Evolutionary strata on the X chromosomes of the dioecious plant Silene latifolia: evidence from new sex-linked genes

¹ University of Edinburgh

^* To whom correspondence should be addressed. E-mail: r.bergero{at}ed.ac.uk.

Submitted on December 20, 2006
Revised on January 23, 2007
Accepted on 1 February 2007

	Abstract

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 eight 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 homologues on the X and Y chromosomes. Synonymous divergence estimated between the X and Y homologue pairs are 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 so far been studied, and that X-Y divergence increases with genetic distance from the pseudo-autosomal 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 about 10-20 MYA.

Key Words: Silene latifolia, X-Y divergence, evolutionary strata, sex chromosomes

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