The scenario we have presented is based on the observation that, for most dioecious species, fitness is a function not only of genotype (gender) but also of genotype-by-environmental interactions; i.e., the evolution of dioecy
in the monoecious pathway depends critically on ecological factors.
A model for the evolution of dioecy
Several species are complex polyploids with a range of breeding systems, including apomixis and dioecy
, that promote interspecific hybridization and introgression (Muntzing, 1940; Clausen, 1961).
Darwin (1851) found that barnacles exhibit a diverse array of sexual systems, including simultaneous hermaphroditism, dioecy
(coexistence of females and males in a population), and androdioecy (hermaphrodites and males).
The gender ratio in 2-5-y-old geoducks was male biased relative to the 1:1 sex ratio observed in wild populations (P [much less than] 0.05), providing evidence for facultative protandric dioecy
. Rates of maturation in cultured populations were synchronous with nearby wild populations.
This system is described as "temporal dioecy
" by pollination biologists.
(1997) suggesting a polyphyletic origin of dioecy
during the evolution of the angiosperms.
Pollinator selectivity and its implications for the evolution of dioecy
and sexual dimorphism.
Andromonoecy has been reported for Cryptanthus subgenus Cryptanthus (Ramirez-Morillo 1996), a Southeastern Brazilian group of terrestrial species; dioecy
has been reported for the genus Hechtia (however, there is a poligamomonoecious species), for the only species of Androlepis Brongn ex Houllet (A.
While strictly genetic or chromosomal dioecy
is widespread, other types of reproduction such as hermaphroditism, parthenogenesis, and biased sex ratio appear to be the rule in many species.
Additionally, species in the Caryophyllaceae exhibit a range of breeding systems from hermaphroditism to dioecy
(Muller 1883, Gleason and Cronquist 1963, Thrall et al.
As pointed out earlier, dioecy
offers problems for a switch to animal pollinators because a similar attractant must exist in individuals of both genders, as is most striking in Gnetales.