Sterile F1 Hybrids Research Articles

Little or no gene flow despite F1 hybrids at two interspecific contact zones.

Hybridization can create the selective force that promotes assortative mating but hybridization can also select for increased hybrid fitness. Gene flow resulting from hybridization can increase genetic diversity but also reduce distinctiveness. Thus the formation of hybrids has important implications for long‐term species coexistence. This study compares the interaction between the tree wētā Hemideina thoracica and its two neighboring species; H. crassidens and H. trewicki. We examined the ratio of parent and hybrid forms in natural areas of sympatry. Individuals with intermediate phenotype were confirmed as first generation hybrids using nine independent genetic markers. Evidence of gene flow from successful hybridization was sought from the distribution of morphological and genetic characters. Both species pairs appear to be largely retaining their own identity where they live in sympatry, each with a distinct karyotype. Hemideina thoracica and H. trewicki are probably reproductively isolated, with sterile F1 hybrids. This species pair shows evidence of niche differences with adult size and timing of maturity differing where Hemideina thoracica is sympatric with H. trewicki. In contrast, evidence of a low level of introgression was detected in phenotypes and genotypes where H. thoracica and H. crassidens are sympatric. We found no evidence of size divergence although color traits in combination with hind tibia spines reliably distinguish the two species. This species pair show a bimodal hybrid zone in the absence of assortative mating and possible sexual exclusion by H. thoracica males in the formation of F1 hybrids.

Two Tightly Linked Genes at the hsa1 Locus Cause Both F1 and F2 Hybrid Sterility in Rice

Molecular mechanisms of hybrid breakdown associated with sterility (F2 sterility) are poorly understood as compared with those of F1 hybrid sterility. Previously, we characterized three unlinked epistatic loci, hybrid sterility-a1 (hsa1), hsa2, and hsa3, responsible for the F2 sterility in a cross between Oryza sativa ssp. indica and japonica. In this study, we identified that the hsa1 locus contains two interacting genes, HSA1a and HSA1b, within a 30-kb region. HSA1a-j (japonica allele) encodes a highly conserved plant-specific domain of unknown function protein (DUF1618), whereas the indica allele (HSA1a-is) has two deletion mutations that cause disruption of domain structure. The second gene, HSA1b-is, encodes an uncharacterized protein with some similarity to a nucleotide-binding protein. Homozygous introgression of indica HSA1a-is–HSA1b-is alleles into japonica showed female gamete abortion at an early mitotic stage. The fact that the recombinant haplotype HSA1a-j–HSA1b-is caused semi-sterility in the heterozygous state with the HSA1a-is–HSA1b-is haplotype suggests that variation in the hsa1 locus is a possible cause of the wide-spectrum sterility barriers seen in F1 hybrids and successive generations in rice. We propose a simple genetic model to explain how a single causal mechanism can drive both F1 and F2 hybrid sterility.

Development of a new CMS system in pigeonpea utilizing crosses with Cajanus lanceolatus (WV Fitgz) van der Maesen

Cytoplasmic male sterility is an important biological tool which is now available to pigeonpea breeders to exploit heterosis/hybrid vigor. A variety of CMS systems have been developed when wild relatives of pigeonpea from different gene pools were crossed as the female parent with cultivated types as the male parent. This paper reports a second source of CMS developed by using the cultivated pigeonpea as the female parent and one of its wild relative Cajanus lanceolatus (WV Fitgz) van der Maesen as the pollen donor, as such the A5 CMS system derived from C. acutifolius. All the F1 hybrids were evaluated to confirm hybridity using 27 simple sequence repeat (SSR) markers. SSR marker analysis of parents provided 17 polymorphic markers from a total of 27 SSR markers used. Subsequently polymorphic SSRs were used to confirm the hybridity of the F1 plants. F1 hybrid plants were crossed with a range of pigeonpea cultivars to identify maintainers of male sterility. Morphology of the F1 and backcross generations, cytology of the sterile as well as fertile floral buds derived from the crosses between sterile F1 hybrids and unrelated pigeonpea cultivars were studied. An important observation made was that male sterility was a post meiotic process. Microsporogenesis was normal until the tetrad stage, but none of them formed pollen grains. Instead, they grouped together within the pollen mother cell wall and the tetrads did not separate into individual pollen grains.

