Dioicy promotes
outcrossing.[3]Sexual dimorphism is commonly found in dioicous species.[4]: 71 [2]: 378 Dioicy is correlated with reduced
sporophyte production,[5] due to spatial separation of
male and
female colonies, scarcity or absence of males.[4]: 65
The term
dioecy is inapplicable to
bryophytes because it refers to the sexuality of
vascular plant sporophytes.[4]: 62 Nonetheless dioecy and dioicy are comparable in many respects.[6]
Etymology
The words dioicous and di(o)ecious are derived from οἶκος or οἰκία and δι- (di-), twice, double. ((o)e is
the Latin way of transliterating Greek οι, whereas oi is a more straightforward modern way.) Generally, the term and "dioicous" have been restricted to description of haploid sexuality (
gametophytic sexuality), and are thus primarily to describe
bryophytes in which the gametophyte is the dominant generation. Meanwhile, "dioecious" are used to describe diploid sexuality (
sporophytic sexuality), and thus are used to describe
tracheophytes (vascular plants) in which the
sporophyte is the dominant generation.[7][8]: 82
The ancestral
sexual system in bryophytes is unknown but it has been suggested
monoicy and dioicy evolved several times.[12] It has also been suggested that dioicy is a
plesiomorphic character for bryophytes.[4]: 71 In order for dioicy to evolve from monoicy it needs two mutations, a male sterility mutation and a female sterility mutation.[12]
Hornworts have gone through twice as many transitions from dioicy to monoicy than monoicy to dioicy.[1]
Among moss species the transition from monoicy to dioicy is more common than dioicy to monoicy[13] with there being at least 133 transitions from monoicy to dioicy in moss.
Sexual specialization has been used as an explanation for this recurring evolution of dioicy in
mosses.[4]: 71
^Buck WR & Goffinet B (2000). "Morphology and classification of mosses". In Shaw AJ & Goffinet B (ed.). Bryophyte Biology. New York: Cambridge University Press.
ISBN978-0-521-66794-4.