Genetic analysis and gene mapping of a rice few-tillering mutant in early backcross populations ( Oryza sativa L.)

A rice mutant,G069, characteristic of few tiller numbers, was found in anther culture progeny from theF ₁ hybrid between anindica-japonica cross, Gui630×02428. The mutant has another two major features: delayed tillering development and yellowing apex and margin on the mature leaves. As a donor parent,G069 was further backcrossed with the recurrent parent,02428, for two turns to develop aBC ₂F₂ population. Genetic analysis in theBC ₂F₂ population showed that the traits of few-tillering and yellowing apex and margin on the mature leaves were controlled by one recessive gene. A pool of equally mixed genomic DNA, from few-tillering individual plants inBC ₂F₂, was constructed to screen polymorphism with simple sequence repeat (SSR) markers in comparison with the02428 genome. One SSR marker and three restriction fragment length polymorphism (RFLP) markers were found possibly linked with the recessive gene. By using these markers, the gene of few-tillering was mapped on chromosome 2 between RFLP marker C424 and S13984 with a genetic distance of 2.4 cM and 0.6 cM, respectively. The gene is designatedft1.     

Fine mapping and candidate gene analysis of <Emphasis Type="Italic">ptgms2-1</Emphasis>, the photoperiod-thermo-sensitive genic male sterile gene in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

Photoperiod-thermo-sensitive genic male sterile (PTGMS) rice exhibits a number of desirable traits for hybrid rice production. The cloning genes responsible for PTGMS and those elucidating male sterility mechanisms and reversibility to fertility would be of great significance to provide a foundation to develop new male sterile lines. Guangzhan63S, a PTGMS line, is one of the most widely used indica two-line hybrid rice breeding systems in China. In this study, genetic analysis based on F₂ and BC₁F₂ populations derived from a cross between Guangzhan63S and 1587, determined a single recessive gene controls male sterility in Guangzhan63S. Molecular marker techniques combined with bulked-segregant analysis (BSA) were used and located the target gene (named ptgms2-1) between two SSR markers RM12521 and RM12823. Fine mapping of the ptgms2-1 locus was conducted with 45 new Insertion–Deletion (InDel) markers developed between the RM12521 and RM12823 region, using 634 sterile individuals from F₂ and BC₁F₂ populations. Ptgms2-1 was further mapped to a 50.4 kb DNA fragment between two InDel markers, S2-40 and S2-44, with genetic distances of 0.08 and 0.16 cM, respectively, which cosegregated with S2-43 located on the AP004039 BAC clone. Ten genes were identified in this region based on annotation results from the RiceGAAS system. A nuclear ribonuclease Z gene was identified as the candidate for the ptgms2-1 gene. This result will facilitate cloning the ptgms2-1 gene. The tightly linked markers for the ptgms2-1 gene locus will further provide a useful tool for marker-assisted selection of this gene in rice breeding programs.     

Two new loci for hybrid sterility in cultivated rice (Oryza sativa L.)

