Mapping and characterization of Rf 5: a new gene conditioning pollen fertility restoration in A1 and A2 cytoplasm in sorghum (Sorghum bicolor (L.) Moench)

Newly Developed Restorer Lines of Sorghum [Sorghum bicolor (L.) Moench] Resistant to Greenbug

Eight lines of grain sorghum [Sorghum bicolor (L.) Moench], which can be used as a promising source material in heterotic hybrid breeding as pollen fertility restorers and donors of resistance to the greenbug (Schizaphis graminum Rondani), are characterized. The new restorer lines (R-lines) were developed by crossing the maternal sterile line Nizkorosloe 81s (CMS A1) with two lines selected from the grain sorghum collection accessions VIR-928 and VIR-929 as the paternal forms. The R-lines were genotyped using PCR markers, and also characterized by height, duration of the seedling-flowering period, and some of the technological properties of flour. With the use of microsatellite markers linked to the Rf genes and by hybridological analysis, it was shown that the new lines carry the dominant allele of the gene Rf2. The PCoA analysis demonstrated clear differences of each R-line from the parents. The genotypes of the new lines and their parental forms for the Rf2 locus were confirmed by applying three allele-specific codominant CAPS markers which detected SNPs in the candidate Rf2 gene. All new lines were highly fertile, as demonstrated by cytological analysis of acetocarmine-stained pollen preparations. A high resistance to the greenbug was demonstrated for each new R-line both in the laboratory and field conditions against a severe aphid infestation. Grain quality parameters such as protein content and dough rheological properties varied widely and were quite satisfactory in some R-lines. Characteristics common to all eight sorghum lines studied, such as the ability to restore pollen fertility in the F1 generation, good pollen quality, greenbug resistance, early ripening, spreading panicle, and low stature, allow us to recommend them for producing commercial F1 hybrids with satisfactory grain quality for both fodder and food purposes.

Fine mapping of Rf5 region for a sorghum fertility restorer gene and microsynteny analysis across grass species

Cytoplasmic male sterility (CMS) is widely used to control pollination in the production of commercial F₁ hybrid seed in sorghum. So far, 6 major fertility restorer genes, Rf1 to Rf6, have been reported in sorghum. Here, we fine-mapped the Rf5 locus on sorghum chromosome 5 using descendant populations of a 'Nakei MS-3A' × 'JN43' cross. The Rf5 locus was narrowed to a 140-kb region in BTx623 genome (161-kb in JN43) with 16 predicted genes, including 6 homologous to the rice fertility restorer Rf1 (PPR.1 to PPR.6). These 6 homologs have tandem pentatricopeptide repeat (PPR) motifs. Many Rf genes encode PPR proteins, which bind RNA transcripts and modulate gene expression at the RNA level. No PPR genes were detected at the Rf5 locus on the corresponding homologous chromosome of rice, foxtail millet, or maize, so this gene cluster may have originated by chromosome translocation and duplication after the divergence of sorghum from these species. Comparison of the sequences of these genes between fertile and CMS lines identified PPR.4 as the most plausible candidate gene for Rf5.

A restorer-of-fertility-like pentatricopeptide repeat protein promotes cytoplasmic male sterility in Arabidopsis thaliana

Pentatricopeptide repeat (PPR) proteins form a large family of proteins targeted to organelles, where they post-transcriptionally modulate gene expression through binding to specific RNA sequences. Among them, the mitochondria-targeted restorer-of-fertility (Rf) PPRs inhibit peculiar mitochondrial genes that are detrimental to male gametes and cause cytoplasmic male sterility (CMS). Here, we revealed three nuclear loci involved in CMS in a cross between two distant Arabidopsis thaliana strains, Sha and Cvi-0. We identified the causal gene at one of these loci as RFL24, a conserved gene encoding a PPR protein related to known Rf PPRs. By analysing fertile revertants obtained in a male sterile background, we demonstrate that RFL24 promotes pollen abortion, in contrast with the previously described Rf PPRs, which allow pollen to survive in the presence of a sterilizing cytoplasm. We show that the sterility caused by the RFL24 Cvi-0 allele results from higher expression of the gene during early pollen development. Finally, we predict a binding site for RFL24 upstream of two mitochondrial genes, the CMS gene and the important gene cob. These results suggest that the conservation of RFL24 is linked to a primary role of ensuring a proper functioning of mitochondria, and that it was subsequently diverted by the CMS gene to its benefit.

