Female Frequencies in Gynodioecious Populations Correlated with Selfing Rates in Hermaphrodites

Gynodioecious populations consist of sebarate hermaphroditic and female individuals. Females are at a selective disadvantage because they contribute genes to the next generation only through ovules, while hermaphrodites contribute genes through ovules and pollen. For females to be maintained in populations they must have some compensating selective advantage. The outcrossing hypothesis postulates that females are maintained because their progeny result from obligate outcrossing, whereas some of the progeny of hermaphrodites result from self-fertilization and are less fit because of inbreeding depression. If correct, the frequency of females should be positively correlated with selfing rates of hermaphrodites in populations. We found a strong positive correlation between female frequency and selfing rates of hermaphrodites (r = 0.91, P l/2, where s are relatively rare. Gynodioecious species are is the selfing rate of hermaphrodites (i.e., 1 estimated to make up about 2% of the flora of minus the outcrossing rate), and d is the inNew Zealand (Godley, 1979), but are much breeding depression of progeny from selfing rarer elsewhere. The successful establishment (Lloyd, 1974, 1975; Ganders, 1978; Charlesof male-sterile mutations in natural populaworth and Charlesworth, 1978; Charlesworth tions is therefore a rare event, but is not limited and Ganders, 1979; Charlesworth, 198 1). The by mutation rate or phylogenetic constraints. frequency of females maintained in populaObviously, male-sterility is at a selective distions increases as sd increases (Lloyd, 1975). advantage because it reduces the male fitness Estimating inbreeding depression of progeny of an individual to zero. Hermaphrodites conin natural populations is technically difficult. tribute genes to the next generation through However, estimation of selfing rates is possible both pollen and ovules, but females contribute from progeny arrays assayed for polymorphic genes only through ovules. Females can be allozyme loci (Clegg, 1980; Ritland and Jain, 198 1). If the outcrossing hypothesis of gynoI Received for publication 4 March 1986; revision acdioecy is a significant factor in the maintenance cepted 20 June 1986. of females in natural populations, then the freThis research was supported by a grant from the Natural quency of females should be positively corSciences and Engineering Research Council of Canada to FRG. We thank Dr. Helen Kennedy for assisting with the related with the selfing rate in hermaphrodites. field work. This prediction has not previously been tested, -1646 AMERICAN JOURNAL OF BOTANY [Vol. 73 TABLE1. Selfing rates of hermaphrodites and percentage of females in gynodioecious populations of Hawaiian species of Bidens B. cervicata SheriT B. forbesii SheriT subsp. forbesii B. menziesii (A. Gray) ShertT subsp. jiliformis (Sherff) Ganders & Nagata B. sandvicensis Less. subsp. confusa Nagata & Ganders B. sandvicensis Less. subsp. sandvicensis Locality Makaha Ridge, Kauai Na Pali Coast, Kauai Ahumoa, Hawaii Puu Koohe, Habaii Iliau Nature TrAil, Kauai Waahila Ridge, Oahu Wailua River, Kauai % Selfing in hemaphrodites (S.E.)