Female gamete abortion in Indica-Japonica crosses of rice was earlier identified to be due to an allelic interaction at the S-5 locus on chromosome 6. Recently, in other crosses of rice, similar allelic interactions were found at loci designated as S-7 and S-8, located on chromosomes 7 and 6 respectively. All of them are independent of each other. At the S-5 locus, Indica and Japonica rice have S-5 ⁱ and S-5 ^j alleles respectively and Javanicas, such as Ketan Nangka, have a neutral allele S-5 ⁿ .The S-5 ⁱ /S-5 ^j genotype is semi-sterile due to partial abortion of female gametes carrying S-5 ^j , but both the S-5 ⁿ /S-5 ⁱ and S-5 ⁿ /S-5 ^j genotypes are fertile. The S-5 ⁿ allele is thus a wide-compatibility gene (WCG), and parents homozygous for this allele are called wide-compatible varieties (WCV). Such parents when crossed with Indica or Japonica varieties do not show F₁ hybrid sterility. Wide-compatible parents have been used to overcome sterility barriers in crosses between Indica and Japonica rice. However, a Javanica variety, Ketan Nangka (WCV), showed typical hybrid sterility when crossed to the Indian varieties N22 and Jaya. Further, Dular, another WCV from India, showed typical hybrid sterility when crossed to an IRRI line, IR2061-628-1-6-4-3(IR2061-628). By genetic analyses using isozyme markers, a new locus causing hybrid sterility in crosses between Ketan Nangka and the Indicas was located near isozyme loci Est-1 and Mal-1 on chromosome 4, and was designated as S-9. Another new locus for hybrid sterility in the crosses between Dular and the IR2061-628 was identified and was found linked to four isozyme loci, Sdh-1, Pox-2, Acp-1 and Acp-2, on chromosome 12. It was designated as S-15. On the basis of allelic interactions causing female-gamete abortion, two alleles were found at S-9, S-9 ^kn in Ketan Nangka and S-9 ⁱ in N22 and Jaya. In the heterozygote, S-9 ^kn /S-9 ⁱ , which was semisterile, female gametes carrying S-9 ^kn were aborted. The hybrid of Dular and IR2061-628, with a genetic constitution of S-15 ^Du /S-15 ⁱ , was semi-sterile and the female gametes carrying S-15 ^Du were aborted. A Japonica tester variety, Akihikari, and an Indica variety, IR36, were found to have neutral alleles, S-9 nand S-15 n, at these loci, in addition to S-7 nand at S-7. The accumulation of three neutral alleles into a breeding line should help solve the hybrid sterility problem in wide crosses of rice.     

Mapping of a broad-spectrum brown planthopper resistance gene, <Emphasis Type="Italic">Bph3</Emphasis>, on rice chromosome 6

The brown planthopper (BPH) is one of the most destructive insect pests of rice in Thailand. We performed a cluster analysis that revealed the existence of four groups corresponding to the variation of virulence against BPH resistance genes in 45 BPH populations collected in Thailand. Rice cultivars Rathu Heenati and PTB33, which carry Bph3, showed a broad-spectrum resistance against all BPH populations used in this study. The resistant gene Bph3 has been extensively studied and used in rice breeding programs against BPH; however, the chromosomal location of Bph3 in the rice genome has not yet been determined. In this study, a simple sequence repeat (SSR) analysis was performed to identify and localize the Bph3 gene derived from cvs. Rathu Heenati and PTB33. For mapping of the Bph3 locus, we developed two backcross populations, BC₁F₂ and BC₃F₂, from crosses of PTB33 × RD6 and Rathu Heenati × KDML105, respectively, and evaluated these for BPH resistance. Thirty-six polymorphic SSR markers on chromosomes 4, 6 and 10 were used to survey 15 resistant (R) and 15 susceptible (S) individuals from the backcross populations. One SSR marker, RM190, on chromosome 6 was associated with resistance and susceptibility in both backcross populations. Additional SSR markers surrounding the RM190 locus were also examined to define the location of Bph3. Based on the linkage analysis of 208 BC₁F₂ and 333 BC₃F₂ individuals, we were able to map the Bph3 locus between two flanking SSR markers, RM589 and RM588, on the short arm of chromosome 6 within 0.9 and 1.4 cM, respectively. This study confirms both the location of Bph3 and the allelic relationship between Bph3 and bph4 on chromosome 6 that have been previously reported. The tightly linked SSR markers will facilitate marker-assisted gene pyramiding and provide the basis for map-based cloning of the resistant gene.     

