Abstract
The outcrossing rate is a key component of the breeding system. While estimates of outcrossing rate exist for some plants, less information is available on variation in outcrossing rate within species and on its causes. Since many features of flowers affect the level of outcrossing, plant breeding systems are often inferred from observations on floral morphology, physiology, and phenology. Typically, such observations include whether plants are unisexual or cosexual, have perfect or unisexual flowers, are self-compatible or selfincompatible (Grant, 1958; Baker, 1959), whether flowers are protandrous, protogynous, or homogamous (Faegri and van der Pijl, 1971), degree of stigma exsertion (Grant, 1954; Breese, 1959), flower size (Rollins, 1963), pollen-ovule ratio (Lloyd, 1965; Cruden, 1977), and amount of fruit or seed set in the absence of pollinators (Lloyd, 1965; Harding et al., 1974). Few studies of plant breeding systems have addressed the problem of how well the presence of these floral features can be used to predict outcrossing rates of plant populations, or whether variation among populations in the expression of these features is heritable. This latter question is especially relevant to studies concerned with the evolution of plant breeding systems (e.g., Lloyd, 1965; Moore and Lewis, 1965; Baker, 1966; Ornduff, 1972; Arroyo, 1973; Rick et al., 1978; Solbrig and Rollins, 1977). This paper presents the results of an investigation of breeding system variation in the California annual plant, Gilia
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