performed at low temperature (18°C) when s is used as maternal parent (5), there is a specific abortion of S spores (spore killing), the s gene exerts a meiotic drive effect. In two-spored asci, the two remaining spores display the s phenotype. Color coding is black for s, grey for s^S and white for S. (after Rizet, 1952, Beisson-Schecroun 1962, Bernet 1965).