Molecular tagging and mapping of the erect panicle gene in rice

Erect panicle (EP) is one of the more important traits of the proposed ideotype of high-yielding rice. Several rice cultivars with the EP phenotype, which has been reported to be controlled by a dominant gene, have been successfully developed and released for commercial production in North China. To analyze the inheritance of the EP trait, we generated segregating F₂ and BC₁F₁ populations by crossing an EP-type variety, Liaojing 5, and a curved-panicle-type variety, Fengjin. Our results confirmed that a dominant gene controls the EP trait. Simple-sequence repeat (SSR) and bulked segregant analyses of the F₂ population revealed that the EP gene is located on chromosome 9, between two newly developed SSR markers, RM5833-11 and RM5686-23, at a genetic distance of 1.5 and 0.9 cM, respectively. Markers closer to the EP gene were developed by amplified fragment length polymorphism (AFLP) analysis with 128 AFLP primer combinations. Three AFLP markers were found to be linked to the EP gene, and the nearest marker, E-TA/M-CTC₂₀₀, was mapped to the same location as SSR marker RM5686-23, 1.5 cM from the EP gene. A local map around the EP gene comprising nine SSR and one AFLP marker was constructed. These markers will be useful for marker-assisted selection (MAS) for the EP trait in rice breeding programs.     

Mapping of a wide compatibility locus in indica rice using SSR markers

Hybrid sterility between indica and japonica subspecies in rice is basically caused by partial abortion of gametes and hybrid fertility could be recovered by a single wide compatibility (WC) allele. In this study, a typical indica germplasm source of rice, UPRI 95-162, with strong wide compatibility in cross with japonica rice was studied for location of its WC locus. Bulked segregant analysis was performed and SSRs (simple sequence repeats) were conducted on a F₁ population derived from a three-way cross (UPRI 95-162/T8//Akihikari). The locus was located on chromosome 1 approximately 0.2 cM to SSR markers RM581 on one side and 1.5 cM to RM292 on another. This WC locus, tentatively designated as S-20 ⁿ (t), and its tight linkage markers, RM581 and RM292, would be very useful for efficient marker-assisted selection for breeding new WC varieties and for map-based cloning of the gene.     

Assessing hybrid sterility in <Emphasis Type="Italic">Oryza glaberrima</Emphasis> × <Emphasis Type="Italic">O. sativa</Emphasis> hybrid progenies by PCR marker analysis and crossing with wide compatibility varieties

Interspecific crossing of the African indigenous rice Oryza glaberrima with Oryza sativa cultivars is hindered by crossing barriers causing 100% spikelet sterility in F₁ hybrids. Since hybrids are partially female fertile, fertility can be restored by back crossing (BC) to a recurrent male parent. Distinct genetic models on spikelet sterility have been developed predicting, e.g., the existence of a gamete eliminator and/or a pollen killer. Linkage of sterility to the waxy starch synthase gene and the chromogen gene C, both located on chromosome 6, have been demonstrated. We selected a segregating BC₂F₃ population of semi-sterile O. glaberrima × O. sativa indica hybrid progenies for analyses with PCR markers located at the respective chromosome-6 region. These analyses revealed that semi-sterile plants were heterozygous for a marker (OSR25) located in the waxy promoter, whereas fertile progenies were homozygous for the O. glaberrima allele. Adjacent markers showed no linkage to spikelet sterility. Semi-sterility of hybrid progenies was maintained at least until the F₄ progeny generation, suggesting the existence of a pollen killer in this plant material. Monitoring of reproductive plant development showed that spikelet sterility was at least partially due to an arrest of pollen development at the microspore stage. In order to address the question whether genes responsible for F₁ sterility in intraspecific hybrids (O. sativa indica × japonica) also cause spikelet sterility in interspecific hybrids, crossings with wide compatibility varieties (WCV) were performed. WCV accessions possess "neutral" S-loci (Sⁿ) improving fertility in intraspecific hybrids. This experiment showed that the tested Sⁿ-loci had no fertility restoring effect in F₁ interspecific hybrids. Pollen development was completely arrested at the microspore stage and grains were never obtained after selfing. This suggests that distinct or additional S-loci are responsible for sterility of O. glaberrima × O. sativa hybrids.Communicated by H.C. Becker     

