Self_incompatibility (SI) is a major genetic mechanism to prevent self_fertilization in flowering plants and, in most cases, controlled by a single multiallelic locus, known as the S locus. In Brassica , the genes mediating both stylar ( SRK, S receptor kinase) and pollen (SCR/SP11, S locus cystein rich protein/ S locus protein 11) expression of self_incompatible reaction have been characterized though the first S locus_encoded gene, SLG (S locus glycoprotein), was isolated nearly fifteen years ago. These findings have finally unveiled the molecular partners in pollen recognition during self_incompatible reaction in Brassica .
Fertilization in flowering plants is completed through several recognitionevents, and the first of which is the recognition of pollen by pistil of female reproductivetissue. Self-incompatibility (SI) is an intraspecific reproductive barrier to prevent selfferitilization and widely distributed in flowering plants. In many species, SI shows simplegenetics and is controlled by a single multi-allelic locus, called the S locus. In gametophyticSI (GSI) exemplified by the Solanaceae, Scrophulariaceae and Rosaceae, a class ofribonucleases, called S RNases, have been shown to mediate the stylar expression of SI butnot the pollen expression of SI. The latter appears to be determined by a gene differentfrom those encoding S RNases, often referred to as pollen S gene. The pollen S gene is thecrucial missing part in understanding the biochemical and molecular mechanisms of self andnon-self pollen recognition in flowering plants. Recent genetic analysis of mutationsaffecting the pollen expression of SI has suggested a possible model of how the pollen S geneinteracts with S RNases to achieve self and non-self pollen recognition. Furthermore, wewill present two approaches, S-locus directed transposon tagging and map-based cloning, forcloning the pollen S in Antirrhinum.
The self-incompatibility ( S) loci from the Solanaceae, Rosaceae and Scrophulariaceae encode a class of ribonucleases, known as S RNases, which have been shown to control the pistil expression of self-incompatible reaction. In the former two families, the S loci have been shown to be located near centromere. However, the chromosomal location of the S locus in Antirrhinum, a species of the Scrophulariaceae, is not known. To determine its chromosomal location and genomic organization, an S-2 RNase gene and its corresponding 63 kb BAC clone were separately used for fluorescence in situ hybridization (FISH) of mitotic metaphase chromosomes of a self-incompatible Antirrhinum line Of S2S5. The results showed that the S-2 RNase detected a doublet signal near the centromere of the smallest chromosome (2n = 16). Two separate doublet signals of the tested BAC sequence were shown on both sides of the centromeres of all eight pairs of the chromosomes, suggesting that the Antirrhinum S locus is located in a pericentromeric region. Furthermore, a retrotransposon, named RIS1 (retrotransposon in the S locus), which has not been identified yet in. Antirrhinum, was found next to S-2 RNase. Taken together, the centromeric location of the S locus from the three S-RNase-based self-incompatible families provides a further support on a common origin of their evolution as well as suppressed recombination.
