S-RNASE AND INTERSPECIFIC POLLEN REJECTION IN THE GENUS NICOTIANA - MULTIPLE POLLEN-REJECTION PATHWAYS CONTRIBUTE TO UNILATERAL INCOMPATIBILITY BETWEEN SELF-INCOMPATIBLE AND SELF-COMPATIBLE SPECIES

In self-incompatible (SI) plants, the S locus acts to prevent growth o f self pollen and thus promotes outcrossing within the species, Inters pecific crosses between SI and self-compatible (SC) species often show unilateral incompatibility that follows the SI x SC rule: SI species reject pollen from SC species, but the reciprocal crosses are usually compatible, The general validity of the SI x SC rule suggests a link b etween SI and interspecific pollen rejection; however, this link has b een questioned because of a number of exceptions to the rule, To clari fy the role of the S locus in interspecific pollen rejection, we trans formed several Nicotiana species and hybrids with genes encoding S-A2 Or S-C10 RNase from Si N. alata, Compatibility phenotypes in the trans genic plants were tested using pollen from three SC species showing un ilateral incompatibility with N. alata. S RNase was implicated in reje cting pollen from all three species. Rejection of N. plumbaginifolia p ollen was similar to S allele-specific pollen rejection, showing a req uirement for both S RNase and other genetic factors from N. alata. In contrast, S RNase-dependent rejection of N. glutinosa and N. tabacum p ollen proceeded without these additional factors. N. alata also reject s pollen from the latter two species through an S RNase-independent me chanism, Our results Implicate the S locus in all three systems, but i t is clear that multiple mechanisms contribute to interspecific pollen rejection.