Transient self-incompatibility confers delayed selfing in Leptosiphon jepsonii (Polemoniaceae)

Experimental self- and cross-pollinations revealed that Leptosiphon jepsonii (formerly Linanthus jepsonii), an annual species, possesses a floral age-dependent form of self- incompatibility (SI) that confers delayed selfing. In field and growth room studies of two populations, self- pollination produced few to no pollen tubes when flowers were first open, but growth of outcross pollen tubes indicated that stigmas were receptive during this phase. By the second day of a flower's duration, however, self pollen tubes grew readily, and pollen tube numbers in self- and outcross-pollinated flowers were not significantly different. Experimental cross-pollinations among full siblings indicated the presence of two mating types, consistent with allelic segregation at a single SI locus. On the basis of these results and the site of pollen rejection, we hypothesize that the floral age-dependent pollen-pistil phenomenon is a modified form of the single-locus sporophytic SI found in a closely related species, Leptosiphon parviflorus. To explore the potential effect of transient SI on the timing and rate of self- fertilization, we used allozyme genetic markers to determine the paternity of seeds produced by mixed self and outcross pollen loads. Consistent with the pollen tube data, nearly all progeny from mixed pollinations of day 1 flowers were outcrossed, and the proportion of selfed progeny increased significantly with floral age. Transient SI may be an evolutionarily stable strategy or a transitional step in the evolution from SI to complete selfing.