水稻抗褐飞虱基因bph2的SSR定位和标记辅助选择

利用综合性状较好对褐飞虱敏感的粳稻恢复系C418为父本,以含有bph2基因的抗褐飞虱品种ASD7为母本构建了包含134个F23家系的群体,利用苗期鉴定法对F2：3家系进行抗性鉴定：用SSR标记技术,将bph2基因定位在第12染色体长臂上,标记RM7102和RM463之间,其遗传距离分别为7．6cM和7．2cM。在进行表型选择的同时,利用与bph2基因连锁的SSR标记RM7102和RM463对BC1F1和BC2F1进行了标记辅助选择,选择效率分别为89．9％和91．2％,为培育高抗褐飞虱水稻品种奠定了基础。     

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.     

Genetic Analysis and Physical Mapping of Lk-4(t), a Major Gene Controlling Grain Length in Rice, with a BC2F2 Population

Grain size and shape are important factors affecting grain quality and yield in rice. Mapping, tagging and identification of their related genes can lead us to understand their expression pattern and mechanism network, which is to their control. In this study we mapped a grain length controlling gene named Lk-4(t) with SSR and CAPs markers by screening 800 recessive plants in a BC₂F₂ population derived from a cross of Shuhui527xXiaoli and backcrossed with Xiaoli as the donor parent. The distribution of grain shape parameters and thousand grain weight in F₂ and BC₂F₂ population showed that backcross can diminish most unnecessary variations to identify the target gene more clearly. There were only two grain length phenotypes found among the 3 209 BC₂F₂ plants, long and short, indicating it is a qualitative trait. The frequency distribution for the grain length showed a typical segregation ratio of 3 : 1, suggesting that only one allele was responsible for the variation. By screening the recessive long grain plants with three CAPs markers, P1-EcoR V, P2-Sac I and P3-Mbo I, we tagged the locus on the arm of chromosome 3 near the centromere. Lk-4(t) was located between P1-EcoRV and P2-Sac I, with genetic distance of 0.90 cM and 0.50 cM from the two markers respectively. Mapping of the gene is a foundation for its final identification and function analysis.     

High-resolution mapping of a new brown planthopper (BPH) resistance gene, Bph18(t), and marker-assisted selection for BPH resistance in rice (Oryza sativa L.)

Brown planthopper (BPH) is a destructive insect pest of rice in Asia. Identification and the incorporation of new BPH resistance genes into modern rice cultivars are important breeding strategies to control the damage caused by new biotypes of BPH. In this study, a major resistance gene, Bph18(t), has been identified in an introgression line (IR65482-7-216-1-2) that has inherited the gene from the wild species Oryza australiensis. Genetic analysis revealed the dominant nature of the Bph18(t) gene and identified it as non-allelic to another gene, Bph10 that was earlier introgressed from O. australiensis. After linkage analysis using MapMaker followed by single-locus ANOVA on quantitatively expressed resistance levels of the progenies from an F₂ mapping population identified with marker allele types, the Bph18(t) gene was initially located on the subterminal region of the long arm of chromosome 12 flanked by the SSR marker RM463 and the STS marker S15552. The corresponding physical region was identified in the Nipponbare genome pseudomolecule 3 through electronic chromosome landing (e-landing), in which 15 BAC clones covered 1.612 Mb. Eleven DNA markers tagging the BAC clones were used to construct a high-resolution genetic map of the target region. The Bph18(t) locus was further localized within a 0.843-Mb physical interval that includes three BAC clones between the markers R10289S and RM6869 by means of single-locus ANOVA of resistance levels of mapping population and marker-gene association analysis on 86 susceptible F₂ progenies based on six time-point phenotyping. Using gene annotation information of TIGR, a putative resistance gene was identified in the BAC clone OSJNBa0028L05 and the sequence information was used to generate STS marker 7312.T4A. The marker allele of 1,078 bp completely co-segregated with the BPH resistance phenotype. STS marker 7312.T4A was validated using BC₂F₂ progenies derived from two temperate japonica backgrounds. Some 97 resistant BC₂F₂ individuals out of 433 screened completely co-segregated with the resistance-specific marker allele (1,078 bp) in either homozygous or heterozygous state. This further confirmed a major gene-controlled resistance to the BPH biotype of Korea. Identification of Bph18(t) enlarges the BPH resistance gene pool to help develop improved rice cultivars, and the PCR marker (7312.T4A) for the Bph18(t) gene should be readily applicable for marker-assisted selection (MAS). K. K. Jena and J. U. Jeung contributed equally to this study.     

