DIOECY AND ITS CORRELATES IN THE FLOWERING PLANTS

Considerable effort has been spent documenting correlations between di oecy and various ecological and morphological traits for the purpose o f testing hypotheses about conditions that favor dioecy. The data anal yzed in these studies, with few exceptions, come from local floras, wi thin which it was possible to contrast the subsets of dioecious and no ndioecious taxa with regard to the traits in question. However, if the re is a strong phylogenetic component to the presence or absence of di oecy, regional sampling may result in spurious associations. Here, we report results of a categorical multivariate analysis of the strengths of various associations of dioecy with other traits over all flowerin g plants. Families were scored for presence of absence of monoecy or d ioecy, systematic position, numbers of species and genera, growth form s, modes of pollination and dispersal, geographic distribution, and tr ophic status. Seven percent of angiosperm genera (959 of 13,500) conta in at least some dioecious species, and approximate to 6% of angiosper m species (14,620 of 240,000) are dioecious. The most consistent assoc iations in the data set relate the presence of dioecy to monoecy, wind or water pollination, and climbing growth. At both the family and the genus level, insect pollination is underrepresented among dioecious p lants. At the family level, a positive correlation between dioecy and woody growth results primarily from the association between dioecy and climbing growth (whether woody or herbaceous) because neither the tre e nor the shrub growth forms alone are consistently correlated with a family's tendency to include dioecious members. Dioecy appears to have evolved most frequently via monoecy, perhaps through divergent adjust ments of floral sex ratios between individual plants. Monoecy itself i s related to abiotic pollination and climbing growth as revealed by mu ltivariate analysis. Dioecy and monoecy are concentrated in the less a dvanced superorders of Thorne (1992) and subclasses of Cronquist (1988 ). The frequency of dioecy found in a local flora therefore reflects t he level of dioecy in its particular pool of families as much as, or m ore than, local selective factors. The positive associations of dioecy with abiotic pollination and monoecy are related to floral developmen tal and morphological attributes, as is the negative association with bird and bat pollination; the positive association of dioecy with clim bing growth is tentatively explained in terms of differential selectio n for optimal resource allocation to sexual function. If rapid upward growth is at a premium in climbers and if fruit set at least temporari ly inhibits growth or requires the production of thicker, more