Journal:Broad-scale genetic diversity of Cannabis for forensic applications

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Full article title Broad-scale genetic diversity of Cannabis for forensic applications
Journal PLOS ONE
Author(s) Dufresnes, Christophe; Jan, Catherine; Bienert, Friederike; Goudet, Jérôme; Fumagalli, Luca
Author affiliation(s) University of Lausanne, Centre Universitaire Romand de Médecine Légale,
Primary contact Email: Luca dot Fumagalli at unil dot ch
Editors Scali, Monica
Year published 2017
Volume and issue 121
Page(s) e0170522
DOI 10.1371/journal.pone.0170522
ISSN 1932-6203
Distribution license Creative Commons Attribution 4.0 International
Website http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0170522
Download http://journals.plos.org/plosone/article/file?id=10.1371/journal.pone.0170522&type=printable (PDF)
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Abstract

Cannabis (hemp and marijuana) is an iconic yet controversial crop. On the one hand, it represents a growing market for pharmaceutical and agricultural sectors. On the other hand, plants synthesizing the psychoactive THC produce the most widespread illicit drug in the world. Yet, the difficulty to reliably distinguish between Cannabis varieties based on morphological or biochemical criteria impedes the development of promising industrial programs and hinders the fight against narcotrafficking. Genetics offers an appropriate alternative to characterize drug vs. non-drug Cannabis. However, forensic applications require rapid and affordable genotyping of informative and reliable molecular markers for which a broad-scale reference database, representing both intra- and inter-variety variation, is available. Here we provide such a resource for Cannabis, by genotyping 13 microsatellite loci (STRs) in 1,324 samples selected specifically for fiber (24 hemp varieties) and drug (15 marijuana varieties) production. We showed that these loci are sufficient to capture most of the genome-wide diversity patterns recently revealed by next-generation sequencing (NGS) data. We recovered strong genetic structure between marijuana and hemp and demonstrated that anonymous samples can be confidently assigned to either plant types. Fibers appear genetically homogeneous whereas drugs show low (often clonal) diversity within varieties, but very high genetic differentiation between them, likely resulting from breeding practices. Based on an additional test dataset that includes samples from 41 local police seizures, we showed that the genetic signature of marijuana cultivars could be used to trace crime scene evidence. To date, our study provides the most comprehensive genetic resource for Cannabis forensics worldwide.

Introduction

Cannabis is one of humanity’s oldest cultivated plant. It is thought to have originated in central Asia and was domesticated as early as 8,000 BP for food, fiber, oil, medicines and as an inebriant. This crop was since distributed across the world during the last two millennia and, due to its recent legalization in several countries, is increasingly exploited by several industrial sectors (hemp) and as a recreational drug (marijuana). The taxonomic status of Cannabis has always been disputed, as it encompasses multiple cultural, geographic, historical, and functional aspects.[1][2][3][4] Whereas most authors now consider it a monotypic panmictic taxon, Cannabis sativa, three species or subspecies (sativa, indica and ruderalis) are often mentioned but without a comprehensive taxonomic grouping so far. The nomenclature may thus differ depending on whether it refers to morphological or chemical variation, geographic distribution, ecotype, as well as crop-use characteristics and intoxicant properties resulting from human selection.[4][5][6][7] Cannabis presumably diversified following selection for traits enhancing fiber and seed production (”hemp”) or psychoactive properties ("drug"). Importantly, Cannabis types differ in their absolute and relative amounts of terpenophenolic cannabinoids, notably Δ1-tetrahydrocannabinol (THC), the well-known psychoactive compound of marijuana, and the non-psychoactive cannabidiol (CBD). In this context, drug-type Cannabis (marijuana) is broadly characterized by a higher overall cannabinoid content than fiber-types. However, the most widely recognized criteria to assign a Cannabis plant to either “drug” or “hemp” type is the THC:CBD ratio, according to which three main chemical phenotype (chemotype) classes are recognized: hemp-type plants with a low ratio (THC:CBD < 1), drug-type plants with a high ratio (THC:CBD > 1), and intermediate-type plants with a ratio close to one.[6][8] The informal designation sativa and indica may have various, controversial meanings. Morphologically, the name sativa designates tall plants with narrow leaves, while indica refers to short plants with wide leaves. Among the marijuana community however, sativa rather refers to equatorial varieties producing stimulating psychoactive effects (THC:CBD ≈ 1), whereas indica-type plants from Central Asia are used for relaxing and sedative drugs (THC:CBD > 1).[8]

