0Dioecy, the presence of separate sexes on distinct individuals, has evolved repeatedly 3 1 in multiple plant lineages. However, the specific mechanisms through which sex 3 2 systems evolve and their commonalities among plant species remain poorly 3 3 understood. With both XY and ZW sex systems, the family Salicaceae provides a 3 4 system to uncover the evolutionary forces driving sex chromosome turnovers. In this 3 5 study, we performed a genome-wide association study to characterize sex 3 6 determination in two Populus species, P. euphratica and P. alba. Our results reveal an 3 7XY system of sex determination on chromosome 14 of P. euphratica, and a ZW 3 8 system on chromosome 19 of P. alba. We further assembled the corresponding sex 3 9 determination regions, and found that their sex chromosome turnovers may be driven 4 0 by the repeated translocations of a Helitron-like transposon. During the translocation, 4 1 this factor may have captured partial or intact sequences that are orthologous to a 4 2 type-A cytokinin response regulator gene. Based on results from this and other 4 3recently published studies, we hypothesize that this gene may act as a master regulator 4 4 of sex determination for the entire family. We propose a general model to explain how 4 5 the XY and ZW sex systems in this family can be determined by the same RR gene.
6Our study provides new insights into the diversification of incipient sex chromosome 4 7 in flowering plants by showing how transposition and rearrangement of a single gene 4 8 can control sex in both XY and ZW systems. 4 9 5 0