Hybrid Sterility Loci Research Articles

Whole genome de novo assemblies of three divergent strains of rice, Oryza sativa, document novel gene space of aus and indica

BackgroundThe use of high throughput genome-sequencing technologies has uncovered a large extent of structural variation in eukaryotic genomes that makes important contributions to genomic diversity and phenotypic variation. When the genomes of different strains of a given organism are compared, whole genome resequencing data are typically aligned to an established reference sequence. However, when the reference differs in significant structural ways from the individuals under study, the analysis is often incomplete or inaccurate.ResultsHere, we use rice as a model to demonstrate how improvements in sequencing and assembly technology allow rapid and inexpensive de novo assembly of next generation sequence data into high-quality assemblies that can be directly compared using whole genome alignment to provide an unbiased assessment. Using this approach, we are able to accurately assess the ‘pan-genome’ of three divergent rice varieties and document several megabases of each genome absent in the other two.ConclusionsMany of the genome-specific loci are annotated to contain genes, reflecting the potential for new biological properties that would be missed by standard reference-mapping approaches. We further provide a detailed analysis of several loci associated with agriculturally important traits, including the S5 hybrid sterility locus, the Sub1 submergence tolerance locus, the LRK gene cluster associated with improved yield, and the Pup1 cluster associated with phosphorus deficiency, illustrating the utility of our approach for biological discovery. All of the data and software are openly available to support further breeding and functional studies of rice and other species.Electronic supplementary materialThe online version of this article (doi:10.1186/s13059-014-0506-z) contains supplementary material, which is available to authorized users.

X Chromosome Control of Meiotic Chromosome Synapsis in Mouse Inter-Subspecific Hybrids

Hybrid sterility (HS) belongs to reproductive isolation barriers that safeguard the integrity of species in statu nascendi. Although hybrid sterility occurs almost universally among animal and plant species, most of our current knowledge comes from the classical genetic studies on Drosophila interspecific crosses or introgressions. With the house mouse subspecies Mus m. musculus and Mus m. domesticus as a model, new research tools have become available for studies of the molecular mechanisms and genetic networks underlying HS. Here we used QTL analysis and intersubspecific chromosome substitution strains to identify a 4.7 Mb critical region on Chromosome X (Chr X) harboring the Hstx2 HS locus, which causes asymmetrical spermatogenic arrest in reciprocal intersubspecific F1 hybrids. Subsequently, we mapped autosomal loci on Chrs 3, 9 and 13 that can abolish this asymmetry. Combination of immunofluorescent visualization of the proteins of synaptonemal complexes with whole-chromosome DNA FISH on pachytene spreads revealed that heterosubspecific, unlike consubspecific, homologous chromosomes are predisposed to asynapsis in F1 hybrid male and female meiosis. The asynapsis is under the trans- control of Hstx2 and Hst1/Prdm9 hybrid sterility genes in pachynemas of male but not female hybrids. The finding concurred with the fertility of intersubpecific F1 hybrid females homozygous for the Hstx2Mmm allele and resolved the apparent conflict with the dominance theory of Haldane's rule. We propose that meiotic asynapsis in intersubspecific hybrids is a consequence of cis-acting mismatch between homologous chromosomes modulated by the trans-acting Hstx2 and Prdm9 hybrid male sterility genes.

Whole genome de novo assemblies of three divergent strains of rice, Oryza sativa, document novel gene space of aus and indica.

The use of high throughput genome-sequencing technologies has uncovered a large extent of structural variation in eukaryotic genomes that makes important contributions to genomic diversity and phenotypic variation. When the genomes of different strains of a given organism are compared, whole genome resequencing data are typically aligned to an established reference sequence. However, when the reference differs in significant structural ways from the individuals under study, the analysis is often incomplete or inaccurate. Here, we use rice as a model to demonstrate how improvements in sequencing and assembly technology allow rapid and inexpensive de novo assembly of next generation sequence data into high-quality assemblies that can be directly compared using whole genome alignment to provide an unbiased assessment. Using this approach, we are able to accurately assess the "pan-genome" of three divergent rice varieties and document several megabases of each genome absent in the other two. Many of the genome-specific loci are annotated to contain genes, reflecting the potential for new biological properties that would be missed by standard reference-mapping approaches. We further provide a detailed analysis of several loci associated with agriculturally important traits, including the S5 hybrid sterility locus, the Sub1 submergence tolerance locus, the LRK gene cluster associated with improved yield, and the Pup1 cluster associated with phosphorus deficiency, illustrating the utility of our approach for biological discovery. All of the data and software are openly available to support further breeding and functional studies of rice and other species.

