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 accompanies 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 provide a model system for the study of the origin of sex chromosomes. The Y chromosomes in Silene will be studied as a test of theoretical predictions concerning their structure, function, and evolutionary development. To trace the evolutionary history of the Y, a collection DNA-based genetic markers will be generated from Y chromosomes isolated using flow cytometry and subsequently digested using restriction enzymes. The resulting Y-specific DNA probes will be cross-hybridized with metaphase chromosomes of S. latifolia as well as related species with and without separate sexes the expectation is that the Y chromosome will have some regions of similarity with the X . Auras of non-homology serve to prevent recombination that would disrupt the 'active Y' sex determination of S. latifolia. Second, the Y chromosome in S. latifolia is believed to be partially dysfunctional. A quantitative genetic analysis of Y chromosome effects on floral phenotypes will be using performed regressions over a range of degrees of relationship (e.g. parent: offspring, grandparent:grand-offspring, half-sib, etc.). This study will integrate diverse approaches, from molecular cytogenetics to quantitative genetics, and will greatly enhance our understanding of the genetic events involved in sex differentiation.
