Factors limiting the reproductive success of Phlox hirsuta, an endangered species

Background/Question/Methods

Poor reproductive success hinders the population growth and range expansion of many threatened and endangered plant species. Identifying the factors that are and are not responsible for decreased seed set is important for devising effective conservation strategies. Self-incompatible species are particularly prone to high ovule-to-seed ratios, as a proportion of pollen grains in the population is incapable of siring seeds. Phlox hirsuta, a self-sterile endangered species endemic to serpentine soils in northern California, suffers from poor reproductive success with ovule-to-seed ratios averaging 32:1 in 2010 and 27:1 in 2011. We combine field, growth chamber, and laboratory studies to identify factors that limit this species’ reproductive success. Specifically, we determine 1) if seed set is affected by the total number of pollen grains on stigmas of naturally-pollinated flowers or the total number of outcross pollen grains on stigmas of hand-pollinated flowers, 2) if pollen viability and autonomous self pollination change as flowers age, and 3) if self pollen on the stigma interferes with the ability of outcross pollen to sire seeds. 

Results/Conclusions

Combined, our data indicate that pollen inviability and autonomous self pollination are major factors limiting the reproductive success of P. hirsuta. Self pollen grains, which were able to grow tubes through styles, interfered with the ability of outcross pollen to sire seeds. The fitness costs associated with the presence of self pollen grains on stigmas can be overcome if P. hirsuta evolves traits that either prevent self pollination or prevent the rejection of self pollen. The increased activity of pollinators that collect pollen before autonomous self pollination and that transport viable pollen grains between compatible plants could also ameliorate this species’ risk of extinction.