Genetic analysis and fine mapping of a rice brown planthopper (Nilaparvata lugens Stål) resistance gene bph19(t)

Genetic analysis and fine mapping of a resistance gene against brown planthopper (BPH) biotype 2 in rice was performed using two F₂ populations derived from two crosses between a resistant indica cultivar (cv.), AS20-1, and two susceptible japonica cvs., Aichi Asahi and Lijiangxintuanheigu. Insect resistance was evaluated using F₁ plants and the two F₂ populations. The results showed that a single recessive gene, tentatively designated as bph19(t), conditioned the resistance in AS20-1. A linkage analysis, mainly employing microsatellite markers, was carried out in the two F₂ populations through bulked segregant analysis and recessive class analysis (RCA), in combination with bioinformatics analysis (BIA). The resistance gene locus bph19(t) was finely mapped to a region of about 1.0 cM on the short arm of chromosome 3, flanked by markers RM6308 and RM3134, where one known marker RM1022, and four new markers, b1, b2, b3 and b4, developed in the present study were co-segregating with the locus. To physically map this locus, the bph19(t)-linked markers were landed on bacterial artificial chromosome or P1 artificial chromosome clones of the reference cv., Nipponbare, released by the International Rice Genome Sequencing Project. Sequence information of these clones was used to construct a physical map of the bph19(t) locus, in silico, by BIA. The bph19(t) locus was physically defined to an interval of about 60 kb. The detailed genetic and physical maps of the bph19(t) locus will facilitate marker-assisted gene pyramiding and cloning.     

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.     

Marker Assisted Transfer of Two Powdery Mildew Resistance Genes PmTb7A.1 and PmTb7A.2 from Triticum boeoticum (Boiss.) to Triticum aestivum (L.)

Powdery mildew (PM), caused by Blumeria graminis f. sp. tritici, is one of the important wheat diseases, worldwide. Two PM resistance genes, designated as PmTb7A.1 and PmTb7A.2, were identified in T. boeoticum acc. pau5088 and mapped on chromosome 7AL approximately 48cM apart. Two resistance gene analogue (RGA)-STS markers Ta7AL-4556232 and 7AL-4426363 were identified to be linked to the PmTb7A.1 and PmTb7A.2, at a distance of 0.6cM and 6.0cM, respectively. In the present study, following marker assisted selection (MAS), the two genes were transferred to T. aestivum using T. durum as bridging species. As many as 12,317 florets of F₁ of the cross T. durum /T. boeoticum were pollinated with T. aestivum lines PBW343-IL and PBW621 to produce 61 and 65 seeds, respectively, of three-way F₁. The resulting F₁s of the cross T. durum/T. boeoticum//T. aestivum were screened with marker flanking both the PM resistance genes PmTb7A.1 and PmTb7A.2 (foreground selection) and the selected plants were backcrossed to generate BC₁F₁. Marker assisted selection was carried both in BC₁F₁ and the BC₂F₁ generations. Introgression of alien chromatin in BC₂F₁ plants varied from 15.4 - 62.9 percent. Out of more than 110 BC₂F₁ plants showing introgression for markers linked to the two PM resistance genes, 40 agronomically desirable plants were selected for background selection for the carrier chromosome to identify the plants with minimum of the alien introgression. Cytological analysis showed that most plants have chromosome number ranging from 40-42. The BC₂F₂ plants homozygous for the two genes have been identified. These will be crossed to generate lines combining both the PM resistance genes but with minimal of the alien introgression. The PM resistance gene PmTb7A.1 maps in a region very close to Sr22, a stem rust resistance gene effective against the race Ug99. Analysis of selected plants with markers linked to Sr22 showed introgression of Sr22 from T. boeoticum in several BC₂F₁ plants. Thus, in addition to PM resistance, these progeny might also carry resistance to stem rust race Ug99.     

