MENTOR EFFECTS IN WILD-SPECIES OF HELIANTHUS (ASTERACEAE)

Self-incompatibility (SI) is an effective method for limiting self-fer tilization in flowering plant species, but there are circumstances in which an otherwise functional SI system may fail. One of the most intr iguing of these is the induction of selfing by mixed loads of self and heterospecific pollen (the mentor effect) because it is likely to occ ur under natural conditions, such as in hybrid zones. Here we conducte d a series of controlled crosses to determine whether mentor effects o perate in two SI annual species, Helianthus annuus and H, petiolaris, and whether the failure of SI results in a decrease in the frequency o f hybridization between these two species. Of the 1396 achenes examine d from pollen mixtures that included varying ratios of self, intraspec ific compatible, and interspecific pollen, 71 (5.1%) were selfed. Self ing frequencies were significantly less than expected based on pollen ratios, except when the proportion of intraspecific compatible pollen was low. Hybridization frequencies from these same pollen ratios avera ged 41.8% with H. annuus as the maternal species and 13.3% with H. pet iolaris as the mother. Analysis of 1404 achenes from pollen mixtures t hat excluded self pollen resulted in hybridization frequencies for H. annuus (42.2%) and H, petiolaris (18.2%) that do not differ significan tly from those including self pollen. Thus, mentor effects do not appe ar to play an important role in reproductive isolation between these s pecies. On the other hand, even a modest increase in self-fertilizatio n in hybrid populations, such as that due to mentor effects, could enh ance the probability of hybrid species establishment.