The commercial interest for Cannabis declined during the twentieth century due, e.g., to the development of synthetic fibers and the stringent policies regarding its exploitation, but this iconic weed is recently regaining attention in many countries for its high medicinal, industrial, and agricultural potentials.[9] However, its usage is still controversial, in particular from agro-economic, public health, and forensic perspectives. Due to its intoxicant properties, the cultivation and possession of Cannabis is under strict legal regulations. High-THC:CBD varieties are prohibited in many countries but remain the most frequently-used illicit drug worldwide[10] (~180 million consumers in 2013[11]), in the form of marijuana (dried inflorescences) or hashish (resin). In contrast, low-THC:CBD hemp crops can be exploited under licensed control for seed oil, fibers, and pharmaceuticals. For instance, quantitative measures of THC content are currently considered by the European Union (EU) for approval as a licensed hemp cultivar (below 0.2% THC weight per weight in the mature dry inflorescences; http://ec.europa.eu/food/plant_en). Yet hemp and marijuana varieties are hardly distinguishable morphologically, and discrimination of drug vs. non-drug chemotypes by quantitative THC dosage has also proven inadequate due to its dependence on environmental factors, to the strong variation during the plant’s life cycle, as well as between individual plants.[12][13] In addition, the qualitative assessment of THC:CBD ratio is also problematic for an unequivocal discrimination between fiber and drug types due to the presence of a largely variable intermediate chemotype class, the occurrence of several exceptions (e.g., hemp accessions with a THC-predominant chemotype[14][15][16]), and the common practice among drug breeders to produce hybrid varieties.

This issue largely impedes crops’ improvement and full-scale industrial development; it even causes a security risk, as licensed crops may be used as a cover for illegal drug production. Moreover, it significantly limits the ability of law enforcement agencies to trace drug seizures and link illegal producers to organized crime syndicates supplying the black market of Cannabis drugs. In addition, Cannabis can have long-distance dispersal capabilities[17], and fiber crops might face cryptic contamination by pollen from drug varieties.

Genetic tools offer a promising avenue to overcome these issues, especially to distinguish between drug vs. non-drug plants.[18] Importantly, genetics requires small amounts of tissues as a DNA source, whereas chemical analyses necessitate inflorescences. A promising aspect has been to genotype loci directly linked to THC synthesis[8][19] in association with chemotype profiling. However, this association is not ubiquitous[14][15], and genotyping may be compromised by complex gene duplications, pseudogenes[20][21][22], and the fact that only a limited number of varieties among the tremendous Cannabis diversity has been validated[15]; moreover, chemotype seem to greatly vary even among genotypes.[20]

A parallel, complementary approach is to discriminate drug vs. hemp plants from their non-adaptive genetic variation. Until the recent past, the genetic diversity of Cannabis has remained surprisingly under-investigated, partly due to the important restrictions imposed by anti-drug policies, even for scientific inquiries. In the last few years, a draft genome of Cannabis was published[22], and high-density Single-Nucleotide-Polymorphism (SNP) data obtained from NGS techniques evidenced genome-wide differentiation between hemp and marijuana plants.[23] However, genetic resources applicable for forensics remain under-developed. Forensic investigations require sets of sufficiently informative loci that can be genotyped in large batches of samples in a rapid and affordable manner, such as microsatellites (Short-Tandem-Repeats, STRs). Another prerequisite is that the species’ diversity is exhaustively represented in reference databases, both within and among varieties, so that investigated samples of unknown origin can be identified with statistical confidence. In Cannabis, these two aspects are challenging given the diversity of varieties, their complex breeding histories, as well as the rapid shifts of the drug varieties available on black markets. In addition, hemp and marijuana diverged during the human era and still largely share a common pool of genetic variation.[23]

