Fig. 5.

Evolution and mechanism of the S1 killer–protector system. a A simplified evolutionary trajectory of the S1 alleles in Oryza. Current S1TPR (TPR) and S1TP (TP) alleles in the Oryza genus might be derived from the ancestral TPR genes from two independent lineages. In one lineage, the diverged TP alleles passed through a bottleneck and migrated into O. rufipogon, eventually being fixed as S1-s in O. sativa. In another lineage, the intermediate structures (A4-TP and TP-A6) carrying the new genes S1A4 (A4) and S1A6 (A6) arose in ancient wild rice, and the A4-TPR-A6 three-gene structure were generated in O. barthii, probably by natural hybridization and allelic recombination; this structure further migrated into O. glaberrima as the functional S1-g allele. In hybrids between O. glaberrima and O. sativa, this gamete killer–protector system causes postzygotic reproductive isolation (PRI). b A working model for the S1 gamete killer–protector system in African-Asian rice hybrids. In the sporophytic cells (megaspore/microspore mother cells) of the hybrids, the three-protein complex, comprising TPR, A4, and A6, expressed from S1-g may produce a sterility-trigging signal. This signal is retained in the post-meiotic male and female gametes and causes the selective abortion of gametes carrying S1-s, whereas TPR in the S1-g gametes eliminates the sterility. S1-g therefore has a strong transmission advantage in the hybrids and acts as a typical ultra-selfish genetic element