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.     

Genetic determination of male sterility in gynodioecious Silene nutans

Gynodioecy, the coexistence of female and hermaphrodite plants within a species, is often under nuclear–cytoplasmic sex determination, involving cytoplasmic male sterility (CMS) genes and nuclear restorers. A good knowledge of CMS and restorer polymorphism is essential for understanding the evolution and maintenance of gynodioecy, but reciprocal crossing studies remain scarce. Although mitochondrial diversity has been studied in a few gynodioecious species, the relationship between mitotype diversity and CMS status is poorly known. From a French sample of Silene nutans, a gynodioecious species whose sex determination remains unknown, we chose the four most divergent mitotypes that we had sampled at the cytochrome b gene and tested by reciprocal crosses whether they carry distinct CMS genes. We show that gynodioecy in S. nutans is under nuclear–cytoplasmic control, with at least two different CMSs and up to four restorers with epistatic interactions. Female occurrence and frequency were highly dependent on the mitotype, suggesting that the level of restoration varies greatly among CMSs. Two of the mitotypes, which have broad geographic distributions, represent different CMSs and are very unequally restored. We discuss the dynamics of gynodioecy at the large-scale meta-population level.     

High-resolution mapping of the Brassica napus Rfp restorer locus using Arabidopsis-derived molecular markers

The two forms of cytoplasmic male sterility (CMS) native to the oilseed rape or canola species Brassica napus, nap and pol, have novel features that may provide insight into the molecular mechanisms through which CMS/nuclear restorer systems evolve. One such feature is the finding that the distinct nuclear restorer genes for the two systems represent different alleles or haplotypes of the same nuclear locus. Improved understanding of how these systems have evolved will require molecular cloning and characterization of this novel locus. We have employed an approach that exploits the regional co-linearity between the Arabidopsis and Brassica genomes to construct a high-resolution genetic map of the nuclear restorer for the pol system, Rfp. Specifically, Arabidopsis-derived sequences have been used as a set of ordered RFLP probes to localize Rfp to a region of the B. napus genome equivalent to a 115 kb interval on Arabidopsis chromosome 1. Based on the known relationship of physical distances between orthologous segments of Arabidopsis and Brassica chromosomes, it is anticipated that the B. napus restorer locus is now mapped to sufficient resolution to permit its isolation and characterization.     

Molecular mapping of a male fertility restorer locus of Brassica oleracea using expressed sequence tag-based single nucleotide polymorphism markers and analysis of a syntenic region in Arabidopsis thaliana for identification of genes encoding pentatricopeptide repeat proteins

An F₂ population was developed from a cross between a mur-cytoplasmic male sterile broccoli line and a restorer Chinese kale line. Phenotypic analysis of F₂ plants indicated that the pollen fertility is controlled by two genes and segregated in a duplicate gene interaction mode with a ratio of 15:1. A total of 236 single nucleotide polymorphism (SNP) markers were developed utilizing 1,448 primers designed for production of expressed sequence tag (EST)-SNP markers of Raphanus sativus and analyzed by the dot-blot technique in 205 F₂ individuals. A linkage map was constructed with a total of 142 markers and these markers were assigned to nine linkage groups together with simple sequence repeat markers mapped previously on the published linkage maps of Brassica oleracea. The linkage map spanned 909 cM with an average marker distance of 6.4 cM. A fertility restorer locus (Rfm1) was mapped on LG1, corresponding to chromosome 3, along with a flower color locus at a distance of 25 cM. SNP markers flanking the Rfm1 locus were BoCL2642s at a distance of 2.5 cM on one side and BoCL2901s at a distance of 7.5 cM on the other side. All the SNP markers showed homology with Arabidopsis thaliana and Brassica rapa genome sequences. Three pentatricopeptide repeat genes of the P-subfamily, particularly expressed in buds of the restorer line, were identified and these genes could be potential candidate fertility restorer genes.     