Additive transgene expression and genetic introgression in multiple green-fluorescent protein transgenic crop × weed hybrid generations

The level of transgene expression in crop × weed hybrids and the degree to which crop-specific genes are integrated into hybrid populations are important factors in assessing the potential ecological and agricultural risks of gene flow associated with genetic engineering. The average transgene zygosity and genetic structure of transgenic hybrid populations change with the progression of generations, and the green fluorescent protein (GFP) transgene is an ideal marker to quantify transgene expression in advancing populations. The homozygous T₁ single-locus insert GFP/Bacillus thuringiensis (Bt) transgenic canola (Brassica napus, cv Westar) with two copies of the transgene fluoresced twice as much as hemizygous individuals with only one copy of the transgene. These data indicate that the expression of the GFP gene was additive, and fluorescence could be used to determine zygosity status. Several hybrid generations (BC₁F₁, BC₂F₁) were produced by backcrossing various GFP/Bt transgenic canola (B. napus, cv Westar) and birdseed rape (Brassica rapa) hybrid generations onto B. rapa. Intercrossed generations (BC₂F₂ Bulk) were generated by crossing BC₂F₁ individuals in the presence of a pollinating insect (Musca domestica L.). The ploidy of plants in the BC₂F₂ Bulk hybrid generation was identical to the weedy parental species, B. rapa. AFLP analysis was used to quantify the degree of B. napus introgression into multiple backcross hybrid generations with B. rapa. The F₁ hybrid generations contained 95–97% of the B. napus-specific AFLP markers, and each successive backcross generation demonstrated a reduction of markers resulting in the 15–29% presence in the BC₂F₂ Bulk population. Average fluorescence of each successive hybrid generation was analyzed, and homozygous canola lines and hybrid populations that contained individuals homozygous for GFP (BC₂F₂ Bulk) demonstrated significantly higher fluorescence than hemizygous hybrid generations (F₁, BC₁F₁ and BC₂F₁). These data demonstrate that the formation of homozygous individuals within hybrid populations increases the average level of transgene expression as generations progress. This phenomenon must be considered in the development of risk-management strategies.Communicated by J. Dvorak     

Mapping and Marker-assisted Selection of a Brown Planthopper Resistance Gene bph2 in Rice (Oryza sativa L.)

Nilaparvata lugens Stål (brown planthopper, BPH), is one of the major insect pests of rice (Oryza sativa L.) in the temperate rice-growing region. In this study, ASD7 harboring a BPH resistance gene bph2 was crossed to a susceptible cultivar C418, a japonica restorer line. BPH resistance was evaluated using 134 F_2:3 lines derived from the cross between “ASD7” and “C418”. SSR assay and linkage analysis were carried out to detect bph2. As a result, the resistant gene bph2 in ASD7 was successfully mapped between RM7102 and RM463 on the long arm of chromosome 12, with distances of 7.6 cM and 7.2 cM, respectively. Meanwhile, both phenotypic selection and marker-assisted selection (MAS) were conducted in the BC₁F₁ and BC₂F₁ populations. Selection efficiencies of RM7102 and RM463 were determined to be 89.9% and 91.2%, respectively. It would be very beneficial for BPH resistance improvement by using MAS of this gene.     

Identification of a new resistance gene Pi-Da(t) from Dacca6 against rice blast fungus (Magnaporthe oryzae) in Jin23B background

Rice blast, caused by the fungal pathogen Magnaporthe oryzae, severely threatens rice production worldwide. A new resistance gene, Pi-Da(t), was found in Dacca6, a local upland rice variety from the Philippines. It was mapped into a region between RM5529 and RM211 on chromosome 2, where no blast resistance gene has been reported, by bulk segregant analysis (BSA) in a BC₁F₂ population from a cross between Dacca6 and Jin23B. The presence of Pi-Da(t) in Jin23B background, an elite parental line preferred for its good grain quality and widely adopted in China??s three-line hybrid rice breeding program over the past 20?years, was verified by BSA and graphical genotyping with additional eight BC₁F₂ bulks. This work presents an example of combining gene mapping work and gene introgression with BSA and graphical genotyping methods in a backcross (BC) breeding scheme. Both the resistant Jin23B line and the linked markers will provide useful information and materials for marker-assisted breeding against blast disease in rice.     

