Evolution of Sex Chromosomes in Silene

In most plant species male and female organs are present in the same plant or even the same flower. What happens at the genome and transcriptome levels when ancestrally cosexual species evolves separate sexes and sex chromosomes? We are addressing this question using a plant species, Silene latifolia, that evolved separate sexes (dioecy) and heteromorphic sex chromosomes only a few million years ago. Despite relatively recent transition to dioecy and evolution of sex chromosomes, many Y-linked genes show signs of genetic degeneration. Y-degeneration in S. latifolia is apparently slower than what would be expected based on our knowledge from animals. This disparity may arise from fundamental difference of plant and animal life cycles: unlike animals, plants have alteration of haploid and diploid generation and widespread haploid expression may result in stronger purifying selection. Loss of expression from Y-linked genes appears to be partly compensated by altered expression of X-linked genes, indicating that S. latifolia has started to evolve dosage compensation. The nature of this dosage compensation system remains unclear, but our preliminary analyses of gene expression in Y-deletion mutants suggest the presence of trans-acting system that up-regulates X-copies of genes for which Y-linked copies are deleted. Evolution of such trans-acting dosage compensation system in relatively short time since the origin of sex chromosomes in this lineage appears surprising and requires further research.