Transmission Genetics of a Sorghum bicolor × S. halepense Backcross Populations

Despite a "ploidy barrier," interspecific crosses to wild and/or cultivated sorghum (Sorghum bicolor, 2n = 2x = 20) may have aided the spread across six continents of Sorghum halepense, also exemplifying risks of "transgene escape" from crops that could make weeds more difficult to control. Genetic maps of two BC₁F₁ populations derived from crosses of S. bicolor (sorghum) and S. halepense with totals of 722 and 795 single nucleotide polymorphism (SNP) markers span 37 and 35 linkage groups, with 2-6 for each of the 10 basic sorghum chromosomes due to fragments covering different chromosomal portions or independent segregation from different S. halepense homologs. Segregation distortion favored S. halepense alleles on chromosomes 2 (1.06-4.68 Mb, near a fertility restoration gene), 7 (1.20-6.16 Mb), 8 (1.81-5.33 Mb, associated with gene conversion), and 9 (47.5-50.1 Mb); and S. bicolor alleles on chromosome 6 (0-40 Mb), which contains both a large heterochromatin block and the Ma1 gene. Regions of the S. halepense genome that are recalcitrant to gene flow from sorghum might be exploited as part a multi-component system to reduce the likelihood of spread of transgenes or other modified genes. Its SNP profile suggests that chromosome segments from its respective progenitors S. bicolor and Sorghum propinquum have extensively recombined in S. halepense. This study reveals genomic regions that might discourage crop-to-weed gene escape, and provides a foundation for marker-trait association analysis to determine the genetic control of traits contributing to weediness, invasiveness, and perenniality of S. halepense.

Reproductive Cold Stress Tolerance in Sorghum F1 Hybrids is a Heterotic Trait

The sensitivity of sorghum to pre-flowering cold stress, resulting in reduced pollen viability and poor seed set, is a major constraint for expanding growing areas into higher altitudes and latitudes. Nevertheless, compared to juvenile cold tolerance, reproductive cold tolerance in sorghum has received much less attention so far, and very little is known about its inheritance in F1-hybrids. We have composed a representative factorial (n = 49 experimental F1-hybrids) for a comprehensive study on heterosis and combining ability for crucial tolerance traits as spikelet fertility (panicle harvest index), seed yield and pollen viability, using field trials in stress- and control environments in Germany and Mexico as well as climate chamber experiments. Our results indicate a heterotic and rather dominant inheritance of reproductive cold tolerance in sorghum, with strong effects of female general combining ability (GCA) on F1-hybrid performance in our material. These findings, together with the comparatively low contribution of specific combining ability (SCA) effects and high heritability estimates, suggest that robust and efficient enhancement of reproductive cold tolerance is feasible via hybrid breeding.

Undesired fertility restoration in msm1 barley associates with two mTERF genes

The novel Rfm3 locus causing undesired fertility restoration in the msm1 cytoplasm of winter barley is located on the short arm of chromosome 6H. Undesired fertility restoration of cytoplasmic male sterile (CMS) mother lines in absence of the functional Rfm1 restorer gene is a significant problem for hybrid breeding in winter barley. Here, we describe that a novel restorer locus on the short arm of chromosome 6H, designated Rfm3, is closely linked to two mitochondrial transcription termination factor family (mTERF) protein coding genes. Genome-wide association studies in a multiparental mapping population revealed that two of the most significantly associated markers are located very close to these genes, with one marker lying directly within one mTERF gene sequence. Sequences of the candidate genes in the parental lines, segregating individuals and an independent set of breeding lines clearly revealed haplotypes discriminating completely sterile, partially fertile and Rfm1-restorer lines. The haplotypes segregate for several single nucleotide polymorphisms, a 6 bp insertion-deletion (InDel) polymorphism and another 2 bp InDel. CMS-unstable genotypes carrying haplotypes associated with undesired fertility restoration showed significantly higher grain setting on bagged spikes when plants were subjected to elevated temperatures during anthesis, indicating a temperature influence on pollen fertility. SNPs associated with desirable Rfm3 haplotypes can be implemented in marker-assisted selection of stable CMS mother lines.

