Unusual Mitochondrial Genome Organization in Cytoplasmic Male Sterile Common Bean and the Nature of Cytoplasmic Reversion to Fertility

Spontaneous reversion to pollen fertility and fertility restoration by the nuclear gene Fr in cytoplasmic male sterile common bean (Phaseolus vulgaris L.) are associated with the loss of a large portion of the mitochondrial genome. To understand better the molecular events responsible for this DNA loss, we have constructed a physical map of the mitochondrial genome of a stable fertile revertant line, WPR-3, and the cytoplasmic male sterile line (CMS-Sprite) from which it was derived. This involved a cosmid clone walking strategy with comparative DNA gel blot hybridizations. Mapping data suggested that the simplest model for the structure of the CMS-Sprite genome consists of three autonomous chromosomes differing only in short, unique regions. The unique region contained on one of these chromosomes is the male sterility-associated 3-kb sequence designated pvs. Based on genomic environments surrounding repeated sequences, we predict that chromosomes can undergo intra- and intermolecular recombination. The mitochondrial genome of the revertant line appeared to contain only two of the three chromosomes; the region containing the pvs sequence was absent. Therefore, the process of spontaneous cytoplasmic reversion to fertility likely involves the disappearance of an entire mitochondrial chromosome. This model is supported by the fact that we detected no evidence of recombination, excision or deletion events within the revertant genome that could account for the loss of a large segment of mitochondrial DNA.     

Identification of a Sterility-Inducing Cytoplasm in a Fertile Accession Line of Phaseolus Vulgaris L

Previous investigations into the genetic mechanism of fertility restoration in cytoplasmic male sterile Phaseolus vulgaris suggested that this is a particularly interesting system for the study of nuclear-mitochondrial interactions. This study was conducted to investigate the nature of nuclear-mitochondrial compatibility in fertile accession line G08063, the reported progenitor to the cytoplasmic male sterile line. Results from genetic analysis indicated that fertile line G08063 carried a sterility-inducing cytoplasm with a fertility restoring nuclear genotype. Mitochondrial DNA analysis indicated that the mechanism of fertility restoration by line G08063 was different from that conditioned by Fr, a previously described restorer gene. A mitochondrial DNA sequence associated with sterility and lost upon fertility restoration by nuclear gene Fr was present in the mitochondrial genome of fertile line G08063; this sequence was not carried within the mitochondrial genome of any other P. vulgaris accession line tested.     

Transfer of wild abortive cytoplasmic male sterility through protoplast fusion in rice

Wild abortive cytoplasmic male sterility has been extensively used in hybrid seed production in the tropics. Using protoplast fusion between cytoplasmic male sterile and fertile maintainer lines; we report here, transfer of wild abortive cytoplasmic male sterility to the nuclear background of RCPL1-2C, an advance breeding line which also served as maintainer of this cytoplasm. In total, 27 putative cybrids between V20A and RCPL1-2C and 23 lines between V20A and V20B were recovered and all of them were sterile. DNA blots prepared from the mitochondrial DNA of the cybrid lines from both the sets were probed with orf155 that is known to exhibit polymorphism between the mitochondrial DNA of the male-sterile and fertile maintainer lines. Hybridization of orf155 to 1.3 kb HindIII-digested mitochondrial DNA fragment of the cybrids showed transfer of mitochondrial DNA from wild abortive cytoplasmic male-sterile line to the maintainers, viz. RCPL 1-2C and V20B. Expression of male sterility was confirmed by the presence of sterile pollen grains and the lack of seed setting due to selfing in all the cybrid lines. These cybrids, on crossing with respective fertile maintainers set seeds that in turn, produced sterile BC1 plants. DNA blots from HindIII-digested mitochondrial DNA of these BC1 plants when probed with orf155 again exhibited localization of orf155 in wild abortive cytoplasm-specific 1.3 kb HindIII-digested mitochondrial DNA fragments. This demonstrated that the cytoplasmic male sterility transferred through protoplast fusion retained intact female fertility and was inherited and expressed in BC1 plants. Fusion-derived CMS lines, on pollination with pollen grains from restorer, showed restoration of fertility in all the lines. The results demonstrate that protoplasts fusion can be used for transferring maternally inherited traits like cytoplasmic male sterility to the desired nuclear background which can, in turn, be used in hybrid seed production programme of rice in the tropical world.     

