Non-functioning of an intensively studied relationship, the one between pistil and pollen in flowering plants, has generally been associated with incompatibility. An evolutionary study in Lycopersicon and related research demonstrated the existence of an independent second mechanism for nonfunctioning.The nature of this second mechanism is explained by a model in which the pistil-pollen relationship is based on matching genic systems. A lack of genetic information in the pollen about some relevant process in the pistil will cause non-functioning of the pistil-poIlen relationship. For this phenomenon the term incongruity is proposed. Evolution and genetics of incongruity are different from those of incompatibility. Whereas incompatibility is an exception and compatibility the rule, incongruity is the rule and congruity an exception.Reinterpretation of earlier research is necessary. It shows that in interspecific crosses incongruity plays a major role, whereas incompatibility is secondary or absent. Evolution of self-compatible species, genetics of crossability in interspecific hybrids and later generations, complex patterns of crossability between populations and S -gene polymorphism can all be reinterpreted on the same basis: the principle of matching genic systems. It is shown that properties have been ascribed to the S -gene, which are in fact based on other genes and other principles.The present model has general applicability for intimate partner relationships as, for example, the host-parasite relationship. Naturally occurring incongruity can be exploited in different ways.
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