Origin of complementary incompatibility systems in flowering plants

How far are we from unravelling self-incompatibility in grasses?

The genetic and physiological mechanisms involved in limiting self-fertilization in angiosperms, referred to as self-incompatibility (SI), have significant effects on population structure and have potential diversification and evolutionary consequences. Up to now, details of the underlying genetic control and physiological basis of SI have been elucidated in two different gametophytic SI (GSI) systems, the S-RNase SI and the Papaver SI systems, and the sporophytic SI (SSI) system (Brassica). In the grass family (Poaceae), which contains all the cereal and major forage crops, SI has been known for half a century to be controlled gametophytically by two multiallelic and independent loci, S and Z. But still none of the gene products for S and Z is known and only limited information on related biochemical responses is available. Here we compare current knowledge of grass SI with that of other well-characterized SI systems and speculate about the relationship between SSI and grass SI. Additionally, we discuss comparative mapping as a tool for the further investigation of grass SI.

Evolution of incompatibility systems in plants: Complementarity and the mating locus in flowering plants and fungi

The restriction of sexual pairing by a specificity gene is considered to be an ancient development in the plant kingdom. The diversity and general parallelism of incompatibility systems seen amongst the phyla at the present time can be rationalized in terms of the association of various derived forms of the ancestral specificity unit with differing spectra of accessory factors controlling sexual physiology in the different phyla. Sexual morphogenesis has become divided into distinct phases under the control of complementary genes. These phases are initiated by a regulatory system of "Co-ordinator genes" which control the order in which groups of morphogenetic genes are expressed during development. The entire sexual cycle will be completed only if all the complementary groups are activated in the appropriate sequence. The present article discusses essential features of the evolution of the breeding locus in different phyla. These features are consistent in themselves with the present data and are not dependent on the proposed ancient origin of the specificity gene.The above hypothesis throws light on the (1) evolution of the complex mating loci in flowering plants and fungi; (2) evolution of complementary incompatibility and heteromorphic incompatibility in flowering plants; (3) anomalous cross-compatibility behaviour of mutants in the fungus Schizophyllum commune; (4) nature of homothallism in higher fungi; (5) mode of origin of new functional self-incompatibility alleles; and (6) "homogenic" and "heterogenic" incompatibility.

Generation or multiple genetic specificities: origin of genetic polymorphism through gene regulation

Based on results of mutation studies in the fungus Schizophyllum commune, a new mechanism of the origin of genetic polymorphism is proposed. This may explain the intractable problems of the rise of multiple allelism controlling incompatibility in plants and the wide array of antibody diversity controlling immunity reaction in animals.