Effect of the number of intercrosses on grain yield in basic rice synthetic populations

The effect of 0, 1, 2, 3 and 4 intercross cycles using genetic male sterility on the mean yield and genetic variability of the CNA 5 irrigated rice synthetic population was assessed studying 300 S_0:2 (1998/99) and S_0:3 (1999/00) families grown in two locations (Lambari, MG and Goianira, GO). Each intercross was investigated using a 8 × 8 triple lattice design experiment containing 60 families and four control cultivars. Five experiments were carried out per location and generation, totaling 20 experiments. Individual, combined over locations, combined over generations and combined over locations and generations analyses of variance were performed. Linear regression equations were fitted to data of each location and generation as well as, genetic and phenotypic parameters were estimated. An average increase of 3.91% in grain yield was obtained with the increase in the number of intercrosses. However, when genetic gains obtained from selection of the five and ten highest yielding families were considered, negative b estimates were always obtained, suggesting a tendency of gain increase reduction with the increase in the number of intercrosses. This was confirmed by the analysis of the means of the best families, since increases in the number of intercrosses did not result in selection of families with greater mean grain yield. The estimates of the genetic variances and heritabilities did not suggest an increase in genetic variability with the increase in intercross cycles. Therefore, carrying out intercrosses on the base population did not improve the results of the irrigated rice recurrent selection program. This revised version was published online in August 2006 with corrections to the Cover Date.     

A new gene for resistance in rice to Asian rice gall midge (Orseolia oryzae Wood Mason) biotype 1 population at Raipur, India

The Asian rice gall midge, Orseolia oryzae Wood Mason (Diptera: Cecidomyiidae), is a major pest of rice in several South and South East Asian countries. The maggots feed internally on the growing tips of the tillers and transform them into tubular galls, onion leaf-like structures called ‘silver shoots’ resulting into severe yield loss to the rice crop. We studied the mode of inheritance and allelic relationships of the resistance genes involved in resistant donor Line 9, a sib of a susceptible cultivar ‘Madhuri’. The segregation behaviour of F₁, F₂ and F₃ populations of the cross between Line 9 and susceptible cultivar MW10 confirmed the presence of a single dominant gene for resistance. Tests of allelism with all the known genes giving resistance to this population indicated that Line 9 possessed a new gene which was designated Gm 9 This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterization of segregation distortion on chromosome 3 induced in wide hybridization between indica and japonica type rice varieties

We previously surveyed chromosomal regions showing segregation distortion of RFLP markers in the F₂ population from the cross between a japonica type variety ‘Nipponbare’ and an indica type variety ‘Milyang23’, and showed that the most skewed segregation appeared on the short arm of chromosome 3. By comparison with the marker loci where distortion factors were previously identified, this region was assumed to be a gametophytic selection-2 (ga2) gene region. To evaluate this region, two near isogenic lines (NILs) were developed. One NIL had the ‘Nipponbare’ segment of this region on the genetic background of ‘Milyang23’ (NIL9-23), and the other NIL had the ‘Milyang23’ segment on the genetic background of ‘Nipponbare’ (NIL33-18). NIL9-23 and ‘Milyang23’, NIL33-18 and ‘Nipponbare’, and ‘Nipponbare’ and ‘Milyang23’ were respectively crossed to produce F₁ and F₂ populations. The F₁ plants of NIL9-23 × ‘Milyang23’ and NIL33-18 × ‘Nipponbare’ showed high seed fertility and the same pollen fertility as their parental cultivars, indicating that ga2 does not reduce seed and pollen fertility. Segregation ratio of a molecular marker on the ga2 region in the three F₂ populations was investigated to clarify whether segregation distortion occurred on the different genetic backgrounds. Segregation distortion of the ga2 region appeared in the both F₂ populations from the NIL9-23 and ‘Milyang23’ cross (background was ‘Milyang23’ homozygote) and the ‘Nipponbare’ and ‘Milyang23’ cross (background was heterozygote), but did notin the F₂ population from the NIL33-18 and ‘Nipponbare’ cross (background was ‘Nipponbare’ homozygote). This result indicates that ga2 interacts with a ‘Milyang23’ allele(s) on the different chromosomal region(s) to cause skewed segregation of the ga2 region. In addition, segregation ratio was the same between the F₂ populations from NIL9-23 × ‘Milyang23’ and ‘Nipponbare’ × ‘Milyang23’ crosses, suggesting that the both genotypes, ‘Milyang23’ homozygote and heterozygote, of gene(s) located on the different chromosomal region(s) have the same effect on the segregation distortion. This revised version was published online in July 2006 with corrections to the Cover Date.     