Fertility Restoration Is Associated with Loss of a Portion of the Mitochondrial Genome in Cytoplasmic Male-Sterile Common Bean

Restoration of pollen fertility to cytoplasmic male-sterile common bean by nuclear gene Fr is accompanied by mitochondrial (mt) DNA rearrangements within restored plants. These rearrangements are also observed upon spontaneous cytoplasmic reversion to fertility. An mtDNA fragment of at least 25 kilobases was lost from the genome upon restoration or reversion. This fragment contained DNA segments that were not repeated elsewhere in the genome and, therefore, were not detected within the genome upon fertility restoration. This result suggested that the particular mtDNA configuration absent from restored plants could not be maintained by a constant process of recombination but rather by autonomous replication. No evidence of excision of this region from the mt genome, in the form of a junction fragment associating flanking DNA regions, was detected in fertile restored plants. DNA gel blot hybridization of this mtDNA region, compared with hybridization to related regions of the mitochondrial genome that shared sequence homology, indicated that the mtDNA region associated with sterility was present in lower copy number. These observations, as well as the occurrence of similar or identical rearrangements upon spontaneous cytoplasmic reversion, indicate that the restoration of pollen fertility may be accompanied by loss of an independently replicating subgenomic DNA molecule from the mitochondrial genome.     

mRNA differential display between sterile and fertile anther of rice and analysis of cDNA differential fragments

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Genetics of fertility restoration in cytoplasmic male sterile Phaseolus vulgaris L.

Summary Restoration of fertility in cytoplasmic male sterile Phaseolus vulgaris by line R-351 was controlled by a single gene. The restorer gene (Fr) displayed incomplete dominance leading to partial restoration of fertility in F₁ generations; full restoration was not achieved until the F₂ generation. Once full restoration of fertility was produced in the F₂ generation, no segregation for sterility was observed in subsequent generations derived from heterozygotes Frfr, either by testcrossing (restored × maintainer) or in F₃ progenies. Implications of the irreversible nature of this restoration are discussed.Florida Agr. Exp. Sta. Journal Series No. 7733     

来自密阳46的籼型新不育胞质H236A的育性鉴定和花粉粒败育形态观察

胞质雄性不育多样性是解决三系杂交稻品质、抗性和产量的主要措施。在以密阳46为母本的杂交后代中发现不育材料H236A,通过杂交和自交,确定其不育类型和不育度;通过碘染和徕卡荧光显微镜DM2500对成熟花粉粒观察,确定花粉粒育性、败育形态和时期。结果表明：H236A是胞质雄性不育,不育度达99.8%以上;花粉粒属典败型的达83.17%,圆败型占16.83%,没有染败型花粉粒,为单核期败育。花粉粒败育形态多种多样,有不规则形,梭形,圆形等。清晰观察到晚期小孢子细胞质定向移动形成细胞质桥现象。本文还讨论了成熟花粉粒败育时期和败育形态的划分。     

Two non-allelic nuclear genes restore fertility in a gametophytic pattern and enhance abiotic stress tolerance in the hybrid rice plant

