Gynodioecy in silene vulgaris (Caryophyllaceae): progeny success, experimental design, and maternal effects

This study documents the comparative success of seeds and seedlings of the perennial gynodioecious-gynomonoecious weed, Silene vulgaris. in the greenhouse. The importance of experimental design is stressed by comparing two different statistical analyses ofthe data. Seeds were obtained from artificial pollinations in the field: self-fertilization of hermaphrodites, and crossfertilizations of both hermaphrodites and females. One-way analysis of variance using progeny type (selfed hermaphrodites, outcrossed hermaphrodites, and outcrossed females) as the treatment effect for each seed and seedling variable showed statistically discernable differences among progeny from different cross types. The significance of this type of ANOV A resulted from a reduced error term and positively-biased F ratios. A factorial design showed no significant differences due to type of progeny in seed mass, days to germination, leaf number, area, or biomass at six weeks of age. There were, however, significant differences among seeds attributable to maternal parent for all seed and seedling variables. A higher proportion of seeds from outcrossed individuals germinated compared to that from self-fertilization. When the data were analyzed to include and partition all sources ofvariation, differences among offspring appeared during germination, rather than during later development. Seed mass, cross type, and sex of maternal parent all significantly affected the likelihood of germination; however, they had decreasing predictive power, respectively. Inbreeding depression in Silene vulgaris may help maintain gynodioecy; however, the pleiotropic effects of both nuclear and cytoplasmic genes for sex expression also may affect fitness and the maintenance of females. THE TERM "gynodioecious" was used by Darwin (1877) for Thymus serpyllum to describe the occurrence offema1es and hermaphrodites within the same population of a plant species. Since that time, many hypotheses have attempted to explain the occurrence of two difI Received for publication 19 September 1988; revision accepted 9 March 1989. We appreciate support from the Naturalist-Ecologist Training Program, Andrew W. Mellon Foundation, ofthe University of Michigan Biological Station, the Research and Creative Activity Committee of the Faculty Senate of East Carolina University, and the Southern Regional Education Board. Special thanks to Chiquita Culberson, Department of Botany, Duke University, Robert J. Downs, Judith J. Thomas, Clyde M. Farrell and the staff of the Southeastern Plant Environment Laboratory, North Carolina State University, and to Wade L. Bryant, Laura K. Gillikin, Mark R. Luttenton, Ann Maureen D. Otte, and Sondra Padley. Additional statistical consultation from Gary Fowler, Kevin F. O'Brien, Donald Holbert, Marsha Shepherd, and Doug Deutschman is very much appreciated. This manuscript was thoughtfully reviewed by Deborah Charlesworth, Michael C. Grant, A. R. Kruckeberg, Ann K. Sakai, Mark A. Schlessman, and Stephen G. Weiler, and completed while CU was the recipient of a Research Opportunity Award, NSF BSR-40464, at the University of Colorado. My colleagues at the Department of Environmental, Population, and Organismic Biology, particularly Jane H. and Carl L. Bock, are most appreciated. ferent forms ofbreeding individuals within the same population. Ifsex expression is controlled by nuclear inheritance, females are at a selective disadvantage relative to hermaphrodites due to their loss of male function. Compensation must exist that enables females to surpass hermaphrodites in some component(s) of fitness (Lewis, 1941; Lloyd, 1975, 1976; Ross, 1978; Charlesworth and Charlesworth, 1978; Charlesworth, 1981). Reported advantages of females over hermaphrodites include greater numbers offlowers per plant, ovules per flower, adult survivorship, seeds per plant, greater seed weight, or progeny success (Darwin, 1877; Connor, 1973; Assouad et al., 1978; Philipp, 1980; Kesseli and Jain, 1984; Van Damme and Van De1den, 1984; Schrader, 1986; Shykoff, 1988). Hypotheses used to explain the evolution and maintenance ofgynodioecy are oftwo general types-those dealing with the adults and those dealing with the progeny. First, adult females may exhibit greater fitness than hermaphrodites due to overdominance (Ross, 1978; Gregorius, Ross, and Gillet, 1982; Van Damme, 1984) or to pleiotropic action ofgenes for sex, i.e., direct effects ofgenetic factors causing sterility on progeny fitness (Van Damme,