In a different approach, based on extensive genotyping-by-sequencing, 14,031
SNPs were identified and used to compare 124Cannabissamples (both marijuana
and hemp; Sawler et al. 2015 ). The results indicated that there was a genome-wide
difference between the two main pools ofCannabis, i.e. those selected for drug use
and those bred forfiber, that could not be solely attributed to the genes directly
involved in the synthesis of cannabinoid type and amount; however, the authors
also concluded that“hemp and marijuana still largely share a common pool of
genetic variation”and that drug strains had a lower heterozigosity compared to
non-drug strains, as expected by the much more stringent selection and inbreeding
to which they are subjected.
Overall, the high degree of intra-accession genetic diversity and heterozygosity
found inCannabisby means of different molecular markers reflects the features of
an obligated outbreeding species; the levels of genetic variability observed by
different marker types and/or by sequencing, seem to strictly reflect the breeding
strategies applied to the different types of varieties ranging from the old, dioecious
fiber varieties, to the progressively more selected monoecious species, and to the
drug varieties, ending up with highly inbred clones destined for pharmaceutical use.
Moreover, in view of the relatively low number of molecular markers with high
discriminating power that has been reported by different groups, and the higher
intra-accession genetic variability when compared to inter-accession variability
(Forapani et al. 2001 ; Gilmore et al. 2003), the existence of a widely shared gene
pool with weak cultivar boundaries inCannabiscan be envisaged.
15.2.3 Genetic Maps
With few recent exceptions, molecular markers only found occasional application
in the construction of genetic maps inC. sativa. The reason for this limited
exploitation may lie in the extreme in-accession variability shown byCannabis,as
described above, which makes mapping of agronomically relevant traits difficult,
and the identification of associated markers too often strictly limited to the popu-
lation where they were developed. The high level of heterozygosity found in most
varieties, however, suggests that mapping could be carried out in F1 populations,
though F2 maps have also been developed. For example, an early RAPD map was
obtained from a cross between a Carmagnola female plant and a monoecious plant
(accession CAN18/86 from Southern Italy), in the frame of a study aimed at
mapping the monoecious trait. The F1 population (in which a 1:1 segregation ratio
of female to monoecious plants was observed) was scored for 674 RAPD markers,
269 of which were polymorphic; of these, 181 showed a 1:1 segregation, i.e. being
heterozygous in one parental. These loci were used to create two different maps for
each parental: the female Carmagnola map consisted of 66 markers distributed
across 11 linkage groups, while the CAN18/86 map included 43 markers distributed
across 9 linkage groups. Unfortunately, none of them included the monoecious trait,
probably because the number of markers was limited, and possibly not evenly
15 Genomics and Molecular Markers inCannabis sativaL. 325