In indica rice, the HongLian (HL)-type combination of cytoplasmic male sterility (CMS) and fertility restoration (Rf) is widely used for the production of commercial hybrid seeds in China, Laos, Vietnam and other Southeast Asian countries. Generally, any member of the gametophytic fertility restoration system, 50% of the pollen in hybrid F(1) plants displays recovered sterility. In this study, however, a HL-type hybrid variety named HongLian You6 had approximately 75% normal (viable) pollen rather than the expected 50%. To resolve this discrepancy, several fertility segregation populations, including F(2) and BC(1)F(1) derived from the HL-CMS line Yuetai A crossed with the restorer line 9311, were constructed and subjected to genetic analysis. A gametophytic restoration model was discovered to involve two non-allelic nuclear restorer genes, Rf5 and Rf6. The Rf5 had been previously identified using a positional clone strategy. The Rf6 gene represents a new restorer gene locus, which was mapped to the short arm of chromosome 8. The hybrid F(1) plants containing one restorer gene, either Rf5 or Rf6, displayed 50% normal pollen grains with I(2)-KI solution; however, those with both Rf5 and Rf6 displayed 75% normal pollens. We also established that the hybrid F(1) plants including both non-allelic restorer genes exhibited an increased stable seed setting when subjected to stress versus the F(1) plants with only one restorer gene. Finally, we discuss the breeding scheme for the plant gametophytic CMS/Rf system.     

A CMS-Related Gene, Ψatp6-2, Causes Increased ATP Hydrolysis Activity of the Mitochondrial F1Fo-ATP Synthase and Induces Male Sterility in Pepper (Capsicum annuum L.)

Pollen formation is a complex process that is strictly controlled by genetic factors. Although many novel mitochondrial genes have been implicated in the dysfunction of mitochondrial enzymes and the cytoplasmic male sterility (CMS), there is little empirical evidence to show that CMS-related genes actually result in the dysfunction of enzyme and little is known about the regulatory mechanisms of the aberrant mitochondrial enzymes in male sterility in CMS lines. Here, we report the characterization of a novel mitochondrial gene, Ψatp6-2, which is hypothesized to play a role in male sterility in pepper. Using virus-induced gene silencing (VIGS), we observed that silencing the atp6-2 gene in the maintainer line resulted in an increase in ATP hydrolysis activity of the mitochondrial F₁F_o-ATP synthase along with pollen abortion, while silencing the truncated Ψatp6-2 gene in the CMS line resulted in an inhibition of ATP hydrolysis activity and restoration of fertility. Altered ATP hydrolysis also affected the tolerance of the gene-silenced plants to abiotic stresses. Localization experiments showed that premature ATP hydrolysis occurred at the tetrad stage of pollen development in the CMS line, but no ATPase activity was observed in the microspores at the later stage. These results suggest that the Ψatp6-2 gene causes the alteration in ATP hydrolysis activity of the mitochondrial F₁F_o-ATP synthase during pollen development, which eventually leads to male sterility in pepper.     

Alloplasmic male sterile Brassica napus with Enarthrocarpus lyratus cytoplasm: introgression and molecular mapping of an E. lyratus chromosome segment carrying a fertility restoring gene.

A cytoplasmic male sterility (CMS) system for Brassica napus (2n = 38; AACC) was developed by backcross substitution of its nucleus into the cytoplasm of a wild crucifer, Enarthrocarpus lyratus. Male sterility was complete, stable, and expressed in small flowers with rudimentary anthers. Since the B. napus germplasm lines were complete or partial maintainers of male sterility, the required fertility restorer gene (Rfl) was introgressed from the cytoplasm donor species. Inheritance studies carried out on F1 and F2 populations derived from hybridizing cytoplasmic male sterile and male fertile near-isogenic (PNILs) lines of B. napus 'Westar', revealed a monogenic dominant control for fertility restoration. Bulked segregant analysis with 215 RAPD primers helped in the identification of putative primers associated with fertility restoration. Co-segregation analysis of eight such primers with Rfl gene revealed two markers, OPK 15700 and OPZ 061300, which flank the Rfl locus on either side at a distance of 8.2 and 2.5 cM, respectively. These DNA markers will be useful in marker-assisted selection for improving the commercial potential of this newly developed CMS-fertility-restorer system for hybrid seed production programs in rapeseed.     