稻属间杂种花粉植株的细胞遗传学研究

以栽培稻０２４２８与紧穗野生稻间不育Ｆ１杂种（２ｎ＝２４，ＡＣ）为材料进行花约培养，共接种花约３２６８８个，获得再生绿苗１３株及白化苗两株，愈伤诱导率与分化率分别为０．１４％和３４．７８％。有９朱花粉绿苗长至成熟，均表现长芒、紫柱头、强感光及易落粒等野生亲本的特性。细胞学结果进一步表明，其中４株为二倍体（２ｎ＝２４），形态上琚的Ｆ１杂种相似，花粉母细胞中期Ⅰ亦只能见到少数二价体；另５株则为四倍体（     

Breeding of widely compatible restorer lines in rice (Oryza sativa L.) through anther culture

Following anther culture of various F₁ hybrids of indica restorer (R) lines/wide compatible varieties (WCVs) and japonica R lines/WCVs, the homozygous diploid plants (2n) generated were test-crossed with indica WA type cytoplasmic-genetic male sterile (CMS) line Zhanshan 97A (WA), indica testers Nanjing 11 and Nante, japonica BT CMS line Hanfeng A (BT), and japonica testers Balilla and Akihikari to identify widely compatible restorer lines. The results of this study showed that among the diploid pollen plants generated from F₁ hybrids of indica R lines/WCVs, 36.7% and 64.7% possessed normal fertility restoration ability (rate of seed-setting > 80%) to Hanfeng A and Zhanshan 97A, respectively. 71.3% and 32.3% had normal fertility restoration ability to Hanfeng A and Zhanshan 97A, respectively, in diploid pollen plants derived from the japonica R lines/WCVs F₁'s anther culture. Several widely compatible R lines were selected from anther culture of F₁ hybrids of indica R lines/WCVs and japonica R lines/WCVs. These widely compatible R lines derived from diploid pollen plants showed good wide compatibility and restoration ability both to WA and BT type CMS lines. Strong standard heterosis of major agronomic traits and yield traits was observed in F₁ test-crosses of widely compatible R lines with Hanfeng A and Zhanshan 97A. The wide compatibility in widely compatible R lines H17, H158 and H281 was studied by a set of three-way crosses. Segregation of the fertile plants with seed-setting rate > 70% and semi-fertile plants with seed-setting rate > 69.9% agreed with a ratio of 1:1, indicating that wide compatibility in these widely compatible R lines is governed by a pair of major genes. This revised version was published online in July 2006 with corrections to the Cover Date.     

QTL mapping and associated marker selection for the efficacy of green plant regeneration in anther culture of rice

Anther culturability of rice is a quantitative trait controlled by nuclear‐encoded genes. The identification of quantitative trait loci (QTL) and associated marker selection for anther culturability is important for increasing the efficiency of green plant regeneration from microspores. QTL associated with the capacity for green plant regeneration in anther culture of rice were mapped on chromosomes 3 and 10 using 164 recombinant inbred (RI) lines from a cross between ‘Milyang 23’ and ‘Gihobyeo’. The quantitative trait locus located on chromosome 10 was detected repeatedly when three anther culture methods were applied and was tightly linked to the markers, RG323, RG241 and RZ400. Associations between these markers and the efficacy of green plant regeneration in 43 rice cultivars and two F₂ populations, ‘MG RI036’/‘Milyang 23’, and ‘MG RI036’;/‘IR 36’ were analysed. One of these markers, RZ400, was able to identify effectively genotypes with good (> 10.0%) and poor (< 3.0%) regenerability, based on the marker genotypes in the cultivars and two F₂ populations. This marker enables the screening of rice germplasm for anther culturability and introgression into elite lines in breeding programmes.     