Two linked genes on rice chromosome 2 for F1 pollen sterility in a hybrid between Oryza sativa and O. glumaepatula.

Hybrid incompatibility plays an important role in establishment of post-zygotic reproductive isolation. To unveil genetic basis of hybrid incompatibilities between diverged species of genus Oryza AA genome species, we conducted genetic dissection of hybrid sterility loci, S22(t), which had been identified in backcross progeny derived from Oryza sativa ssp. japonica (recurrent parent) and South American wild rice O. glumaepatula near the end of the short arm of chromosome 2. The S22(t) region was found to be composed of two loci, designated S22A and S22B, that independently induce F1 pollen sterility. Pollen grains containing either of the sterile alleles (S22A-glums or S22B-glums) were sterile if produced on a heterozygous plant. No transmission of the S22A-glums allele via pollen was observed, whereas a low frequency of transmission of S22B-glums was observed. Cytological analysis showed that the sterile pollen grains caused by S22A could reach the bicellular or tricellular stage, and the nearly-sterile pollen grains caused by S22B could reach the tricellular stage. Our genetic analysis showed repulsion linkage effect is possible to induce strong reproductive barrier by high pollen sterility based on recombination value and transmission ratio of hybrid sterility gene to the progeny was influenced by frequency of competitors on fertilization.

DISSECTING THE GENETIC ARCHITECTURE OF F1HYBRID STERILITY IN HOUSE MICE

Hybrid sterility as a postzygotic reproductive isolation mechanism has been studied for over 80 years, yet the first identifications of hybrid sterility genes in Drosophila and mouse are quite recent. To study the genetic architecture of F(1) hybrid sterility between young subspecies of house mouse Mus m. domesticus and M. m. musculus, we conducted QTL analysis of a backcross between inbred strains representing these two subspecies and probed the role of individual chromosomes in hybrid sterility using the intersubspecific chromosome substitution strains. We provide direct evidence that the asymmetry in male infertility between reciprocal crosses is conferred by the middle region of M. m. musculus Chr X, thus excluding other potential candidates such as Y, imprinted genes, and mitochondrial DNA. QTL analysis identified strong hybrid sterility loci on Chr 17 and Chr X and predicted a set of interchangeable autosomal loci, a subset of which is sufficient to activate the Dobzhansky-Muller incompatibility of the strong loci. Overall, our results indicate the oligogenic nature of F(1) hybrid sterility, which should be amenable to reconstruction by proper combination of chromosome substitution strains. Such a prefabricated model system should help to uncover the gene networks and molecular mechanisms underlying hybrid sterility.

A major locus qS12, located in a duplicated segment of chromosome 12, causes spikelet sterility in an indica-japonica rice hybrid

Chromosome segment duplications are integral in genome evolution by providing a source for the origin of new genes. In the rice genome, besides an ancient polyploidy event known in the rice common ancestor, it had been identified that there was a special segmental duplication involving chromosomes 11 and 12, but the biological role of this duplication remains unknown. In this study, by using a set of chromosome segment substitution lines (CSSLs) and near isogenic lines (NILs) derived from the indica cultivar 9311 and japonica cultivar Nipponbare, a major QTL (qS12) resulting in hybrid male sterility was mapped within ~400 kb region adjacent to the special duplicated segment on the short arm of chromosome 12. Compared to the japonica cultivar Nipponbare, the two sides of the qS12 candidate region were inverted in the indica cultivar 9311. Among 47 of the 111 rice genotypes evaluated by molecular markers, the inverted sides were detected, and found completely homologous to indica cultivar 9311. These results suggested that the two inverted sides protect the sequence in the qS12 regions from recombination. On the short-arm of chromosome 12, two QTLs S-e and S25, in addition to qS12, were previously detected as a distinct segregation distortion and pollen semi-sterility loci. We propose these three hybrid sterility loci are the same locus, and the duplicated segment on chromosome 12 may play a prominent role in diversification, i.e., sub-speciation of cultivated rice.