叶用芥菜细胞质雄性不育系0912A的胞质效应和杂种优势分析

通过异地多代的育性鉴定及花器形态观察来调查叶用芥菜细胞质雄性不育系0912A的不育性及结籽能力;同时分别以不育系0912A和其保持系0912B为母本,与5个典型叶用芥菜自交系配成10个同核异质的杂交种,以研究hau胞质的胞质效应并测定杂种优势.结果显示:该不育系败育彻底稳定,不育株率和不育度均为100％,有正常的结籽能力;不育系杂种一代与产最相关性状的杂种优势明显,但品质性状无优势;hau胞质在单株重上的负效应较明显,在其他性状上的负效应相对较弱,但hau胞质组合F1的单株重杂种优势依然显著,且胞质效应受杂交父本核影响,通过选择合适的杂交父本配组,细胞质的负效应可以减轻或消除.     

Pollen fertility restoration by nuclear gene Fr in CMS common bean: an Fr linkage map and the mode of Fr action

The Fr gene in common bean, Phaseolus vulgaris L., is a unique gene for the study of plant nuclear-mitochondrial interactions because it appears to directly influence plant mitochondrial genome structure, resulting in the restoration of pollen fertility in cytoplasmic male sterile plants. This gene action is distinct from other pollen fertility restoration systems characterized to date. As a first step towards the map-based cloning of this unusual nuclear gene, we identified RAPD markers linked to Fr using bulked segregant analysis of near-isogenic lines. Using DNA gel blot hybridization, we localized the identified RAPD markers to a linkage group on the common bean RFLP map and constructed a linkage map of the Fr region using both RAPD markers and RFLP markers. Analysis of the mode of Fr action with the aid of identified Fr-linked DNA markers indicated that Fr functions in a semidominant fashion, showing dosage effect in controlling the dynamics of a heteroplasmic mitochondrial population. We also present our observations on the developmental distinctions, crucial in the accurate mapping of the Fr gene, between spontaneous cytoplasmic reversion and Fr-driven fertility restoration, two phenomena that are phenotypically indistinguishable.     

Alloplasmic male-sterile <Emphasis Type="Italic">Brassica juncea</Emphasis> with <Emphasis Type="Italic">Enarthrocarpus lyratus</Emphasis> cytoplasm and the introgression of gene(s) for fertility restoration from cytoplasm donor species

A new cytoplasmic male sterility (CMS) source in Brassica juncea (2n = 36; AABB) was developed by substituting its nucleus into the cytoplasm of Enarthrocarpus lyratus (2n = 20; E(l)E(l)). Male sterility was complete, stable and manifested in either petaloid- or rudimentary-anthers which were devoid of fertile pollen grains. Male sterile plants resembled the euplasmic B. juncea except for slight leaf yellowing and delayed maturity. Leaf yellowing was due mainly to higher level of carotenoids rather than a reduction in chlorophyll pigments. Female fertility in male-sterile plants varied; it was normal in lines having rudimentary anthers but poor in those with petaloid anthers. Each of the 62 evaluated germplasm lines of B. juncea was a functional maintainer of male sterility. The gene(s) for male-fertility restoration ( Rf) were introgressed from the cytoplasm donor species through homoeologous pairing between A and E(l) chromosomes in monosomic addition plants (2n = 18II+1E(l)). The percent pollen fertility of restored F(1) ( lyr CMS x putative restorer) plants ranged from 60 to 80%. This, however, was sufficient to ensure complete seed set upon by bag selfing. The CMS ( lyr) B. juncea compared favourably with the existing CMS systems for various productivity related characteristics. However, the reduced transmission frequency of the Rf gene(s) through pollen grains, which was evident from the sporadic occurrence of male-sterile plants in restored F(1) hybrids, remains a limitation.     

A new fertility restorer locus linked closely to the Rfo locus for cytoplasmic male sterility in radish

