Mating systems and interfertility of swamp milkweed (Asclepias incarnata ssp. incarnata and ssp. pulchra)

We investigated the breeding system and interfertility of both subspecies o f Asclepias incarnata. We performed hand-pollinations in the glasshouse to compare fruit-set from self- vs, cross-pollinations and to assess interfert ility in crosses between the subspecies. We also used horizontal starch-gel electrophoresis to infer mating-system parameters from open-pollinated pro geny 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 significantl y in both self- and cross-fertility. Fertile offspring resulted from inters ubspecific crosses, but hybrid plants showed reduced pollen fertility relat ive to parental plants, which is similar to what has been reported for wide r hybrid crosses among milkweeds. Reduced pollen fertility in hybrids, alon g with geographical barriers, may contribute to maintaining subspecific dif ferentiation. Based on allozyme markers, naturally pollinated populations w ere largely outcrossed (t(m) ranged from 0.881 to 0.986). Plants varied sig nificantly with respect to outcrossed male fertility (r(p) ranged from 0.18 6 to 0.396) but not self-fertility (r(t) was not significantly different fr om 0). Outcrossing rate did not differ significantly among populations or b etween years. We found swamp milkweed to be neither fully self-compatible n or self-incompatible, which adds to growing evidence of the complexity of m ilkweed breeding systems. Variation in frequencies of alleles modifying the expression of the self-incompatibility system could explain some of this c omplexity.