Department of Biology, West Virginia University, Morgantown, WV, 26506-6057, USA.
Horticulture Section, School of Integrative Plant Science, Cornell University, New York State Agricultural Experiment Station, Geneva, NY, 14456, USA.
Genome Biol. 2020 Feb 14;21(1):38. doi: 10.1186/s13059-020-1952-4.
Sex 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.
Here 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.
The 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.
性染色体在广泛的物种中独立出现，但它们具有共同的特征，包括在性别决定基因座周围存在抑制重组。哺乳动物性染色体在非重组区域包含多个回文重复序列，通过基因转换显示序列保守性，并包含对有性生殖至关重要的基因。在植物中，在没有同源重组的情况下，回文重复是否在维持序列保守性方面发挥作用尚不清楚。
基于雌雄异株灌木柳树的 W 染色体的高度连续组装，我们首次提供了植物性染色体上存在大回文结构的证据。W 染色体由于从常染色体转座而增加了大量基因。它还包含两个连续的回文，跨越 200kb 的区域，四个臂之间有明显的 20kb 长的高度保守序列，显示出基因转换的证据。回文内的四个基因与亲缘关系密切的杨属性别决定区域的基因同源，而杨属位于不同的染色体上。与常染色体上的同源拷贝相比，这些基因表现出明显的、偏向花的表达模式。
哺乳动物和植物性染色体上回文的存在突出了这些特征在缺乏重组的适应性进化中的内在重要性。趋同进化不仅驱动了性染色体的独立建立，还驱动了其精细的序列结构。
