Cell-cell recognition and compatibility between heterogenic and homogenic incompatibility

Cell-cell recognition systems are involved in a variety of reactions. Among these, heterogenic (somatic) and homogenic (sexual) incompatibility appear fundamentally different, yet they are induced by the same gene in some species. A review of different systems leads to the view that associative recognition of identity and difference may resolve this paradox by providing a simple explanation of homogenic incompatibility. A similar mechanism is suggested for the gene-for-gene relationship evolved in plant host/microbial pathogen systems. It is inferred that the extreme gene polymorphism in angiosperm self-incompatibility may be caused by gene conversion, utilizing information in S-pseudogenes.