The rare Asclepias texana and its widespread sister species, A-perennis, are self-incompatible and interfertile

Like most other species of Asclepias, A. texana and A. perennis are entirely self-incompatible. They therefore differ from A. incarnata and A. curassavica, which are also members of the supposedly primitive series Incarnatae, but which are self-compatible Interspecific crosses of A. texana and A. perennis showed that the two taxa are freely and reciprocally interfertile, with fruit-set levels (26.5%) as high as for intraspecific cross-pollinations. Crosses that used A. texana as the female parent had significantly fewer seeds per fruit (19.8) than did those in which A. perennis served as the female parent (42.8), suggesting some form of unilateral interspecific incongruity. In contrast, interspecific crosses between A. perennis and A. curassavica (also Incarnatae) were totally unsuccessful, as were interspecific crosses of A. incarnata with both A. perennis and A. texana. F-1 interspecific hybrids of A. perennis xA, texana were male-sterile. The occurrence of both self-incompatible (A. perennis, A. texana, and A. verticillata) and self-compatible (A. curassavica and A. incarnata) species in Woodson's series Incarnatae may reflect problems in the taxonomic delimitation of subgroups within Asclepias. We believe, however, that self-compatibility has probably arisen independently numerous times within the genus.