MAINTENANCE OF GYNODIOECY IN SILENE ACAULIS (CARYOPHYLLACEAE): STAGE‐SPECIFIC FECUNDITY AND VIABILITY SELECTION

Through a combination of field and greenhouse experiments, I examined the factors contributing to the maintenance of female individuals in natural populations of the gynodioecious alpine plant Silene acaulis (Caryophyllaceae). Hand-pollinatedfemale plants produced 3.8 times as many seeds as did either selfor outcross-pollinated hermaphrodites, and open-pollinated females produced 2.9 times as many seeds as open-pollinated hermaphrodites. Females and hermaphrodites produce seeds of similar mass, and the same number of seeds per fruit, but females set fruit from a higher proportion of their flowers. This fecundity difference alone is sufficient to sustain equilibrium frequencies of females in natural populations. Greenhouse germination trials revealed no difference in the timing or probability of germination among seeds produced from females, selfed, and outcrossed hermaphrodites. Seedlings produced by females had the highest juvenile survivorship followed by those from outcrossed hermaphrodites and then those from selfed hermaphrodites. Differences between selfed hermaphrodites and the other treatments provide evidence for inbreeding depression at the establishment stage. The discrepancy between success of seedlings from female plants and those from outcrossed hermaphrodites suggests either that superior resources are allocated to seeds by females (matemal effect) or that seedlings of females are genetically superior to those of hermaphrodites. THE PERSISTENCE of female individuals in populations consisting mainly of hermaphrodites has puzzled evolutionary biologists since Darwin (1877). Female plants contribute genes to the subsequent generation through seed production only, while hermaphrodite plants pass on copies through both seed and pollen production. Tn addition, most gynodioecious species are self-compatible, allowing hermaphrodite plants to produce some progeny through selfing, further increasing their genomic contribution over that of the females (Lewis, 1941). Thus, in order for female plants to persist in populations containing hermaphrodites, they must enjoy a compensatory advantage, such as higher seed production (Darwin, R Received for publication 1 May 1987; revision accepted 25 August 1987. This study benefited from discussions with Drs. C. Galen, M. H. Gromko, R. L. Lowe, J. T. Rotenberry and M. Bradie all of whom provided me with constructive comments on the manuscript. Further invaluable comments on a previous draft of this paper were provided by B. Speiser and Drs. P. Bierzychudek and G. Muenchow. S. Blau and C. Galen helped with field work. I thank the University of Colorado at Colorado Springs for sharing research facilities. This work represents part of the requirements for the degree of Master of Science from Bowling Green State University. Funding was provided by the Faculty Research Council of Bowling Green State University, and NSF Grant BSR-8604726 to Dr. C. Galen. 2 Present address: Zoologisches Institut der Universitat, Rheinsprung 9, CH-405 1 Basel (Switzerland). 1877; Lewis, 1941; Kesseli and Jain, 1984) or avoidance of inbreeding depression (Lloyd, 1975; Charlesworth and Charlesworth, 1978). Many theoretical studies have examined conditions necessary for the maintenance of females at predictable equilibrium frequencies through either fertility advantage or enforced outbreeding. If the compensatory advantage to females arises only through fecundity differences, then females must produce at least twice as many seeds as the hermaphrodites, and female frequency can never be greater than 50% (Lewis, 1941). Other models that invoke inbreeding depression in the selfed progeny of hermaphrodites predict that when hermaphrodites experience high selfing rates and strong inbreeding depression, females can achieve equilibrium frequencies even in the absence of seed set differences (Lloyd, 1 9 7 5; Charlesworth and Charlesworth, 1978). However, inbreeding depression is unlikely to be intense in populations with individuals that regularly selffertilize (Valdeyron, Dommee, and Valdeyron, 1973; Wright, 1977; Falconer, 1981). Female frequencies in natural populations are often higher than predicted either by their relative fecundity values (Lloyd, 1976; Webb, 1981) or by inbreeding depression in hermaphrodites (Lloyd, 1974) with frequencies of over 50% reported in some gynodioecious populations (Baker, 1963; Dommee, Assouad, and Valdeyron, 1978). This suggests that females