The genus Cannabis was formerly placed in the Nettle (Urticaceae)
or Mulberry (Moraceae) family, but is now considered along with hops
(Humulus sp.) to belong to the Hemp family (Cannabaceae). Recent
phylogenetic studies based on cpDNA restriction site analysis and
gene sequencing strongly suggest that the Cannabaceae arose from within
the Celtidaceae clade, and that the two families should be merged
to form a single monophyletic group.
Cannabis plants produce a unique family of terpeno-phenolic
compounds called cannabinoids, which produce the "high" one experiences
from smoking marijuana. The two cannabinoids usually produced in greatest
abundance are cannabidiol (CBD) and/or Δ9-tetrahydrocannabinol
(THC), but only THC is psychoactive. Since the early 1970s, Cannabis
plants have been categorized by their chemical phenotype or "chemotype,"
based on the overall amount of THC produced, and on the ratio of THC
to CBD. Although overall cannabinoid production is influenced by
environmental factors, the THC/CBD ratio is genetically determined
and remains fixed throughout the life of a plant.
Non-drug plants produce relatively low levels of THC and high levels
of CBD, while drug plants produce high levels of THC and low levels
of CBD. When plants of these two chemotypes cross-pollinate, the plants
in the first filial (F1) generation have an intermediate
chemotype and produce similar amounts of CBD and THC. Female plants
of this chemotype may produce enough THC to be utilized for drug production.
Cannabis is predominantly dioecious, although many monoecious
varieties have been described. Subdioecy (the occurrence of monoecious
individuals and dioecious individuals within the same population)
is widespread. Many populations have been described as sexually labile.
As a result of intensive selection in cultivation, Cannabis
exhibits many sexual phenotypes that can be described in terms of
the ratio of female to male flowers occurring in the individual, or
typical in the cultivar.
Dioecious varieties are preferred for drug production, where only
the female plants are used. Dioecious varieties are also preferred
for textile fiber production, whereas monoecious varieties are preferred
for pulp and paper production. It has been suggested that the presence
of monoecy can be used to differentiate licit crops of monoecious
hemp from illicit drug crops. However, the so-called "sativa" drug
strains often produce monoecious individuals, probably as a result
of inbreeding.
Mechanisms of sex determination
Cannabis has been described as having one of the most complicated mechanisms
of sex determination among the dioecious plants.
Many models have been proposed to explain sex determination in Cannabis.
Based on studies of sex reversal in hemp, it was first reported by
K. Hirata in 1924 that an XY sex-determination system is present.
At the time, the XY system was the only known system of sex determination.
The X:A system was first described in Drosophila spp in 1925.
Soon thereafter, Schaffner disputed Hirata's interpretation,
and published results from his own studies of sex reversal in hemp,
concluding that an X:A system was in use and that furthermore sex
was strongly influenced by environmental conditions. Since then, many different
types of sex determination systems have been discovered, particularly in plants.
Dioecy is relatively uncommon in the plant kingdom, and a very low percentage of
dioecious plant species have been determined to use the XY system. In most cases
where the XY system is found it is believed to have evolved recently and independently.
Since the 1920s, a number of sex determination models have been proposed for
Cannabis. Ainsworth describes sex determination in the genus as using "an X/autosome dosage type".
The question of whether heteromorphic sex chromosomes are indeed
present is most conveniently answered if such chromosomes were clearly
visible in a karyotype. Cannabis was one of the first plant
species to be karyotyped; however, this was in a period when karyotype
preparation was primitive by modern standards (see History of Cytogenetics).
Heteromorphic sex chromosomes were reported to occur in staminate
individuals of dioecious "Kentucky" hemp, but were not found in pistillate
individuals of the same variety. Dioecious "Kentucky" hemp was assumed
to use an XY mechanism. Heterosomes were not observed in analyzed
individuals of monoecious "Kentucky" hemp, nor in an unidentified
German cultivar. These varieties were assumed to have sex chromosome
composition XX. According to other researchers, no modern karyotype
of Cannabis had been published as of 1996. Proponents of the
XY system state that Y chromosome is slightly larger than the X, but
difficult to differentiate cytologically.
More recently, Sakamoto and various co-authors
have used RAPD to isolate several genetic marker sequences that they
name Male-Associated DNA in Cannabis (MADC), and which they interpret
as indirect evidence of a male chromosome. Several other research
groups have reported identification of male-associated markers using
RAPD and AFLP.
Environmental sex determination is known to occur in a variety of
species. Many researchers have suggested that sex in Cannabis
is determined or strongly influenced by environmental factors. Ainsworth
reviews that treatment with auxin and ethylene have feminizing effects,
and that treatment with cytokinins and gibberellins have masculinizing
effects. It has been reported that sex can be reversed in Cannabis using
chemical treatment. A PCR-based method for the detection of female-associated DNA polymorphisms
by genotyping has been developed.
Uses
Cannabis plants, more commonly known as hemp for this end,
is used for a wide variety of purposes, including the manufacture
of cordage of varying tensile strength, clothing, and nutritional
products. Industrial hemp products are made from Cannabis plants
selected to produce an abundance of fiber and minimal levels of THC
(?9- tetrahydrocannabinol). The bast fibers can
be used in 100% hemp products, but are commonly blended with fibers
such as flax, cotton or silk, for apparel and furnishings, most commonly
at a 55%/45% hemp/cotton blend. The inner two fibers of hemp are more
woody, and are more often used in non-woven items and other industrial
applications, such as mulch, animal bedding and litter. The oil from
the fruits ("seeds") dries on exposure to air (similar to linseed
oil) and is sometimes used in the manufacture of oil-based paints,
in creams as a moisturising agent, for cooking, and in plastics. Hemp
seeds have been used in bird seed mix. Due to the a psychoactive molecule
that produces the "high" associated with marijuana, the cultivation
and possession of Cannabis for recreational use is outlawed
in most countries. Various extracts including hashish and hash oil
are also produced from the plant.