Homozygous transgenic rice lines expressing GNA with enhanced resistance to the rice sap-sucking pest Laodelphax striatellus

Homozygous transgenic rice lines expressing snowdrop lectin, Galanthus nivalis agglutinin (GNA), were investigated for resistance to the rice small brown planthopper (SBPH), Laodelphax striatellus, an important rice sap‐sucking pest causing yield losses and serving as vector for some important rice viruses. Insect bioassay results revealed that both the GNA‐expressing homozygous lines tested significantly inhibited SBPH by decreasing its survival, its overall fecundity and retarding development. This is the first report on homozygous transgenic rice lines expressing GNA conferring enhanced resistance to SBPH. This result suggests that transgenic rice expressing GNA will be useful in rice pest resistance breeding, as an alternative to conventional breeding for the control of SBPH, and potentially alleviating damage caused by the viruses it transmits.     

Chromosomal localization of five mutant genes in rice, Oryza sativa, using primary trisomics

The chromosomal locations of five mutant genes in rice were determined by crossing the marker stocks with the 12 primary trisomics. Genetic segregation of each gene was studied in the F₂ or backcross populations. Out of the 60 possible cross combinations, 43 F₂ or BC₁ populations were studied. Segregation data indicated that spl11 was located on chromosome 12 while wp2 and eg2_(t) were located on chromosome 6. The genes v12_(t) and Bc₆ were located on chromosomes 8 and 9, respectively, which are sparsely populated with genetic markers.     

Optimizing selection for rough rice yield, head rice, and total milled rice

High rough rice yields are important for the profitable production of rice (Oryza sativa L.). The value of rough rice is determined primarily by head rice (whole milled grains) and broken rice, together referred to as total milled rice. The objective of this study was to optimize selection for rough rice yield, head rice, and total milled rice using path-coefficient analysis and selection indices. Thirty-seven long grain pure lines were grown in an experiment at three Arkansas locations during 1993 and 1994. In another test, twenty-eight F₁ hybrids obtained from an eight-parent half diallel of long-grain rices were grown at two locations in Arkansas during 1991. For the pure lines, path analysis revealed that rough rice yield was approximately twice as important in determining total milled rice/ha than head rice. Path analysis for the hybrids revealed that rough rice yield was an even greater component in determining total milled rice/ha. Based on yield components for hybrid rice, panicle density had the largest direct effect determining total milled rice/ha. Filled grain/panicle, grain weight, and head rice, in that order, were secondary, but positive, factors determining total milled rice/ha. A selection index for pure lines based only on selection for head rice was 19.2 percent as effective as selection for total milled rice/ha directly. When selection for total milled rice/ha was based solely on rough rice yield, selection was 82.7 percent as effective as selecting for total milled rice/ha directly. Selection to improve total milled rice/ha should concentrate on increasing both rough rice yield and head rice. Rough rice yield should receive greater priority than head rice in selection strategies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Heterosis in inter and intrasubspecific crosses over three-environments in rice

Moderate to high heterosis for yield and ten related characters was studied in 45 crosses involving six indica (I) and four tropical japonica (J) varieties of rice under three (E₁-optimum sowing and high fertility, E₂-both optimum sowing and fertility and E₃- late sowing and high fertility) environments. Trends of magnitude of heterosis for grain yield and plant height were I/J > I/I > J/J and for days to 50% flowering I/J > J/J > I/I hybrids. Estimates of standard heterosis (%) for grain yield were –64.5 to 146.1 in E₁, –70.4 to 82.2 in E₂ and –67.2 to 63.8 in E₃. Environment E₁ seemed to be more favorable for higher heterosis expression than others. Higher heterosis in yield also accompanied heterosis in panicle number, dry matter and spikelet and grain number per panicle. Most estimates for days to flower were negative. Heterotic I/J hybrids also recorded maximum heterosis for earliness. Moderate to low standard heterosis for plant height across environments (2.0 to 13.7&percnt;) was recorded. Hybrids were identified in specific environments for direct exploitation in hybrid breeding. Hybrids B4116 / Sarjoo 52, B4122 / Pant Dhan 4 and B4122 / Narendera 359 were more stable than others over three environments. This revised version was published online in July 2006 with corrections to the Cover Date.