A Belated Green Revolution for Cannabis: Virtual Genetic Resources to Fast-Track Cultivar Development

Abstract

Cannabis is a predominantly diecious phenotypically diverse domesticated genus with few if any extant natural populations. International narcotics conventions and associated legislation have constrained the establishment, characterization, and use of Cannabis genetic resource collections. This has resulted in the underutilization of genepool variability in cultivar development and has limited the inclusion of secondary genepools associated with genetic improvement strategies of the Green Revolution. The structured screening of ex situ germplasm and the exploitation of locally-adapted intraspecific traits is expected to facilitate the genetic improvement of Cannabis. However, limited attempts have been made to establish the full extent of genetic resources available for pre-breeding. We present a thorough critical review of Cannabis ex situ genetic resources, and discuss recommendations for conservation, pre-breeding characterization, and genetic analysis that will underpin future cultivar development. We consider East Asian germplasm to be a priority for conservation based on the prolonged historical cultivation of Cannabis in this region over a range of latitudes, along with the apparent high levels of genetic diversity and relatively low representation in published genetic resource collections. Seed cryopreservation could improve conservation by reducing hybridization and genetic drift that may occur during Cannabis germplasm regeneration. Given the unique legal status of Cannabis, we propose the establishment of a global virtual core collection based on the collation of consistent and comprehensive provenance meta-data and the adoption of high-throughput DNA sequencing technologies. This would enable representative core collections to be used for systematic phenotyping, and so underpin breeding strategies for the genetic improvement of Cannabis.

Keywords: cannabinoids; genetic conservation; industrial hemp; marijuana; phenotypic variation; plant breeding.

Figure 1

Phenotypic variability in cannabinoid composition in exotic Cannabis germplasm (A) variability of acidic and neutral forms of the penty-alkyl-cannabinoids THC and CBD within the cannabinoid fraction. Data points correspond to 66 plants derived from 22 accessions; Accessions were sourced from different geographical locations and represent different stages of domestication; Accessions were grown in environmentally-controlled conditions and harvested at similar developmental stages; redrawn from Welling et al. (2015); (B) chromatogram at 272 nm of a single cultivated and wild-type plant; Top chromatogram of an individual European cultivar Futura 77 exhibiting acidic forms of the pentyl-alkyl-cannabinoids CBD and THC as the predominant cannabinoids; Bottom chromatogram of an individual East Asian landrace exhibiting acidic forms of the propyl-alkyl-cannabinoids CBDV and THCV as the predominant cannabinoids; CBD, cannabidiol; CBDV, cannabidivarin; THC, delta-9-tetrahydrocannabinol; THCV, delta-9-tetrahydrocannabivarin.

Figure 2

Phenotypic variation in the xylem: phloem ratio for a range of Cannabis germplasm from the Ecofibre Global Germplasm Collection. Stereo microscopy images taken from internode 3–4 across a representative sub-set of 20 accessions; Accessions were grown in environmentally-controlled conditions and harvested at similar developmental stages; Letters correspond to accessions and numbers to replicate plant; Scale bar: 500μm; redrawn from (Stanger, 2015).

Figure 3

Summary of CPRO, INF&MP, and VIR published Cannabis germplasm resources. Number and associated country of origin of accessions listed in CPRO, INF&MP, and VIR germplasms. CPRO accessions listed in De Meijer and van Soest (1992) and Gilmore et al. (2007); INF&MP listed in Mankowska and Silska (2015); VIR accessions listed in Clarke (1998); Center for Plant Breeding and Reproduction Research, CPRO; Institute of Natural Fibers and Medicinal Plants, INF∓ Vavilov Research Institute gene bank, VIR.