The molecular and genetic bases of S-RNase-based self-incompatibility.

The majority of flowering plants produce perfect flowers that contain both the male and female reproductive organs in close proximity; consequently, they would have a strong tendency to self-fertilize if there were no mechanisms to prevent them from doing so. Because inbreeding can result in reduced fitness in the progeny, hermaphroditic plants have adopted a variety of reproductive strategies, including self-incompatibility (SI), by which inbreeding is prevented and outcrosses are promoted. SI allows the pistil of a flower to distinguish between genetically related (self) and unrelated (non-self) pollen. This self/non-self recognition results in the inhibition of germination of self-pollen on the stigmatic surface or the inhibition of growth of self-pollen tubes in the style. Thus, SI is a prezygotic reproductive barrier by which incompatible pollen/pollen tubes are prevented from delivering the sperm cells to the ovary to effect double fertilization. SI can be classified into homomorphic and heteromorphic types based on whether it is associated with floral polymorphism. In species that exhibit homomorphic SI, all individuals produce the same type of flower and the outcome of pollination depends only on the genetic identity of the male and female partners. In contrast, species that exhibit heteromorphic SI produce two or three different flowermorphologies (e.g., a flower with short anthers and long style or a flower with long anthers and short style). For successful pollination, pollen must come from genetically unrelated individuals whose anthers are of the same height as the style of the flower being pollinated. To date,much of what we know about themolecular basis of SI has been deduced from studies of homomorphic SI, which will be the focus of this review. A monograph by de Nettancourt (2001) provides a comprehensive treatise on SI, including a discussion of the heteromorphic type. For homomorphic SI (hereafter referred to as SI), self/non-self discrimination between pollen and pistil is determined by one or more polymorphic loci, and this type of SI is further classified into gametophytic and sporophytic types based on the genetic control of pollen behavior. To date, four of the families that exhibit gametophytic SI (GSI), Solanaceae, Rosaceae, Scrophulariaceae, and Papaveraceae, and one of the families that exhibit sporophytic SI (SSI), Brassicaceae, have been studied extensively at themolecular level (Table 1). A single polymorphic locus, termed the S-locus, controls the SI response in all five of these families. As described below, other loci often are required for the full manifestation of the SI response, but by definition, the S-locus determines the specificity of the response. It is now known that two separate genes at the S-locus control male and female specificities. Thus, the term ‘‘haplotypes’’ is used to describe variants of the S-locus, whereas the term ‘‘alleles’’ is used to describe variants of an S-locus gene. For the four GSI families, SI occurs when the S-haplotype of the pollen matches either of the two S-haplotypes carried by the pistil. That is, the SI phenotype of the pollen (gametophyte) is determined by its own S-genotype. For the SSI family, in the simplest case, SI occurs when the pollen-producing parent shares one or both S-haplotypes with the pistil. That is, the SI phenotype of the pollen is determined by the S-genotype of its diploid parent. For SSI, complex relationships often exist between the different S-haplotypes of the pollen and pistil parents. One S-haplotype could be dominant over or recessive to another, or it could interact with another to result in mutual weakening or in an entirely new S-haplotype specificity (Thompson and Taylor, 1966).