异源四倍体小麦形成早期实现细胞学二倍体化的分子机制

Upon allopolyploidization, two or more sets of homeologous chromosomes are merged into one nucleus and cytoplasm. Thus, allopolyploidy increases the complexity of cell division (mitosis and meiosis) in which precisely orchestrated managing and partitioning of chromosomes are essential. In newly formed allopolyploids, aneuploidy often occurs due to mis-segregation of chromosomes during mitosis and/or meiosis. In particular, meiosis, which is more sophisticated than mitosis, represents one of the most severe bottlenecks for survival and establishment of newly formed allopolyploid individuals. It is believed that aneuploidy is generally associated with reduced cellular and/or organismal fitness (a phenomenon termed aneuploidy syndrome), and hence, aneuploid cells or individuals will be rapidly eliminated by natural selection. As a result, newly formed allopolyploids must undergo cytological diploidization to restore diploid-like meiotic behavior and disomic inheritance to avoid the fate of extinction. Recent studies in Arabidopsis have revealed that some meiosis related genes were under strong selection and tightly associated with polyploidization. Based on these new findings, it is more clear that established polyploidy can overcome meiotic crises achive cytological diploidization by genetic mutations. However, the issue of how newly formed allopolyploids accomplish diploid-like meiotic behavios and disomic inheritance is unknown. The wheat group plants (the Triticum-Aegilops complex) provide an ideal system to study polyploid evolution, because in many cases neo-allopolyploid plants mimicking those of natural allopolyploid species can be resynthesized. We have found recently that dramatic difference exists among newly synthesized allotetraploid wheat lines with different genome combinations (e.g., SSAA, SSDD and AADD) for the occurrence of aneuploidy, and some combinations showed normal diploid-like meiotic behavior and full disomic inheritance, suggesting immediate cytological diploididzation. This proposal is aimed to investigate the molecular basis for the occurrence cytological diploidization in these newly formed allotetraploid wheats. By comparing expression differences among these nascent allotetraploid lines and their parents for all possible meiosis-related genes, and then integrating this difference with the rapidly occurred genetic and epigenetic changes, we hope to gain new insights into the molecular basis for the occurrence of immediate cytological diploidization in neo-synthetic allotetraploid wheat.

异源多倍体形成过程至少有两个不同染色体组整合到一个细胞核中，增加了减数分裂的复杂性，故此易于产生非整倍体，进而对新形成多倍体的适合度产生不利影响。因此，多倍体必须在早期即进化出相应机制来保证细胞学水平的二倍体化，否则会灭绝。在拟南芥中已检测到一些减数分裂相关基因在多倍体成种过程被强烈选择。然而，这种基于"基因突变+选择"的遗传机制实现细胞学二倍体化需要足够的进化时间，因此只适合已经在进化上"成功"的多倍体物种。对于形成早期的多倍体如何实现细胞学二倍体化，目前尚一无所知；但可以推测基因表达模式和表观遗传的变化起了重要作用。小麦是研究异源多倍化的模式植物。我们最近研究发现，基因组构成不同的新合成异源四倍体小麦，在减数分裂稳定性上存在巨大差异。本研究拟以此为基础，通过对全基因组范围内减数分裂基因的表达模式变化及特定基因组区段遗传及表观遗传变化，揭示异源多倍体形成早期细胞学二倍体化的分子机制。

多倍体或全基因组加倍是所有显花植物基因组进化中的重要事件，包括了一些重要的作物，例如，小麦，玉米，棉花，油菜，咖啡。随着越来越多的动物及植物基因组测序的完成，科学家估计在现存的种子植物和被子植物分化之前，就已经发生了两轮古基因组加倍事件，而这为对种子及花发育相关基因及相应通路的进化及选择提供了可能性，从而使被子植物在整个地球上成为优势物种。人们对多倍体进化命运的认识也发生了变化，多倍体不再被认为是进化的盲端或者进化的偶然事件，而被认为是进化中的创新。.异源多倍体形成过程至少有两个不同染色体组整合到一个细胞核中，增加了减数分裂的复杂性，故此易于产生非整倍体，进而对新形成多倍体的适合度产生不利影响。因此，多倍体必须在早期即进化出相应机制来保证细胞学水平的二倍体化，否则便会灭绝。在拟南芥中已检测到一些减数分裂相关基因在多倍体成种过程被强烈选择。然而，这种基于"基因突变+选择"的遗传机制实现细胞学二倍体化需要足够的进化时间，因此只适合解释已经在进化上"成功"的多倍体物种。.小麦是研究异源多倍化的模式植物。我们的研究发现，基因组构成不同的新合成异源四倍体小麦，在减数分裂稳定性上存在巨大差异。通过对不同亲本组合产生的多倍体以及同一组合产生多倍体的株系研究表明，表观遗传变异，包括DNA甲基化，组蛋白修饰都参与了这一过程。和遗传变异相比，表观遗传变异发生频率更高，遗传变异对性状的影响常常是获得或者丢失，而表观遗传变异对性状的影响经常是控制性状的表现程度，从而使得某些性状呈现一定的分布范围，过去我们称这些性状为数量性状。我们研究结果表明，表观遗传变异在异源多倍体形成早期迅速完成细胞学的二倍化过程中起了重要的作用。