determined by its own gametophytic haploid genotype. This is the most common type of SI. Two different mechanisms of GSI have been described in detail at the molecular level, and their description follows. The RNase mechanism In this mechanism, pollen tube elongation is halted when it has proceeded approximately one third of the way through the style. The female component ribonuclease protein, termed S-RNase probably causes degradation of the ribosomal RNA (rRNA) inside the pollen tube, in the case of identical male and female S alleles, and consequently pollen tube elongation is arrested, and the pollen grain dies. Within a decade of the initial confirmation their role in GSI, proteins belonging to the same RNase gene family were also found to cause pollen rejection in species of Rosaceae and Plantaginaceae. Despite initial uncertainty about the common ancestry of RNase-based SI in these distantly related plant families, phylogenetic studies and the finding of shared male determinants (F-box proteins) strongly supported homology across eudicots. Therefore, this mechanism likely arose approximately 90 million years ago, and is the inferred ancestral state for approximately 50% of all plant species. In the past decade, the predictions about the wide distribution of this mechanism of SI have been confirmed, placing additional support of its single ancient origin. Specifically, a style-expressed T2/S-RNase gene and pollen-expressed F-box genes are now implicated in causing SI among the members of Rubiaceae, Rutaceae, and Cactaceae. Therefore, other mechanisms of SI are thought to be recently derived in eudicots plants, in some cases relatively recently. One particularly interesting case is the Prunus SI systems, which functions through self-recognition (the cytotoxic activity of the S-RNAses is inhibited by default and selectively activated by the pollen partner SFB upon self-pollination), while SI in the other species with S-RNAse functions through non-self recognition (the S-RNAses are selectively detoxified upon cross-pollination). The S-glycoprotein mechanism In this mechanism, pollen growth is inhibited within minutes of its placement on the stigma. The mechanism is described in detail for Papaver rhoeas and so far appears restricted to the plant family Papaveraceae. The female determinant is a small, extracellular molecule, expressed in the stigma; the identity of the male determinant remains elusive, but it is probably some cell membrane receptor. The interaction between male and female determinants transmits a cellular signal into the pollen tube, resulting in strong influx of calcium cations; this interferes with the intracellular concentration gradient of