Format: ORAL

Niels A. Mller1, Birgit Kersten1, Matthias Fladung1

1 Thünen Institute of Forest Genetics, Grosshansdorf, Germany

Although hundreds of plant lineages have independently evolved dioecy, i.e. separation of the sexes, the underlying genetic basis has been resolved only in a small subset of species. Additionally, transitions between sexual systems, such as monoecy and dioecy, remain largely elusive. Here we show that diverse poplar species carry partial duplicates of an ARABIDOPSIS RESPONSE REGULATOR 17 (ARR17)-like gene in the male-specific region of the Y chromosome. These duplicates give rise to small RNAs apparently causing male-specific DNA methylation and silencing of the ARR17 gene. Excitingly, CRISPR/Cas9-induced mutations demonstrate that ARR17 functions as a single-gene sex switch triggering female development when on and male development when off. Despite repeated turnovers, including transitions between XY and ZW systems, sex-specific regulation of ARR17 is conserved across the Populus genus. Interestingly, ARR17 appears to be involved in sex determination in the sister genus Salix as well. To address the question of when ARR17 evolved its sex-determining function in the Salicaceae family and whether the same gene may control monoecy and dioecy, we analyzed Poliothyrsis sinensis, the closest monoecious relative of poplars and willows. I will present first functional genomics results of Poliothyrsis and discuss their implications for the evolution of sexual systems in the Salicaceae. In summary, our work illustrates how single-gene sex determination in combination with minimal sexual conflict can facilitate repeated turnovers and prevent expanding recombination suppression and sex chromosome degeneration.