普通小麦D2型CMS系的育性基因及其遗传特性

通过研究普通小麦D^2型CMS－育性恢复体系中育性基因的种类及其遗传特性。结果表明：（1）D^2型不育系具有较好的不育性保持与恢复特征,在一般的普通小麦品种（系）中具有广泛的恢复（基因）源、可恢复度高（恢复度超过50％的品种或品质占到33．61％）,也能较容易地转育出新的不育系（完全保持不育性的品种或品系占到25．21％）,这一特征明显优于现有T、K、V型等不育系。（2）D^2型不育系的不育性受核内不育基因和抑制基因控制,相应的核基因型分为Al（不育基因）、A2（不育基因＋抑制基因）两类;恢复纱的恢复性受核内主效恢复基因、微效恢复基因和抑制基因控制,相应的核基因型分为C1（主效恢复基因）、C2(驻效恢复基因＋微效恢复基因）、C3（微效恢复基因）、C4（主效恢复基因＋抑制基因）、C5（主效恢复基因＋微效恢复基因＋抑制基因）、C6（微效恢复基因＋抑制基因）6种。环境条件的变化对育性基因、尤其是微效恢复基因和抑制基因的遗传效应有不同程度的影响。D^2型不育有效杂交组合的模型为：A1＋C1`A1 C2、A2＋C2。（3）D^2型不育系等位恢复基因的遗传表现为不完全显性,非等位恢复基因的遗传表现出积效应,这正是强恢复系德育的理论依据之一。     

Development of a candidate gene marker for Rf 1 based on a PPR gene in cytoplasmic male sterile CMS-D2 Upland cotton

The cytoplasmic male sterility (CMS) system is convenient and efficient for hybrid seed production in Upland cotton (Gossypium hirsutum L.). However, it has not been widely used because of limited restorer lines carrying the restorer gene Rf ₁ in the CMS-D2 system. In this study, the fertility segregation in a backcross (BC8F1) population of 409 individuals and an F2 population of 695 plants confirmed that the fertility restoration was determined by one dominant restorer gene (Rf ₁ ). A sequence alignment showed that 13 Rf ₁ -linked simple sequence repeat marker sequences were distributed on nine scaffolds of chromosome 9 in the sequenced D5 genome of G. raimondii Ulbrich. Ten pentotricopeptide repeat (PPR)-like genes were identified on two scaffolds, including Scaffold 333 where nine PPR-like genes were clustered in a region of about 160 kb. Among them, PPR-like gene Cotton_D_gene_10013437 was identified as the candidate for the Rf ₁ gene through a comparative sequence analysis of the homologous gene among sterile (A), maintainer (B) and restorer (R) lines, and co-segregation analysis. Compared with the non-restoring lines, the restorer had a 9-nucleotide (nt) insertion and a single nucleotide polymorphism (SNP) 8 nt upstream of the insertion at the 3′ untranslated regions (3′ UTRs) in this gene. A cleaved amplified polymorphic sequence (CAPS) marker named CAPS-R was developed from the SNP site using the restriction enzyme DraI, and was further used to track the restorer gene and its homozygous or heterozygous status in molecular breeding for restorer lines. A marker-assisted selection system using the Rf ₁ -specific CAPS-R marker and a CMS-D2 cytoplasm-specific SCAR marker was established to distinguish the three-line hybrids from other genotypes.     

Mapping of genes for male-fertility restoration in ’Pampa’ CMS winter rye (Secale cereale L.)

Hybrid rye breeding and seed production is based on the cytoplasmic male sterility (CMS)-inducing Pampa (P)-cytoplasm. For restoring male fertility in the hybrids, dominant, nuclear restorer genes are necessary. However, current pollinator lines are only partial restorers. Effective restorers were recently detected in the German inbred line L18 and in materials originating from the Argentinian rye cultivar Pico Gentario and an Iranian primitive rye accession called IRAN IX. F₂ populations were developed for each of these three restorer sources to map the responsible genes by means of RFLP (restriction fragment length polymorphism) markers. For this purpose, homo- and heterologous DNA probes were used leading to 101 polymorphic marker loci in total. For phenotypic evaluation, 100 to 134 randomly chosen plants from each of the populations were cloned and grown at two or three locations with two plants each. Segregation ratios of pollen fertility in the F₂ populations with L18 and IRAN IX were in accordance with a monogenic dominant inheritance. The segregation pattern for Pico Gentario indicated complementary gene action. Major dominant restorer genes were detected on chromosomes 1RS (L18) and 4RL (Pico Gentario, IRAN IX). The gene on 1RS explained 54% of the phenotypic variation and that on 4 RL 59% and 68% in the Pico Gentario and IRAN IX populations, respectively. Additionally, three minor genes from L18 were identified on chromosomes 3RL, 4RL and 5R. In the Pico Gentario population, a dominant modifier gene contributed by the female parent was found on chromosome 6R. This gene significantly enhanced the expression of the major restorer gene but on its own was not able to restore any degree of fertility. The map-distances between the major restorer loci and at least one flanking marker were small in all three F₂ populations (5–6 cM). In Pico Gentario an unfavorable linkage exists between the major restorer gene and a QTL for plant height. Since highly effective restorers are scarce in actual breeding populations, the major restorer genes detected on chromosomes 1 RS and 4RL should be introgressed into actual restorer lines. This is facilitated by using the closely linked molecular markers described. Received: 10 February 2000 / Accepted: 31 March 2000<@head-com-p1a.lf>Communicated by G. Wenzel     

