An analysis of hybrid sterility in rice using a diallel cross of 21 parents involving indica, japonica and wide compatibility varieties

Summary There is much interest in the inter-varietal fertility of Asian cultivated rice. In this study, we analyzed the fertility of hybrids in a diallel set of 210 crosses involving 21 parents representing a broad range of the cultivated rice germplasm including landraces, primitive cultivars, modern elite cultivars and parents of hybrid rice. The materials were also carefully selected with respect to indica-japonica classification including typical indica/japonica, more or less intermediate and wide compatibility varieties. The level of hybrid fertility varied widely among the crosses from almost completely sterile to fully fertile. In general, hybrid fertility of intra-subspecific crosses, i.e., indica by indica (I×I) and japonica by japonica (J×J), is much higher than inter-subspecific crosses (I×J or J×I). The fertility varied widely in hybrids involving wide compatibility varieties and also in inter-subspecific crosses not involving wide compatibility varieties. An analysis of variance showed that both the main effects of indica and japonica parents and the interaction between the parents are highly significant in determining hybrid fertility. We speculate that, in addition to wide compatibility, the overall genetic difference between the indica and japonica parents, that may involve a series of minor mutations, play an important role in determining the hybrid fertility. There are also genes of sizable effect that influence fertility in hybrids resulting from specific combinations of the parents.Abbreviations I×I indica by indica cross - I×J indica by japonica cross - J×I japonica by indica cross - J×J japonica by japonica cross - WCV wide compatibility variety     

Cytoplasm and cytoplasm-nucleus interactions affect agronomic traits in japonica rice

Some sources of cytoplasm have been favoredin rice breeding for unknown reasons. Tostudy cytoplasmic effects on agronomictraits, five core japonica parents inOryza sativa, Xinan 175, Reimei,Keqing No. 3, Todorokiwase, and Toride No.1, which are ancestors of 75 % of cultivarsbred in Yunnan, China, were used as femaleparents in crosses with 3 distinctjaponica rice cultivars, 8-126,Lijiangxintuanheigu, and Norinmochi No.20as males. These nuclear genomes weresubstituted into the five cytoplasms viaseven backcrosses using the original maleas recurrent parent. Fifteen combinationsof BC₇F₂ and their parents weresown in Jinghong during the late season(July-October) for agronomic evaluation in1999. Meanwhile, all materials werescreened for low temperature tolerancebased upon two methods in Kunming (1916 mabove sea level): natural field and lowtemperature water (19 °C) cyclingirrigation at booting stage. Spikeletfertility was used as an indication of lowtemperature tolerance. Effects of cytoplasmon yield, width of flag leaf, and lowtemperature tolerance were significant. Significant cytoplasm-nucleus interactionon yield, plant height, and low temperaturetolerance were also observed. These resultsindicated that cytoplasm andcytoplasm-nucleus interaction playedimportant roles in yield, low temperaturetolerance, and some important agronomictraits in japonica rice. The role ofcytoplasm and cytoplasm-nucleus interactionshould be considered in future ricebreeding and resources work.     

A gamete abortion locus detected by segregation distortion of isozyme locus Est-9 in wide crosses of rice (Oryza sativa L.)

Summary A locus for male gamete abortion in hybrids for Japonica and Indica rice was identified with the aid of marker genes Rc and Est-9 on chromosome 7. In an Indica-Japonica cross, AKAMAI 1/IR50, the Indica allele Est-9 ² was transmitted via the male gamete with a ratio of 0.29 instead of the normal 0.5, whereas no segregation distortion was observed for the Rc locus. The recombination value (p) for Est-9 and Rc was estimated to be 0.38 by a least square method after adjusting Mendelian segregation ratios with the male transmission ratios of 0.29 (Tr) for Est-9 ² and 0.71 (1-Tr) for Est-9 ¹. The recombination value (q) for the new locus for male gamete abortion, ga-11, and Est-9 was estimated to be 0.23 by using 56 F₃ lines from F₂ plants which were heterozygous for the Est-9 locus. No linkage for Rc and ga-11 was found. Therefore, the two markers and ga-11 were located in the order of ga-11-Est-9-Rc. Using the estimated recombination value (q), the male transmission rate (k) of ga-11 ^a was estimated to be 0.11 with the F₂ data and-0.07 with the F₃ line data. Thus, it was apparent that male gametes possessing ga-11 ^a were frequently aborted in the Indica-Japonica hybrid.     

