Format: ORAL

Quentin Cronk1, MaKenzie Drowns2, C.J. Tsai2

1 Department of Botany, University of British Columbia, Vancouver, Canada 2 Warnell School of Forestry and Natural Resources, University of Georgia, Athens, USA

The Salicaceae, and especially Salix and Populus, are becoming a leading model for the clade biology of sex determination, as dioecious, monoecious and polygamous forms are common. In angiosperms,dicliny (the production of single sex flowers), evolves through one of two contrasting evolutionary developmental mechanisms: organ deletion or organ abortion. Organ deletion, under which sex organs never form, may involve MADS-box genes, as these genes are required to produce floral organs of a given identity. Organ abortion, under which sex organs form but during development become vestigial or non-functional, are less likely to involve MADS-box genes, and more likely to involve late-stage developmental modifiers of organ development. Phylogenetic studies strongly suggest that there are necessary pre-adaptations for plant lineages to adopt either the deletion or abortion pathway. Populus and Salix are both examples of the deletion pathway, in which no trace of stamens is ever seen in female flowers, and no trace of carpels in male flowers. In poplar we show that the B-class MADS-box genes PISTILLATA (PI) and APETALA3 (AP3) are strongly down-regulated in early female flower development. This is expected as these genes are required for stamen development under the canonical ABC model. A powerful way of dissecting the role of the MADS-box developmental module in dioecious species is provided by CRISPR/Cas9 gene editing. Recent developments in efficient poplar transformation coupled with engineered hyper-precocious flowering have facilitated novel experiments in this area. Preliminary experiments on MADS-box genes in poplar show the potential for functional dissection of pathways to dioecy. The development of CRISPR/Cas9 technology in other systems could greatly accelerate understanding of the evolutionary developmental genetics of dioecy in diverse lineages.