The breeding system of desert milkweed, Asclepias subulata

WYATr, R. (Institute of Ecology, University of Georgia, Athens, GA 30602), C. T IVEY AND S. R. LiPOW (Department of Botany, University of Georgia, Athens, GA 30602). The breeding system of desert milkweed, Asclepias subulata. Bull. Torrey Bot. Club 123: 180-183. 1996.-Like most other species of Asclepias, A. subulata is largely genetically self-incompatible; of 99 cross-pollinations, 15 were successful, versus only 1 of 99 self-pollinations. The number of apparently viable seeds per cross-pollinated fruit was unexpectedly variable and low, averaging 98.5, as was percent seed germination, which averaged 58.3%. Fluorescence microscopy revealed that self-pollen tubes grew as rapidly as outcross-pollen tubes and entered the micropyles of ovules within the ovary. It appears that this unusual late-acting form of self-incompatibility, which is characteristic of milkweeds, arose early in the basal Asclepiadaceae/Apocynaceae clade and has been retained in both morphologically plesiomorphic and apomorphic lineages.