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We investigated the breeding system and interfertility of both subspecies of Asclepias incarnata. We performed hand-pollinations in the glasshouse to compare fruit-set from self- vs. cross-pollinations and to assess interfertility in crosses between the subspecies. We also used horizontal starch-gel electrophoresis to infer mating-system parameters from open-pollinated progeny arrays in three natural populations over two consecutive years. Plants of ssp. incarnata were about 1/6 as likely to mature hand self-pollinated fruits as were ssp. pulchra plants. Furthermore, plants varied significantly in both self- and cross-fertility. Fertile offspring resulted from intersubspecific crosses, but hybrid plants showed reduced pollen fertility relative to parental plants, which is similar to what has been reported for wider hybrid crosses among milkweeds. Reduced pollen fertility in hybrids, along with geographical barriers, may contribute to maintaining subspecific differentiation. Based on allozyme markers, naturally pollinated populations were largely outcrossed (tm ranged from 0.881 to 0.986). Plants varied significantly with respect to outcrossed male fertility (rp ranged from 0.186 to 0.396) but not self-fertility (rt was not significantly different from 0). Outcrossing rate did not differ significantly among populations or between years. We found swamp milkweed to be neither fully self-compatible nor self-incompatible, which adds to growing evidence of the complexity of milkweed breeding systems. Variation in frequencies of alleles modifying the expression of the self-incompatibility system could explain some of this complexity. [ABSTRACT FROM AUTHOR]