The Sorghum QTL Atlas: a powerful tool for trait dissection, comparative genomics and crop improvement

We describe the development and application of the Sorghum QTL Atlas, a high-resolution, open-access research platform to facilitate candidate gene identification across three cereal species, sorghum, maize and rice. The mechanisms governing the genetic control of many quantitative traits are only poorly understood and have yet to be fully exploited. Over the last two decades, over a thousand QTL and GWAS studies have been published in the major cereal crops including sorghum, maize and rice. A large body of information has been generated on the genetic basis of quantitative traits, their genomic location, allelic effects and epistatic interactions. However, such QTL information has not been widely applied by cereal improvement programs and genetic researchers worldwide. In part this is due to the heterogeneous nature of QTL studies which leads QTL reliability variation from study to study. Using approaches to adjust the QTL confidence interval, this platform provides access to the most updated sorghum QTL information than any database available, spanning 23 years of research since 1995. The QTL database provides information on the predicted gene models underlying the QTL CI, across all sorghum genome assembly gene sets and maize and rice genome assemblies and also provides information on the diversity of the underlying genes and information on signatures of selection in sorghum. The resulting high-resolution, open-access research platform facilitates candidate gene identification across 3 cereal species, sorghum, maize and rice. Using a number of trait examples, we demonstrate the power and resolution of the resource to facilitate comparative genomics approaches to provide a bridge between genomics and applied breeding.

QTL mapping and validation of fertility restoration in West African sorghum A1 cytoplasm and identification of a potential causative mutation for Rf2

Key message Major A ₁ cytoplasm fertility restoration loci, Rf ₂ and Rf ₅ , were found in the West African sorghum. A potential causative mutation for Rf ₂ was identified. KASP markers were validated on independent material. To accelerate the identification and development of hybrid parental lines in West African (WA) sorghum, this study aimed to understand the genetics underlying the fertility restoration (Rf) in WA A₁ cytoplasmic male sterility system and to develop markers for a routine use in WA breeding programs. We genotyped by sequencing three F₂ populations to map the Rf quantitative trait loci (QTL), validated the molecular KASP markers developed from those QTL in two F_2:3 populations, and assessed the most promising markers on a set of 95 R- and B-lines from WA breeding programs. Seven QTL were found across the three F₂ populations. On chromosome SBI-05, we found a major fertility restorer locus (Rf₅) for two populations with the same male parent, explaining 19 and 14% of the phenotypic variation in either population. Minor QTL were detected in these two populations on chromosomes SBI-02, SBI-03, SBI-04 and SBI-10. In the third population, we identified one major fertility restorer locus on chromosome SBI-02, Rf₂, explaining 31% of the phenotypic variation. Pentatricopeptide repeat genes in the Rf₂ QTL region were sequenced, and we detected in Sobic.002G057050 a missense mutation in the first exon, explaining 81% of the phenotypic variation in a F_2:3 population and clearly separating B- from R-lines. The KASP marker developed from this mutation stands as a promising tool for routine use in WA breeding programs.

In Silico Identification of Candidate Genes for Fertility Restoration in Cytoplasmic Male Sterile Perennial Ryegrass (Lolium perenne L.)

Perennial ryegrass (Lolium perenne L.) is widely used for forage production in both permanent and temporary grassland systems. To increase yields in perennial ryegrass, recent breeding efforts have been focused on strategies to more efficiently exploit heterosis by hybrid breeding. Cytoplasmic male sterility (CMS) is a widely applied mechanism to control pollination for commercial hybrid seed production and although CMS systems have been identified in perennial ryegrass, they are yet to be fully characterized. Here, we present a bioinformatics pipeline for efficient identification of candidate restorer of fertility (Rf) genes for CMS. From a high-quality draft of the perennial ryegrass genome, 373 pentatricopeptide repeat (PPR) genes were identified and classified, further identifying 25 restorer of fertility-like PPR (RFL) genes through a combination of DNA sequence clustering and comparison to known Rf genes. This extensive gene family was targeted as the majority of Rf genes in higher plants are RFL genes. These RFL genes were further investigated by phylogenetic analyses, identifying three groups of perennial ryegrass RFLs. These three groups likely represent genomic regions of active RFL generation and identify the probable location of perennial ryegrass PPR-Rf genes. This pipeline allows for the identification of candidate PPR-Rf genes from genomic sequence data and can be used in any plant species. Functional markers for PPR-Rf genes will facilitate map-based cloning of Rf genes and enable the use of CMS as an efficient tool to control pollination for hybrid crop production.

