Format: ORAL

Giacomo Potente1, Yasui Yukiko2, Rachel N. Walstead3, Jane Grimwood3, Jeremy Schmutz3, Jim Leebens-Mack4, Eita Shimokawa2, Tomoha Tanaka2, Yuka Umeya2, Shogo Kawamura2, Katsuyuki T. Yamato5, Katsushi Yamaguchi6, Shuji Shigenobu6, Masaki Shimamura7, Takayuki Kohchi2and Peter Szovenyi1

1 Dept. of Systematic and Evolutionary Botany, University of Zurich, Zurich, Switzerland 2 Kyoto University, Kyoto, Japan 3 HudsonAlpha Institute for Biotechnology, Huntsville, AL 4 University of Georgia, Athens, GA 5 Kindai University, B.O.S.T., Kinokawa, Japan 6 Trans-Scale Biology Center, National Institute for Basic Biology, Okazaki, Japan 7 Hiroshima University, Hiroshima, Japan

Sex chromosomes have evolved independently in many groups of plants and animals. Genetic sex determination ensures obligate outcrossing that may confer advantages against selfing. In liverworts, sex is expressed in the haploid phase and genetically determined by the presence of a single U (female) or V (male) sex chromosome. The UV sex determination is ancestral in liverworts and represents the oldest sex chromosome system ever described (430 million years old). Although the transition to monoicy occurred repeatedly in liverworts, its genomic basis has been investigated only in one species (Ricciocarpos natans). Here we report on the chromosome-scale genome assembly of the monoicous liverwort Marchantia quadrata, which diverged from the dioicous M. polymorpha 125 million years ago. We show that the M. quadrata genome consists of eight large and one (9th) small chromosome (1/10th of the size of the large eight chromosomes). The eight large chromosomes are collinear with the eight autosomes of M. polymorpha showing many intra- but few inter-chromosomal rearrangements. By contrast, the small 9th chromosome contains genes homologous to the M. polymorpha V chromosome. Many genes homologous to U-chromosome genes of M. polymorpha are absent from the M. quadrata genome or are relocated to the autosomes, including the major sex determining gene (Feminizer, BPCU). These results imply that the transition from dioecy to monoicy was achieved by the translocation of few crucial U-genes to the autosomes and the loss of the U chromosome, while a larger proportion of V-linked genes was retained on a segment of the V chromosome as chromosome 9. Our findings are remarkably similar to the recent observation made on R. natans, raising the possibility that genomic changes underlying the transition from dioecy to monoecy in liverworts may be highly predictable.