SUMMARY: Incompatibility may occur in strains which were previously considered homothallic. A cross between two incompatible strains is completely sterile. Such strains are termed F -. Strains which are self-compatible are termed F + and are productive when crossed either with other F + strains or with F - strains. The F + state is transmissible by infection due to a virus-like agent (F) which is not readily separable from the cells. Thus, both in vegetative and sexual reproduction, infection must be mediated by cell to cell contact. No changes other than those of compatibility have been correlated with the F + state. F is independent of λ, the latent phage of the K-12 strain of A small proportion of other strains of are fertile when mated with K-12 and, among these, F + and F - strains are found with about equal frequency. In K-12, extreme variations in fertility are found which are only partly associated with F and partly depend on the residual genotype. The cross between two F + strains is usually less fertile than F + x F - and in such a cross one of the two strains behaves predominantly as F -, the other as F +. The F + state has a definite effect on segregation in the sense that the genetical contribution of the F + parent to the zygote, or at least to the resulting recombinants, is less than that of the F - parent.


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