Genetic mapping of nuclear fertility restorer genes for the ‘Polima’ cytoplasmic male sterility in canola (Brassica napus L.) using DNA markers

 Co-segregation of male fertility with DNA markers selected by targeted mapping approaches as being potentially linked to the Rfp1 restorer gene for the pol cytoplasmic male sterility (CMS) was analyzed using two canola (Brassica napus L.) backcross populations. Eleven DNA markers (10 RFLP markers and one RAPD marker) directly linked to the Rfp1 locus were identified. The linkage group containing the Rfp1 locus was found to correspond to B. napus linkage group 18 of Landry et al. (1991). A similar pattern of co-segregation between DNA markers and male fertility was observed in a backcross population segregating for the pol restorer gene Rfp2 from line ‘UM2383’; one RFLP marker, cRF1b, showed perfect linkage with both Rfp1 and Rfp2 and detected identical polymorphic fragments in both the Rfp1 and Rfp2 restorer lines. Our findings indicate that restoration of pol CMS is controlled by a single nuclear genetic locus on linkage group 18 and that Rfp1 and Rfp2 are likely allelic. Received: 2 October 1996 / Accepted: 20 December 1996     

Genetic analysis of male fertility restoration in wild cytoplasmic male sterility<Emphasis Type="Italic"> G</Emphasis> of beet

Cytoplasmic male sterility (CMS) has been used in the breeding of sugar beet for decades but is also more generally an important feature of the reproductive system in its wild relative, Beta vulgaris ssp. maritima. Among the several CMSs found in wild populations, the G CMS is a mitochondrial variant of the respiratory chain. The segregants derived from a cross between a restored plant and a female (male-sterile) plant on G cytoplasm exhibited three sexual phenotypic classes: female, hermaphrodite and intermediate. The pattern of segregation suggests a genetic inheritance with two loci in epistatic interaction. Nevertheless, it was possible to apply a bulk segregant analysis approach to search for AFLP and microsatellite markers linked to the restorer locus (RfG₁) which controls the capacity to produce pollen [female versus non female (i.e. intermediates and hermaphrodites)] in the segregating population. A linkage group was constructed with four AFLP markers and nine microsatellites, and a total size of 40 cM (Kosambi). The closest marker, a microsatellite, was totally linked to RfG1, which was also flanked by two AFLP markers delimiting a 5 cM window. This linkage group was identified as being chromosome VIII where neither of the restorer loci of the Owen CMS are located.Communicated by H.C. Becker     

SEX RATIO VARIATION AMONG GYNODIOECIOUS POPULATIONS OF SEA BEET: CAN IT BE EXPLAINED BY NEGATIVE FREQUENCY‐DEPENDENT SELECTION?

This study is devoted to assess sex ratio variation among 33 populations of the gynodioecious Beta vulgaris ssp. maritima in Brittany (France) and to explore the causes of this variation. We showed that three different CMS (cytoplasmic male sterility) cytotypes occurred in populations, but strongly differed for their frequencies and the frequency of their associated nuclear restorer alleles (which counteract the effect of CMS and restore male fertility). No correlation was found between CMS and restorer frequencies within populations, which has been previously interpreted as a result of stochasticity. However, neutral genetic variation did not indicate recent population bottlenecks in studied populations. Moreover, no significant correlation was found between female frequency or variance and current population size. Consequently, stochastic processes could not be the major cause of sex ratio variation. Alternatively, empirical estimations of the variation of females, CMS genes and nuclear restorer allele's frequencies were compared to theoretical predictions based on a frequency‐dependent selection model of gynodioecy. In particular, we showed that an absence of correlation between CMS and restorer frequencies could also occur without stochasticity. The large variation of sex ratio in Beta vulgaris could thus be explained by frequency‐dependent selection acting on CMS genes and restorer alleles.     

