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.