Sharbel Timothy F, Voigt Marie-Luise, Corral José María, Thiel Thomas, Varshney Alok, Kumlehn Jochen, Vogel Heiko, Rotter Björn
Apomixis Research Group, Leibniz-Institut für Pflanzengenetik und Kulturpflanzenforschung (IPK), Corrensstrasse 3, Gatersleben, Germany.
Plant J. 2009 Jun;58(5):870-82. doi: 10.1111/j.1365-313X.2009.03826.x. Epub 2009 Feb 10.
Apomixis, a natural form of asexual seed production in plants, has evolved independently in various taxa, and represents an important potential technology for agriculture. The switch to apomixis is based on de-regulation of developmental pathways originally leading to sexual seed formation. Hybridization and polyploidy, both typical characteristics of asexual plants and animals, are mechanisms that could trigger de-regulation. Here we show that up-regulation of alleles in apomeiotic ovules is mediated by genomic duplication, heterochrony and the residual effects of ancient hybridization in diploid apomicts of the Boechera holboellii complex. Using SuperSAGE, we have identified over 4000 differentially expressed mRNA tags between micro-dissected ovules from two diploid sexual (Boechera stricta and B. holboellii) and two diploid apomictic (Boechera divaricarpa) accessions. Pairwise sequence comparisons between tags enabled identification of allelic variants of the same loci. Up-regulated candidate apomeiosis alleles consistently have more than three related alleles, thus demonstrating transcription from duplicated loci. A further 543 alleles were heterochronically expressed between sexual and apomeiotic ovules at developmental stages 2-II to 2-IV. Intriguingly, 69 B. holboellii specific alleles were preferentially up-regulated in apomeiotic ovules, thus showing a remnant'parent of origin' effect stemming from the Pleistocene origin of the hybrid B. divaricarpa from taxa related to B. holboellii and B. stricta. These data implicate polyploid gene dosage in the expression of asexual seed formation, and support hypotheses of de-regulation of the sexual pathway. The observed 'parent of origin' effect suggests that the genomic memory of hybridization has somehow been maintained after hundreds, if not thousands, of asexual generations.
无融合生殖是植物中一种自然的无性种子生产形式，在不同的分类群中独立进化，是农业领域一项重要的潜在技术。向无融合生殖的转变基于原本导致有性种子形成的发育途径的去调控。杂交和多倍体是无性植物和动物的典型特征，它们是可能引发去调控的机制。在这里，我们表明，在波氏岩荠复合体的二倍体无融合生殖植物中，无融合生殖胚珠中等位基因的上调是由基因组复制、发育异时性以及古代杂交的残留效应介导的。使用超级SAGE技术，我们在两个二倍体有性生殖（狭叶波氏岩荠和波氏岩荠）和两个二倍体无融合生殖（叉状波氏岩荠）材料的显微切割胚珠之间鉴定出了4000多个差异表达的mRNA标签。标签之间的成对序列比较能够识别同一基因座的等位基因变体。上调的候选无融合生殖等位基因始终有三个以上相关等位基因，从而证明了来自复制基因座的转录。在发育阶段2-II至2-IV，另外543个等位基因在有性生殖和无融合生殖胚珠之间异时表达。有趣的是，69个波氏岩荠特异性等位基因在无融合生殖胚珠中优先上调，从而显示出来自叉状波氏岩荠与波氏岩荠和狭叶波氏岩荠相关类群的更新世杂交起源的“起源亲本”残留效应。这些数据表明多倍体基因剂量参与无性种子形成的表达，并支持有性途径去调控的假说。观察到的“起源亲本”效应表明，即使经过数百代（如果不是数千代）无性繁殖，杂交的基因组记忆仍以某种方式得以保留。