Genetic base of Japonica rice varieties released in Taiwan

Summary The pedigrees of 99 Japonica rice (Oryza sativa L.) varieties released between 1940 and 1987 were traced back to 65 ancestors, of which 44 were plant introductions from Japan and only 11 from Taiwan. Japanese introductions collectively contributed more than 85% of the parentage of the varieties. Although more ancestors were integrated into recent breeding programs, as few as 10 ancestors comprised 66% of the genetic background. Extensive use of superior genotypes from the same sources explained why the integration of new germplasm did not result in the diversification of the released varieties. The 10 most important ancestral contributors of the 99 varieties were from Japan except Oloan-chu from Taiwan. Two introductions, Sinriki and Kameji, had the highest mean relative genetic contributions of the genes with 21.3% and 16.7%, respectively. Shinriki was the most frequently used ancestor and occurred in 83 of the 99 pedigrees. These results clearly revealed the narrow genetic base in current Japonica rice varieties of Taiwan.     

Anther culture of tropical japonica × indica hybrids of rice (Oryza sativa L.)

Summary Nine japonica × indica F1 hybrids of rice involving 6 indica and 3 japonica tropical varieties, were large scale anther cultured. The frequency of callusing anthers averaged 18.7%. The microspore-derived calli produced green plants with a mean frequency of 8.7%. Albino plants represented 61% of the shoot forming calli. Monitoring of the green and albino plant regenerating capabilities of calli arising between week 4 and week 8 of incubation of the anthers showed no increase of the albino/green ratio and a slow decrease of the shoot forming ability of the transferred calli after the sixth week of culture. Spontaneous doubled haploids (SDH) represented 46% of the regenerated green plants in 4 hybrids. However, a high frequency of partially sterile regenerants was noticed among 132 SDH plants generated from a hybrid.     

Genetic studies on the interspecific cytoplasm substitution lines of japonica varieties of Oryza sativa L. and O. glaberrima Steud.

Summary Interspecific cytoplasm substitution lines of Oryza sativa and O. glaberrima, i.e. (sativa)-glaberrima and (glaberrima)-sativa, have been bred by means of successive backcrosses, using three japonica varieties of sativa and two glaberrima strains.In all the six substitution lines with the cytoplasm of the glaberrima strains, the fertility increased with succeeding backcrosses, and eventually completely fertile plants whith the characteristics of the parental japonica variety appeared. This indicates that the glaberrima cytoplasm exerted no effect on the genome manifestation of these japonica varieties. Of the five substitution lines with the cytoplasm of each of the japonica varieties, four lines produced male sterile (M.S.) plants only in the backcross generations. In the remaining substitution line with the cytoplasm of the japonica variety Akebono, there was simultaneous segregation for male sterile (M.S.) and pollen fertile plants bearing indehiscent anthers (ID.M.F.) in the backcross generations. In the compulsively selfed progeny of ID.M.F. plants, pollen fertile plants with dehiscent anthers (D.M.F.) occurred with M.S- and ID.M.F. plants. Morphologically, these three types were supposed to have the same genetic background as the glaberrima parent. It was established that D.M.F.-and ID.M.F. plants were homozygous and heterozygous for a dominant nuclear gene restoring pollen fertility, respectively, and the M.S. plants and the two glaberrima strains used in this study carried a recessive gene for pollen sterility in homozygous condition. The restorer gene was assumed to derive from the japonica variety Akebono. The expression of the restorer gene was of the sporophytic type. The pollen sterility of the substitution lines that possessed the cytoplasm of the japonica varieties was of cytoplasmon-genic nature.     

Molecular mapping of S-c, an F1 pollen sterility gene in cultivated rice

Hybrids between indica and japonica rice varieties usually show partial sterility, and are a major limiting factor in the utilization of heterosis at subspecific level. When studying male-gamete (pollen) abortion, a possibly important cause for sterility, six loci (S-a, S-b, S-c, S-d, S-e and S-f) for F₁ pollen sterility were identified. Here we report genetic and linkage analysis of S-c locus using molecular markers in a cross between Taichung 65, a japonica variety carrying allele S-c ^j, and its isogenic line TISL5, carrying alleleS-c ^j. Our results show that pollen sterility occurring in the hybrids is controlled by one locus. We used 208 RFLP markers, as well as 500 RAPD primers, to survey the polymorphism between Taichung 65 and TISL5. Six RFLP markers located on a small region of chromosome 3, detected different RFLP patterns. Co-segregation analysis of fertility and RFLP patterns with 123 F₂ plants confirmed that the markers RG227, RG391, R1420 were completely linked with the S-c locus. The genetic distances between the markers C730, RG166 and RG369 and the S-c locus were 0.5 cM, 3.4 cM, and 3.4 cM respectively. Distorted F₂ ratios were also observed for these 4 RFLP markers in the cross. This result suggests that the `one locus sporo-gametophytic' model could explain F₁ hybrid pollen sterility in cultivated rice. RG227, the completely linked marker, has been converted to STS marker for marker-assisted selection. This revised version was published online in August 2006 with corrections to the Cover Date.     

