Evaluation of the breeding system and inbreeding depression in the rare plant Astragalus peckii

Background/Question/Methods

Astragalus peckii, a rare species endemic to south- central Oregon, tends to have low seed set in the field.  Many factors can contribute to low reproductive success including the breeding system and self-compatibility of a species.  Little research has been conducted on the reproductive biology of A. peckii and the objectives of this study are to: 1) Describe the breeding system of A. peckii, with particular emphasis on its capacity for self-compatibility, and 2) Test for inbreeding depression in seed germination and plant survival, growth, and reproduction.  Breeding system studies were conducted at three field sites in 2007 with the following treatments:  bagging of flowers without manipulation (autogamy), bagging of flowers after manual self-pollination, and bagging of flowers after manual cross-pollination.  Controls were also established to record fruit and seed set without manipulation.  Seeds from self-pollinated and open-pollinated plants were collected in 2008 and germinated in the greenhouse.  Germination rates were recorded along with plant survival and size.

Results/Conclusions

Reproduction in A. peckii was low at all three field sites in 2007, with between 0.15 and 0.32 mature seeds/flower.  Field results show that A. peckii is self-compatible but does not produce fruit or seed without pollinators present.  Average seed set per flower was higher in cross-pollinated flowers compared to self-pollinated flowers.  Seeds from self- and open-pollinated plants had similar germination rates in the greenhouse.  Preliminary results, after 9 weeks of growth, show different survival rates for the two groups, with 81% of plants from open pollinated seeds and 67% of the plants from self-pollinated seeds, surviving.  Seedlings from open-pollinated seeds were also 82% taller than those from self-pollinated seeds but had no difference in the number of leaves produced.  Overall, results from this study show that A. peckii is self-compatible and susceptible to inbreeding depression.  Results also suggest that plants in shrinking or increasingly isolated populations will be most at risk of inbreeding depression if transfer of pollen from closely related plants increases.