A Willow Sex Chromosome Reveals Convergent Evolution of Complex Palindromic Repeats

Background: Sex chromosomes in a wide variety of species share common characteristics, including the presence of suppressed recombination surrounding sex determination loci. They have arisen independently in numerous lineages, providing a conclusive example of convergent evolution. Mammalian sex chromosomes contain multiple palindromicrepeats across the non-recombining region that facilitate sequence conservation through gene conversion, and contain genes that are crucial for sexual reproduction. Plant sex chromosomes are less well understood, and in particular it is not clear how coding sequence conservation is maintained in the absence of homologous recombination. Results: Here we present the first evidence of large palindromicstructures in a plant sex chromosome, based on a highly contiguous assembly of the W chromosomeof the dioecious shrub Salix purpurea. Two consecutive palindromesspan over a region of 200 kb, with conspicuous 20 kb stretches of highly conserved sequencesamong the four arms. The closely-related species S. suchowensis also has two copies of a portion of the palindromearm and provides strong evidence for gene conversion. Four genes in the palindromeare homologous to genes in the SDR of the closely-related genus Populus , which is located on a different chromosome. These genes show distinct, floral-biased expression patterns compared to paralogous copies on autosomes. Conclusion: The presence of palindromicstructures in sex chromosomes of mammals and plants highlights the intrinsic importance of these features in adaptive evolution in the absence of recombination. Convergent evolutionis driving both the independent establishment of sex chromosomes as well as their fine-scale sequence structure.