Savers and spenders: Water-use strategies of two milkweed species and the consequences of their hybridization — 3a — Sneha Acharya, Gabriela Rabboni

Herbaceous, perennial plant species utilize an abundance of strategies in order to maximize survival and reproduction, and contrasting environmental influences may require even closely related species to employ different solutions. Two such related species are the common (Asclepias syriaca) and the showy milkweed (A. speciosa) that occupy varying environments on either end of a precipitation gradient crossing the continental US: A. syriaca in the wet east and A. speciosa in the dry west. Both species meet and hybridize in the region of sympatry along the 100th meridian. Based on differences in annual rainfall, it was expected that the two species should employ different water-use strategies. Taking advantage of a natural drought, we provided supplemental water to half of the plants during establishment using a block design within a common garden in Eastern South Dakota. We measured above-ground biomass, number of seeds, seed viability, and overall survival between milkweed species and hybrids. A. syriaca was found to have higher biomass and greater fecundity and exhibited a faster growth response strategy in response to drought compared with A. speciosa. A. syriaca also had greater seed viability. In contrast, A. speciosa exhibited lower biomass and fecundity and displayed traits consistent with drought avoidance. Their hybrids exhibited the lowest survival under drought, however, they had immense gains in survival in the watered treatment and displayed high heterogeneity in all traits measured. These contrasting water-use and growth strategies likely reflect adaptations allowing the two species to survive and reproduce on opposing sides of a precipitation gradient. The low survival of the hybrids under drought may reflect postzygotic barriers selecting against the hybrids. 

Augustana University
Dr. Olson-Manning