Occurrence of self-compatibility, self-incompatibility and unilateral incompatibility after crossing diploid S-tuberosum (SI) with S-verrucosum (SC):I. Expression and inheritance of self-compatibility

Diploid Solanum tuberosum (tbr), 2n=2x=24, can be crossed with S. verrucosu m (ver) only when the latter is used as a pistillate parent but not recipro cally. This conforms to the phenomenon of unilateral incompatibility (UI) w here a self-compatible species, like ver (SC) cannot be used as a male pare nt to cross with a self-incompatible (SI) parent like tbr. Even if ver x tb r hybrids are made, the F1 hybrids possess cytoplasmic male sterility and t hus hinder genetic analysis of crossing barriers. Exceptionally, however, s ome diploid genotypes of tbr (SI) can be used as pistillate parents to cros s with ver, and such exceptional tbr clones are called 'acceptors'. Repeate d backcrossing of acceptors to ver have resulted in male fertile genotypes that possess tbr cytoplasm and ver nucleus. These genotypes were used for t he genetic analysis of 'acceptance' and UI in thse experiments. It was foun d that acceptance of ver-pollen by tbr-pistils is based on a dominant gene A that expresses only in the absence of an inhibitor I. In the F-1 hybrids, only the S-allele of tbr was expressed but not that of ver. Concomitant wi th this observation, it was shown that ver does not produce style-specific S-glycoproteins that are responsible for self-incompatible reaction in dipl oid potato. Although the the F-1 populations were SC, they segregated into SC and SI genotypes giving skewed segregation ratios for this trait. Becaus e of this as well as the disappearance and re-appearance of SC trait in the offspring generations, it was necessary to postulate a more complex intera ction between A and I. Models are presented in order to explain acceptance, non-acceptance and the expression of UI. It is concluded that at least fou r different loci are involved in the expression of UI.