In this study, we have investigated a new fertility restorer (Rf) locus for cytoplasmic male sterility (CMS) in radish. We have obtained a CMS-Rf system consisting of sterile line '9802A1', maintainer line '9802B1' and restorer line '9802H'. F(1) plants from cross between sterile line '9802A1' and restorer line '9802H' were all male fertile, self pollination of F(1) plants produced an F(2) segregating population consisting of 600 individuals. The segregating population was found to fit a segregation ratio 3:1 for male fertile and sterile types, indicating that male fertility is restored by a single dominant gene (termed Rfo2) in the CMS-Rf system. Based on the DNA sequence of Rfo/Rfk1 (AJ535623), just one full length gene in the sterile line '9802A1', in the restorer line '9802H' and in the male fertile line '2006H', was cloned, respectively. The three sequences correspond to the same gene with two alleles: Rfob in '9802H' and rfob in '9802A1' and '2006H'. These two alleles differ from Rfo/Rfk1 and rfk1 (AJ535624) alleles by two synonymous base substitutions, respectively. Based on the differences between the Rfob and rfob genes, one PCR-based marker was developed, and designated Marker 1, which is identical to the corresponding region of Rfob by sequence analysis. In the F(2) segregating population described above, the Marker 1 was present in 5 sterile plants and in 453 fertile plants, absent in 4 fertile plants and in 138 sterile plants, and was found to fit a segregation ratio 3:1 indicating that Rfob was single copy in '9802H'. Linkage analysis showed that the Rfo2 locus for our CMS-Rf system was distant from the Rfo locus by about 1.6 cM. The sterile line '9802A1' was pollinated by the male fertile line '2006H' and the resulting F(1) plants were all male fertile. These results indicated that the male fertility of radish CMS can be restored by a new Rf locus, which linked tightly to the Rfo locus.     

大白菜新型细胞质雄性不育系6w-9605A的育性鉴定和花药败育的细胞学观察

采用石蜡切片技术,研究了大白菜(Brassica campestris L.ssp.pekinensis)细胞质雄性不育系6w-9605A及其保持系6w-9605B的花药发育过程的细胞形态学特征,确定不育系花药败育时期及方式,并对不育系6w-9605A进行花器官观察和育性鉴定.结果表明:保持系6w-9605B花药发育正常;不育系6w-9605A花药发育受阻于孢原分化时期,占总败育花药的66.7％,不形成花粉囊和花粉粒,属于无花粉囊型败育;另外33.3％的败育花药可形成花粉囊,小孢子均受阻于单核靠边期或者二胞期,败育特点为绒毡层细胞异常肥大,挤压小孢子,导致小孢子和绒毡层解体;6w-9605A的不育性稳定、彻底,不育株率和不育度均为100％.     

Map-based cloning of a fertility restorer gene, Rf-1, in rice (Oryza sativa L.)

A rice nuclear gene, Rf-1, restores the pollen fertility disturbed by the BT-type male sterile cytoplasm, and is widely used for commercial seed production of japonica hybrid varieties. Genomic fragments carrying Rf-1 were identified by conducting chromosome walking and a series of complementation tests. Isolation and analysis of cDNA clones corresponding to the fragments demonstrated that Rf-1 encodes a mitochondrially targeted protein containing 16 repeats of the 35-aa pentatricopeptide repeat (PPR) motif. Sequence analysis revealed that the recessive allele, rf-1, lacks one nucleotide in the putative coding region, presumably resulting in encoding a truncated protein because of a frame shift. Rice Rf-1 is the first restorer gene isolated from cereal crops that has the property of reducing the expression of the cytoplasmic male sterility (CMS)-associated mitochondrial gene like many other restorer genes. The present findings may facilitate not only elucidating the mechanisms of male sterility by the BT cytoplasm and its restoration by Rf-1 but also isolating other restorer genes from cereal crops, especially rice.     

水稻温敏核不育突变体0A15-1的育性表现及遗传学研究

水稻细胞质型雄性不育系IR69700A的幼穗经离体培养,获得一个体细胞克隆突变体0A15-1。经短日照和低温处理表明,0A15-1具有在高温下不育和低温下转为可育的特性,是一例温敏不育突变体。花粉染色显示0A15-l属于典败型不育。通过与明恢63、优B和广陆矮等多个父本的杂交,其F2和BC1群体的育性分离比都揭示0A15-1的不育性状受一对隐性核基因控制,并且为孢子体型雄性不育。该新种质可以用于对相关温敏核不育基因进行分子标记及用于两系法生产杂交水稻。