Chromosomal rearrangements directly cause underdominant F1 pollen sterility in Mimulus lewisii-Mimulus cardinalis hybrids.

Chromosomal rearrangements can contribute to the evolution of postzygotic reproductive isolation directly, by disrupting meiosis in F1 hybrids, or indirectly, by suppressing recombination among genic incompatibilities. Because direct effects of rearrangements on fertility imply fitness costs during their spread, understanding the mechanism of F1 hybrid sterility is integral to reconstructing the role(s) of rearrangements in speciation. In hybrids between monkeyflowers Mimulus cardinalis and Mimulus lewisii, rearrangements contain all quantitative trait loci (QTLs) for both premating barriers and pollen sterility, suggesting that they may have facilitated speciation in this model system. We used artificial chromosome doubling and comparative mapping to test whether heterozygous rearrangements directly cause underdominant male sterility in M. lewisii-M. cardinalis hybrids. Consistent with a direct chromosomal basis for hybrid sterility, synthetic tetraploid F1 s showed highly restored fertility (83.4% pollen fertility) relative to diploids F1 s (36.0%). Additional mapping with Mimulus parishii-M. cardinalis and M. parishii-M. lewisii hybrids demonstrated that underdominant male sterility is caused by one M. lewisii specific and one M. cardinalis specific reciprocal translocation, but that inversions had no direct effects on fertility. We discuss the importance of translocations as causes of reproductive isolation, and consider models for how underdominant rearrangements spread and fix despite intrinsic fitness costs.

Domestication and Long-Distance Dissemination of Rice: A Revised Version

Historically, in 1930s two sub-species, indica and japonica, were proposed on the basis of sterility in F1 hybrids between them. Subsequently, the two types were classified by associations of a number of genetically independent traits. The characteristic associations of traits have been explained by a series of hybrid sterility or reproductive barriers, in which a duplicate recessive gene model was proposed to cause varietal differentiation. In 1990s, most of hybrid sterility genes were proved to be caused by an interaction of allelic genes at each locus. In a well-known case of hybrid sterility, the indica, japonica, and wide-compatibility type which gives fertile hybrids in its cross to indica and japonica type, are proved to contain an allele, S5-i, S5-j and S5-n, respectively at a locus on Chromosome 6. Those gametes having S5-j allele is found to be partially aborted in the hybrid genotypes of S5-i/ S5-j while no gamete abortion occurred in S5-i/S5-n and S5-j/S5-n genotype. Since then, the S5-n has been used in hybrid rice breeding to obtain fertile and vigorous hybrids between indica and japonica types. For the last decade the action of S5-n is found to be due to a deletion in its gene sequence, while functional enzymes are produced by other alleles. Thus, the earlier assumed role of hybrid sterility genes as to lead to varietal differentiation are no more supported. Today, the characteristic association of traits found in each of varietal groups is better explained by founder effects. Recently , genetic analyses have been advanced for the contrasting traits between indica and japonica group. For an instance, genetic bases for long slender grain of indica and short wide grain of japonica are analyzed, and the short wide type is found to be caused by a deletion in its gene sequence. Thus, the short wide grain is considered to be mutational origin during domestication. On the other hand, a large number of native cultivars of rice were surveyed with enzyme polymorphism in 1980s, and later with molecular markers. As the results, profound genetic diversity is found in the genetic background of rice and wild rice. As the results, some scientist proposed the idea of multiple independent domestications of rice. However, before reaching to such a conclusion, at least two factors, i.e., long-distance-dissemination of some genotypes and possibilities of introgression by local wild rice to primitive cultivars need to be examined. Taking the two factors as well as historical events into consideration, the author considered that perennial japonica cultivars were first domesticated in Changjiang river basin, and disseminated to East India through Assam or along Bengal Bay, meanwhile their genetic background were transformed under introgressions of local wild rice which resulted to form the indica types. Thus–formed indica types seem to be disseminated to Southeast Asia, for an instance to Champa, under the expansion of Hinduism. Later some of the genotypes were introduced into China. Recently, this explanation has been verified by extensive analyses of gene sequence for wild and cultivated rice.

