Cannabis uses X chromosomes, not Y genes, to determine plant sex

This groundbreaking study reveals the fundamental genetic mechanisms controlling sex determination in cannabis and hop plants by sequencing and analyzing their complete genomes, including their sex chromosomes. Researchers discovered that cannabis uses an X-linked sex determination system rather than a traditional Y-chromosome system, with an X-linked gene homologous to aminocyclopropane-1-carboxylate synthase (ACS) controlling female inflorescence development. This ACS gene is involved in the ethylene biosynthesis pathway, which regulates plant growth and reproductive development—a completely different genetic mechanism than in many other plants.

The findings show that the Y chromosome in cannabis is severely degenerated and gene-poor, having lost substantial genetic material since diverging from hop approximately 36 million years ago. Rather than containing active sex-determining genes, the Y chromosome's deterioration suggests that female sex determination is controlled dominantly by genes on the X chromosome, particularly through the ethylene signaling pathway. This X-linked system explains why cannabis and hop evolved differently from species with traditional male-determining Y chromosomes.

Beyond sex determination alone, the research demonstrates that sex chromosomes in cannabis contribute to broader sexual dimorphism in ecology, physiology, and breeding characteristics—traits that have been crucial to human domestication and selective breeding of the species. Understanding these genetic mechanisms has significant implications for cannabis cultivation, breeding programs aimed at producing specific plant phenotypes, and predicting how plants will develop under different environmental conditions.