5.2. Monoecy, dioecy and evolutionary considerations
To add further complications to the study of sex determination inCannabis , monecious varieties exist, in addition to the canonical
dioecious varieties. Monoecious cultivars develop male and female
flowers on the same plant and are particularly popular for fibre
production (Figure 2). This is because in dioecious varieties, male
plants flower earlier than female plants, whereas for monoecious
cultivars flowering time is more synchronized, thus facilitating the
determination of an optimal harvest time
(Faux et al., 2016).
Genetically, monoecious cultivars carry two X chromosomes and no Y
chromosome, indicating that the XY sex determination system can be
‘leaky’ in Cannabis(Faux et al., 2014;
Razumova et al., 2016). In addition, the monoecious cultivars express
‘femaleness’ and ‘maleness’ to different degrees, i.e. the ratio of
female to male flowers one plant develops differs between cultivars but
also between different environmental conditions
(Faux et al., 2014).
At least some genetic loci relevant for the sex expression in monoeciousCannabis plants seem to be located on the X chromosome
(Faux et al., 2016).
It will be interesting to see whether the distinction between dioecious
and monoecious cultivars and between different degrees of sex expression
(femaleness or maleness) in monoecious cultivars can be traced back to
the same molecular circuits.
From an evolutionary point of view, the sexual system in the entire
family of Cannabaceae is complex. In contrast to angiosperms in general,
some 85 % of which are bisexual
(Renner, 2014), true
bisexual flowers are conspicuously rare in Cannabaceae
(Yang et al., 2013).
Several shifts of the sex determination system occurred in the
Cannabaceae, and ancestral character state reconstructions indicate that
monoecy, or, with a lesser likelihood, dioecy, is the ancestral state in
the family (Yang et
al., 2013). Interestingly, one of the closest relatives ofCannabis sativa , Humulus lupulus (common hop), is
dioecious with an XY sex determination system in which the X to autosome
ratio determines the sex
(Parker and Clark,
1991). This may be taken as additional evidence that alsoCannabis has an X to autosome ratio sex determination system.
However, it should be kept in mind that that Humulus andCannabis may have separated some 25 million years ago
(Jin et al., 2020)
and that sex determination systems can frequently and rapidly change
during evolution
(Bachtrog et al.,
2014). Nevertheless, available data on the sex determination systems in
Cannabaceae as a whole may be taken as indication that all family
members share an ancestral molecular mechanism for sex determination,
but that this mechanism is relatively labile and manifests differently
in different species.