J Plant Genetics
Plants, therefore, offer opportunities to study the most interesting early stages of the evolution of sex separation and sex chromosome evolution. The only other comparable case where evolution of sex chromosomes is open to study is the situation when autosomal genomic segments are translocated onto sex chromosomes, generating neo-sex chromosome systems. This has happened a number of times in the genus Drosophila (Patterson & Stone, 1952) and it has been discovered that genes on the neo-Y chromosomes undergo genetic degeneration and become inactivated, and dosage compensation then evolves (Bone & Kuroda, 1996). The repeatability of these evolutionary events can be exam ined in plants too, with the added interest that sex chromo somes in dioecious plants from different angiosperm families are probably entirely independent evolutionary replicates.