A new gene controlling hybrid sterility in rice (Oryza sativa L.)

Hybrids between rice subspecies indica and japonica display strong heterosis. However, semi-sterility of inter-subspecific hybrids between indica and japonica varieties is a major obstacle for application of hybrid vigor in rice production. Semi-sterility was previously ascribed to allelic interaction at a number of different loci, whereas, wide-compatibility varieties can overcome hybrid sterility. Variety Nekken 2, which is a source of wide compatibility genes, showed sterility when crossed to the Korean variety Yeong Pung. Genetic and cytological analyses revealed that the semi-sterility was caused by partial abortion of the embryo sac. Genome-wide analysis of the backcross population, Nekken 2/Yeong Pung//Nekken 2 identified two independent loci for hybrid sterility on chromosomes 1 and 12, explaining 18.99 and 18.03% of the phenotypic variance, respectively. To confirm this result, another population of the same backcross containing 216 individuals was tested at a different site in a different year. The locus on chromosome 12 was detected again. Based on the study, the stable QTL on chromosome 12 appeared to be different from previously reported genes for this trait, and was designated as S35(t).

Genetic dissection of a chromosomal region conferring hybrid sterility using multi-donors from Oryza glaberrima

A chromosomal region containing a hybrid sterility locus, S1, from Oryza glaberrima, located on the short arm of chromosome 6, was investigated in 15 BC7F1 and one BC3F2 populations. As a gamete eliminator, S1 induced both male and female gamete abortion and responded to typical, spherical, or empty pollen abortion when it was heterozygous. The gametes carrying S1a (Oryza sativa allele in S1) in the heterozygous S1 locus had been eliminated in previous studies. We found here that both male and female gametes carrying the S1a allele were killed incompletely in heterozygous (S1gS1a) plants, with 26.22% female and 9.06% male gametes with the S1a genotype remaining when the BC3F1 sterile plant produced male and female gametes, which fits with the hypothesis that the ratio of S1a gametes of the female that were killed is higher than that of the male gametes. In addition, in sterile BC6F1 plants with the heterozygous S1 locus, not only a majority of the female gametes carrying the S1a allele, but also a quite number of those carrying the S1g allele (O. glaberrima allele in S1) could not be transmitted to their offspring. This abnormal transmission of female gametes illustrates the complexity of the reproductive isolation mechanism caused by S1 and can not be explained by the ‘one locus, sporo-gametal interaction’ model alone. Interaction for female sterility between S1 and two other loci on chromosomes 1 and 4 that suppress the expression of S1 was identified. S1 is the main reproductive isolation locus when mining favorable traits/genes from O. glaberrima due to its ubiquity between these two species, and the development of bridge parents carrying S1 should be a useful method for overcoming or abating this obstacle between these two species. The chromosomal region of S1 may be the hot region of genetic diversity for speciation because several hybrid sterility loci between species, subspecies, and even intra-subspecies have been identified in the same region.

Analysis on the spectra and level of wide‐compatibility conferred by three neutral alleles in inter‐subspecific hybrids of rice (Oryza sativa L.) using near isogenic lines

AbstractWide‐compatibility varieties are a special class of rice germplasm that is able to produce fertile hybrids when crossed to either indica or japonica subspecies. Previous studies determined the f5 allele from ‘Dular’ (f5‐Du), f6 allele from ‘Dular’ (f6‐Du) and S5 allele from ‘02428’ (S5‐08) as neutral alleles conferring wide‐compatibility. However, the possible extent of the effect of the three neutral alleles has not been fully characterized because of the narrow range of the tester varieties used and the highly complex differentiation in Asian cultivated rice. In this study, we further developed the five near‐isogenic lines with higher recovery rates of the recurrent parent genome, and testcrossed to 14 japonica varieties, which have been widely used in rice breeding programmes in China. The results clearly revealed that all three neutral alleles exhibited substantial effects on spikelet fertility in most of the indica–japonica testcrosses, which indicated that these hybrid sterility loci have been extensively differentiated between indica and japonica varieties. The magnitudes of effects on spikelet fertility averaged over various crosses seem to be similar among the three neutral alleles, with f5‐Du, f6‐Du and S5‐08 alleles increasing spikelet fertility by 15.09%, 13.99% and 14.25%, respectively. The testcrosses involving f5‐Du allele generally showed much smaller variation in pollen fertility than others. The pyramiding lines with two neutral alleles showed a wider spectrum and a higher level of wide compatibility than others, whereas most of the increases in hybrid fertility couldn’t be simply explained by additive effects, suggesting the very complexity of wide compatibility and hybrid sterility. The indica–japonica hybrids involving restorer lines as one of their parents showed much higher pollen fertility (almost normal) and also higher spikelet fertility. The implications of the findings in rice breeding programmes are also discussed.