High-resolution mapping of the brown planthopper resistance gene Bph6 in rice and characterizing its resistance in the 9311 and Nipponbare near isogenic backgrounds

Brown planthopper (Nilaparvata lugens Stål, BPH) is one of the most destructive insect pests of rice. Exploring resistance genes from diverse germplasms and incorporating them into cultivated varieties are critical for controlling this insect. The rice variety Swarnalata was reported to carry a resistance gene (designated Bph6), which has not yet been assigned to a chromosome location and the resistance mechanism is still unknown. In this study, we identified and mapped this gene using the F₂ and backcrossing populations and characterized its resistance in indica 9311 and japonica Nipponbare using near isogenic lines (NILs). In analysis of 9311/Swarnalata F₂ population, the Bph6 gene was located on the long arm of chromosome 4 between the SSR markers RM6997 and RM5742. The gene was further mapped precisely to a 25-kb region delimited between the STS markers Y19 and Y9; and the distance between these markers is 25-kb in Nipponbare genome. The Bph6 explained 77.5% of the phenotypic variance of BPH resistance in F₂ population and 84.9% in BC₂F₂ population. Allele from Swarnalata significantly increased resistance to the BPH, resulted in a reduced damage score. In characterization of Bph6-mediated resistance, the BPH insects showed significant preference between NIL-9311 and 9311 in 3 h and between NIL-NIP and Nipponbare in 120 h after release. BPH growth and development were inhibited, and the insect’s survival rates were lower on Bph6-NIL plants, compared with the parents 9311 and Nipponbare. The results indicate that the Bph6 exerted prolonged antixenotic and antibiotic effects in Bph6-NIL plants, and NIL-9311 plants showed a quicker and stronger effect toward BPH than NIL-NIP plants.     

Identification and fine mapping of <Emphasis Type="Italic">Pi39</Emphasis>(t), a major gene conferring the broad-spectrum resistance to <Emphasis Type="Italic">Magnaporthe oryzae</Emphasis>

Blast, caused by the ascomycete fungus Magnaporthe oryzae, is one of the most devastating diseases of rice worldwide. The Chinese native cultivar (cv.) Q15 expresses the broad-spectrum resistance to most of the isolates collected from China. To effectively utilize the resistance, three rounds of linkage analysis were performed in an F₂ population derived from a cross of Q15 and a susceptible cv. Tsuyuake, which segregated into 3:1 (resistant/susceptible) ratio. The first round of linkage analysis employing simple sequence repeat (SSR) markers was carried out in the F₂ population through bulked-segregant assay. A total of 180 SSR markers selected from each chromosome equally were surveyed. The results revealed that only two polymorphic markers, RM247 and RM463, located on chromosome 12, were linked to the resistance (R) gene. To further define the chromosomal location of the R gene locus, the second round of linkage analysis was performed using additional five SSR markers, which located in the region anchored by markers RM247 and RM463. The locus was further mapped to a 0.27 cM region bounded by markers RM27933 and RM27940 in the pericentromeric region towards the short arm. For fine mapping of the R locus, seven new markers were developed in the smaller region for the third round of linkage analysis, based on the reference sequences. The R locus was further mapped to a 0.18 cM region flanked by marker clusters 39M11 and 39M22, which is closest to, but away from the Pita/Pita ² locus by 0.09 cM. To physically map the locus, all the linked markers were landed on the respective bacterial artificial chromosome clones of the reference cv. Nipponbare. Sequence information of these clones was used to construct a physical map of the locus, in silico, by bioinformatics analysis. The locus was physically defined to an interval of ≈37 kb. To further characterize the R gene, five R genes mapped near the locus, as well as 10 main R genes those might be exploited in the resistance breeding programs, were selected for differential tests with 475 Chinese isolates. The R gene carrier Q15 conveys resistances distinct from those conditioned by the carriers of the 15 R genes. Together, this valuable R gene was, therefore, designated as Pi39(t). The sequence information of the R gene locus could be used for further marker-based selection and cloning. Xinqiong Liu and Qinzhong Yang contributed equally to this work.     

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.     

