Laboratory of Plant Genetics, Graduate School of Agricultural Science, Kobe University, Kobe, Japan.
PLoS One. 2010 Jun 25;5(6):e11326. doi: 10.1371/journal.pone.0011326.
Hybrid speciation is classified into homoploid and polyploid based on ploidy level. Common wheat is an allohexaploid species that originated from a naturally occurring interploidy cross between tetraploid wheat and diploid wild wheat Aegilops tauschii Coss. Aegilops tauschii provides wide naturally occurring genetic variation. Sometimes its triploid hybrids with tetraploid wheat show the following four types of hybrid growth abnormalities: types II and III hybrid necrosis, hybrid chlorosis, and severe growth abortion. The growth abnormalities in the triploid hybrids could act as postzygotic hybridization barriers to prevent formation of hexaploid wheat.
METHODOLOGY/PRINCIPAL FINDINGS: Here, we report on the geographical and phylogenetic distribution of Ae. tauschii accessions inducing the hybrid growth abnormalities and showed that they are widely distributed across growth habitats in Ae. tauschii. Molecular and cytological characterization of the type III necrosis phenotype was performed. The hybrid abnormality causing accessions were widely distributed across growth habitats in Ae. tauschii. Transcriptome analysis showed that a number of defense-related genes such as pathogenesis-related genes were highly up-regulated in the type III necrosis lines. Transmission electron microscope observation revealed that cell death occurred accompanied by generation of reactive oxygen species in leaves undergoing type III necrosis. The reduction of photosynthetic activity occurred prior to the appearance of necrotic symptoms on the leaves exhibiting hybrid necrosis.
CONCLUSIONS/SIGNIFICANCE: Taking these results together strongly suggests that an autoimmune response might be triggered by intergenomic incompatibility between the tetraploid wheat and Ae. tauschii genomes in type III necrosis, and that genetically programmed cell death could be regarded as a hypersensitive response-like cell death similar to that observed in Arabidopsis intraspecific and Nicotiana interspecific hybrids. Only Ae. tauschii accessions without such inhibiting factors could be candidates for the D-genome donor for the present hexaploid wheat.
杂种形成根据倍性水平分为同源多倍体和异源多倍体。普通小麦是一种异源六倍体物种，起源于四倍体小麦和二倍体野生小麦节节麦之间的自然发生的异倍体杂交。节节麦提供了广泛的自然遗传变异。有时，它与四倍体小麦的三倍体杂种表现出以下四种杂种生长异常：类型 II 和 III 杂种坏死、杂种黄化和严重生长败育。三倍体杂种中的生长异常可以作为合子后杂交障碍，防止六倍体小麦的形成。
方法/主要发现：在这里，我们报告了诱导杂种生长异常的节节麦品系的地理和系统发育分布，并表明它们广泛分布在节节麦的生长生境中。对 III 型坏死表型进行了分子和细胞学特征分析。导致杂种异常的品系广泛分布在节节麦的生长生境中。转录组分析表明，一些防御相关基因，如病程相关基因，在 III 型坏死系中高度上调。透射电子显微镜观察表明，在发生 III 型坏死的叶片中，伴随着活性氧的产生，细胞死亡发生。在表现出杂种坏死的叶片出现坏死症状之前，光合作用活性降低。
结论/意义：综合这些结果强烈表明，在 III 型坏死中，四倍体小麦和节节麦基因组之间的基因组间不兼容性可能触发自身免疫反应，并且遗传编程的细胞死亡可以被视为类似于拟南芥种内和烟草原种间杂种中观察到的过敏反应样细胞死亡。只有没有这种抑制因子的节节麦品系才能成为当前六倍体小麦的 D 基因组供体的候选者。
