Abstract In some species, the Y is a tiny chromosome but the dioecious plant Silene latifolia has a giant ∼550 Mb Y chromosome, which has remained unsequenced so far. Here we used a hybrid approach to obtain a high-quality male S. latifolia genome. Using mutants for sexual phenotype, we identified candidate sex-determining genes on the Y. Comparative analysis of the sex chromosomes with outgroups showed the Y is surprisingly rearranged and degenerated for a ∼11 MY-old system. Recombination suppression between X and Y extended in a stepwise process, and triggered a massive accumulation of repeats on the Y, as well as in the non-recombining pericentromeric region of the X, leading to giant sex chromosomes. One-Sentence Summary This work uncovers the structure, function, and evolution of one of the largest giant Y chromosomes, that of the model plant Silene latifolia , which is almost 10 times larger than the human Y, despite similar genome sizes.

The Silene latifolia genome and its giant Y chromosome