Identification of near-isogenic lines: an innovative approach,validated for root and shoot morphological characters in a mapping population of rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

Near-isogenic lines (NILs) constitute valuable tools in genetic investigations and plant breeding programs. Conventional methods for developing these are time consuming and tedious. An innovative method for identifying NILs is proposed and validated. The method involves computation of simple correlation coefficients of all possible pairs of genotypes within a mapping population using molecular marker data, and phenotypic characterization of those pairs with very high positive correlation. The pairs having both genomic and phenotypic similarity except for a single trait are considered as NILs. This strategy was tested with a doubled haploid mapping population involving CT9993 and IR62266. This population was saturated with 315 markers and comprised 154 lines. The pairs showing very high correlation coefficients (0.70–0.97) and differing for less than 10% of the markers were considered as Genotypically Closely Related Pairs (GCRPs). Graphical genotyping was employed to visualize the genome of the closely related lines. A total of 39 such pairs were subjected to rigorous evaluation for root and shoot morphological traits in two contrasting moisture regimes. Four GCRPs under well-watered condition and ten GCRPs under low moisture stress condition are statistically significant for a single phenotypic trait and are considered as NILs for their respective traits and would be the valuable materials for genetic studies. Mapped QTLs and candidate genes were employed to explain the probable cause of phenotypic difference in NILs.     

Inheritance and mapping of male sterility restoration gene in upland japonica restorer lines

Several upland Japonica breeding lines, WAB450-11-1-3-P40-HB (Abbreviated as WAB450-11), WAB450-11-1-2-P61-HB (WAB450-13), WAB450-l-B-P-91-HB (WAB450-14), IRAT216, IRAT359, and IRAT104, possessing restoring ability for the Dian 1 type cms (cms-D) line Dianyu 1A were recently identified at Food Crops Research Institute, Yunnan Academy of Agricultural Sciences, P. R. China. In this study, the inheritance of restoring ability in these lines was characterized through the production of backcross populations to the male-sterile and maintainer Dianyu 1 lines. Each of the restorer lines was used to pollinate Dianyu 1A to form a F₁ hybrid which was then backcrossed (1) with Dianyu 1B producing a BC₁F₁ population and (2) to the female parent Dianyu 1A producing a BC₅F₂ population. The lines were also crossed with the japonica restorer line C57, carrying the restorer gene Rf1 that was introgressed from indica, to form F₁ hybrids, these hybrids were then testcrossed with Dianyu 1A to study the allelic relationship of their restorer genes to Rf1. The inheritance in these testcross populations indicated that the complete restoring ability of WAB450-11, WAB450-13, WAB450-14, IRAT216, IRAT359, and the partial restoring ability of IRAT104 were controlled by dominant genes, and the gene in WAB450-13, WAB450-14, and IRAT216 was allelic or identical to Rf1. When 136 SSR markers were used to score 143 BC₁F₁ individuals from Dianyu 1A/WAB450-13//Dianyu 1B, the japonica Rf1 allele was found to be located between RM171 and RM6100 on the long arm of chromosome 10, an interval corresponding to that known for the indica Rf1 allele. The distance between RM171 and Rf1 is 2.8 cM, and that between Rf1 and RM6100 is 4.9 cM. Similar linkage results were obtained from mapping 89 individuals of the corresponding BC₅F₂ population (Dianyu 1A/6/Dianyu 1A/WAB450-13).     

Genome differentiation between Lycopersicon esculentum and L. pennellii as revealed by genomic in situ hybridization

The interaction between the Se1 and the Ef1 loci, which chiefly control the photoperiod sensitivity (PS) and the basic vegetative growth (BVG) period of rice (Oryza sativa L.) respectively, was investigated using four tester lines different in genotype for the two heading time loci from each other. The four tester lines were grown under 10, 13, 14, 15, and16h day lengths to estimate their BVG period and PS. The Taiwanese cultivar Taichung 65(T65), one of the tester lines, has an extremely long BVG period that has been considered to be conferred by a late heading-time allele ef1 at the Ef1 locus. Experimental results, however, showed that the extremely long BVG of T65was conferred not by a single effect ofef1 but by a complementary effect ofef1 and Se1-e, a photoperiod insensitivity allele, at theSe1 locus. It was also found that a complementary effect of a PS allele Se1-n at the Se1 locus and ef1stimulates the PS of rice. Gene analysis for heading time under an optimum daylength (10 h) as well as under natural day length confirmed the presence of the complementary effect of the two nonallelic genes on BVG, which was found only with homozygosity of both the genes. Based on these results and earlier reports on the Se1 locus, the roles of the Se1 andEf1 loci on the durations of pre-flowering developmental phases in rice were discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Interaction between the eui gene and thermo-sensitive genic male sterility in rice