Molecular mapping and inheritance of restoration of fertility (<Emphasis Type="Italic">Rf</Emphasis>) in A4 hybrid system in pigeonpea (<Emphasis Type="Italic">Cajanus cajan</Emphasis> (L.) Millsp.)

Key message

We report molecular mapping and inheritance of restoration of fertility (Rf) in A4 hybrid system in pigeonpea. We have also developed PCR-based markers amenable to low-cost genotyping to identify fertility restorer lines.

Abstract

Commercial hybrids in pigeonpea are based on A4 cytoplasmic male sterility (CMS) system, and their fertility restoration is one of the key prerequisites for breeding. In this context, an effort has been made to understand the genetics and identify quantitative trait loci (QTL) associated with restoration of fertility (Rf). One F₂ population was developed by crossing CMS line (ICPA 2039) with fertility restorer line (ICPL 87119). Genetic analysis has shown involvement of two dominant genes in regulation of restoration of fertility. In parallel, the genotyping-by-sequencing (GBS) approach has generated ~?33 Gb data on the F₂ population. GBS data have provided 2457 single nucleotide polymorphism (SNPs) segregating across the mapping population. Based on these genotyping data, a genetic map has been developed with 306 SNPs covering a total length 981.9 cM. Further QTL analysis has provided the region flanked by S8_7664779 and S8_6474381 on CcLG08 harboured major QTL explained up to 28.5% phenotypic variation. Subsequently, sequence information within the major QTLs was compared between the maintainer and the restorer lines. From this sequence information, we have developed two PCR-based markers for identification of restorer lines from non-restorer lines and validated them on parental lines of hybrids as well as on another F₂ mapping population. The results obtained in this study are expected to enhance the efficiency of selection for the identification of restorer lines in hybrid breeding and may reduce traditional time-consuming phenotyping activities.

     

Molecular markers linked to the Rf2 fertility restorer gene in cotton.

Cytoplasmic male sterility (CMS) is a maternally inherited trait in which plants do not produce viable pollen. Fertility in plants with CMS can be recovered by nuclear restorer genes. Most restorer genes cloned so far are members of the pentatricopeptide repeat (PPR) protein family. The objective of our study was to use the CMS-D8 and restoration (Rf2) system of cotton (Gossypium hirsutum L.) to develop more DNA markers for the Rf2 gene. In a backcross population with 112 plants, segregation of male fertility was 1 fertile : 1 sterile. Three new RAPD markers were identified for Rf2, one of which was converted to a CAPS marker. In addition, 2 AFLP markers and 1 SSR marker were identified to be linked to the fertility restorer gene (Rf2). PPR motif primers were designed based on the conserved PPR motifs and used in combination with AFLP primers to test the mapping population, and 1 PPR-AFLP marker was identified. A linkage map with 9 flanking markers including 1 from a previous study was constructed.     

Genetic analysis of fertility-restorer genes in rice

Wild-abortive (WA), Honglian (HL) and Baro-II (BT) are three important cytoplasmic male sterility (CMS) types in rice. It is essential to investigate genetic mode and allelism of fertility restorer (Rf) genes and the relationship between Rf and CMS. Fertility of the all test-cross F₁ plants shows that the restorer-maintainer relationship is similar for HL-CMS and BT-CMS, while that is variance for WA-CMS and HL-CMS (or BT-CMS), respectively. Genetic analysis of Rf genes indicates that HL-or BT-CMS are controlled by single dominant Rf gene and WA-CMS is controlled by one or two pairs of dominant Rf genes, which reflects the characters of the gametophytic and sporophytic restoration CMS type. It is concluded that there are at least three Rf loci in different accessions with Rf genes for each CMS type.     

