Population differences in self-fertility in the "self-incompatible" milkweed Asclepias exaltata (Asclepiadaceae)

Individual plants of Asclepias exaltata (Asclepiadaceae) typically express an unusual self-incompatibility system under single-gene control. Hand-poll inations performed in six natural populations detected occasional self-fert ile plants. The frequency of self-fertile individuals ranged from 0 to 34.0 % and differed significantly among populations. Self-Fertility appears to b e under genetic control, as the ability of most plants (80.0%) to set fruit following self-pollinations was identical under natural and greenhouse con ditions. Seed- and fruit-set, however, were significantly lower from self- vs. cross-pollinations. Allozyme analysis of the population with the highes t frequency of self-fertility revealed that adult plants were not significa ntly inbred. Finally. fruit-set following within-population cross-pollinati ons did not differ from that following wide, between-population cross-polli nations.