Pollinator Maintenance vs. Fruit Production: Partitioned Reproductive Effort in Subdioecious Fuchsia Lycioides

Abstract

Populations of Fuchsia lycioides Andrews are composed of small-flowered female plants and an equal number of larger-flowered hermaphrodites, which may be female fertile, or morphologically or physiologically female sterile. A major selective force driving the evolution of separate sexes in F. lycioides is suggested to be the partitioning of limited resources associated with pollinator maintenance and fruit production in a semi-arid Mediterranean climate. The hummingbird Rhodopsis vesper atacamensis is the only known pollinator, and appears to be energetically dependent upon F. lycioides. Hermaphrodites are facultative in their fruit production, produce as much as six times more nectar than females, and feed birds both prior to flower opening and during anthesis. Females bear abundant fruit and produce only a relatively small amount of nectar during anthesis. Nectar production is extremely variable in both sexes but is unpredictable in hermaphrodites and apparently predictable in females. The unpredictability of hermaphrodite nectar production may be a key factor permitting the evolution of resource partitioning into large-flowered bird-feeding pollen plants and small-flowered reproductive individuals. Subdioecy, a state of dioecy in which populations regularly contain imperfectly differentiated individuals in addition to strictly unisexual individuals, has evolved by at least five evolutionary pathways (Ross, 1982). The genus Fuchsia is characterized by the hermaphroditism-gynodioecy-subdioecy pathway, wherein the male sterile mutants among the hermaphrodites produce the gynodioecious state, followed by a gradual reduction of seed fertility of hermaphrodites so that these come to function largely or completely as males (Godley, 1955; Breedlove, 1969; Arroyo & Raven, 1975). The selective forces apparently responsible for the evolution of dioecy have recently been reviewed by Ross (1982), who emphasized that many of the proposed selection models (fertility variation, sexual selection, overdominance, and resource allocation) are scarcely or not at all distinct from each other. Cases involving pollinator influence, fruit dispersal, and predation (Bawa, 1980; Givnish, 1980) are also likely to involve differential resource allocation. If so, perhaps the most generalized model for the evolution of dioecy is simply an energetic argument: that female reproductive effort is largely limited by the availability of physiological resources, and that separation of thesexes increases fitness by allowing more efficient use or allocation of limiting factors. The resource allocation model need not exclude the traditionally considered hypothesis of outbreeding advantage (inbreeding depression). Ross (1982) concludes that outbreeding advantage is not always a factor in the evolution of dioecy, and even where it occurs, it is probably accompanied by other selective forces. Freeman et al. (1980a), and Bawa (1980) have expressed similar views. In this paper we report observations on the floral biology of a facultatively I Department of Ecology and Evolutionary Biology, University of California, Irvine, California 92717. ANN. MISSOURI BOT. GARD. 69: 199-208. 1982. 0026-6493/82/0199-0208$01.15/0 This content downloaded from 157.55.39.4 on Fri, 09 Sep 2016 04:36:39 UTC All use subject to http://about.jstor.org/terms 200 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 69 XCuesta Buenas Aires . Arraydn