COS 192-2 - Inbreeding depression matters more than outbreeding for conservation of an endangered plant

Abstract: As biodiversity loss accelerates globally, conservation of at-risk species increasingly relies on strategic genetic management to support remnant isolated populations and reintroductions. In endangered plants, the breeding system can provide crucial information on the relative importance of inbreeding and outbreeding depression as well as the role of pollinators in successful reproduction. We performed controlled crosses to document the breeding system of Willamette daisy (Erigeron decumbens, Asteraceae), an endangered plant endemic to western Oregon prairies. Crosses included no pollen addition (to test for autogamy), geitonogamous self-pollination (to test for self-compatibility), local (individuals in the same population), and distant (plants from populations separated by over 80 km). All crosses were performed at a wild population west of Eugene, Oregon on lands managed by the Army Corps of Engineers. Distant crosses used fresh pollen transported from a population near Salem, Oregon. All cross types were performed on 30 individuals (four flower heads were randomly selected on each plant for treatment) that were caged to eliminate natural cross pollination. Crosses were conducted three times during a ~6 day period when flowers opened serially within each compound flower head. Filled seeds from each treated flower head were counted, germinated, and grown in potting soil in a heated, lighted greenhouse at Oregon State University for 3 months. We used ANOVA to test for a treatment effect on seed set, germination, and F1 plant size (number of leaves). Pollination treatment had a significant effect on mean seed set (p< 0.0001) but not on mean seed germination (p=0.532). Seed set was < 5% in autogamous and self-pollinations, 50% in local pollinations, and 51% in distant crosses. Plants grown from distant crosses (mean 64.6 leaves) outgrew local crosses (53.4 leaves) by 21% (p=0.008). Willamette daisy is therefore an outcrossing species that relies on insect pollinators for reproduction. In addition, inbreeding depression (as demonstrated by low seed set) is a significant barrier to reproduction, while outbreeding results in substantial increases in plant growth in the F1 generation. These results suggest that genetic mixing of isolated populations may enhance fitness and population growth in this species, and habitat management should emphasize conservation of native bees and other pollinators that this species relies on for reproduction. Future population introductions or conservation translocations may benefit from using seeds from multiple remnant populations to avoid inbreeding depression and leverage the fitness benefits of outbreeding.