Mitochondrial Respiration Associated with Cytoplasmic Male Sterility in Pearl Millet

Respiratory activities in vegetative tissues and the isolated mitochondria of etiolated seedlings and reproductive tissues were studied in two cytoplasmic male sterile (CMS) lines having the same A, cytoplasm with different nuclear backgrounds of pearl millet along with their maintainer, restorer and restored lines. In addition, the assays of cytochrome c oxidase and succinate dehydrogenase were performed in isolated mitochondria. Cyanide-sensitive, cyanide-insensitive and total respirations in vegetative tissues and isolated mitochondria were lower in both the CMS lines than their respective maintainers, restorers and restored hybrids. Except in CMS lines, uptake of 0₂ during anther development increased from premeiotic to postmeiotic stage in all the lines. Consumption of 0₂,however, declined from meiotic to postmeiotic stage in the anthers of CMS lines. Pure mitochondrial preparations were obtained which were 92-94% intact. Enzymes, cytochrome c oxidase (complex IV) and succinate dehydrogenase (complex II) showed lower activities in both the CMS lines than their respective maintainer, restorer and restored hybrids. The CMS lines also displayed lower levels of nuclear encoded enzymes, viz., alternative oxidase and succinate dehydrogenase.     

Multiple CMS-restorer gene polymorphism in gynodioecious Plantago coronopus

The mode of inheritance of the male sterility trait is crucial for understanding the evolutionary dynamics of the sexual system gynodioecy, which is the co-occurrence of female and hermaphrodite plants in natural populations. Both cytoplasmic (CMS) and nuclear (restorer) genes are known to be involved. Theoretical models usually assume a limited number of CMS genes with each a single restorer gene, while reality is more complex. In this study, it is shown that in the gynodioecious species Plantago coronopus two new CMS-restorer polymorphisms exist in addition to the two that were already known, which means four CMS-restorer systems at the species level. Furthermore, three CMS types were shown to co-occur within a single population. All new CMS types showed a multilocus system for male fertility restoration, in which both recessive and dominant restorer alleles occur. Our finding of more than two co-occurring CMS-restorer systems each with multiple restorer genes raises the question how this complex of male sterility systems is maintained in natural populations.     

Marker-assisted identification of restorer gene(s) in iso-cytoplasmic restorer lines of WA cytoplasm in rice and assessment of their fertility restoration potential across environments

Iso-cytoplasmic restorers possess the same male sterile cytoplasm as the cytoplasmic male sterile (CMS) lines, thereby minimizing the potential cyto-nuclear conflict in the hybrids. Restoration of fertility of the wild abortive CMS is governed by two major genes namely, Rf3 and Rf4. Therefore, assessing the allelic status of these restorer genes in the iso-cytoplasmic restorers using molecular markers will not only help in estimating the efficiency of these genes either alone or in combination, in fertility restoration in the hybrids in different environments, but will also be useful in determining the efficacy of these markers. In the present study, the efficiency of molecular markers in identifying genotypes carrying restorer allele of the gene(s) Rf3 and Rf4, restoring male fertility of WA cytoplasm in rice was assessed in a set of 100 iso-cytoplasmic rice restorers using gene linked as well as candidate gene based markers. In order to validate the efficacy of markers in identifying the restorers, a sub-set of selected 25 iso-cytoplasmic rice restorers were crossed with four different cytoplasmic male sterile lines namely, IR 79156A, IR 58025A, Pusa 6A and RTN 12A, and the pollen and spikelet fertility of the F₁s were evaluated at three different locations. Marker analysis showed that Rf4 was the predominant fertility restorer gene in the iso-cytoplasmic restorers and Rf3 had a synergistic effect on fertility restoration. The efficiency of gene based markers, DRCG-RF4-14 and DRRM-RF3-10 for Rf4 (87%) and Rf3 (84%) genes was higher than respective gene-linked SSR markers RM6100 (80%) and RM3873 (82%). It is concluded that the gene based markers can be effectively used in identifying fertility restorer lines obviating the need for making crosses and evaluating the F₁s. Though gene based markers are more efficient, there is a need to identify functional polymorphisms which can provide 100% efficiency. Three iso-cytoplasmic restorers namely, PRR 300, PRR 363 and PRR 396 possessing both Rf4 and Rf3 genes and good fertility restoration have been identified which could be used further in hybrid rice breeding.     