Identification of SSR markers for a broad-spectrum blast resistance gene Pi20(t) for marker-assisted breeding

The Pi20(t) gene was determined to confer a broad-spectrum resistance against diverse blast pathotypes (races) in China based on inoculation experiments utilizing 160 Chinese Magnaporthe oryzae (formerly Magnaporthe grisea) isolates, among which isolate 98095 can specifically differentiate the Pi20(t) gene present in cv. IR24. Two flanking and three co-segregating simple sequence repeat (SSR) markers for Pi20(t), located near the centromere region of chromosome 12, were identified using 526 extremely susceptible F₂ plants derived from a cross of Asominori, an extremely susceptible cultivar, with resistant cultivar IR24. The SSR OSR32 was mapped at a distance of 0.2 cM from Pi20(t), and the SSR RM28050 was mapped to the other side of Pi20(t) at a distance of 0.4 cM. The other three SSR markers, RM1337, RM5364 and RM7102, co-segregated with Pi20(t). RM1337 and RM5364 were found to be reliable markers of resistance conditioned by Pi20(t) in a wide range of elite rice germplasm in China. As such, they are useful tags in marker-assisted rice breeding programs aimed at incorporating Pi20(t) into advanced rice breeding lines and, ultimately, at obtaining a durable and broad spectrum of resistance to M. oryaze. Wei Li and Cailin Lei contributed equally to this work.     

Transfer of bacterial blight and blast resistance from the tetraploid wild rice Oryza minuta to cultivated rice,Oryza sativa

Summary Oryza minuta J. S. Presl ex C. B. Presl is a tetraploid wild rice with resistance to several insects and diseases, including blast (caused by Pyricularia grisea) and bacterial blight (caused by Xanthomonas oryzae pv. oryzae). To transfer resistance from the wild species into the genome of cultivated rice (Oryza sativa L.), backcross progeny (BC₁, BC₂, and BC₃) were produced from interspecific hybrids of O. sativa cv IR31917-45-3-2 (2n=24, AA genome) and O. minuta Acc. 101141 (2n=48, BBCC genomes) by backcrossing to the O. sativa parent followed by embryo rescue. The chromosome numbers ranged from 44 to 47 in the BC₁ progeny and from 24 to 37 in the BC₂ progeny. All F₁ hybrids were resistant to both blast and bacterial blight. One BC₁ plant was moderately susceptible to blast while the rest were resistant. Thirteen of the 16 BC₂ progeny tested were resistant to blast; 1 blast-resistant BC₂, plant 75-1, had 24 chromosomes. A 3 resistant: 1 susceptible segregation ratio, consistent with the action of a major, dominant gene, was observed in the BC₂F₂ and BC₂F₃ generations. Five of the BC₁ plants tested were resistant to bacterial blight. Ten of the 21 BC₂ progeny tested were resistant to Philippine races 2, 3, and 6 of the bacterial blight pathogen. One resistant BC₂, plant 78-1, had 24 chromosomes. The segregation of reactions of the BC₂F₂, BC₂F₃, and BC₂F₄ progenies of plant 78-1 suggested that the same or closely linked gene(s) conferred resistance to races 2, 3, 5, and 6 of the bacterial blight pathogen from the Philippines.     

Genetic characterization and fine mapping of a novel thermo-sensitive genic male-sterile gene tms6 in rice (Oryza sativa L.)

