Abstract
BackgroundSex chromosomes have arisen independently in a wide variety of species, yet they share common characteristics, including the presence of suppressed recombination surrounding sex determination loci. Mammalian sex chromosomes contain multiple palindromic repeats across the non-recombining region that show sequence conservation through gene conversion and contain genes that are crucial for sexual reproduction. In plants, it is not clear if palindromic repeats play a role in maintaining sequence conservation in the absence of homologous recombination.ResultsHere we present the first evidence of large palindromic structures in a plant sex chromosome, based on a highly contiguous assembly of the W chromosome of the dioecious shrub Salix purpurea. The W chromosome has an expanded number of genes due to transpositions from autosomes. It also contains two consecutive palindromes that span a region of 200 kb, with conspicuous 20-kb stretches of highly conserved sequences among the four arms that show evidence of gene conversion. Four genes in the palindrome are homologous to genes in the sex determination regions 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.ConclusionThe presence of palindromes in sex chromosomes of mammals and plants highlights the intrinsic importance of these features in adaptive evolution in the absence of recombination. Convergent evolution is driving both the independent establishment of sex chromosomes as well as their fine-scale sequence structure.
Highlights
Sex chromosomes carry genes that confer or control sex-specific traits [1]
We demonstrate that gene content is expanded on the W chromosome, and homologous genes occur in the Salix and Populus sex determination regions (SDRs), suggesting that there may be some overlap in the underlying mechanisms of sex determination in this family
It is possible that the repressed recombination in this region pre-dated the transposition of a relatively small SDR cassette, as has been observed in octoploid Fragaria [17]. This is consistent with the apparently small size of the region in Populus (~ 100 kb), which is located on a different chromosome [24]. This is consistent with the structure and composition of the palindromic repeats that we discovered in S. purpurea, which are excellent candidates as sex determination loci, as detailed below
Summary
Sex chromosomes carry genes that confer or control sex-specific traits [1]. In theory, the heterogametic (sexspecific) sex chromosome evolved from an autosome. Ampliconic sequences on the human Y chromosome were acquired through transpositions from diverse sources, and amplified [4]. These ampliconic sequences account for about 30% of the Y euchromatin [4]. Mammalian sex chromosomes contain multiple palindromic repeats across the non-recombining region that show sequence conservation through gene conversion and contain genes that are crucial for sexual reproduction. In plants, it is not clear if palindromic repeats play a role in maintaining sequence conservation in the absence of homologous recombination
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