A single gene underlies the dynamic evolution of poplar sex determination

Although hundreds of plant lineages have independently evolved dioecy, i.e. separation of the sexes, the underlying genetic basis remains largely elusive. Here we show that various poplar species carry partial duplicates of the ARABIDOPSIS RESPONSE REGULATOR 17 (ARR17) ortholog within the male-specific region of the Y chromosome. These duplicates are arranged as inverted repeats giving rise to non-coding RNA and RNA-directed DNA methylation of the ARR17 locus. Using CRISPR/Cas9, we demonstrate that ARR17 represents a sex switch triggering female development when ‘on’ and male development when ‘off’. Interestingly, in male white poplars ARR17 is absent altogether, changing the XY into a ZW-system. Our data reveal that a single-gene-based mechanism of dioecy can enable high dynamics of the sex-linked region and contribute to maintaining sex chromosome recombination and integrity.