The application of genetic male sterility in hybrid rice production has great potential to revolutionize hybrid seed production methodology. The two-line breeding system by using thermo-sensitive genic male sterility (TGMS) has been discovered and successfully developed as a breeding strategy in rice. One TGMS gene was investigated by a spontaneous rice mutant line, Sokcho-MS, originated from a Korean japonica variety. It was shown that Sokcho-MS is completely sterile at a temperature higher than 27°C and/or lower than 25°C during the development of spikelets, but fertile at the temperature ranging from 25 to 27°C regardless of the levels of day-length. Genetic analysis and molecular mapping based on SSR, STS and EST markers revealed that a single recessive gene locus involved the control of genic male sterility in Sokcho-MS. By using an F₂ mapping population derived from a cross between Sokcho-MS and a fertile indica variety Neda, the new TGMS gene, designated as tms6, was mapped primarily to the long arm of chromosome 5 of Oryza sativa at the interval between markers E60663 (2.0 cM) and RM440 (5.8 cM). Subsequently, tms6 was fine mapped to the interval between markers RM3351 (0.1 cM) and E60663 (1.9 cM). As tms6 appeared to be independent of other mapped TGMS genes in rice, the genetic basis of Sokcho-MS was further discussed.     

Characterization and mapping of <Emphasis Type="Italic">Rpi1</Emphasis>, a gene that confers dominant resistance to stalk rot in maize

The maize inbred lines 1145 (resistant) and Y331 (susceptible), and the F₁, F₂ and BC₁F₁ populations derived from them were inoculated with the pathogen Pythium inflatum Matthews, which causes stalk rot in Zea mays. Field data revealed that the ratio of resistant to susceptible plants was 3:1 in the F₂ population, and 1:1 in the BC₁F₁population, indicating that the resistance to P. inflatum Matthews was controlled by a single dominant gene in the 1145×Y331 cross. The gene that confers resistance to P. inflatum Matthews was designated Rpi1 for resistance to P. inflatum) according to the standard nomenclature for plant disease resistance genes. Fifty SSR markers from 10 chromosomes were first screened in the F₂ population to find markers linked to the Rpi1 gene. The results indicated that umc1702 and mmc0371 were both linked to Rpi1, placing the resistance gene on chromosome 4. RAPD (randomly amplified polymorphic DNA) markers were then tested in the F₂population using bulked segregant analysis (BSA). Four RAPD products were found to show linkage to the Rpi1 gene. Then 27 SSR markers and 8 RFLP markers in the region encompassing Rpi1 were used for fine-scale mapping of the resistance gene. Two SSR markers and four RFLP markers were linked to the Rpi1 gene. Finally, the Rpi1 gene was mapped between the SSR markers bnlg1937 and agrr286 on chromosome 4, 1.6 cM away from the former and 4.1 cM distant from the latter. This is the first time that a dominant gene for resistance to maize stalk rot caused by P. inflatum Matthews has been mapped with molecular marker techniques.     

Fine mapping of a grain weight quantitative trait locus on rice chromosome 8 using near-isogenic lines derived from a cross between Oryza sativa and Oryza rufipogon

A quantitative trait locus (QTL) for grain weight (GW) was detected near SSR marker RM210 on chromosome 8 in backcross populations derived from a cross between the Korean japonica cultivar Hwaseongbyeo and Oryza rufipogon (IRGC 105491). The O. rufipogon allele increased GW in the Hwaseongbyeo background despite the fact that O. rufipogon was the small-seeded parent. Using sister BC₃F₃ near-isogenic lines (NILs), gw8.1 was validated and mapped to a 6.1 cM region in the interval between RM42 and RM210 (P≤0.0001). Substitution mapping with eight BC₃F₄ sub-NILs further narrowed the interval containing gw8.1 to about 306.4 kb between markers RM23201.CNR151 and RM30000.CNR99. A yield trial using homozygous BC₃F₄ sister sub-NILs and the Hwaseongbyeo recurrent parent indicated that the NIL carrying an O. rufipogon chromosome segment across the entire gw8.1 target region out-yielded its sister NIL (containing Hwaseongbyeo chromosome in the RM42–RM210 interval) by 9% (P=0.029). The higher-yielding NIL produced 19.3% more grain than the Hwaseongbyeo recurrent parent (P=0.018). Analysis of a BC₃F₄ NIL indicated that the variation for GW is associated with variation in grain shape, specifically grain length. The locus, gw8.1 is of particular interest because of its independence from undesirable height and grain quality traits. SSR markers tightly linked to the GW QTL will facilitate cloning of the gene underlying this QTL as well as marker-assisted selection for variation in GW in an applied breeding program.     

