In Neraudia, dioecy appears to have evolved from monoecy. In the two Psychotria colonists, separate sexes probably were derived from heterostyly.
Dioecious and dimorphic breeding systems apparently have been derived from heterostyly, from monoecy, or most commonly directly from hermaphroditism or from hermaphroditism via gynodioecy.
These are clearly upper limits to the mean time to extinction, because no sources of demographic or environmental stochasticity imperil the populations in our simulations for monoecy.
Under monoecy, the CV of extinction time tends to be higher, more on the order of 0.1 to 0.2, decreasing with increasing fecundity (table 4).
Shrubs 1-4 m tall, dioecious with occasional deviations to monoecy
, indumentum with stellate trichomes, latex absent.
Here I compare the effects of two breeding systems, dioecy and monoecy, on reproductive characters and life histories of two subspecies of the Mediterranean cucurbit, Ecballium elaterium.
The evolutionary pathway from monoecy to dioecy has been described as a gradual divergence in the relative proportions of male and female flowers in the two incipient "sexes" (Charlesworth and Charlesworth 1978b, Lloyd 1980b, Ross 1982).
The distribution of monoecy, protogyny, and self-incompatibility in grasses, when compared with the phylogenetic structures resolved here, are suggestive of the possibility that self-incompatibility evolved only once or twice in the grasses.
It is possible that both protogyny and monoecy preceded the evolution of self-incompatibility.
may originally have evolved as a mechanism to promote outcrossing (e.g., Lloyd, 1972; Charlesworth and Charlesworth, 1978) and/or to reduce interference between male and female function (Webb and Lloyd, 1986).
Pronounced spatial separation of male and female organs in the flower (herkogamy), or separate male and female flowers (monoecy
), are very effective outbreeding devices (Breese, 1959; Moore & Lewis, 1965; Faegri & van der Pijl, 1971; Shoen, 1977; Vasek, 1977; Rick et al., 1978; Thomas & Murray, 1981; Ritland & Ritland, 1989; Holtsford & Ellstrand, 1992).
Plants showing any form of hermaphroditism (i.e., monoecy
, sexual lability, or perfect flowers) were labeled as "hermaphrodites." Twelve male, 11 female, and 9 hermaphroditic plants were chosen at random from the 200 tagged plants of an earlier study (Freeman and McArthur, 1984b).