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.
