Gynodioecy Charlesworth, 1978) is that gynodioecy is normally

Gynodioecy pathway: Besides
monoecy, the sexual system most often discussed as a possible pathway to dioecy
is gynodioecy (Darwin, 1877; Charlesworth and Charlesworth, 1978; Bachtrog et
al., 2014). About 275 (1.8%) of the 14,559 genera of angiosperms have one or
more gynodioecious species, and 59 genera have gynodioecious as well as
dioecious species (Dufay et al., 2014; Renner, 2014). However, from herbarium
material these systems cannot be reliably distinguished, and this is true even
in some living populations (Spigler and Ashman, 2012). Herbarium-based
descriptions of species’ sexual systems still constitute the vast majority of
the available data on plant sex, and they often resort to Linnaeus’s term
polygamy, which refers to the presence of unisexual and bisexual flowers on
some or all individuals without precise data on frequencies and function.    


Especially important has been a model of the gynodioecy pathway proposed
by the population geneticists Brian and Deborah Charlesworth (1978). The
Charlesworths’ model assumes two sequential mutations affecting male and female
fertility in a closely linked region, followed by a recombination suppressor
(possibly an inversion) that would take populations of hermaphrodites to
chromosomally determined dioecy via a gynodioecious intermediate phase. Because
in flowering plants loss of function mutations are almost 50 times more frequent
than gain of function mutations, pathways starting with the loss of male
function or the loss of female function are thought to predominate (Bachtrog et
al., 2014). A loss of male function in some individuals (i.e., gynodioecy),
results in plants with functionally female flowers that may benefit from
avoiding self-pollination, and the resulting inbreeding depression. A loss of
female function (resulting in plants with only male flowers, androdioecy) is
more costly because males compete with hermaphrodites for female flowers to
fertilize, while being unable to benefit from reproductive assurance through
pollination of own ovules.. These theoretical arguments where thought to
account for a supposed prevalence in flowering plants of a gynodioecy/dioecy
pathway as well as for a prevalence of the XY chromosome system (Charlesworth
and Charlesworth, 1978; Bachtrog et al., 2014).

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So far, there is little
evidence from comparative-phylogenetic work for the gynodioecy/dioecy pathways,
which is expected to play a role in temperate herbaceous plants, where
gynodioecy is more common than in trees (although this may result from poor
knowledge of sexual systems in tropical trees). A promising group to
investigate would be the New Zealand Veronica section
Hebe, with several suspected switches
between these sexual systems (Delph, 1990). A problem with the
gynodioecy/dioecy pathway (pointed out by Charlesworth and Charlesworth, 1978)
is that gynodioecy is normally cytoplasmically controlled while dioecy os
nuclear-controlled (McCauley and Bailey 2009).  The gynodioecious species with the best evidence on the genetics of
its sexual system is Fragaria vesca
subsp. bracteata in which both
cytoplasmic and nuclear genes are involved in sex determination (Tennessen et al., 2015; Ashman et al., 2015). (709 words)