Panicle enclosure is a typical phenotype of almost all male-sterile rice lines. An elongated uppermost internode (eui) mutant exhibited notably rapid elongation of the uppermost internode at the heading stage; this is considered as a potential mechanism to eliminate panicle enclosure. We developed thermo-sensitive genic male-sterile (TGMS) eui mutants that were characterized by notably elongated uppermost internodes. The elongation of the uppermost internode in the TGMS eui mutant Changxuan 3S (CX) is mainly attributed to an increase in cell number and cell elongation, the latter being the more significant process. Temperature treatments revealed that the effects of temperature on panicle exsertion were similar to those on fertility and that the most temperature-sensitive stage coincides with the period from the formation of the pollen mother cell to meiosis during panicle initiation. These results indicate that elongation increases as temperature decreases and that the expression of the eui gene is more efficient at low temperatures than at high temperatures. In hybrid rice seed production using the TGMS eui mutant, the temperature range should be optimized at 24–28°C in order to preserve the development of completely male-sterile pollen and to eliminate panicle enclosure. Consequently, by using TGMS eui rice lines, gibberellin application can be avoided, thereby reducing the cost of hybrid seed production. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Isolation and characterization of a rice mutant insensitive to cool temperatures on amylose synthesis

The Wx ^b gene, one of the alleles at the rice waxy(wx) locus, is activated at cool temperatures during seed development, andas a result, a large amount of amylose is accumulated causing a reductionin rice grain quality. We found that the seeds of a du mutant couldbe visibly distinguished depending on whether they matured at cool ornormal temperatures. Using these characteristics, we isolated a mutantcandidate insensitive to cool temperatures. While the amylose content inthe original line was about 2% at a normal temperature (28 °C)and 12% at a cool temperature (21 °C), in the mutant candidate(coi) the amylose content was not affected by temperatures, i.e. theamylose content was about 3% at both temperatures. This finding incombination with the results of an immunoblot analysis indicated that theabsence of an increase in the amylose content in this mutant was caused bya constant level of Wx gene expression at normal and cooltemperature. Genetic analysis revealed that this insensitivity to cooltemperatures was caused by a single recessive mutation. This mutantshould be useful in breeding programs designed to produce rice of desiredquality at cool temperatures and in understanding genetic and molecularmechanisms that respond to slight changes in temperature.     

QTL-based analysis of genotype-by-environment interaction for grain yield of rice in stress and non-stress environments

Use of DNA-based markers can accelerate cultivar development in variable cultivation environments since, in contrast to phenotype, DNA markers are environment-independent. In an effort to elucidate the genetic basis of genotype-by-environment interaction (G × E) for yield of rice (Oryza sativa L.), the associations between 139 AFLP markers and grain yield were determined for rice grown in fresh water (EC of 0.65 dS m⁻¹) and saline conditions (EC of 4–8 dS m⁻¹) with 0 kg ha⁻¹ or 100 kg ha⁻¹ nitrogen fertilizer in the years 2000 and 2001. A population of recombinant inbred lines of rice, developed from an IR29 × Pokkali cross, was used in the study. Both genotype × salinity and genotype × nitrogen level interactions were significant, with the genotype × salinity interaction being stronger. Through multiple regression analysis using a stepwise procedure for selecting markers, 36 markers were detected for grain yield in the four test conditions and of these 28 were detected in only one test condition implying strong environmental specificity for yield QTL expression. However, the fact that eight QTLs were detected in more than one test condition points to the existence of wide-adaptability genes in this cross. Markers with significant associations with yield explained between 37% and 48% of the yield variation in each test condition. Superior genotypes of rice were identified in all four test conditions based on their marker signatures. Furthermore, across N fertilizer regimes, yield predicted from summed additive effects of QTLs were significantly correlated with observed yield in the same year and across years. Thus marker-assisted selection can help breeders overcome the problem of low selection efficiency encountered during phenotypic selection for yield in stress environments.     