Advances in the understanding of inter-subspecific hybrid sterility and wide-compatibility in rice

Hybrid sterility is a major form of postzygotic reproductive isolation and frequently occurs in hybrids between divergent populations, such as the indica and japonica subspecies of Asian cultivated rice (Oryza sativa L.). It has been a major barrier for utilization of the strong heterosis expressed in hybrids between indica and japonica. A large number of loci for rice inter-subspecific hybrid sterility have been identified by genetic analysis. Cytological studies revealed that male and female gamete abortions and reduced affinity between the uniting gametes all occurred in indica-japonica hybrids, suggesting the complexity of the causes for inter-subspecific hybrid sterility. Two genes conditioning embryo-sac and pollen sterility respectively in indica-japonica hybrids have been cloned recently, providing opportunities for molecular characterization of the indica-japonica hybrid sterility and wide-compatibility. Future studies should aim at cloning more genes for indica-japonica hybrid sterility, characterizing the underlying molecular mechanism, and utilization of the findings for the development of inter-subspecific hybrids to increase rice productivity.

QTL analysis for hybrid sterility and plant height in interspecific populations derived from a wild rice relative, Oryza longistaminata

Hybrid sterility is a serious barrier in the utilization of wild rice for breeding, but little is known regarding hybrid sterility between the cultivated rice, Oryza sativa, and its wild relative, O. longistaminata. In order to understand further the nature of interspecific hybrid sterility, pollen and spikelet fertility were investigated in two BC7F2 populations derived from a semisterile individual of BC5F1 between Oryza sativa L. and O. longistaminata. One main-effect QTL for pollen and spikelet fertility qpsf6 was detected on the short arm of chromosome 6 close to RM587, around it favoring O. longistaminata allele was found. Comparing the position and effect with other studies indicated that this QTL coincides with the gamete eliminator, S1. It suggests that there exists an orthologous hybrid sterility locus that controls the reproduction barrier between O. sativa and its AA genome relatives. QTL mapping for plant height was also conducted in one of the BC7F2 populations. One QTL, qph1, was detected on the long arm of chromosomes 1 close to RM6333 and coincides with the semi-dwarf gene, sd-1. These new QTL information will increase the efficiency of cultivar development via interspecific hybridization involving O. longistaminata, and offset the stage for fine mapping of these QTL.

Hybrid male sterility in rice controlled by interaction between divergent alleles of two adjacent genes

Sterility is common in hybrids between divergent populations, such as the indica and japonica subspecies of Asian cultivated rice (Oryza sativa). Although multiple loci for plant hybrid sterility have been identified, it remains unknown how alleles of the loci interact at the molecular level. Here we show that a locus for indica-japonica hybrid male sterility, Sa, comprises two adjacent genes, SaM and SaF, encoding a small ubiquitin-like modifier E3 ligase-like protein and an F-box protein, respectively. Most indica cultivars contain a haplotype SaM(+)SaF(+), whereas all japonica cultivars have SaM(-)SaF(-) that diverged by nucleotide variations in wild rice. Male semi-sterility in this heterozygous complex locus is caused by abortion of pollen carrying SaM(-). This allele-specific gamete elimination results from a selective interaction of SaF(+) with SaM(-), a truncated protein, but not with SaM(+) because of the presence of an inhibitory domain, although SaM(+) is required for this male sterility. Lack of any one of the three alleles in recombinant plants does not produce male sterility. We propose a two-gene/three-component interaction model for this hybrid male sterility system. The findings have implications for overcoming male sterility in inter-subspecific hybrid rice breeding.

