A Cryptic Cytoplasmic Male Sterility Unveils a Possible Gynodioecious Past for <i>Arabidopsis thaliana</i>

Gynodioecy, the coexistence of hermaphrodites and females (i.e. male-sterile plants) in natural plant populations, most often results from polymorphism at genetic loci involved in a particular interaction between the nuclear and cytoplasmic genetic compartments (cytonuclear epistasis): cytoplasmic male sterility (CMS). Although CMS clearly contributes to the coevolution of involved nuclear loci and cytoplasmic genomes in gynodioecious species, the occurrence of CMS genetic factors in the absence of sexual polymorphism (cryptic CMS) is not easily detected and rarely taken in consideration. We found cryptic CMS in the model plant Arabidopsis thaliana after crossing distantly related accessions, Sha and Mr-0. Male sterility resulted from an interaction between the Sha cytoplasm and two Mr-0 genomic regions located on chromosome 1 and chromosome 3. Additional accessions with either nuclear sterility maintainers or sterilizing cytoplasms were identified from crosses with either Sha or Mr-0. By comparing two very closely related cytoplasms with different male-sterility inducing abilities, we identified a novel mitochondrial ORF, named orf117Sha, that is most likely the sterilizing factor of the Sha cytoplasm. The presence of orf117Sha was investigated in worldwide natural accessions. It was found mainly associated with a single chlorotype in accessions belonging to a clade predominantly originating from Central Asia. More than one-third of accessions from this clade carried orf117Sha, indicating that the sterilizing-inducing cytoplasm had spread in this lineage. We also report the coexistence of the sterilizing cytoplasm with a non-sterilizing cytoplasm at a small, local scale in a natural population; in addition a correlation between cytotype and nuclear haplotype was detected in this population. Our results suggest that this CMS system induced sexual polymorphism in A. thaliana populations, at the time when the species was mainly outcrossing.雌全异株（gynodioecy）指自然植物种群中两性株与雌性株（即雄性不育植株）的共存现象，其成因多为细胞核与细胞质遗传组分间特定互作相关基因座的多态性，即核质互作上位性（cytonuclear epistasis），核心为细胞质雄性不育（cytoplasmic male sterility, CMS）。尽管细胞质雄性不育可显著推动雌全异株物种中相关核基因座与细胞质基因组的协同进化，但在缺乏生殖多态性的情境下，细胞质雄性不育遗传因子的检出难度极大，且极少被纳入研究范畴。 本研究通过对远缘种质生态型Sha与Mr-0进行杂交实验，在模式植物拟南芥（Arabidopsis thaliana）中成功检出隐性细胞质雄性不育（cryptic CMS）现象。该雄性不育由Sha细胞质与Mr-0基因组中位于1号染色体和3号染色体的两个区域互作所介导。通过与Sha或Mr-0进行杂交，本研究还鉴定出分别携带核不育保持基因或不育细胞质的额外种质材料。 通过比对两个雄性不育诱导能力存在差异的近缘细胞质，本研究鉴定出一个全新的线粒体开放阅读框（mitochondrial open reading frame, ORF），并将其命名为orf117Sha，该序列极有可能是Sha细胞质的不育因子。我们对全球自然种质材料中orf117Sha的分布情况进行了检测，发现其主要与一类单一叶绿体型相关联，该类叶绿体型存在于主要起源于中亚的进化枝所属的种质材料中。该进化枝中超过三分之一的种质材料携带orf117Sha，这表明该不育诱导细胞质已在该支系中发生扩散。 本研究还报道了在一个小型局域自然种群中，不育细胞质与可育细胞质共存的现象；此外，在该种群中检测到细胞质型与核单倍型之间存在显著相关性。本研究结果表明，在拟南芥主要以异交方式繁殖的时期，该细胞质雄性不育系统可促使其种群产生生殖多态性。