Self‐Sterility in the Understory Herb Clintonia borealis (Liliaceae)

Abstract

In the understory perennial Clintonia borealis, self‐pollination yields lower seed number per fruit than cross‐pollination. To measure the magnitude of this partial self‐sterility and identify the pre‐ and postzygotic mechanisms on which it is based, we conducted self‐ and outcross‐pollinations in 34 patches in a natural population and estimated the effects on seed and fruit production, frequency of fertilization, and postzygotic endosperm development. Plants in nine of the 34 patches set no fruit after pollination. In the remaining patches, 22% of self‐pollinated flowers and 54% of outcrossed flowers set fruit. Mean seed set per fruit was 0.38 ($$\mathrm{SE}\,=0.04$$ ) for outcrossed flowers compared with 0.09 ($$\mathrm{SE}\,=0.04$$ ) after self‐pollination. The self : outcross ratio of seed set ranged from 1.88 to 0.0 among patches, but the effects of selfing did not differ significantly among patches. Observations of cleared ovules from 1, 2, 3, 4, 6, 8, and 10 d after pollination showed that fertilization does not occur until day 3. The proportion of ovules fertilized increased with time and, at 10 d, was 0.44 ($$\mathrm{SE}\,=0.18$$ ) for outcross‐pollinations and 0.14 ($$\mathrm{SE}\,=0.06$$ ) for selfs. In addition, the number of endosperm nuclei in fertilized ovules increased more rapidly and the size of full seeds was larger after outcross‐pollination than self‐pollination. Our results indicate that prezygotic mechanisms preventing fertilization can account for much of the self‐sterility observed in C. borealis in the field. However, some postzygotic differences were also evident, which may influence the number and size of selfed versus outcrossed offspring.