A Novel Approach Identifying Hybrid Sterility QTL on the Autosomes of Drosophila simulans and D. mauritiana

When species interbreed, the hybrid offspring that are produced are often sterile. If only one hybrid sex is sterile, it is almost always the heterogametic (XY or ZW) sex. Taking this trend into account, the predominant model used to explain the genetic basis of F1 sterility involves a deleterious interaction between recessive sex-linked loci from one species and dominant autosomal loci from the other species. This model is difficult to evaluate, however, as only a handful of loci influencing interspecies hybrid sterility have been identified, and their autosomal genetic interactors have remained elusive. One hindrance to their identification has been the overwhelming effect of the sex chromosome in mapping studies, which could ‘mask’ the ability to accurately map autosomal factors. Here, we use a novel approach employing attached-X chromosomes to create reciprocal backcross interspecies hybrid males that have a non-recombinant sex chromosome and recombinant autosomes. The heritable variation in phenotype is thus solely caused by differences in the autosomes, thereby allowing us to accurately identify the number and location of autosomal sterility loci. In one direction of backcross, all males were sterile, indicating that sterility could be entirely induced by the sex chromosome complement in these males. In the other direction, we identified nine quantitative trait loci that account for a surprisingly large amount (56%) of the autosome-induced phenotypic variance in sterility, with a large contribution of autosome-autosome epistatic interactions. These loci are capable of acting dominantly, and thus could contribute to F1 hybrid sterility.

Interspecific hybridization betweenCajanus cajan(L.) Millsp. andC. lanceolatus(WV Fitgz) van der Maesen

Cultivated pigeonpea has a narrow genetic base. Wild relatives play an important role in the efforts to broaden its genetic base. In this report, we present a successful wide-cross between the cultivated pigeonpea andCajanus lanceolatus, a wild relative from the secondary gene pool, native to Australia, with desirable traits such as frost and drought resistance. A range of F1progeny were obtained and the resultant F1hybrid plants set mature pods and seeds. The hybrids had intermediate morphology, sharing the traits of both the parents. All the F1hybrids flowered profusely. Some of the hybrids were completely male sterile and some were partially fertile with pollen fertility ranging from 35 to 50 %. Meiotic analysis of the fertile F1hybrids revealed a high degree of meiotic chromosome pairing between the two parental genomes. Meiotic analysis of the sterile F1hybrids revealed that the breakdown of microsporogenesis occurred at the post-meiotic stage after the formation of tetrads. Fertile plants formed regular bivalents with normal disjunction, except for occasional asynchrony at meiotic II division.

IDENTIFICATION OF WIDE COMPATIBILITY VARIETIES IN SOME TROPICAL JAPONICA RICE

The F1 hybrid sterility in indica/japonica crosses is the major barrier in developing hybrid rice varieties between these two diverse germplasm. The sterility problem in japonica/indica hybrids can be overcome by using wide compatibility genes. The objective of this study was to identify wide compatibility varieties (WCVs) in some tropical japonica rice. Twenty five tropical japonica varieties as male parents were crossed with indica (IR64) and japonica (Akitakomachi) testers as female parents. The crosses were planted following a randomized complete block design with three replications. Varieties having average spikelet fertility of more than 70% with both the indica and japonica testers were rated as WCVs. Result from this study showed that six tropical japonica varieties were classified as WCVs, i.e., Cabacu, Grogol, Kencana Bali, Klemas, Lampung Lawer, and Napa. Hybrid sterility is caused by partial sterility of male and female gametes. The WCVs from the present study can be used in hybrid rice breeding program to solve hybrid sterility in indica/japonica hybrids.

