SIY1, the first active gene cloned from a plant Y chromosome, encodes a WD-repeat protein

Unlike the majority of flowering plants, which possess hermaphrodite flower s, white campion (Silene latifolia) is dioecious and has flowers of two dif ferent sexes. The sex is determined by the combination of heteromorphic sex chromosomes: XX in females and XY in males. The Y chromosome of S. latifol ia was microdissected to generate a Y-specific probe which was used to scre en a young male flower cDNA library. We identified five genes which represe nt the first active genes to be cloned from a plant Y chromosome. Here we r eport a detailed analysis of one of these genes, SlY1 (S. larifolia Y-gene 1), SlY1 is expressed predominantly in male flowers, A closely related gene , SlX1, is predicted to be located on the X chromosome and is strongly expr essed in both male and female flowers. SlY1 and SlX1 encode almost identica l proteins containing WD repeats. Immunolocalization experiments showed tha t these proteins are localized in the nucleus, and that they are most abund ant in cells that are actively dividing or beginning to differentiate. Inte restingly, they do not accumulate in arrested sexual organs and represent p otential targets for sex determination genes. These genes will permit inves tigation of the origin and evolution of sex chromosomes in plants.