Plant sex determining systems and sex chromosomes are often evolutionarily young. Here, we present the early stage of sex chromosome in a fully dioecious plant, P. deltoides, by determining separate sequences of the physically small X- and Y-linked regions. Intriguingly, two Y genes are absent from the X counterpart. One gene represses female structures by producing siRNAs that block expression of a gene necessary for development of female structures, via RNA-directed DNA methylation and

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... uided mRNA cleavage. The other gene generates long non-coding RNA transcripts that, in males, soak up miRNAs that specifically inhibit androecium development. Transformation experiments in Arabidopsis thaliana show that the two genes affect gynoecium and androecium development independently and antagonistically. Sex determination in the poplar therefore has the properties proposed for the first steps in the evolution of dioecy in flowering plants, with two genes whose joint effects favor close linkage, as is observed in poplar.