Variation on a Theme: Diploid and Hexaploid Persimmon Regulate Sex in Slightly Different Ways

Plants have evolved many different strategies to prevent self-fertilization and maintain genetic diversity. For example, approximately 5% of plant species are dioecious and bear male and female flowers on separate individuals. One of these species is diploid persimmon, Diospyros lotus, a species with heterogametic sex chromosomes similar to humans, with XY male trees and XX female trees. In contrast, commercial persimmons, D. kaki, are hexaploid and trees are either fully female or monoeocious, i.e. carrying both male and female flowers. We have determined that sex determination in D. lotus relies on a Y-encoded pseudogene, OGI, which is expressed in male developing flowers and negatively regulates the expression of an autosomal feminizing determinant called MeGI. We will discuss ongoing experiments aimed at deciphering the exact role of MeGI in flower development. In D. lotus, MeGI repression occurs via transitive smRNA from OGI. Surprisingly OGI expression is elusive in D. kaki, but the targeting of MeGI by smRNAs is still visible, as is the resulting repression of MeGI. Instead, we observed long-term methylation of the MeGI promoter and this methylation pattern was fully associated with flower gender. In other words, flowers within a single tree exhibited differential methylation signal at the MeGI promoter that was associated with the gender of the flower. This was true even in the case of “sex reversal”, cases where a male flower appears on a previously female branch, or vice-versa. In those cases, MeGI promoter methylation was reversed as well, suggesting a role for de novo methylation in this process. This suggests that that diploid and hexaploid persimmons are using a slightly different approach to regulating sex determination.