IDENTIFICATION OF WIDE COMPATIBILITY VARIETIES IN SOME TROPICAL JAPONICA RICE

The F1 hybrid sterility in indica/japonica crosses is the major barrier in developing hybrid rice varieties between these two diverse germplasm. The sterility problem in japonica/indica hybrids can be overcome by using wide compatibility genes. The objective of this study was to identify wide compatibility varieties (WCVs) in some tropical japonica rice. Twenty five tropical japonica varieties as male parents were crossed with indica (IR64) and japonica (Akitakomachi) testers as female parents. The crosses were planted following a randomized complete block design with three replications. Varieties having average spikelet fertility of more than 70% with both the indica and japonica testers were rated as WCVs. Result from this study showed that six tropical japonica varieties were classified as WCVs, i.e., Cabacu, Grogol, Kencana Bali, Klemas, Lampung Lawer, and Napa. Hybrid sterility is caused by partial sterility of male and female gametes. The WCVs from the present study can be used in hybrid rice breeding program to solve hybrid sterility in indica/japonica hybrids.

<I>Carex viridistellata</I> sp. nov. (Cyperaceae), a New Cryptic Species from Prairie Fens of the Eastern United States

Divergence between evolutionary lineages is not always marked by the development of obvious species-specific characters, whether morphological, physiological, or ecological. Consequentially, extant biodiversity can easily be overlooked. These cryptic species are often not recognized until genetic data are in hand, as is the case for the novel taxon we describe here. Carex viridistellata in Carex section Ceratocystis is an endemic species restricted to calcareous wetlands of Michigan, Ohio, and Indiana, where it has previously been collected as Carex cryptolepis. Crosses between Carex viridistellata and Carex cryptolepis produce sterile F1 hybrids, and the two species are differentiated by a number of subtle morphological characters, as well as aspects of their respective ecologies. Phylogenetic analyses of nrDNA strongly indicate monophyly of Carex viridistellata and its sister species relationship with the North Carolina narrow endemic Carex lutea. Both species constitute a distinct lineage within a generally poorly resolved section Ceratocystis. This work highlights the broad importance of cryptic taxa, with implications for fields ranging from population genetics to conservation and restoration.

Inheritance of the fertility restoration and genotyping of rice lines at the restoring fertility (Rf) loci using molecular markers

The combination of cytoplasmic male sterility (CMS) in one parent and a restorer gene (Rf) to restore fertility in another are indispensable for the development of hybrid varieties. To genotype rice lines at the restoring fertility (Rf) loci, 38 lines were crossed with a sterile tester (rfrf) line. Pollen fertility test was performed to identify sterile and fertile F1 hybrids. Seven lines were identified as restorer lines. Then these F1 hybrids were self-pollinated to obtain F2 seeds, and at flowering stage pollen fertility test was performed to identify sterile and fertile individuals. Bulked segregant analysis (BSA) in F2 showed that fertility in rice WA system is controlled by more than two loci, one on the short arm of chromosome 1, one on the short arm of chromosome 10, one on the long arm of chromosome 10 and an unknown Rf gene in the rice genome. Results also show that lines IR28, Amol1 and Amol2 carry Rf4 gene linked with SSR marker RM171 on the long arm of chromosome 10, lines IR36 and IR60966 carry Rf3 gene linked with SSR marker RM1 on the short arm of chromosome 1, line IR62030 carries Rf5 gene on the short arm of chromosome 10, and finally line IR24 carries Rf4 gene on the long arm of chromosome 10 and an unknown Rf gene, respectively. The results suggested the use of the method deployed here to identify restorer lines and also allelic location of fertility restoration genes in rice lines.