Several microsatellite analyses were previously performed on Cannabis. Some loci became available in the early 2000s[24][25][26] but remained scarcely tested at the individual or population level. The first STR multiplex kit for forensics was validated years later[27], and subsequently trialed to distinguish fibers from confiscated drug seizures in Australia, with moderate success.[28] Another STR kit was developed by Köhnemann et al.[29], although without reference data. Using transcriptomic sequences (EST), Gao et al.[30] isolated >100 STRs, allowing them to discriminate between Chinese and European hemp samples according to their geographic origin. Other studies genotyped Cannabis, notably from police seizures, using new or published markers.[31][32][33][34][35] However, although these studies are regionally and timely relevant, they rely on limited sample sets (i.e., few varieties and few individuals per variety, and/or only representing plants available on a regional black market at the time of confiscations), thus hardly accounting for the different levels of genetic variation of Cannabis stocks. So far no comprehensive database of Cannabis diversity exists for broad-scale forensic enquiries.

Considering these limitations, we developed a new STR resource for Cannabis forensics. We analyzed intra- and inter-populational variation at 13 published STR markers in >1,300 Cannabis samples from 48 fiber and drug accessions, broadly representative of known hemp and marijuana varieties (see Table S1 in "Supporting information"), and characterized unknown samples of various origins, notably police seizures. We aimed at (i) showing that these loci fully recover the genetic structure between marijuana and hemp; (ii) demonstrating that anonymous samples can be confidently assigned to either plant types; and (iii) documenting the genetic diversity among and within samples and its potential for forensic investigations.

Results and discussion

The selected STR markers (see Table S2 in "Supporting information") unanimously recovered the strong structure between fibers and drug Cannabis samples. This is clearly depicted by a principal component analysis (PCA, Fig. 1A), genetic distances between accessions (Fst, Fig. S1 in "Supporting information") and genotype clustering by STRUCTURE (Fig. 1B), where two groups appears as the best clustering solution (ΔK2 = 1205.6). As recently evidenced from NGS data[23], this pattern reflects differentiation between hemp and marijuana over the entire genome, not only at genes underlying THC and fiber synthesis. Some drugs and fibers show weak signs of genetic admixture (intermediate PCA scores and STRUCTURE probabilities, Fig 1; lower Fst, Fig. S1 in "Supporting information"), which might stem from introgressive crossbreeding, as reported elsewhere.[23] Interestingly, except for RI (indica/ruderalis hybrid), all drug varieties closely related to hemps are of sativa ancestry (HMW, HA, SWA, MS; based on available information from suppliers). This would support the common assumption that hemp varieties selected for fiber and seed production derived from sativa, although this view has been challenged by other studies that found more similarities between hemp and indica.[7][23][36] Alternatively, sativa drugs, which are nowadays distributed in more equatorial regions, may be frequently crossbred with indica and agricultural varieties to facilitate their cultivation in temperate countries. In any case, marijuana genetic diversity seems weakly associated with the documented breeding history. We also performed a PCA solely on drugs, which only marginally clustered according to their main sativa and indica pedigree (Fig. S2 in "Supporting information"). Some cultivars of the same appellation appear genetically distinct (e.g., Alpine Rocket, ARa and ARb, FST = 0.36) whereas others harboring different names are genetically identical (e.g., PM, T44, BS, FST = 0.00; identical clones shared by ARa and B52, Table S1 in "Supporting information"). Overall, these observations are in line with the general conclusions of Sawler et al.[23] that drug varieties are often misinformed due to the clandestine nature of Cannabis breeding over the last century, and that names do not necessarily reflect a meaningful genetic identity. In addition, hemp varieties were grouped according to reproductive characteristics, as expected (dioecious versus monoecious; Table S1 in "Supporting information"), as a result of their breeding history (illustrated on the PCA, Fig 1; Fst tree, Fig. S1 in "Supporting information").