Morphological Characterization of a New and Easily Recognizable Nuclear Male Sterile Mutant of Sorghum (Sorghum bicolor)

Sorghum (Sorghum bicolor L. Moench) is one of the most important grain crops in the world. The nuclear male sterility (NMS) trait, which is caused by mutations on the nuclear gene, is valuable for hybrid breeding and genetic studies. Several NMS mutants have been reported previously, but none of them were well characterized. Here, we present our detailed morphological characterization of a new and easily recognizable NMS sorghum mutant male sterile 8 (ms8) isolated from an elite inbred BTx623 mutagenized by ethyl methane sulfonate (EMS). Our results show that the ms8 mutant phenotype was caused by a mutation on a single recessive nuclear gene that is different from all available NMS loci reported in sorghum. In fertile sorghum plants, yellow anthers appeared first during anthesis, while in the ms8 mutant, white hairy stigma emerged first and only small white anthers were observed, making ms8 plants easily recognizable when flowering. The ovary development and seed production after manual pollination are normal in the ms8 mutant, indicating it is female fertile and male sterile only. We found that ms8 anthers did not produce pollen grains. Further analysis revealed that ms8 anthers were defective in tapetum development, which led to the arrest of pollen formation. As a stable male sterile mutant across different environments, greenhouses, and fields in different locations, the ms8 mutant could be a useful breeding tool. Moreover, ms8 might be an important for elucidating male gametophyte development in sorghum and other plants.

Evolutionary plasticity of restorer-of-fertility-like proteins in rice

Hybrid seed production in rice relies on cytoplasmic male sterility (CMS) induced by specific mitochondrial proteins, whose deleterious effects are suppressed by nuclear Restorer of Fertility (RF) genes. The majority of RF proteins belong to a specific clade of the RNA-binding pentatricopeptide repeat protein family. We have characterised 'restorer-of-fertility-like' (RFL) sequences from 13 Oryza genomes and the Brachypodium distachyon genome. The majority of the RFL sequences are found in genomic clusters located at two or three chromosomal loci with only a minor proportion being present as isolated genes. The RFL genomic cluster located on Oryza chromosome 10, the location of almost all known active rice RF genes, shows extreme variation in structure and gene content between species. We show evidence for homologous recombination events as an efficient mechanism for generating the huge repertoire of RNA sequence recognition motifs within RFL proteins and a major driver of RFL sequence evolution. The RFL sequences identified here will improve our understanding of the molecular basis of CMS and fertility restoration in plants and will accelerate the development of new breeding strategies.

A Gene Encoding Pentatricopeptide Repeat Protein Partially Restores Fertility in RT98-Type Cytoplasmic Male-Sterile Rice

Cytoplasmic male sterility (CMS) lines in rice, which have the cytoplasm of a wild species and the nuclear genome of cultivated rice, are of value for the study of genetic interactions between the mitochondrial and nuclear genomes. The RT98-type CMS line RT98A and the fertility restorer line RT98C carry the cytoplasm of the wild species Oryza rufipogon and the nuclear genome of the Taichung 65 cultivar (Oryza sativa L.). Based on a classical crossing experiment, fertility is reported to be restored gametophytically by the presence of a tentative single gene, designated Rf98, which is derived from the cytoplasm donor. Fine mapping of Rf98 revealed that at least two genes, which are closely positioned, are required for complete fertility restoration in RT98A. Here, we identified seven pentatricopeptide repeat (PPR) genes that are located within a 170 kb region as candidates for Rf98 Complementation tests revealed that the introduction of one of these PPR genes, PPR762, resulted in the partial recovery of fertility with a seed setting rate up to 9.3%. We conclude that PPR762 is an essential fertility restorer gene for RT98-type CMS. The low rate of seed setting suggested that some other genes near the Rf98 locus are also necessary for the full recovery of seed setting.

Cytoplasmic male sterility (CMS) in hybrid breeding in field crops

A comprehensive understanding of CMS/Rf system enabled by modern omics tools and technologies considerably improves our ability to harness hybrid technology for enhancing the productivity of field crops. Harnessing hybrid vigor or heterosis is a promising approach to tackle the current challenge of sustaining enhanced yield gains of field crops. In the context, cytoplasmic male sterility (CMS) owing to its heritable nature to manifest non-functional male gametophyte remains a cost-effective system to promote efficient hybrid seed production. The phenomenon of CMS stems from a complex interplay between maternally-inherited (mitochondrion) and bi-parental (nucleus) genomic elements. In recent years, attempts aimed to comprehend the sterility-inducing factors (orfs) and corresponding fertility determinants (Rf) in plants have greatly increased our access to candidate genomic segments and the cloned genes. To this end, novel insights obtained by applying state-of-the-art omics platforms have substantially enriched our understanding of cytoplasmic-nuclear communication. Concomitantly, molecular tools including DNA markers have been implicated in crop hybrid breeding in order to greatly expedite the progress. Here, we review the status of diverse sterility-inducing cytoplasms and associated Rf factors reported across different field crops along with exploring opportunities for integrating modern omics tools with CMS-based hybrid breeding.

