ABSTRACT Sex-ratio variation is common among gynodioecious species. One argument predicts that when sex is determined by a combination of nuclear and cytoplasmic factors, the frequency of females will be determined by genetic rather than ecological factors. An alternative argument suggests that the relative seed fitness of the female and hermaphroditic morphs will control female frequency. Hence, sex-ratio variation can be influenced by any factor that affects relative seed fitness, including ecological factors such as variation in pollination or site quality. In this study, we investigated sex-ratio variation in the gynodioecious species Silene acaulis, which has nuclear-cytoplasmic sex determination. We determined whether the frequency of females in 10 sites on Niwot Ridge, in the Front Range of Colorado, was correlated with the quality of the site or the relative seed fitness of the two morphs. Furthermore, we determined whether the two morphs differed consistently in investment in flowers, ovules, seeds and fruits. We found significant variation in sex ratio, site quality and relative seed fitness of the two morphs across sites. Although ovule number was greater in flowers on females, seed number per fruit did not vary in a consistent manner between morphs. The morphs differed consistently only in their propensity to produce fruit, with hermaphrodites exhibiting highly variable, and relatively low, fruit set compared to females. Female frequency was not significantly correlated with site quality. However, the relative seed fitness of the morphs was significantly correlated with site quality and the frequency of females. These results suggest that ecological factors do play a role in determining female frequency in S. acaulis and, consequently, can impact breeding-system evolution in this long-lived species
