Join Date: Sep 2017
( Part - 1 )
Cannabis, an erect annual herb and member of the Cannabaceae family, is monotypic and characterized by a single species Cannabis sativa L. (Small and Cronquist, 1976). Plants are diploid (2n = 20) with an estimated haploid genome of ~830 Mb (Van Bakel et al., 2011). The extant genepool is thought to be comprised primarily of domesticated or feral populations, cultivars, and selections (Small and Cronquist, 1976), with a subset having been subject to steep selective gradients toward phenotypes for specific end-uses (Mandolino and Carboni, 2004; Potter, 2014; Small, 2015a). Cannabis has been cultivated in Eurasia over several thousand years (Li, 1974; Bradshaw et al., 1981; Murphy et al., 2011; Herbig and Sirocko, 2013) and has since radiated from this region and been subject to prolonged artificial selective pressures in Africa (Duvall, 2016) and North and South America (Small and Marcus, 2002), and is now cultivated globally (Salentijn et al., 2014). Plants are diecious and obligate outbred, although some fiber forms are monecious (Faeti et al., 1996). This has contributed to a high level of hybridization between pre-, post-, and de-domesticated populations (Gilmore et al., 2007), and therefore few if any intact wild populations are thought to exist (Small and Cronquist, 1976).
Transcriptome analysis (Van Bakel et al., 2011; McKernan et al., 2015; Onofri et al., 2015; Weiblen et al., 2015) and QTL mapping (Weiblen et al., 2015) further support the historical selection pressure that has led to chemotypic separation between contemporary fiber and marijuana groupings. Cannabinoids accumulate in plants in their carboxylic acid forms, such as tetrahydrocannabinolic acid (THCA) and cannabidiolic acid (CBDA) which form neutral cannabinoids THC and CBD in a non-enzymatic reaction when exposed to heat (Dussy et al., 2005). The marijuana variety Purple Kush was found to have higher expression levels of genes encoding cannabinoid biosynthetic pathway intermediates than the fiber cultivar Finola, with Purple Kush only expressing functional sequence variants of genes coding for THCA and Finola those for CBDA synthase (Van Bakel et al., 2011). Linkage mapping in 62 F2 individuals derived from a cross between full-sib inbred contemporary fiber cultivar Carmen and marijuana variety Skunk#1 revealed QTL for THCA and CBDA composition, as well as putative QTL for cannabinoid content, with differences in composition associated with a CBDA synthase locus and loss of CBDA synthase functionality in Skunk#1 (Weiblen et al., 2015).
The occurrence of contemporary fiber cultivars which have relatively high levels of CBDA (Small, 2015a) can be attributed to breeding efforts within France and other European countries toward the middle to latter part of the twentieth century (Amaducci et al., 2014), where techniques such as the Bredemann method, an in vivo fiber evaluation method, and the counter selection for THC using marker assisted selection (MAS), were employed (Ranalli, 2004). Fifty one fiber cultivars are currently registered for use within the European Union (Salentijn et al., 2014) and these cultivars have been exported to North America (Small, 2015a) and the Northern provinces of China (Salentijn et al., 2014). Despite the scarcity of published data relating to ancestry of such accessions, it is believed that a large number of contemporary fiber cultivars are descendants from Central Russian and Mediterranean landraces and derivative cross-progenies (De Meijer and van Soest, 1992).
Phylogenetic relationships between domesticated Cannabis germplasm have recently been examined using reduced representation DNA sequencing. Genotyping by sequencing (GBS) analysis of 195 accessions using 2894 single nucleotide polymorphisms (SNPs) inferred close relatedness and shared ancestry between contemporary fiber accessions, with the latter forming a separate clade (Lynch et al., 2015). These accessions were also observed to exhibit lower levels of heterozygosity than other intraspecific taxa (Mann-Whitney U-test p < 0.001; Lynch et al., 2015), suggesting that recent domestication of fiber traits has resulted in a genetic bottleneck and reduction in allelic diversity. However, these varieties were not well-represented in the sample collection, with only 16 analyzed (Lynch et al., 2015). Moreover, a separate GBS study using 14,031 SNPs in 43 contemporary fiber and 81 marijuana varieties produced results that conflicted with this more recent study, and showed significantly lower levels of heterozygosity in marijuana varieties compared with fiber cultivars (Mann-Whitney U-test p = 8.64 × 10−14; Sawler et al., 2015).
Despite this lack of congruence between GBS analyses, domestication for either industrial hemp or marijuana traits has likely resulted in a loss of genetic and allelic diversity, potentially brought about by changes in breeding systems. Processes such as linkage drag can be associated with complex polygenic flowering QTL (Mace et al., 2013) in relation to latitudinal and environmental adaption (Gao et al., 2014). Regardless of reductions in allelic diversity that may have arisen either from clonal propagation in marijuana (Russo, 2007), or from the propagation of monoecious varieties in industrial hemp (Forapani et al., 2001), it is unclear to what extent contemporary Cannabis germplasm deviates from the broader genepool. Analysis of 45 SNPs in both GBS sample sets reveals an overall limited genetic distance between 22 industrial hemp and 173 marijuana groupings (Lynch et al., 2015). Resequencing and mapping of 30 billion sequence reads from 302 domesticated and wild soybean (Glycine max) accessions identified selective sweeps associated with domestication events (Zhou et al., 2015). By comparison, resequencing of various species of Citrus has also revealed a complex arrangement of large haplotype blocks and admixture between ancestral and domesticated species (Wu et al., 2014). On completion of a fully annotated Cannabis genome (Van Bakel et al., 2011), it may be possible to resequence diverse germplasm (Scossa et al., 2015) to quantify differences in genetic diversity and to determine the contribution wild ancestors have conferred to contemporary forms. However, access to wild and landrace accessions may be a limiting factor to exploring Cannabis phylogeny.
Last edited by Kankakee; 02-28-2018 at 11:46 PM..