Kudoh, A.; Natsume, S.; Sugihara, Y.; Kato, H.; Abe, A.; Oikawa, K.; Shimizu, M.; Itoh, K.; Tsujimura, M.; Takano, Y.; Sakai, T.; Adachi, H.; Ohta, A.; Ohtsu, M.; Ishizaki, T.; Terachi, T.; Terauchi, R.

Dioecious plants, which have distinct male and female individuals, constitute ~5% of angiosperm species and have emerged frequently and independently from hermaphroditic ancestors. Although recent molecular studies of sex determination have started to reveal the diversity of the genetic systems underlying dioecy, research on the evolution of dioecy is limited, especially in monocots. Here, we describe the molecular basis of sex determination in the monocot Dioscorea tokoro, a dioecious wild yam endemic to East Asia. Chromosome-scale and haplotype-resolved genome assemblies and linkage analysis suggested that this plant has a male heterogametic sex determination (XY) system, with sex determination regions located on chromosome 3. Sequence read coverage analysis of the sex chromosomes revealed X- and Y-specific regions in putative pericentromeric chromosome regions. Within the Y-specific region, we identified two candidate genes that are likely involved in sex determination: BLH9, encoding a homeobox protein, and HSP90, encoding a molecular chaperone. BLH9 has similar functions to AtBLH9 in Arabidopsis thaliana. BLH9 is thought to suppress female organ development, whereas HSP90 might be required for pollen development. These results shed light on the complex evolution of dioecy in plants.