Transfer of resistance to the beet cyst nematode (Heterodera Schachtii Schm.) from Sinapis alba L. (white mustard) to the Brassica napus L. gene pool by means of sexual and somatic hybridization

Summary Sexual and somatic hybrid plants have been produced between Sinapis alba L. (white mustard) and Brassica napus L. (oil-seed rape), with the aim to transfer resistance to the beet cyst nematode Heterodera schachtii Schm. (BCN) from white mustard into the oil-seed rape gene pool. Only crosses between diploid accessions of S. alba (2n = 24, S_a1S_a1) as the pistillate parent and several B. napus accessions (2n = 38, AACC) yielded hybrid plants with 31 chromosomes. Crosses between tetraploid accessions of S. alba (2n = 48, S_a1S_a1S_a1S_a1) and B. napus were unsuccessful. Somatic hybrid plants were also obtained between a diploid accession of S. alba and B. napus. These hybrids were mitotically unstable, the number of chromosomes ranging from 56 to more than 90. Analysis of total DNA using a pea rDNA probe confirmed the hybrid nature of the sexual hybrids, whereas for the somatic hybrids a pattern identical to that of B. napus was obtained. Using chloroplast (cp) and mitochondrial (mt) DNA sequences, we found that all of the sexual F₁ hybrids and somatic hybrids contained cpDNA and mtDNA of the S. alba parent. No recombinant mtDNA or cpDNA pattern was observed. Three BC₁ plants were obtained when sexual hybrids were back-crossed with B. napus. Backcrossing of somatic hybrids with B. napus was not successful. Three sexual hybrids and one BC₁ plant, the latter obtained from a cross between a sexual hybrid and B. napus, were found to show a high level of BCN resistance. The level of BCN resistance of the somatic hybrids was in general high, but varied between cuttings from the same plant. Results from cytological studies of chromosome association at meiotic metaphase I in the sexual hybrids suggest partial homology between chromosomes of the AC and S_a1 genomes and thus their potential for gene exchange.     

Genetic and physical mapping of <Emphasis Type="Italic">Pi36</Emphasis>(t), a novel rice blast resistance gene located on rice chromosome 8

Blast resistance in the indica cultivar (cv.) Q61 was inherited as a single dominant gene in two F₂ populations, F₂-1 and F₂-2, derived from crosses between the donor cv. and two susceptible japonica cvs. Aichi Asahi and Lijiangxintuanheigu (LTH), respectively. To rapidly determine the chromosomal location of the resistance (R) gene detected in Q61, random amplified polymorphic DNA (RAPD) analysis was performed in the F₂-1 population using bulked-segregant analysis (BSA) in combination with recessive-class analysis (RCA). One of the three linked markers identified, BA1126₅₅₀, was cloned and sequenced. The R gene locus was roughly mapped on rice chromosome 8 by comparison of the BA1126₅₅₀ sequence with rice sequences in the databases (chromosome landing). To confirm this finding, seven known markers, including four sequence-tagged-site (STS) markers and three simple-sequence repeat (SSR) markers flanking BA1126₅₅₀ on chromosome 8, were subjected to linkage analysis in the two F₂ populations. The locus was mapped to a 5.8 cM interval bounded by RM5647 and RM8018 on the short arm of chromosome 8. This novel R gene is therefore tentatively designated as Pi36(t). For fine mapping of the Pi36(t) locus, five additional markers including one STS marker and four candidate resistance gene (CRG) markers were developed in the target region, based on the genomic sequence of the corresponding region of the reference japonica cv. Nipponbare. The Pi36(t) locus was finally localized to an interval of about 0.6 cM flanked by the markers RM5647 and CRG2, and co-segregated with the markers CRG3 and CRG4. To physically map this locus, the Pi36(t)-linked markers were mapped by electronic hybridization to bacterial artificial chromosome (BAC) or P1 artificial chromosome (PAC) clones of Nipponbare, and a contig map was constructed in silico through Pairwise BLAST analysis. The Pi36(t) locus was physically delimited to an interval of about 17.0 kb, based on the genomic sequence of Nipponbare.     

