A stable genetic mechanism for sex determination is a prerequisite for the maintenance of separate sexes in a species. This in turn requires that those genes promoting or suppressing female and male expression be located on the same chromosome. Chromosomal sex determination, uncommon in plants, is widespread in animals and is almost invariably accompanied by morphological differentiation of the X and Y chromosomes. The relatively recent origin of sex chromosomes in plants means that, in many cases, closely related species which lack sex chromosomes, also exist. The existence of variation in sex chromosome differentiation within a closely related group of plants provides a model system for the study of the origin of sex chromosomes. We are studying Y chromosomes in Silene to test theoretical predictions concerning their structure and function. A DNA library will be generated from Y chromosomes of Silene alba isolated using flow cytometry, a technique applied with great success to the human genome. The resulting Y-specific DNA probes will be cross hybridized with chromosomes of male and female S. alba, as well as the other close relatives. Our expectation is that the Y chromosome will have some regions which are structurally similar to the X. The existence of structurally different regions serves to prevent recombination that would disrupt the "active Y" sex determination found in Silene.