A codominant molecular marker in linkage disequilibrium with a restorer-of-fertility gene (Ms) and its application in reevaluation of inheritance of fertility restoration in onions

To reveal the linkage relationship between the Ms locus, a restorer-of-fertility gene for cytoplasmic male-sterility (CMS) caused by CMS-S cytoplasm in onion (Allium cepa L.) and previously reported molecular markers linked to the Ms locus, 11 recombinants selected from 4,273 segregating plants originating from the cross between male-sterile maternal and male-fertile paternal lines were analyzed. Results showed that genotypes of a codominant marker, jnurf12, were perfectly matched with the male-fertility phenotypes in all recombinants, but that this marker was not applicable in diverse breeding lines due to multiple band patterns. For the development of more reliable markers, a 12-bp indel was identified from the sequences which were obtained by genome walking, and was used to develop a simple PCR marker which was designated jnurf13. When 104 diverse breeding lines containing CMS-S cytoplasm were analyzed with the jnurf13 marker, male-fertility phenotypes of all breeding lines were perfectly matched with marker genotypes. To our surprise, phenotypes of 153 breeding lines containing CMS-T-like cytoplasm were also matched with genotypes of the jnurf13 marker which was linked to the Ms locus for the CMS-S system. Furthermore, phenotypes of four F₂ populations containing CMS-T-like cytoplasm co-segregated perfectly with jnurf13 genotypes. Allelic segregation distortion was detected in two F₂ populations using the jnurf13 maker. The results of this study were in conflict with a previous model for inheritance of fertility restoration in the CMS-T system. Therefore, we proposed a new model based on the data analyzed with the jnurf13 marker, which was in linkage disequilibrium with restorer-of-fertility genes for both CMS systems.     

A Case of Gene Fragmentation in Plant Mitochondria Fixed by the Selection of a Compensatory Restorer of Fertility-Like PPR Gene

The high mutational load of mitochondrial genomes combined with their uniparental inheritance and high polyploidy favors the maintenance of deleterious mutations within populations. How cells compose and adapt to the accumulation of disadvantageous mitochondrial alleles remains unclear. Most harmful changes are likely corrected by purifying selection, however, the intimate collaboration between mitochondria- and nuclear-encoded gene products offers theoretical potential for compensatory adaptive changes. In plants, cytoplasmic male sterilities are known examples of nucleo-mitochondrial coadaptation situations in which nuclear-encoded restorer of fertility (Rf) genes evolve to counteract the effect of mitochondria-encoded cytoplasmic male sterility (CMS) genes and restore fertility. Most cloned Rfs belong to a small monophyletic group, comprising 26 pentatricopeptide repeat genes in Arabidopsis, called Rf-like (RFL). In this analysis, we explored the functional diversity of RFL genes in Arabidopsis and found that the RFL8 gene is not related to CMS suppression but essential for plant embryo development. In vitro-rescued rfl8 plantlets are deficient in the production of the mitochondrial heme–lyase complex. A complete ensemble of molecular and genetic analyses allowed us to demonstrate that the RFL8 gene has been selected to permit the translation of the mitochondrial ccmF_N2 gene encoding a heme–lyase complex subunit which derives from the split of the ccmF_N gene, specifically in Brassicaceae plants. This study represents thus a clear case of nuclear compensation to a lineage-specific mitochondrial genomic rearrangement in plants and demonstrates that RFL genes can be selected in response to other mitochondrial deviancies than CMS suppression.     

Molecular mapping of the fertility restorer gene for ms-CW-type cytoplasmic male sterility of rice

Cytoplasmic male sterility (CMS) of rice (Oryza sativa L.) was first reported using the cytoplasm of a Chinese wild rice, Oryza rufipogon Griff. strain W1. However, it was not possible to characterize this ms-CW-type CMS in more detail until a restorer line had been developed due to the lack of restorer genes among cultivars thus far tested. The breeding of a restorer line (W1-R) was eventually achieved by transferring the restorer gene(s) of W1 to a cultivar. We report here the characterization of the ms-CW pollen grains and mapping of the restorer gene for ms-CW-type CMS. Pollen grains of the male-sterile plants appeared to be normal and viable based on the fluorochromatic reaction test, but they did not germinate on normal stigmas. The 1:1 segregation of fertile and sterile plants in a BC₁F₁ population from a cross between W1-R and a maintainer line demonstrated that fertility restoration is controlled by a single gene. The fertile seed set of all the F₂ plants examined indicated that the fertility restoration functions gametophytically. We designated the fertility restorer gene Rfcw. Using cleaved amplified polymorphic sequence (CAPS) and simple sequence repeat (SSR) markers, we localized Rfcw to chromosome 4 with a genetic distance of 0.6 cM from the nearest SSR marker.