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-pollinations performed in six natural populations detected occasional self-fertile plants. The frequency of self-fertile individuals ranged from 0 to 34.0% and differed significantly among populations. Self-fertility appears to be under genetic control, as the ability of most plants (80.0 %) to set fruit following self-pollinations was identical under natural and greenhouse conditions. Seed- and fruit-set, however, were significantly lower from self- vs. cross-pollinations. Allozyme analysis of the population with the highest frequency of self-fertility revealed that adult plants were not significantly inbred. Finally, fruit-set following within-population cross-pollinations did not differ from that following wide, between-population cross-pollinations.