Mechanisms of sex determination
Cannabis has been described as having one of the most complicated
mechanisms of
sex determination among the dioecious plants.
[60] 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.
[58] 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.
[61] Soon thereafter, Schaffner disputed Hirata's interpretation,
[62] 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.
[59]
Since then, many different types of
sex determination systems have been discovered, particularly in plants.
[53] 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.
[63]
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".
[53]
Dense raceme of carpellate flowers typical of drug-type varieties of
Cannabis
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.
[64] According to other researchers, no modern karyotype of
Cannabis had been published as of 1996.
[65] Proponents of the XY system state that Y chromosome is slightly larger than the X, but difficult to differentiate cytologically.
[66]
More recently, Sakamoto and various co-authors
[67][68] 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.
[69][41][17] Ainsworth commented on these findings, stating,
It is not surprising that male-associated markers are relatively abundant. In dioecious plants where
sex chromosomes have not been identified, markers for maleness indicate either the presence of
sex chromosomes which have not been distinguished by cytological methods or that the marker is tightly linked to a gene involved in
sex determination.
[53]
Environmental
sex determination is known to occur in a variety of species.
[70] Many researchers have suggested that
sex in
Cannabis is determined or strongly influenced by environmental factors.
[59] Ainsworth reviews that treatment with
auxin and
ethylene have feminizing effects, and that treatment with
cytokinins and
gibberellins have masculinizing effects.
[53] It has been reported that
sex can be reversed in
Cannabis using chemical treatment.
[71] A
PCR-based method for the detection of female-associated
DNA polymorphisms by
genotyping has been developed.
[72]