Fine location of theS-5locus responsible for wide compatibility in rice using SSR markers

Partial abortion of gametes possessing S-5j in S-5i/S-5j genotype at locus S-5 is responsible for hybrid sterility between indica and japonica subspecies in rice (Oryza sativa L.), while a single wide compatibility (WC) allele S-5n can restore normal hybrid fertility between the two groups. In this study, Pei’ai 64S, one of the most popular WC line widely used for subspecific hybrid rice breeding program in South China was studied for location of its S-5 locus. Twenty SSR (Simple Sequence Repeat) markers derived from Cornell SSR linkage map and 9 developed using sequences from GenBank database were employed to perform bulked segregant analysis of the mapping population derived from a three-way cross (Pei’ai 64S/T8//Akihikari) to tag fine location of the hybrid sterility locus, S-5. This S-5 locus was mapped on chromosome 6 approximately 0.2 cM from GXR6 and RM276 SSR markers. This tight linkage of the markers and the S-5 locus would be very useful for efficient marker-assisted selection for WC varieties and for map-based cloning of the gene.

Identification of a new hybrid sterility gene in rice (bi Oryza sativa L.)

Partial female gamete abortion which is revealed as hybrid sterility of panicles in most of indica/japonica crosses of rice (Oryza sativa L.) is due to an allelic interaction between indica-type allele S5-i and japonica-type allele S5-j at S5 locus on chromosome 6, while some varieties, such as Dular, have a neutral allele S5-n, and both the S5-n/S5-i and S5-n/S5-j genotypes are fertile. We report here that a new hybrid sterility locus was found in a hybrid of former Soviet Union japonica rice USSR5 and Chinese indica rice Guangjie 9. A back-crossed population (USSR5/Guangjie 9//USSR5) was made to analyze the spikelet fertility in the B1F1 population of 120 plants with 119 simple sequence repeat (SSR) markers on all the 12 chromosomes. As the result, two loci conferring the hybrid fertility were found, which acted independently of each other. Interestingly, the locus on chromosome 4 coincided with the previously identified S9, while the locus on chromosome 5 was distinct from all the previously reported ones. The new locus was thus designated as S31(t). Based on allelic interaction causing female gamete abortion, two alleles were found at S31(t), i.e. S31(t)-un in USSR5 and S31(t)-gi in Guangjie 9. An Aus variety, Dular, was assumed to have a neutral allele, S31(t)-n. The SSR markers of RM5586 and RM13 were considered to be useful for marker-aided transfer of the wide compatibility genes in the hybrid rice breeding.

Molecular Mapping of a Pollen Killer Gene S29(t) in Oryza Glaberrima and Co-Linear Analysis with S22 in O. Glumaepatula

Two species in genus Oryza, O. glaberrima and O. glumaepatula, are valuable and potential sources of useful genes of interest for rice improvement. However, the hybrid sterility between O. sativa and these two species is a main reproduction barrier when transferring the favorable traits/genes to mbox{O. sativa.} To overcome it, the nature of hybrid sterility should be understood further. The objective in the report is to map a new hybrid sterility gene as a Mendelian factor from O. glaberrima and analyze the co-linear of hybrid sterility S loci mbox{between} mbox{O. glaberrima} and mbox{O. glumaepatula} via comparative mapping approach. A BC2F2 population, derived from a single semi-sterility plant of BC2F1 of WAB56-104/ WAB450-11-1-2-P41-HB (WAB450-6) //WAB56-104///WAB56-104 was employed to map this pollen killer in O. glaberrima since WAB450-6 is a progeny of interspecific hybrid between O. sativa and O. glaberrima. A new pollen killer locus, S29(t) in O. glaberrima, was identified and mapped to interval between SSR marker RM7033 (1.1 cM) and RM7562 (1.3 cM) on rice chromosome 2. Comparative mapping indicated that S29(t) closely corresponded to S22 which is also a pollen killer gene in O. glumaepatula and is tightly linked with RFLP marker S910 on the short arm of rice chromosome 2. The good co-linear between S29(t) and S22 implied that there might exist common (orthologous) hybrid sterility loci controlled the reproduction barrier among AA genome species of genus Oryza, which will contribute significantly to our understanding of speciation and operation of hybrid sterility between O. sativa and its AA genome relatives.

