DEVELOPMENTAL AND GENETIC SOURCES OF SEED WEIGHT VARIATION IN RAPHANUS RAPHANISTRUM L. (BRASSICACEAE)

Seed weight is known to have a marked impact on emergence and post-emergence productivity in wild radish (Raphanus raphanistrum). In this paper, I describe several levels of seed weight variation in plants taken from a natural population in Hamden, Connecticut. Six maternal plants from the 1981 season were analyzed in detail: the weights and positions of all seeds within a fruit were recorded, and some of these seeds were used the following summer for competition studies and progeny analysis. Within a plant, average seed weight decreased as the number of seeds within a fruit increased, suggesting that developing embryos compete for maternal resources. Seed weight also varied significantly among the six maternal plants used in the study. Comparison of the average weights of seeds produced by offspring of those six plants with the average weights of seeds borne by the maternal plant revealed a significant genetic component to seed weight variation. Seed weight varied up to six-fold within single fruits of R. raphanistrum; large seeds tend to occur near the pedicel or in the middle positions. Seed size variation seen within single fruits is of sufficient magnitude to result in differential reproductive output among closely related seeds under competitive field conditions. DEMOGRAPHIC STUDIES of natural plant populations have consistently demonstrated the importance of early seedling size differences in determining ultimate seedling fitness (Harper, 1977; Cook, 1979; Gross, 1981; Cook and Lyons, 1983). Individuals in a cohort can gain an early size advantage in several ways: through more rapid relative growth rate (Rabinowitz, 1979), earlier emergence (Howell, 1981; Cook and Lyons,1983), or greater seed weight (Black, 1958; Howe and Richter,1982; Stanton,1984). Despite the potential importance of seed size . . . . ln competltlve envlronments we know relatively little about environmental and genetic sources of seed weight variation in non-agricultural crops. Ecological research on intraspecific seed size variation has been discouraged by two observations. First, an influential survey of seed ' Received for publication 17 October, 1983; revision accepted June 1984. 2 Current address: Botany Department, University of California, Davis, CA 95616. This study was initiated with support from the Brown Post-doctoral Fellowship in Botany at Yale University. S. N. Handel gave encouragement and guidance throughout. J. Mishkin, D. Levine, and K. McFadden provided essential field assistance. Later seed weighing and analysis was supported at U. C. Davis by NSFGrant DEP-82 14508. C. McGee, J. Damey, and H. Hasbrouck were of great assistance during the latter stages of the project. An earlier draft of this paper was much improved by the careful criticisms of N. Ellstrand, S. Handel, D. Marshall, A. Snow, and two anonymous reviewers. weight variation, largely using data from crop species, found that flower number and stem number are many times more plastic than seed size (Harper, Lowell and Moore, 1970). Second, in species that do exhibit substantial seed weight variation, much ofthat variation occurs within individual maternal plants (Cavers and Harper,1966; Maun and Cavers,1971; Janzen, 1977a,b,1982;Primack,1978;Waller,1982). These patterns suggest that seed weight should show low levels of heritability in natural populations and be relatively resistant to directional change through natural selection. Given this background, it is not surprising that most research into the effects of seed size on seedling fitness has focused on interspecific, rather than intraspecific, comparisons (e.g. Wilbur, 1977; Rabinowitz, 1978). Studies on intraspecific seed weight variation may prove to be of great ecological value, despite the above problems. First, surveys of non-agricultural plants commonly report considerable range in seed size for a species (e.g., Berggren, 1981). Second, intraspecific seed weight variation may provide real insights into life history models dealing with maternal resource allocation. The basic premise of such models is that maternal resources are finite, leading to a tradeoW between the number of offispring produced and investment in each oSspring (Smith and Fretwell, 1974; Wilbur, 1977). Because maternal investment in animal offspring often includes hard-to-measure pa-