Originally published as Genetics Published Articles Ahead of Print on February 7, 2007.

Genetics, Vol. 175, 1945-1954, April 2007, Copyright © 2007
doi:10.1534/genetics.106.070110

Evolutionary Strata on the X Chromosomes of the Dioecious Plant Silene latifolia: Evidence From New Sex-Linked Genes

Roberta Bergero1, Alan Forrest, Esther Kamau and Deborah Charlesworth

Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3JT, United Kingdom

1 Corresponding author: Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Ashworth Lab, King's Bldgs., West Mains Rd., Edinburgh EH9 3JT, United Kingdom.
E-mail: r.bergero{at}ed.ac.uk

Despite its recent evolutionary origin, the sex chromosome system of the plant Silene latifolia shows signs of progressive suppression of recombination having created evolutionary strata of different X–Y divergence on sex chromosomes. However, even after 8 years of effort, this result is based on analyses of five sex-linked gene sequences, and the maximum divergence (and thus the age of this plant's sex chromosome system) has remained uncertain. More genes are therefore needed. Here, by segregation analysis of intron size variants (ISVS) and single nucleotide polymorphisms (SNPs), we identify three new Y-linked genes, one being duplicated on the Y chromosome, and test for evolutionary strata. All the new genes have homologs on the X and Y chromosomes. Synonymous divergence estimated between the X and Y homolog pairs is within the range of those already reported. Genetic mapping of the new X-linked loci shows that the map is the same in all three families that have been studied so far and that X–Y divergence increases with genetic distance from the pseudoautosomal region. We can now conclude that the divergence value is saturated, confirming the cessation of X–Y recombination in the evolution of the sex chromosomes at ~10–20 MYA.




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