Identification of simple sequence repeat markers for utilizing wide-compatibility genes in inter-subspecific hybrids in rice (Oryza sativa L.)

Although pronounced heterosis in inter-subspecific hybrids was known in rice for a long time, its utilization for hybrid rice breeding has been limited due to their hybrid sterility (HS). For the last two decades, however, a few inter-subspecific hybrids have been developed by incorporating wide-compatibility genes (WCG) that resolve HS, into parental lines of these inter-subspecific hybrids. For effective use of WCG, it is necessary to find convenient markers linked to WCG of practical importance. In this paper, initially a set of simple sequence repeat (SSR) markers in the vicinity of known WCG loci identified based on comparative linkage maps have been surveyed in a population derived from the three-way cross- IR36/Dular//Akihikari, where a known donor of WCG Dular was crossed to a representative indica and japonica cultivar. Of the five parental polymorphic markers, RM253 and RM276 were found to be closely linked to the WCG locus S5 at a distance of 3.0 and 2.8 cM, respectively. Later, loci for HS were examined in three F(2) populations derived from inter-subspecific crosses, with same set of SSR markers. The locus S8 was confirmed to have major influence on HS in the F(2 )population derived from CHMRF-1/Taichung65 since two SSR markers in its vicinity, RM412 and RM141, co-segregated with HS at a map distance of 7.6 and 4.8 cM, respectively. In the F(2) population derived from the cross BPT5204/Taipei309, three SSR markers in the vicinity of S5, RM50, RM276 and RM136 co-segregated with HS at a map distance of 4.2, 3.2 and 7.8 cM, respectively. In the third F(2 )population derived from Swarna/Taipei309, the SSR markers in the vicinity of S5, RM225, RM253, RM50, RM276 and RM136 were identified to co-segregate with HS at a map distance of 3.2, 2.6, 3.4, 2.6 and 6.6 cM, respectively. These results indicated a clear picture of WCG in Dular as well as the predominant role of HS alleles at S5 locus. The identified SSR markers are expected to be used for incorporation of WCG into parental lines in hybrid rice breeding to solve HS in inter-subspecific hybrids.

Signatures of Reproductive Isolation in Patterns of Single Nucleotide Diversity Across Inbred Strains of Mice

Reproductive isolation is often caused by the disruption of genic interactions that evolve in geographically separate populations. Identifying the genomic regions and genes involved in these interactions, known as "Dobzhansky-Muller incompatibilities," can be challenging but is facilitated by the wealth of genetic markers now available in model systems. In recent years, the complete genome sequence and thousands of single nucleotide polymorphisms (SNPs) from laboratory mice, which are largely genetic hybrids between Mus musculus and M. domesticus, have become available. Here, we use these resources to locate genomic regions that may underlie reproductive isolation between these two species. Using genotypes from 332 SNPs that differ between wild-derived strains of M. musculus and M. domesticus, we identified several physically unlinked SNP pairs that show exceptional gametic disequilibrium across the lab strains. Conspecific alleles were associated in a disproportionate number of these cases, consistent with the action of natural selection against hybrid gene combinations. As predicted by the Dobzhansky-Muller model, this bias was differentially attributable to locus pairs for which one hybrid genotype was missing. We assembled a list of potential Dobzhansky-Muller incompatibilities from locus pairs that showed extreme associations (only three gametic types) among conspecific alleles. Two SNPs in this list map near known hybrid sterility loci on chromosome 17 and the X chromosome, allowing us to nominate partners for disrupted interactions involving these genomic regions for the first time. Together, these results indicate that patterns produced by speciation between M. musculus and M. domesticus are visible in the genomes of lab strains of mice, underscoring the potential of these genetic model organisms for addressing general questions in evolutionary biology.

