Matyášek Roman, Dobešová Eva, Húska Dalibor, Ježková Ivana, Soltis Pamela S, Soltis Douglas E, Kovařík Aleš
Laboratory of Molecular Epigenetics, Institute of Biophysics, Academy of Sciences of the Czech Republic, v.v.i., Brno, Czech Republic.
Florida Museum of National History, University of Florida, Gainesville, FL, 32611, USA.
Plant J. 2016 Feb;85(3):362-77. doi: 10.1111/tpj.13110.
Uniparental silencing of 35S rRNA genes (rDNA), known as nucleolar dominance (ND), is common in interspecific hybrids. Allotetraploid Tragopogon mirus composed of Tragopogon dubius (d) and Tragopogon porrifolius (p) genomes shows highly variable ND. To examine the molecular basis of such variation, we studied the genetic and epigenetic features of rDNA homeologs in several lines derived from recently and independently formed natural populations. Inbred lines derived from T. mirus with a dominant d-rDNA homeolog transmitted this expression pattern over generations, which may explain why it is prevalent among natural populations. In contrast, lines derived from the p-rDNA dominant progenitor were meiotically unstable, frequently switching to co-dominance. Interpopulation crosses between progenitors displaying reciprocal ND resulted in d-rDNA dominance, indicating immediate suppression of p-homeologs in F1 hybrids. Original p-rDNA dominance was not restored in later generations, even in those segregants that inherited the corresponding parental rDNA genotype, thus indicating the generation of additional p-rDNA and d-rDNA epigenetic variants. Despite preserved intergenic spacer (IGS) structure, they showed altered cytosine methylation and chromatin condensation patterns, and a correlation between expression, hypomethylation of RNA Pol I promoters and chromatin decondensation was apparent. Reversion of such epigenetic variants occurred rarely, resulting in co-dominance maintained in individuals with distinct genotypes. Generally, interpopulation crosses may generate epialleles that are not present in natural populations, underlying epigenetic dynamics in young allopolyploids. We hypothesize that highly expressed variants with distinct IGS features may induce heritable epigenetic reprogramming of the partner rDNA arrays, harmonizing the expression of thousands of genes in allopolyploids.
35S核糖体RNA基因（rDNA）的单亲沉默，即核仁显性（ND），在种间杂种中很常见。由刺萼龙葵（d）和蒜叶婆罗门参（p）基因组组成的异源四倍体刺萼龙葵表现出高度可变的核仁显性。为了研究这种变异的分子基础，我们研究了来自最近独立形成的自然种群的几个品系中rDNA同源基因的遗传和表观遗传特征。来自具有显性d-rDNA同源基因的刺萼龙葵的自交系在几代中都传递了这种表达模式，这可能解释了为什么它在自然种群中普遍存在。相比之下，来自p-rDNA显性祖本的品系减数分裂不稳定，经常转变为共显性。显示相互核仁显性的祖本之间的种群间杂交导致d-rDNA显性，表明F1杂种中p同源基因被立即抑制。即使在那些继承了相应亲本rDNA基因型的分离株中，原始的p-rDNA显性在后代中也没有恢复，因此表明产生了额外的p-rDNA和d-rDNA表观遗传变异。尽管基因间隔区（IGS）结构得以保留，但它们显示出胞嘧啶甲基化和染色质凝聚模式的改变，并且表达、RNA聚合酶I启动子的低甲基化与染色质解凝聚之间的相关性很明显。这种表观遗传变异的逆转很少发生，导致具有不同基因型的个体维持共显性。一般来说，种群间杂交可能产生自然种群中不存在的表观等位基因，这是年轻异源多倍体中表观遗传动态的基础。我们假设具有独特IGS特征的高表达变异体可能诱导伴侣rDNA阵列的可遗传表观遗传重编程，协调异源多倍体中数千个基因的表达。