The Propensity of Pentatricopeptide Repeat Genes to Evolve into Restorers of Cytoplasmic Male Sterility

Cytoplasmic male sterility (CMS) is a widespread phenotype in plants, which present a defect in the production of functional pollen. The male sterilizing factors usually consist of unusual genes or open reading frames encoded by the mitochondrial genome. CMS can be suppressed by specific nuclear genes called restorers of fertility (Rfs). In the majority of cases, Rf genes produce proteins that act directly on the CMS conferring mitochondrial transcripts by binding them specifically and promoting processing events. In this review, we explore the wide array of mechanisms guiding fertility restoration. PPR proteins represent the most frequent protein class among identified Rfs and they exhibit ideal characteristics to evolve into restorer of fertility when the mechanism of restoration implies a post-transcriptional action. Here, we review the literature that highlights those characteristics and help explain why PPR proteins are ideal for the roles they play as restorers of fertility.

Molecular Breeding of Sorghum bicolor, A Novel Energy Crop

Currently, molecular breeding is regarded as an important tool for the improvement of many crop species. However, in sorghum, recently heralded as an important bioenergy crop, progress in this field has been relatively slow and limited. In this review, we present existing efforts targeted at genetic characterization of sorghum mutants. We also comprehensively review the different attempts made toward the isolation of genes involved in agronomically important traits, including the dissection of some sorghum quantitative trait loci (QTLs). We also explore the current status of the use of transgenic techniques in sorghum, which should be crucial for advancing sorghum molecular breeding. Through this report, we provide a useful benchmark to help assess how much more sorghum genomics and molecular breeding could be improved.

Inheritance and molecular mapping of Rf6 locus with pollen fertility restoration ability on A1 and A2 cytoplasms in sorghum

Of the several male sterility cytoplasms available as an alternative to the widely exploited A1 (milo) cytoplasm in sorghum, A2 is more suitable for commercial exploitation. Diversification of genetic and cytoplasmic base of hybrids involving A2 cytoplasm necessitates mapping of fertility restorer (Rf) genes for use in marker-assisted restorer development. We mapped a major male fertility restoration locus on sorghum chromosome 4 tightly linked with SSR markers, SB2387 and SB2388. This new fertility locus, Rf6, was able to restore male fertility on both A1 and A2 cytoplasms. Analysis of the genomic region around the Rf6 locus identified six genes including a pentatricopeptide repeat (PPR) gene, Sobic.004G004100. With its similar restoration ability to Rf1, Rf2 and Rf5 loci in sorghum, it is most likely that the Rf6 is a member of the PPR gene family, and the PPR gene Sobic.004G004100 could be a candidate for fertility restoration on A1 and A2 cytoplasms.

Genetic differentiation analysis for the identification of complementary parental pools for sorghum hybrid breeding in Ethiopia

The potential for exploiting heterosis for sorghum hybrid production in Ethiopia with improved local adaptation and farmers preferences has been investigated and populations suitable for initial hybrid development have been identified. Hybrids in sorghum have demonstrated increased productivity and stability of performance in the developed world. In Ethiopia, the uptake of hybrid sorghum has been limited to date, primarily due to poor adaptation and absence of farmer's preferred traits in existing hybrids. This study aimed to identify complementary parental pools to develop locally adapted hybrids, through an analysis of whole genome variability of 184 locally adapted genotypes and introduced hybrid parents (R and B). Genetic variability was assessed using genetic distance, model-based STRUCTURE analysis and pair-wise comparison of groups. We observed a high degree of genetic similarity between the Ethiopian improved inbred genotypes and a subset of landraces adapted to lowland agro-ecology with the introduced R lines. This coupled with the genetic differentiation from existing B lines, indicated that these locally adapted genotype groups are expected to have similar patterns of heterotic expression as observed between introduced R and B line pools. Additionally, the hybrids derived from these locally adapted genotypes will have the benefit of containing farmers preferred traits. The groups most divergent from introduced B lines were the Ethiopian landraces adapted to highland and intermediate agro-ecologies and a subset of lowland-adapted genotypes, indicating the potential for increased heterotic response of their hybrids. However, these groups were also differentiated from the R lines, and hence are different from the existing complementary heterotic pools. This suggests that although these groups could provide highly divergent parental pools, further research is required to investigate the extent of heterosis and their hybrid performance.

