Switches in heterogamety are known to occur in both animals and plants. Although plant sex determination systems probably often evolved more recently than those in several well-studied animals, including mammals, and have had less time for switches to occur, we previously detected a switch in heterogamety in the plant genus Silene: section Otites has both female and male heterogamety, whereas S. latifolia and its close relatives, in a different section of the genus, Melandrium (subgenus Behenantha), all have male heterogamety. Here we analyse the evolution of sex chromosomes in section Otites, which is estimated to have evolved only about 0.55 MYA. Our study confirms female heterogamety in S. otites and newly reveals female heterogamety in S. borysthenica. Sequence analyses and genetic mapping show that the sex-linked regions of these two species are the same, but the region in S. colpophylla, a close relative with male heterogamety, is different. The sex chromosome pairs of S. colpophylla and S. otites each correspond to an autosome of the other species, and both differ from the XY pair in S. latifolia. Silene section Otites species are suitable for detailed studies of the events involved in such changes, and our phylogenetic analysis suggests a possible change from female to male heterogamety within this section. Our analyses suggest a possibility that has so far not been considered, change in heterogamety through hybridization, in which a male-determining chromosome from one species is introgressed into another one, and over-rides its previous sex-determining system.

Centre of the Region Haná for Biotechnological and Agricultural Research Institute of Experimental Botany of the Czech Academy of Sciences 78371 Olomouc Czech Republic

Department of Biological and Environmental Sciences University of Gothenburg 40530 Gothenburg Sweden Sweden

Department of Plant Developmental Genetics Institute of Biophysics of the Czech Academy of Sciences Kralovopolska 135 61265 Brno Czech Republic

Department of Plant Developmental Genetics Institute of Biophysics of the Czech Academy of Sciences Kralovopolska 135 61265 Brno Czech Republic Centre of the Region Haná for Biotechnological and Agricultural Research Institute of Experimental Botany of the Czech Academy of Sciences 78371 Olomouc Czech Republic

Department of Plant Developmental Genetics Institute of Biophysics of the Czech Academy of Sciences Kralovopolska 135 61265 Brno Czech Republic Institute for Biological Physics University of Cologne Zülpicher Straße 77 Cologne Germany

Institute of Evolutionary Biology EH9 3FL University of Edinburgh Edinburgh UK