Summary We recently described, in Cannabis sativa , the oldest sex chromosome system documented so far in plants. Based on our estimate of its age, we predicted that it should be shared by its sister genus Humulus , which is known to also possess XY sex chromosomes. Here, we used transcriptome sequencing of a F1 family of Humulus lupulus to identify and study the sex chromosomes in this species using the probabilistic method SEX-DETector. We identified 265 sex-linked genes in H. lupulus , located on the chromosome that is also the C. sativa sex chromosome pair. Using phylogenies of sex-linked genes, we show that a region of these chromosomes had already stopped recombining in the common ancestor of the two species. Furthermore, as in C. sativa , Y gene expression was reduced in correlation to the position on the X chromosome, and strongly Y degenerated genes showed dosage compensation. Here we report, for the first time in the Angiosperms, a sex chromosome system that is shared by two different genera. Recombination suppression started at least 21-25 My ago, and then (either gradually or step-wise) spread to a large part of the sex chromosomes, leading to a strongly degenerated Y.

Plant generaCannabisandHumulusshare the same pair of well-differentiated sex chromosomes