References

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  3. ↑ Small, E. (2015). "Evolution and Classification of Cannabis sativa (Marijuana, Hemp) in Relation to Human Utilization". The Botanical Review 81 (3): 189–294. doi:10.1007/s12229-015-9157-3. 
  4. ↑ 4.0 4.1 Welling, M.T.; Shapter, T.; Rose, T.J. et al. (2016). "A Belated Green Revolution for Cannabis: Virtual Genetic Resources to Fast-Track Cultivar Development". Frontiers in Plant Science 7: 1113. doi:10.3389/fpls.2016.01113. PMC PMC4965456. PMID 27524992. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4965456. 
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  15. ↑ 15.0 15.1 15.2 Staginnus, C.; Zörntlein, S.; de Meijer, E. (2014). "A PCR marker linked to a THCA synthase polymorphism is a reliable tool to discriminate potentially THC-rich plants of Cannabis sativa L.". Journal of Forensic Sciences 59 (4): 919-26. doi:10.1111/1556-4029.12448. PMID 24579739. 
  16. ↑ Tipparat, P.; Natakankitkul, S.; Chamnivikaipong, P.; Chutiwat, S. (2012). "Characteristics of cannabinoids composition of Cannabis plants grown in Northern Thailand and its forensic application". Forensic Science International 215 (1–3): 164-70. doi:10.1016/j.forsciint.2011.05.006. PMID 21636228. 
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  24. ↑ Alghanim, H.J.; Almirall, J.R. (2003). "Development of microsatellite markers in Cannabis sativa for DNA typing and genetic relatedness analyses". Analytical and Bioanalytical Chemistry 376 (8): 1225-33. doi:10.1007/s00216-003-1984-0. PMID 12811461. 
  25. ↑ Gilmore, S.; Peakall, R.; Robertson, J. (2003). "Short tandem repeat (STR) DNA markers are hypervariable and informative in Cannabis sativa: Implications for forensic investigations". Forensic Science International 131 (1): 65-74. PMID 12505473. 
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  27. ↑ Howard, C.; Gilmore, S.; Robertson, J. et al. (2008). "Developmental validation of a Cannabis sativa STR multiplex system for forensic analysis". Journal of Forensic Sciences 53 (5): 1061-7. doi:10.1111/j.1556-4029.2008.00792.x. PMID 18624889. 
  28. ↑ Howard, C.; Gilmore, S.; Robertson, J. et al. (2009). "A Cannabis sativa STR genotype database for Australian seizures: Forensic applications and limitations". Journal of Forensic Sciences 54 (3): 556-63. doi:10.1111/j.1556-4029.2009.01014.x. PMID 19302382. 
  29. ↑ Köhnemann, S.; Nedele, J.; Schwotzer, D. et al. (2012). "The validation of a 15 STR multiplex PCR for Cannabis species". International Journal of Legal Medicine 126 (4): 601–6. doi:10.1007/s00414-012-0706-6. PMID 22573357. 
  30. ↑ Gao, C.; Xin, P.; Cheng, C. et al. (2014). "Diversity analysis in Cannabis sativa based on large-scale development of expressed sequence tag-derived simple sequence repeat markers". PLOS ONE 9 (10): e110638. doi:10.1371/journal.pone.0110638. PMC PMC4203809. PMID 25329551. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4203809. 
  31. ↑ Chandra, S.; Lata, H.; Techen, N. et al. (2011). "Analysis of Genetic Diversity using SSR Markers and Cannabinoid Contents in Different Varieties of Cannabis sativa L.". Planta Medica 77: P_5. doi:10.1055/s-0031-1273534. 
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  34. ↑ Presinszka, M.; Stiasna, K.; Vyhnanek, T. et al. (2015). "Analysis of microsatellite markers in hemp (Cannabis sativa L.)". In Polák, O.; Cerkal, R.; Belcredi, N.B. (PDF). MendelNet 2015: Proceedings of International PhD Students Conference. Mendel University in Brno. pp. 434–438. ISBN 9788075093639. https://mnet.mendelu.cz/mendelnet2015/mnet_2015_full.pdf. 
  35. ↑ Houston, R.; Birck, M.; Hughes–Stamm, S. et al. (2016). "Evaluation of a 13-loci STR multiplex system for Cannabis sativa genetic identification". International Journal of Legal Medicine 130 (3): 635-47. doi:10.1007/s00414-015-1296-x. PMID 26661945. 
  36. ↑ Pilluza, G.; Delogu, G.; Cabras, A. et al. (2013). "Differentiation between fiber and drug types of hemp (Cannabis sativa L.) from a collection of wild and domesticated accessions". Genetic Resources and Crop Evolution 60 (8): 2331–2342. doi:10.1007/s10722-013-0001-5. 

Notes

This presentation is faithful to the original, with only a few minor changes to presentation. In some cases important information was missing from the references, and that information was added.

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