Inheritance of resistance to Bacillus thuringiensis Cry1Ab protein in the sugarcane borer (Lepidoptera: Crambidae)

Inheritance traits of a Cry1Ab-resistant strain of the sugarcane borer, Diatraea saccharalis (F.) were analyzed using various genetic crosses. Reciprocal parental crosses between Cry1Ab-susceptible and Cry1Ab-resistant populations, F₁ by F₁ crosses, and backcrosses of F₁ with the Cry1Ab-resistant population were successfully completed. Larval mortality of the parental and cross-populations were assayed on Cry1Ab diet and Bacillus thuringiensis (Bt)-corn leaf tissue. Maternal effects and sex linkage were examined by comparing the larval mortality between the two F₁ populations. Dominance levels of resistance were measured by comparing the larval mortality of the Cry1Ab-resistant, -susceptible, and -heterozygous populations. Number of genes associated with the resistance was evaluated by fitting the observed mortality of F₂ and backcross populations with a Mendelian monogenic inheritance model. Cry1Ab resistance in D. saccharalis was likely inherited as a single or a few tightly linked autosomal genes. The resistance was incompletely recessive on Bt corn leaf tissue, while the effective dominance levels (D_ML) of resistance increased as Cry1Ab concentrations decreased with Cry1Ab-treated diet. D_ML estimated based on larval mortality on intact Bt corn plants reported in a previous study ranged from 0.08 to 0.26. This variability in D_ML levels of Cry1Ab resistance in D. saccharalis suggests that Bt corn hybrids must express a sufficient dose of Bt proteins to make the resistance genes functionally recessive. Thus, Bt resistant heterozygous individuals can be killed as desired in the “high/dose refuge” resistance management strategy for Bt corn.     

Genetic Analysis and Molecular Tagging on a Novel Excessive Tillering Mutant in Rice

A rice (Oryza sativa L.) mutant with an excessive tiller number, designated ext-M1B, was found in the F₂ progenies generated from the cross between M1B and GMS-1 (a genetic male sterile), whose number of tillers was 121. The excessive tillering mutant also resulted in significant changes in plant height, flag leaf, stem, filled grains per panicle, and productive panicles per plant. The inbreeding progenies of ext-M1B exhibited the same mutant phenotype. The crosses from ext-M1B/M1B, M1B/ext-M1B, 2480B/ext-M1B, D62B/ext-M1B, G46B/ext-M1B, and G683B/ext-M1B expressed normal tillering in F₁, and segregated into two different phenotypes of normal tillering type and excessive tillering type in a ratio of 3:1 in F₂. Inheritance analysis indicated that the excessive tillering character was controlled by a single recessive nucleic gene. By BSA (bulked segregants analysis) and microsatellite makers with the F₂ population of 2480B/ext-M1B as the mapping population, RM197, RM584, and RM225, all of which were located on the short arm of rice chromosome 6, were identified to be linked with the excessive tillering gene with genetic distance of 3.8 cM, 5.1 cM, and 5.2 cM, respectively. This gene is probably a new excessive tillering gene in rice and is designated tentatively ext-M1B (t).     

Characterization of resistance to Bacillus thuringiensis toxin Cry1Ac in Plutella xylostella from China

A field population (SZ) of Plutella xylostella, collected from the cabbage field in Shenzhen, Guangdong Province of China in 2002, showed 2.3-fold resistance to Cry1Aa, 110-fold to Cry1Ab, 30-fold to Cry1Ac, 2.1-fold to Cry1F, 5.3-fold to Cry2Aa and 6-fold resistance to Bacillus thuringiensis var. kurstaki (Btk) compared with a susceptible strain (ROTH). The SZBT strain was derived from the SZ population through 20 generations of selection with activated Cry1Ac in the laboratory. While the SZBT strain developed 1200-fold resistance to Cry1Ac after selection, resistance to Cry1Aa, Cry1Ab, Cry1F, and Btk increased to 31-, 1900-,>33- and 17-fold compared with the ROTH strain. However, little or no cross-resistance was detected to Cry1B, Cry1C and Cry2Aa in the SZBT strain. Genetic cross analyses between the SZBT and ROTH strains revealed that Cry1Ac-resistance in the SZBT strain was controlled by a single, autosomal, incompletely recessive gene. Binding studies with ¹²⁵I-labeled Cry1Ac showed that the brush border membrane vesicles (BBMVs) of midguts from the resistant SZBT insects had lost binding to Cry1Ac. Allelic complementation tests demonstrated that the major Bt resistance locus in the SZBT strain was same as that in the Cry1Ac-R strain which has “mode 1” resistance to Bt. An F₁ screen of 120 single-pair families between the SZBT strain and three field populations collected in 2008 was carried out. Based on this approach, the estimated frequencies of Cry1Ac-resistance alleles were 0.156 in the Yuxi population from Yunnan province, and 0.375 and 0.472 respectively in the Guangzhou and Huizhou populations from Guangdong province.     