Mitochondrion role in molecular basis of cytoplasmic male sterility

Cytoplasmic male sterility and its fertility restoration via nuclear genes offer the possibility to understand the role of mitochondria during microsporogenesis. In most cases rearrangements in the mitochondrial DNA involving known mitochondrial genes as well as unknown sequences result in the creation of new chimeric open reading frames, which encode proteins containing transmembrane domains. So far, most of the CMS systems have been characterized via restriction fragment polymorphisms followed by transcript analysis. However, whole mitochondrial genome sequence analyses comparing male sterile and fertile cytoplasm open options for deeper insights into mitochondrial genome rearrangements. We more and more start to unravel how mitochondria are involved in triggering death of the male reproductive organs. Reduced levels of ATP accompanied by increased concentrations of reactive oxygen species, which are produced more under conditions of mitochondrial dysfunction, seem to play a major role in the fate of pollen production. Nuclear genes, so called restorer-of-fertility are able to restore the male fertility. Fertility restoration can occur via pentatricopeptide repeat (PPR) proteins or via different mechanisms involving non-PPR proteins.

The Rf and Rf-like PPR in higher plants, a fast-evolving subclass of PPR genes

In the last years, a number of nuclear genes restoring cytoplasmic male sterility (CMS) have been cloned in various crop species. The majority of these genes have been shown to encode pentatricopeptide repeat proteins (PPR) that act by specifically suppressing the expression of sterility-causing mitochondrial transcripts. Functional analysis of these proteins has indicated that the inhibitory effects of restoring PPR (Rf-PPR) proteins involve various mechanisms, including RNA cleavage, RNA destabilization, or translation inhibition. Cross-species sequence comparison of PPR protein complements revealed that most plant genomes encode 10-30 Rf-like (RFL) proteins sharing high-sequence similarity with the identified Rf-PPRs from crops. Evolutionary analyses further showed that they constitute a monophyletic group apart in the PPR family, with peculiar evolution dynamic and constraints. Here we review recent data on RF-PPRs and present the latest discoveries on the RFL family, with prospects on the functionality and evolution of this peculiar subclass of PPR.

An evolutionarily conserved mitochondrial orf108 is associated with cytoplasmic male sterility in different alloplasmic lines of Brassica juncea and induces male sterility in transgenic Arabidopsis thaliana

Nuclear-mitochondrial gene interactions governing cytoplasmic male sterility (CMS) in angiosperms have been found to be unique to each system. Fertility restoration of three diverse alloplasmic CMS lines of Brassica juncea by a line carrying the fertility-restorer gene introgressed from Moricandia arvensis prompted this investigation to examine the molecular basis of CMS in these lines. Since previous studies had found altered atpA transcription associated with CMS in these lines, the atpA genes and transcripts of CMS, fertility-restored, and euplasmic lines were cloned and compared. atpA coding and downstream sequences were conserved among CMS and euplasmic lines but major differences were found in the 5' flanking sequences of atpA. A unique open reading frame (ORF), orf108, co-transcribed with atpA, was found in male sterile flowers of CMS lines carrying mitochondrial genomes of Diplotaxis berthautii, D. catholica, or D. erucoides. In presence of the restorer gene, the bicistronic orf108-atpA transcript was cleaved within orf108 to yield a monocistronic atpA transcript. Transgenic expression of orf108 with anther-specific Atprx18 promoter in Arabidopsis thaliana gave 50% pollen sterility, indicating that Orf108 is lethal at the gametophytic stage. Further, lack of transmission of orf108 to the progeny showed for the first time that mitochondrial ORFs could also cause female sterility. orf108 was found to be widely distributed among wild relatives of Brassica, indicating its ancient origin. This is the first report that shows that CMS lines of different origin and morphology could share common molecular basis. The gametic lethality of Orf108 offers a novel opportunity for transgene containment.