Hybridization of Asclepias Species for the Creation of Novel Cultivars

Milkweed (Asclepias sp.) is a vital pollinator genus across North America and is a host plant for many butterfly species, notably the monarch butterfly (Danaus plexippus). However, commercial production of Asclepias is limited because most species lack commercial traits, having minimal branching habits, excessive height, and minimal color variation. Using a commercially viable Asclepias species, Asclepias tuberosa, as a maternal parent, three different pollination methods were trialed to create interspecific hybrids. Pollination methods included a traditional method, a pollen-solution-based method, and a novel inverted pollinia method. Using the inverted pollinia method increased pollination success rates fourfold among intraspecific crosses of A. tuberosa. Once pollination methods were optimized, A. tuberosa was used as the maternal parent, and one-way crosses were made to seven other Asclepias species. Four species successfully developed hybrid seeds of the seven species used as pollen donors; Asclepias hirtella, Asclepias purpurascens, Asclepias speciosa, and Asclepias syriaca. This is the first documented case of a controlled interspecific hybridization event among these species. Three germination techniques were then trialed: direct seeding, embryo rescue, and stratification. Of the three methods, stratification for 30 days had the highest germination rates across the four hybrid groups compared to the other treatments. After germinating seeds to grow out the F1 hybrids, inheritance of commercially desirable traits was also investigated. Pubescence was found to be a recessively inherited trait across all hybrid groups, although specific inheritance ratios did vary depending on the pollen parent used. White sap color was found to be dominantly inherited across all hybrid groups at a ubiquitous 1:0 ratio. Flower color inheritance varied depending on the parents used but generally trended towards the dominant inheritance of a pink coloration across the hybrid groups. Quantitative traits such as height, leaf length, leaf width, branching, and flower height were inherited at values intermediate of the parents, typical of an F1 hybrid cross. Thus, interspecific hybridization of Asclepias is possible, and commercially desirable traits can be observed in the hybrid progeny. To our knowledge, this is the first documentation of intentional hybridizations between Asclepias species.