Cannabis has one of the plant world's most tangled ways of deciding sex
Most cannabis plants are either male or female, a rarity among flowering plants. Yet scientists have argued for a century over how that sex is set. Chromosomes play a part, but so do hormones and growing conditions, and some populations switch sex readily. The name itself came from the Scythians.
Cannabis is an Asian genus in the family Cannabaceae, and even its species count is disputed: botanists recognise anywhere from one to three, sativa, indica and ruderalis, or fold them all into one. Varieties grown for fibre, seed oil and food are usually called hemp. Others are bred for high or low levels of THC, the main psychoactive compound, and interest has grown in other cannabinoids such as CBD. The Greeks learned of the plant by watching Scythians use it at funerals.
It is an annual herb pollinated by the wind. Its hand-shaped leaves are distinctive: leaflets increase from one on the first leaf pair up to about thirteen, then dwindle again near the top. Even the veins give it away, since the vein feeding each tooth branches off lower than usual and runs past the neighbouring notch, letting experts identify tiny fragments. Most strains flower as days shorten; the so-called autoflowering ruderalis may ignore day length.
Male and female flowers usually grow on separate plants, something Chinese scholars recognised early; the Erya dictionary of about the 3rd century BCE has distinct words for male and female plants. Breeding has multiplied the patterns, including plants carrying both kinds of flowers. Growers pick accordingly: separate-sex varieties for drug crops and textile fibre, mixed-flower varieties for pulp and paper.
The mechanism remains contested. In 1924 K. Hirata proposed an XY system like that of mammals; soon afterwards Schaffner argued for a ratio between X chromosomes and other chromosomes, heavily influenced by the environment. Cannabis was among the first plants to have its chromosomes examined, but early techniques were crude, and later researchers found male-linked DNA markers without clearly seeing a Y. Hormone experiments add another layer: auxin and ethylene push plants toward femaleness, while cytokinins and gibberellins push them toward maleness.
Source: Cannabis
