The complexity of phytocannabinoid synthases does not end there, though. Copy number variation of CBDA and THCA synthase genes might be involved in phytocannabinoid level and composition (Vergara et al., 2019) and most likely, the number of synthase (pseudo)genes might be different for each cultivar sequenced (Grassa et al., 2018; Laverty et al., 2019; McKernan et al., 2020).
High throughput assays for BT and BDmarkers have been developed and show that many plants actually contain both loci (Cascini et al., 2019; McKernan et al., 2020; Toth et al., 2020). Moreover, many BD/BD plants, especially those with higher CBDA levels, have THCA levels of above 0.3 % of dry flower mass, despite the absence of a functional BT allele (Toth et al., 2020). This residual THCA is probably at least to some extent a by-product of the CBDA synthase itself. The THCA and CBDA synthase have a relatively high sequence similarity (83.85 %, Figure 5) and process the same precursor molecule, CBGA (Figure 4). In vitro studies have shown that the CBDA synthase produced CBDA and THCA at roughly a ratio of 20:1 (Zirpel et al., 2018). This is similar to ratios observed in planta in high-CBD hemp varieties as well (Toth et al., 2020; Weiblen et al., 2015). This potentially results in the problem that, if CBDA production is increased, THCA also increases as a by-product, even if plants do not express a functional THCA synthase. Cannabis varieties with very high CBD levels may thus be at risk of exceeding legal THC thresholds.
Understanding the exact genetics underlying the different chemotypes will be important for future targeted breeding approaches. Tight restrictions across the world make it difficult for farmers to grow chemotype III, IV and V varieties, because the presence of residual THC creates regulatory problems and uncertainties. Especially type III plants often have THCA/THC levels slightly above the legal THC limit (Aizpurua-Olaizola et al., 2016; Toth et al., 2020). Hence, one important breeding goal is going to be the generation of zero-THC lines which still produce high levels of CBD in the range of 15 to 20 % of dry flower mass. Whether this is possible to achieve is difficult to say, since even in the absence of a THCA synthase, CBDA synthases produce THCA as a by-product (Toth et al., 2020; Zirpel et al., 2018). This will, therefore, require identification of a CBDA synthase that does produce only very low or no amounts of THCA.In vitro experiments show that point mutations can alter the amount of by-products (Zirpel et al., 2018). Natural variation in synthase genes exists and have been linked to altered phytocannabinoid compositions (Onofri et al., 2015). Hence, naturally occurring or artificially generated CBDA synthase varieties could be used for targeted breeding in this direction.
In addition, Cannabis varieties used for fibre or seed production could be selectively bred and genotyped to have 0 % overall phytocannabinoids (chemotype V), as currently even the farming of these kinds of varieties is heavily restricted in many countries.
Other phytocannabinoids like CBG(A) and CBC(A) as well as the manifold variants of terpenes produced in Cannabis flowers are increasingly coming into focus in the medical research fields (reviewed in Booth and Bohlmann, 2019; Deiana, 2017; Pollastro et al., 2018), hence generating lines with specific phytocannabinoid profiles might be of interest in further research.

4. A hairy topic: Flower development and morphology in Cannabis

The flower is the reproductive structure of flowering plants (angiosperms), which represent one of the most successful and diverse groups of organisms on this planet (Krizek and Fletcher, 2005). While the characteristic shape of the Cannabis leaf is often used as a symbol for the whole plant, Cannabis female flowers are of particular interest because they are the main site of production of pharmacologically active compounds (phytocannabinoids) (Spitzer-Rimon et al., 2019). Understanding the morphology of Cannabis flowers and their developmental genetics is therefore especially important.
The typical angiosperm flower consists of four different organ types, which are organized in concentric whorls: sepals, petals, stamens and carpels (Endress, 1992; Krizek and Fletcher, 2005). Sepals are in the outermost whorl and usually green and leaflike in appearance. Petals are in the second whorl and often coloured to attract pollinators. Petals together with sepals are termed the perianth and constitute the non-reproductive part of a flower. Stamens are typically located in the third floral whorl. They are the male reproductive organs and are composed of an anther and a filament. The anthers grow on top of the stalk-like filaments and are the site of pollen production. Finally, carpels develop in the fourth and central whorl of a typical flower. Carpels are the reproductive organs that contain an ovary inside which ovules develop. The tip of the carpel, the stigma, receives the pollen. The style connects the stigma to the ovary (Becker, 2020; Endress, 1992; Krizek and Fletcher, 2005).
Notably, the number, arrangement, and morphology of the floral organs varies substantially between different species of flowering plants (Endress, 2011; Theissen and Melzer, 2007). Most flowers contain, as described above, both carpels and stamens, and are therefore termed bisexual flowers (Renner, 2014). However some 15 % of flowering plant species are monoecious or dioecious and have unisexual flowers that develop only stamens or carpels (Renner, 2014). In dioecious plants, female and male flowers develop on separate individuals. In contrast, in monoecious plants male and female flowers develop on the same individual (Renner, 2014).