Is it Appropriate to Rely on Seed Set to Assess Candidate Plant Populations for Genetic Rescue? A Case Study with a Threatened Species

Abstract

Management of rare plants living in small populations requires life history and breeding system knowledge for informed conservation decisions, yet often management and recovery planning proceeds without breeding system information. We performed a series of hand pollination treatments to determine the breeding system of Lupinus sulphureus ssp. kincaidii (Kincaid's lupine), a Threatened plant of western Oregon, and followed its most active pollinator, Bombus californica, to estimate selfing rates. We found that Kincaid's lupine is self-compatible and can also produce seeds through facilitated selfing, a condition without pollen transfer between different flowers on the same plant. Bombus spent at least 75% of their total foraging flight time performing geitonogamous flights, foraging between male and female phase flowers on the same plant, but selfing rates were likely underestimated because Kincaid's lupine is capable of vegetative spread. Moreover, the majority of Bombus foraged contrary to conventional flight patterns on protandrous vertical inflorescences, likely increasing selfing rates. Our results indicate that while it is easy to rely on population seed to assess whether or not a population is inbred, and therefore a candidate for augmentation, a molecular study and/or hand pollination experiments are required to assess population inbreeding depression severity, because other plausible explanations for low seed set have been demonstrated.