A new gene located on chromosome 2 causing hybrid sterility in a remote cross of rice

Abstract‘Ketan Nangka’, the donor of the wide compatibility gene (WCG) showed typical hybrid sterility when crossed to a landrace, ‘Bai Mi Fen’, of Yunnan province in China. A genome‐wide analysis was performed for a backcrossed population of ‘Ketan Nangka’/‘Bai Mi Fen’//‘Ketan Nangka’ using a total of 143 simple sequence repeat markers and an expressed sequence tagged marker to cover the entire rice linkage map. As a result, two independent loci were found to cause hybrid sterility via female gamete abortion. The locus on chromosome 4 may correspond to S9, but the other, on chromosome 2, was different from all the previously reported hybrid sterility loci and was designated as S29(t) following the hybrid sterility nomenclature. On the basis of allelic interaction which causes female gamete abortion, two alleles were found: S29kn(t) in ‘Ketan Nangka’ and S29bi(t) in ‘Bai Mi Fen’. In the heterozygote, S29kn(t)/S29bi(t), which was semi‐sterile, female gametes carrying S29bi(t) were aborted. An Aus variety from the Indian subcontinent, ‘Dular’, was found to have a neutral allele, S29n(t). Two molecular markers, RM185 and RM425, linked to S9 and S29(t), respectively, will be useful for marker‐aided transfer of WCGs in hybrid rice breeding.

The Origin of Weedy Rice Ludao in China Deduced by Genome Wide Analysis of Its Hybrid Sterility Genes

The authors report here a case of hybrid sterility between an indica cultivar, IR36 and a weedy strain, Ludao, which naturally grows in and around farms in Jiangsu province, China. A genome-wide analysis was performed with a backcross population of IR36/Ludao//IR36 using a total of 151 simple sequence repeat (SSR) markers covering the entire rice linkage map. As a result, two loci were found to induce independently the hybrid sterility via female gamete abortion. Of the two loci, the locus on chromosome 6 corresponded to S10 according to its chromosomal location, while the other one on chromosome 7 was different from all the previously reported hybrid sterility loci, and was designated as S30(t). Based on the allelic interaction which causes female gamete abortion, two alleles were identified: S30(t)-i in IR36 and S30(t)-j in Ludao. In the heterozygote, S30(t)-i/S30(t)-j, which was semi-sterile, female gametes carrying S30(t)-j were partially aborted. A javanica variety, Ketan Nangka, and an Aus variety, N22, carried the neutral allele, S30(t)-n. Thus, Ludao was found to harbor a hybrid sterility gene different from that of existing cultivars in China, as an evidence of its origin from an ancient wild rice variety.

Hst7: a male sterility mutation perturbing sperm motility, flagellar assembly, and mitochondrial sheath differentiation.

Hst7, a mouse hybrid sterility locus, has been mapped in close linkage to four other hybrid sterility loci, on proximal chromosome 17 within the t complex. When an allele (s) of Hst7 from the species Mus spretus is crossed into the Mus musculus domesticus (laboratory mouse) background, all male offspring are sterile. This occurs regardless of whether the Hst7 allele on the other chromosome 17 homolog is wild-type (+) or an allele (t) derived from the structurally variant homolog known as a t haplotype. Males of the Hst7 genotype s/+ produce sperm that, after release from the cauda epididymis, display moderate asthenospermia (straight line velocity = 49 +/- 4 microm/second, significantly lower than 102 +/- 7 microm/second for congenic wild-type controls) and normal morphology. However, males of the Hst7 genotype s/t produce sperm whose forward movement is below the detectable limit of the sperm motion analysis system. In addition, these sperm exhibit a variety of flagellar abnormalities, with about one third having normal heads attached to sacklike caudal regions. These sacks consist of membrane-delimited cytoplasm containing disorganized and/or misshapen axonemal elements. The remainder of the sperm from s/t mice have flagella with seemingly normal axonemes, although many exhibit enlarged areas of cytoplasm in their midpieces with extra layers of misaligned mitochondria. The s/t sperm mitochondria also display diffuse and vacuolated matrices reminiscent of meiotic germ cell and spermatid mitochondria. Observations of developing spermatids in the s/t testis reveal an unusual phenotype in which gaps of varying length occur in the mitochondrial wrapping of the midpiece. These data suggest that both the s and t alleles of Hst7 are defective alleles that contribute differentially to the severe asthenospermia phenotype and interact genetically to perturb flagellar development.