Abstract
Cannabis (Cannabis sativa L.) is an influential yet controversial agricultural plant with a very long and prominent history of recreational, medicinal, and industrial usages. Given the importance of this species, we deepened some of the main challenges—along with potential solutions—behind the breeding of new cannabis cultivars. One of the main issues that should be fixed before starting new breeding programs is the uncertain taxonomic classification of the two main taxa (e.g., indica and sativa) of the Cannabis genus. We tried therefore to examine this topic from a molecular perspective through the use of DNA barcoding. Our findings seem to support a unique species system (C. sativa) based on two subspecies: C. sativa subsp. sativa and C. sativa subsp. indica. The second key issue in a breeding program is related to the dioecy behavior of this species and to the comprehension of those molecular mechanisms underlying flower development, the main cannabis product. Given the role of MADS box genes in flower identity, we analyzed and reorganized all the genomic and transcriptomic data available for homeotic genes, trying to decipher the applicability of the ABCDE model in Cannabis. Finally, reviewing the limits of the conventional breeding methods traditionally applied for developing new varieties, we proposed a new breeding scheme for the constitution of F1 hybrids, without ignoring the indisputable contribution offered by genomics. In this sense, in parallel, we resumed the main advances in the genomic field of this species and, ascertained the lack of a robust set of SNP markers, provided a discriminant and polymorphic panel of SSR markers as a valuable tool for future marker assisted breeding programs.
Highlights
DAFNAE–Department of Agronomy, Food, Natural Resources, Animals and Environment, University of Padova, Campus of Agripolis, Legnaro, Italy
This is intensified by the increasing number of cannabis varieties sold through the black market, along with the parallel development of legal, registered, and patented materials
The classification of Cannabis through approaches involving both chloroplast DNA barcoding based on the standard genes maturase K (matK) and rbcL and nuclear DNA haplotyping based on the ITS1 and ITS2 regions makes the scenario as complicated as expected
Summary
With the cannabis market showing increases in both demand and availability and cannabis seed companies arising wherever national law allows it, the necessity for a reliable molecular based-taxonomic system for this species is urgent. One gene (Cannbio_057002) showed a different behavior from what was expected, being highly expressed in root organs The fact that this MADS box did not clearly cluster with a specific group of homeotic genes in the phylogenetic tree (Figure 3C) and was not expressed in reproductive tissues allowed us to exclude a possible role in flower determination. The only way to successfully use seeds of cannabis varieties is the one based on the development of true F1 hybrids by crossing genetically divergent but individually uniform parental inbreds In addition to this strategy for selfing, the production of highly homozygous genotypes can be achieved from full-sibling crosses performed by hand between sister-brother individuals that belong to the same progeny and share the same two parental lines. The quality of cannabinoids is strongly dependent on the genotype, whereas cannabinoid quantity is affected by agronomic practices, environmental conditions, and genotype x environment interactions
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