Plant sex chromosomes.
Dioecious species are known in plants and, as in many animals, some have distinguishable sex chromosomes. Genetic maps have identified sex-determining regions in several plants, and mapped male-specific Y (MSY) regions of the chromosome in which crossing over and genetic recombination do not occur, allowing sequence divergence between the X and Y. Divergence values of the few X-Y gene pairs so far available show that recombination between different genes of Silene latifolia stopped at different times. Once recombination stops, MSY genome regions are predicted to accumulate repetitive sequences, including transposable elements, resulting in low gene density. This has been documented in papaya but not yet in other plants. Y-linked genes should also accumulate deleterious mutations, eventually being lost as dosage compensation evolves. The few available data suggest that many plant MSY genes are functional, perhaps because genes required for male gametophyte functions degenerate slowly. Detailed studies of sex-linked genes are needed to test for deleterious substitutions in Y genes, and to date the origins of plant sex chromosomes.
Charlesworth
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Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Ashworth Lab. King's Buildings, Edinburgh, UK.