Hybrid Sterility over Tens of Meters Between Ecotypes Adapted to Serpentine and Non-Serpentine Soils

The development of hybrid sterility is an important step in the process of speciation, however the role of adaptive evolution in triggering these postzygotic barriers is poorly understood. We show that, in the California endemic plant Collinsia sparsiflora, ecotypic adaptation to two distinct soil types is associated with the expression of intrinsic F1 hybrid sterility between ecotypes, over spatial scales of less than 1 km. First, we show that hybrids between soil-adapted ecotypes are less fertile than hybrids within soil ecotypes. Second, we show that between-ecotype postzygotic incompatibility is insensitive to soil growth environment, and can therefore operate under conditions relevant to both ecotypes in the wild. Third, we confirm there is little genetic differentiation between ecotypes using molecular markers, indicating that these postzygotic barriers are recently evolved. Finally, we explore specific soil attributes that might be the source of selective differentiation that confers hybrid sterility. Our results indicate that hybrid barriers are developing in response to strong adaptive differentiation between adjacent and very recently diverged lineages, despite likely ongoing gene exchange.

Irradiation of Adult Epiphyas postvittana (Lepidoptera: Tortricidae): Egg Sterility in Parental and F1 Generations

Adult Epiphyas postvittana Walker were irradiated using a Cobalt 60 source to determine the dose needed to achieve complete egg sterility of mated female moths, and egg sterility of female moths mated to F1 generation males. Adult male and female E. postvittana were irradiated at 100, 200, 250, and 300 Gy and their fertility (when crossed with normal moths) was compared with nonirradiated moths. Viable progeny (determined by egg hatch) were found at doses of 100 and 200 Gy, but very little at 250 and 300 Gy. In particular, there was no survival of female progeny into the F1 generation. Males irradiated at 250 and 300 Gy had very low egg eclosion rates (2.25 and 1.86% at 250 and 300 Gy, respectively) when mated with normal females. The F2 generation from those male progeny had a mean percent hatched of < 1.02%. Based on our results, a dose of 250-300 Gy is recommended for irradiation of E. postvittana adults used for sterile insect technique (SIT) if sterility of parental moths is the desired outcome. Our data also suggests that inclusion of F1 hybrid sterility rather than parental generation sterility into programs using the SIT may allow for doses lower than what we have reported, especially during initial phases of an eradication program where increase fitness of moths might be desirable. Further research is needed to verify the use of F1 hybrid sterility in light brown apple moth SIT programs.

Genetic Architecture of Male Sterility and Segregation Distortion in Drosophila pseudoobscura Bogota–USA Hybrids

Understanding the genetic basis of reproductive isolation between recently diverged species is a central problem in evolutionary genetics. Here, I present analyses of the genetic architecture underlying hybrid male sterility and segregation distortion between the Bogota and USA subspecies of Drosophila pseudoobscura. Previously, a single gene, Overdrive (Ovd), was shown to be necessary but not sufficient for both male sterility and segregation distortion in F(1) hybrids between these subspecies, requiring several interacting partner loci for full manifestation of hybrid phenomena. I map these partner loci separately on the Bogota X chromosome and USA autosomes using a combination of different mapping strategies. I find that hybrid sterility involves a single hybrid incompatibility of at least seven interacting partner genes that includes three large-effect loci. Segregation distortion involves three loci on the Bogota X chromosome and one locus on the autosomes. The genetic bases of hybrid sterility and segregation distortion are at least partially--but not completely--overlapping. My results lay the foundation for fine-mapping experiments to identify the complete set of genes that interact with Overdrive. While individual genes that cause hybrid sterility or inviability have been identified in a few cases, my analysis provides a comprehensive look at the genetic architecture of all components of a hybrid incompatibility underlying F(1) hybrid sterility. Such an analysis would likely be unfeasible for most species pairs due to their divergence time and emphasizes the importance of young species pairs such as the D. pseudoobscura subspecies studied here.

