A Cryptic Cytoplasmic Male Sterility Unveils a Possible Gynodioecious Past for Arabidopsis thaliana

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花个体与雌性个体（即雄性不育植株）共存的现象，其形成大多源于核与细胞质遗传区域间特定互作相关的遗传位点多态性——核质互作上位性（cytonuclear epistasis）：细胞质雄性不育（cytoplasmic male sterility, CMS）。尽管CMS显然推动了雌全异株物种中相关核位点与细胞质基因组的协同演化，但在无性别多态性的情况下，CMS遗传因子的存在（即隐性CMS，cryptic CMS）却难以被检测，且极少被纳入研究考量。我们在对远缘种质系（accession）Sha与Mr-0进行杂交后，于模式植物拟南芥（Arabidopsis thaliana）中发现了隐性CMS。该雄性不育现象源于Sha细胞质与Mr-0基因组中位于1号染色体和3号染色体上的两个区域之间的互作。通过与Sha或Mr-0进行杂交，我们还鉴定出了分别携带核不育保持因子或不育细胞质的其他种质系。通过比较两个亲缘关系极近但雄性不育诱导能力各异的细胞质，我们鉴定出了一个全新的线粒体开放阅读框（open reading frame, ORF）orf117Sha，其极有可能是Sha细胞质中的不育因子。我们对全球范围内的自然种质系开展了orf117Sha的存在情况检测，研究发现orf117Sha主要与单一叶绿体型（chlorotype）相关联，且仅存在于主要起源于中亚的演化支（clade）所属的种质系中。该演化支中超过三分之一的种质系携带orf117Sha，这表明具有不育诱导能力的细胞质已在该演化支中发生扩散。我们还报道了在一个小型自然种群中，不育细胞质与可育细胞质共存的现象；此外，该种群中还检测到了细胞质型与核单倍型（haplotype）之间的相关性。我们的研究结果表明，在该物种主要进行异交的时期，这套CMS系统曾在拟南芥种群中诱导产生了性别多态性。