THE REPRODUCTIVE BIOLOGY OF RHUS INTEGRIFOLIA AND RHUS OVATA (ANACARDIACEAE).

Abstract

Rhus integrifolia (Nutt. ex. T. & G.) Benth. and Hook. ex. Rothr. and Rhus ovata Wats. are perennial shrubs of the coastal sage scrub and chaparral communities of coastal southern and Baja California (Munz, 1949). Rhus ovata occurs also in western and central Arizona. Usually the two species are described as polygamodioecious (Barkley, 1937; Munz and Keck, 1959). However, several populations of the two taxa have been found which more closely approach a gynodioecious breeding system. Gynodioecism is characterized by the presence of female plants (often called male steriles) together with hermaphrodites (Darwin, 1889; Mathers, 1940), and is an outbreeding mechanism, since male steriles must be pollinated by hermaphrodites to produce seed. Several mechanisms have been established for maintaining male steriles in a population. Ross and Shaw (1971) indicated that male steriles, under monogenic or digenic inheritance, are maintained in a population only when they are at least twice as seed-fertile as hermaphrodites. In the case of cytoplasmic inheritance, only a slight difference in seed-fertility is required to maintain male steriles (Lewis, 1941). In Origanumn vulgare (Labiatae) a completely different mechanism, heterozygote advantage, has been shown to maintain male steriles (Lewis and Crowe, 1956). The objective of this study was to determine if the reproductive biology of R. integrifolia and R. ovata fits the models of Ross and Shaw and others for the maintenance of gynodioecism in natural plant populations.