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.[17][41][69] 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.