Coupling- and repulsion-phase RAPDs for marker-assisted selection of PI 181996 rust resistance in common bean

The Guatemalan black bean (Phaseolus vulgaris L.) plant introduction (PI) 181996 is resistant to all known US races of the bean rust fungus Uromyces appendiculatus (Pers. ex Pers.) Unger var. appendiculatus [syn. U. phaseoli (Reben) Wint.]. We report on two random amplified polymorphic DNA (RAPD) markers OAC20₄₉₀ tightly linked (no recombinants) in coupling phase and OAE19₈₉₀ linked in repulsion phase (at 6.2±2.8 cM) to PI 181996 rust resistance. These RAPDs, generated by single decamer primers in the polymerase chain reaction, were identified in near-isogenic bulks of non-segregating resistant and susceptible BC₄F₂ (NX-040*4/PI 181996) lines. Linkage of the RAPD markers was confirmed by screening 19 BC₄F₂ and 57 BC₄F₃ individuals segregating for PI 181996 resistance. Utility of the RAPDs OAC20₄₉₀ and OAE19₈₉₀ was investigated in a diverse group of common bean cultivars and lines. All cultivars into which the PI 181996 resistance was introgressed had the RAPD OAC20₄₉₀. A RAPD similar in size to OAC20₄₉₀, observed in some susceptible common bean lines, was confirmed by Southern blotting to be homologous to the RAPD OAC20₄₉₀. Use of the RAPDs OAC20₄₉₀ and OAE19₈₉₀ in marker-assisted selection (MAS) is proposed. The coupling-phase RAPD is most useful for MAS of resistant BC_nF₁individuals during traditional backcross breeding. The repulsion-phase RAPD has greatest utility in MAS of homozygous-resistant individuals in F₂ or later-segregating generations.Mention of a trademark or a proprietary product does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products that may also be suitable.     

Phenotypic Expression of rkn1-Mediated Meloidogyne incognita Resistance in Gossypium hirsutum Populations

The root-knot nematode Meloidogyne incognita is a damaging pest of cotton (Gossypium hirsutum) worldwide. A major gene (rkn1) conferring resistance to M. incognita was previously identified on linkage group A03 in G. hirsutum cv. Acala NemX. To determine the patterns of segregation and phenotypic expression of rkn1, F₁, F₂, F_2:3, BC₁F₁ and F_2:7 recombinant inbred lines (RIL) from intraspecific crosses between Acala NemX and a closely related susceptible cultivar Acala SJ-2 were inoculated in greenhouse tests with M. incognita race 3. The resistance phenotype was determined by the extent of nematode-induced root galling and nematode egg production on roots. Suppression of root galling and egg production was highly correlated among individuals in all tests. Root galling and egg production on heterozygous plants did not differ from the susceptible parent phenotype 125 d or more after inoculation, but were slightly suppressed with shorter screening (60 d), indicating that rkn1 behaved as a recessive gene or an incompletely recessive gene, depending on the screening condition. In the RIL, rkn1 segregated in an expected 1 resistant: 1 susceptible ratio for a major resistance gene. However, within the resistant class, 21 out of 34 RIL were more resistant than the resistant parent Acala NemX, indicating transgressive segregation. These results suggest that rkn1-based resistance in G. hirsutum can be enhanced in progenies of crosses with susceptible genotypes. Allelism tests and molecular genetic analysis are needed to determine the relationship of rkn1 to other M. incognita resistance sources in cotton.     