Reproductive Isolation between Two Populations ofSpodoptera frugiperda(Lepidoptera: Noctuidae) Collected in Corn and Rice Fields from Central Colombia

AbstractSpodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) is a Neotropical moth that has diverged into corn, Zea mays L., and rice, Oryza sativa L., host strains because these plants are their most frequently used hosts. The corn strain also has been found in cotton, Gossypium hirsutum L., and sorghum, Sorghum bicolor (L.) Moench, and the rice strain in small grasses and pasture grasses. Studies of the reproductive isolation between these two strains have provided ambiguous results from populations in the United States. In Colombia, we tested pre- and postzygotic isolation in these strains. Both strains showed postzygotic isolation for several life-history traits, including number of egg masses, number of larvae, number of females, pupal developmental time, female and male longevity, and female and male pupal weight. We observed a reduction of the number of hybrid females and a reduction in fertility in hybrids in S. frugiperda. These results suggest the possibility of Haldane's rule. Heterosis in the F1(2) and F2(1) generations was observed for number of larvae and adult longevity. This line presented a high standard deviation, suggesting instability in this cross. A possible effect of the X chromosome may explain the reduction in viability and sterility in F1 hybrids of host strains of S. frugiperda. No temporal isolation was observed between the corn and rice strains. Differences in longevity between corn and rice strains might be another form of temporal isolation between these strains, because differences in adulthood time might reduce the encounters between them and thus hybridization.

The evolution of reproductive isolation in a simultaneous hermaphrodite, the freshwater snail Physa

BackgroundThe cosmopolitan freshwater snail Physa acuta has recently found widespread use as a model organism for the study of mating systems and reproductive allocation. Mitochondrial DNA phylogenies suggest that Physa carolinae, recently described from the American southeast, is a sister species of P. acuta. The divergence of the acuta/carolinae ancestor from the more widespread P. pomilia appears to be somewhat older, and the split between a hypothetical acuta/carolinae/pomilia ancestor and P. gyrina appears older still.ResultsHere we report the results of no-choice mating experiments yielding no evidence of hybridization between gyrina and any of four other populations (pomilia, carolinae, Philadelphia acuta, or Charleston acuta), nor between pomilia and carolinae. Crosses between pomilia and both acuta populations yielded sterile F1 progeny with reduced viability, while crosses between carolinae and both acuta populations yielded sterile F1 hybrids of normal viability. A set of mate-choice tests also revealed significant sexual isolation between gyrina and all four of our other Physa populations, between pomilia and carolinae, and between pomilia and Charleston acuta, but not between pomilia and the acuta population from Philadelphia, nor between carolinae and either acuta population. These observations are consistent with the origin of hybrid sterility prior to hybrid inviability, and a hypothesis that speciation between pomilia and acuta may have been reinforced by selection for prezygotic reproductive isolation in sympatry.ConclusionsWe propose a two-factor model for the evolution of postzygotic reproductive incompatibility in this set of five Physa populations consistent with the Dobzhansky-Muller model of speciation, and a second two-factor model for the evolution of sexual incompatibility. Under these models, species trees may be said to correspond with gene trees in American populations of the freshwater snail, Physa.

Introgression of group 4 and 7 chromosomes of Ae. peregrina in wheat enhances grain iron and zinc density

Dietary deficiency of iron and zinc micronutrients affects more than two billion people worldwide. Breeding for micronutrient-dense crops is the most sustainable and cost-effective approach for alleviation of micronutrient malnutrition. Three accessions of Aegilops peregrina (Hack.) Maire & Weill (2n = 28, UPUPSPSP), selected for high grain iron and zinc concentration were crossed with Triticum aestivum L. cv. Chinese Spring (Ph I ). The sterile F1 hybrids were backcrossed with elite wheat cultivars to get fertile BC2F2 derivatives. Some of the fertile BC2F2 derivatives showed nearly 100% increase in grain iron and more than 200% increase in grain zinc concentration compared to the recipient wheat cultivars. The development of derivatives with significantly higher grain micronutrients, high thousand-grain weight and harvest index suggests that the enhanced micronutrient concentration is due to the distinct genetic system of Ae. peregrina and not to the concentration effect. Genomic in situ hybridization, comparison of introgressed chromosomes with the standard karyotype of Ae. peregrina and simple sequence repeat marker analysis revealed the introgression of 7SP chromosomes in five selected derivatives, 7UP in four, group 4 and 4SP in three and a translocated 5UP of Ae. peregrina in one of the selected derivatives. Molecular marker analysis using the introgressed chromosome markers indicated that two of the BC2F3 progenies were stabilized as disomic addition lines. It could, therefore, be concluded that the group 4 and 7 chromosomes of Ae. peregrina carry the genes for high grain iron and zinc concentration.

