Elucidating the genetic basis and molecular mechanism of dioecy in Populus

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While most plant species are hermaphrodites, the separation of female and male flowers onto different individuals, called dioecy, has evolved independently hundreds if not thousands of times. Still, the underlying genetic basis remains mostly elusive. In the proposed project, we want to elucidate the genetic and molecular basis of dioecy in poplars to address overarching questions concerning the evolution of plant sexual systems. Despite an ancient origin of dioecy in the genus Populus, the underlying sex chromosomes appear to have diverged only recently. Different locations of the sex-determining regions (SDRs) and varying heterogametic parents (XY- vs. ZW-system) in different poplar species further indicate independent evolution. However, our preliminary analyses suggest that the observed complexity represents variations on the same theme. A shared single-gene-based mechanism, with important implications for sex chromosome evolution in general, appears to be underlying dioecy across different Populus species.In the proposed project we want to experimentally test whether (i) the SDRs of white poplar and aspen lack a region of suppressed recombination and show opposite heterogamety (XY vs. ZW), whether (ii) the poplar homolog of the ARABIDOPSIS RESPONSE REGULATOR 17 (ARR17) gene is controlling sex determination across the genus via a single-gene-based mechanism, and whether (iii) the molecular mechanisms causing differential sex manifestation are conserved between different poplar species.We will do this by (i) mapping the white poplar SDR, determining recombination frequencies along the SDRs of males and females in large F1 populations and examining reciprocal P. alba x P. tremula crosses, (ii) generating early-flowering transgenic poplar lines knocking out or complementing ARR17 expression and molecularly characterizing the ARR17 locus in different poplar species, and (iii) performing RNA-sequencing of male and female flower buds and integrating the results with other functional genomics data to identify the gene networks causing differential sex manifestation.The results will provide valuable information for breeding programs involving dioecious crop plants, and advance our understanding of the genetic, molecular and evolutionary processes shaping unisexuality and dioecy in plants.