Genetic Analysis and Molecular Mapping of a Novel Gene Conferring Resistance to Rice Stripe Virus

Rice stripe virus (RSV) is one of the most damaging diseases affecting rice in East Asia. Rice variety 502 is highly resistant to RSV, while variety 5112 is extremely susceptible. Field statistical data revealed that all “502 × 5112” F₁ individuals were resistant to RSV and the ratio of resistant to susceptible plants was 3:1 in the F₂ population and 1:1 in the BC₁F₁ population. These results indicated that a dominant gene, designated RSV1, controlled the resistance. Simple sequence repeat (SSR) analysis was subsequently carried out in an F₂ population. Sixty SSR markers evenly distributed on the 12 rice chromosomes were screened and tested. Two markers, RM229 and RM206, showed linkage with RSV1. Based on this result, six SSR markers flanking RM229 and RM206 were further selected and tested. Results indicated that SSR markers RM457 and RM473E were linked to RSV1 with a genetic distance of 4.5 and 5.0 cM, respectively. All of the four SSR markers (RM229, RM473E, RM457 and RM206) linked to RSV1 were all located on chromosome 11, therefore RSV1 should be located on chromosome 11 also. In order to find some new markers more closely linked to the RSV1 gene, sequence-related amplified polymorphism (SRAP) analysis was performed. A total of 30 SRAP primer-pairs were analyzed, and one marker SR1 showed linkage with RSV1 at a genetic distance of 2.9 cM. Finally, RSV1 gene was mapped on chromosome 11 between SSR markers RM457 and SRAP marker SR1 with a genetic distance of 4.5 cM and 2.9 cM, respectively.     

Identification of Quantitative Trait Loci Controlling Drought Tolerance at Seedling Stage in Chinese Dongxiang Common Wild Rice (Oryza rufipogon Griff.)

Common wild rice (Oryza rufipogon Griff.) is the ancestor of cultivated rice (O. sativa L.), which has a greater genetic diversity and important traits that remain to be employed in cultivated rice. In this study, a set of introgression lines (BC₄F₅ and/or BC₄F₆) carrying various introgressed segments from common wild rice, collected from Dongxiang county, Jiangxi Province, China, in the background of an Indica (O. sativa L. ssp. indica) cultivar, Guichao 2, was used. A total of 12 drought-related quantitative trait loci (QTL) were identified by investigating drought tolerance of introgression lines under 30% PEG treatment at the young seedlings stage. Of these QTLs, the alleles of 4 QTLs on chromosome 2, 6 and 12 from Dongxiang common wild rice were responsible for increased drought tolerance of the introgression lines. In particular, a QTL qSDT12-2, near RM17 on chromosome 12, was consistently detected in different replications, and expressed stably under PEG stress throughout the study. It was also found that the QTLs located on different chromosomes might express at different stages.     

Soybean aphid resistance genes in the soybean cultivars Dowling and Jackson map to linkage group M

The soybean aphid [Aphis glycines Matsumura] is an important pest of soybean [Glycine max (L.) Merr.] in North America. Single dominant genes in the cultivars ‘Dowling’ and ‘Jackson’ control resistance to the soybean aphid. The gene in Dowling was named Rag1, and the genetic relationship between Rag1 and the gene in Jackson is not known. The objectives of this study were to map the locations of Rag1 and the Jackson gene onto the soybean genetic map. Segregation of aphid resistance and simple sequence repeat (SSR) markers in F _2:3 populations developed from crosses between Dowling and the two susceptible soybean cultivars ‘Loda’ and ‘Williams 82’, and between Jackson and Loda, were analyzed. Both Rag1 and the Jackson gene segregated 1:2:1 in the F _2:3 populations and mapped to soybean linkage group M between the markers Satt435 and Satt463. Rag1 mapped 4.2 cM from Satt435 and 7.9 cM from Satt463. The Jackson gene mapped 2.1 cM from Satt435 and 8.2 cM from Satt463. Further tests to determine genetic allelism between Rag1 and the Jackson gene are in progress. The SSR markers flanking these resistance genes are being used in marker-assisted selection for aphid resistance in soybean breeding programs. Trade and manufacturers’ names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable.