Early events in the evolution of the Silene latifolia Y chromosome:: Male specialization and recombination arrest

Understanding the origin and evolution of sex chromosomes requires studying recently evolved X-Y chromosome systems such as those in some flowering plants. We describe Y chromosome deletion mutants of Silene latifolia, a dioecious plant with heteromorphic sex chromosomes. The combination of results from new and previously described deletions with histological descriptions of their stamen development defects indicates the presence of two distinct Y regions containing loci with indispensable roles in male reproduction. We determined their positions relative to the two main sex determination functions (female suppressing and the other male promoting). A region proximal to the centromere on the Yparm containing the putative stamen promoting sex determination locus includes additional early stamen developmental factors. A medial region of the Yqarm carries late pollen fertility factors. Cytological analysis of meiotic X-Y pairing in one of the male-sterile mutants indicates that the Y carries sequences or functions specifically affecting sex chromosome pairing.