Overcoming hybrid incompatibility between Nicotiana africana Merxm. and N. tabacum and development of cytoplasmically male sterile tobacco forms

The incompatibility between the wild species N. africana Merxm. and the cultivated species N. tabacum has been overcome by in vitro techniques. Underdeveloped F₀ seeds, placed on MS medium with supplements, produced plants which upon reaching the stage of anthesis proved to be completely sterile. Female sterility of F₁ hybrids was overcome by applying tissue culture methods. Explants of stem parenchyma were grown in vitro. In every passage investigations were made of their callus production, organogenesis and cell polyploidization. The regenerants showed a great diversity in their morphological and cytological characters. Pollination of the R₁ plants (N. africana × N. tabacum) with N. tabacum produced normally seeded capsules. BC₁ plants were male sterile. The male sterility of the first backcross generation was preserved in BC₂ and BC₃, proving its cytoplasmic origin.     

Fertile somatic hybrids between Petunia hybrida and a wild species,Petunia variabilis

Somatic hybrid plants were regenerated following electrofusion between leaf mesophyll protoplasts of P. hybrida (2n = 14) and a wild sexually incompatible species, P. variabilis (2n = 18). The selection of hybrids was based on the hybrid vigour, expressed both in the growth of the callus and at the shoot formation stage, resulting from the combination of parental genomes. Calli exhibiting vigorous growth were selected, and upon transfer to regeneration medium gave rise to shoots. Four regenerated plants from three calli had morphological characteristics intermediate between those of the parents. The hybrid nature of these plants was confirmed by chromosome counts as well as isozyme and DNA analyses. They had amphidiploid chromosome numbers (2n = 32) and were fertile. Following self-pollination and backcrossing with P. variabilis, large numbers of F₂ and BC₁ seedlings were obtained.     

Expression of Thinopyrum distichum NORs in wheat×Thinopyrum amphiploids and their backcross generations

Summary The C-banding pattern of the satellited chromosomes in Thinopyrum distichum and Triticum durum was established. Both T. durum and Th. distichum contained two pairs of satellited chromosomes, which could be distinguished from one another. In the amphiploids [T. durum×Th. distichum (2x=56)] and in the backcross T. durum/(T. durum×Th. distichum)², BC₁F₃, and BC₁F₅ (2n = 42) the satellite was visible on only 1B and 6B of T. durum. The vector pTa 71 containing the rRNA gene from wheat hybridized to two pairs of chromosomes (four hybridization sites) in T. durum and Th. distichum, to eight sites in the amphiploid hybrid (2n=56), and to six sites in the backcross populations BC₁F₁. BC₁F₃, and BC₁F₅ (2n=42). The two satellite pairs in Th. distichum could be distinguished by the chromosomal location of the rRNA site (median or subterminal) and by the centromere position. One copy of each pair was present in the BC₁F₁, but in the BC₁F₃ and BC₁F₅ populations the pair with the subterminal location of rRNA genes was absent. Silver nitrate staining indicated that the rRNA genes of T. durum did not completely suppress those of Th. distichum. The octoploid amphiploid (2n = 56) contained a maximum of four large and four small nucleoli and the hexaploid BC₁s (2n=42), four large and two small nucleoli.