Widespread Over-Expression of the X Chromosome in Sterile F1 Hybrid Mice

The X chromosome often plays a central role in hybrid male sterility between species, but it is unclear if this reflects underlying regulatory incompatibilities. Here we combine phenotypic data with genome-wide expression data to directly associate aberrant expression patterns with hybrid male sterility between two species of mice. We used a reciprocal cross in which F1 males are sterile in one direction and fertile in the other direction, allowing us to associate expression differences with sterility rather than with other hybrid phenotypes. We found evidence of extensive over-expression of the X chromosome during spermatogenesis in sterile but not in fertile F1 hybrid males. Over-expression was most pronounced in genes that are normally expressed after meiosis, consistent with an X chromosome-wide disruption of expression during the later stages of spermatogenesis. This pattern was not a simple consequence of faster evolutionary divergence on the X chromosome, because X-linked expression was highly conserved between the two species. Thus, transcriptional regulation of the X chromosome during spermatogenesis appears particularly sensitive to evolutionary divergence between species. Overall, these data provide evidence for an underlying regulatory basis to reproductive isolation in house mice and underscore the importance of transcriptional regulation of the X chromosome to the evolution of hybrid male sterility.

An unedited 1.1 kb mitochondrial orfB gene transcript in the Wild Abortive Cytoplasmic Male Sterility (WA-CMS) system of Oryza sativa L. subsp. indica

BackgroundThe application of hybrid rice technology has significantly increased global rice production during the last three decades. Approximately 90% of the commercially cultivated rice hybrids have been derived through three-line breeding involving the use of WA-CMS lines. It is believed that during the 21st century, hybrid rice technology will make significant contributions to ensure global food security. This study examined the poorly understood molecular basis of the WA-CMS system in rice.ResultsRFLPs were detected for atp6 and orfB genes in sterile and fertile rice lines, with one copy of each in the mt-genome. The RNA profile was identical in both lines for atp6, but an additional longer orfB transcript was identified in sterile lines. 5' RACE analysis of the long orfB transcript revealed it was 370 bp longer than the normal transcript, with no indication it was chimeric when compared to the genomic DNA sequence. cDNA clones of the longer orfB transcript in sterile lines were sequenced and the transcript was determined unedited. Sterile lines were crossed with the restorer and maintainer lines, and fertile and sterile F1 hybrids were respectively generated. Both hybrids contained two types of orfB transcripts. However, the long transcript underwent editing in the fertile F1 hybrids and remained unedited in the sterile lines. Additionally, the editing of the 1.1 kb orfB transcript co-segregated with fertility restoring alleles in a segregating population of F2 progeny; and the presence of unedited long orfB transcripts was detected in the sterile plants from the F2 segregating population.ConclusionThis study helped to assign plausible operative factors responsible for male-sterility in the WA cytoplasm of rice. A new point of departure to dissect the mechanisms governing the CMS-WA system in rice has been identified, which can be applied to further harness the opportunities afforded by hybrid vigor in rice.

Mitochondrial gene in the nuclear genome induces reproductive barrier in rice

Hybrid incompatibility in F(1) hybrids or later generations is often observed as sterility or inviability. This incompatibility acts as postzygotic reproductive isolation, which results in the irreversible divergence of species. Here, we show that the reciprocal loss of duplicated genes encoding mitochondrial ribosomal protein L27 causes hybrid pollen sterility in F(1) hybrids of the cultivated rice Oryza sativa and its wild relative O. glumaepatula. Functional analysis revealed that this gene is essential for the later stage of pollen development, and distribution analysis suggests that the gene duplication occurred before the divergence of the AA genome species. On the basis of these results, we discuss the possible contribution of the "founder effect" in establishing this reproductive barrier.