Self-incompatibility in the grasses

The grasses represent one of the most important families of plants since they constitute our major crops and pastures and dominate many natural ecosystems. Self-incompatibility (SI) is widespread in the grasses and is under the control of a series of alleles at each of two unlinked loci, S and Z. Specification of the pollen grain is gametophytic and depends upon the combination of S and Z alleles in the pollen grain. Both of these must be matched in the pistil for the pollen grain to be incompatible. Several lines of research suggest that this two-locus SI system is shared by all grass species of the Pooideae and possibly by the entire Graminae. Firstly, genetic studies demonstrated the presence of the S-Z system in the Triticeae, the Poeae and Avenae. Secondly, linkage analyses showed common markers linked to the S-gene in grass species from all three tribes. Recent molecular studies revealed remarkable synteny of chromosomes and conservation of gene order within the Poaceae. Molecular markers have also been used to confirm syntenous localization of S and Z in several species. Despite the complexity of the grass incompatibility system relative to that in other families, the detailed genetic and physical maps for the crop species in the Triticeae and the availability of large insert libraries and mutations at the incompatibility loci, makes this a particularly promising family for molecular studies. (C) 2000 Annals of Botany Company.