Agronomic and morphological characterization of Agrobacterium-transformed Bt rice plants

The agronomic and morphological characteristics of Agrobacterium-transformed rice plants carrying the synthetic cry1Ab or cry1Ac gene were investigated. Tremendous variations in plant height, seed fertility, grain size and other traits were seen in 80 T₁ lines, derived from 80 T₀ plants of 9 rice varieties. On average, about 33% T₁ lines had either morphological or agronomic variant plants. Most of the variations in T₁ plants had no significant correlation with transgene insertion and were proved heritable to their progenies. Genetic analysis in T₃ or T₄ generations showed some simple mutations such as chlorophyll deficiency and stunted plants were independent of transgene insertion and seemed to be controlled by a pair of single genes. However, in two independent transgenic progenies of Xiushui 11, all plants homozygous for transgenes showed dwarfism while all hemizygous and null segregants had normal plant heights. Two advanced homozygous Bt lines, KMD1 and KMD2, were developed from these two progenies. Comparison of the agronomic traits of KMD1 and KMD2 with their parent displayed marked differences among them in terms of seedling growth, tillering ability, yield components and yield potential. The genetic variation observed was generally not linked to the transgene locus and was ascribed to somaclonal variation, but other causes might also exist in particular cases. The results are discussed in the context of choosing appropriate transformation methodology for rice breeding programs. This revised version was published online in August 2006 with corrections to the Cover Date.     

Assessment of genetic diversity within and among Basmati and non-Basmati rice varieties using AFLP,ISSR and SSR markers

Molecular markers provide novel tools to differentiate between the various grades of Basmati rice, maintain fair-trade practices and to determine its relationship with other rice groups in Oryza sativa. We have evaluated the genetic diversity and patterns of relationships among the 18 rice genotypes representative of the traditional Basmati, cross-bred Basmati and non-Basmati (indica and japonica) rice varieties using AFLP, ISSR and SSR markers. All the three marker systems generated higher levels of polymorphism and could distinguish between all the 18 rice cultivars. The minimum number of assay-units per system needed to distinguish between all the cultivars was one for AFLP, two for ISSR and five for SSR. A total of 171 (110 polymorphic), 240 (188 polymorphic) and 160 (159 polymorphic) bands were detected using five primer combinations of AFLP, 25 UBC ISSR primers and 30 well distributed, mapped SSR markers, respectively. The salient features of AFLP, ISSR and SSR marker data analyzed using clustering algorithms, principal component analysis, Mantel test and AMOVA analysis are as given below: (i) the two traditional Basmati rice varieties were genetically distinct from indica and japonica rice varieties and invariably formed a separate cluster, (ii) the six Basmati varieties developed from various indica × Basmati rice crosses and backcrosses were grouped variably depending upon the marker system employed; CSR30 and Super being more closer to traditional Basmati followed by HKR228, Kasturi, Pusa Basmati 1 and Sabarmati, (iii) AFLP, ISSR and SSR marker data-sets showed moderate levels of positive correlation (Mantel test, r = 0.42–0.50), and (iv) the partitioning of the variance among and within rice groups (traditional Basmati, cross-bred Basmati, indica and japonica) using AMOVA showed greater variation among than within groups using SSR data-set, while reverse was true for both ISSR and AFLP data-sets. The study emphasizes the need for using a combination of different marker systems for a comprehensive genetic analysis of Basmati rice germplasm. The high-level polymorphism generated by SSR, ISSR and AFLP assays described in this study shall provide novel markers to differentiate between traditional Basmati rice supplies from cheaper cross-bred Basmati and long-grain non-Basmati varieties at commercial level.The first two authors have equal contribution     

Photoperiod insensitivity gene essential to the varieties grown in the northern limit region of paddy rice (Oryza sativa L.) cultivation

Summary The release of extremely early maturing varieties has made it possible to cultivate rice in Hokkaido (NL45-42°) in Japan, the northern limit region of paddy rice cultivation. Until then, rice cultivation in this region has been impracticable due to the climate condition, especially short summer and long-day more than 15 hours during summer. Experiment results confirmed that the success of rice cultivation in this area depends on raising photoperiod insensitivity varieties with short basic vegetative growth period. Moreover, in this study, the genetic factors controlling the photoperiod insensitivity of Hokkaido varieties were analyzed by using 8 kinds of tester lines for three loci, E1, E2, and E3, controlling photoperiod sensitivity. It was found out that all the varieties examined carry el, a photoperiod insensitivity allele of E1 locus, but as for the other loci, E2 and E3, the existence of plural alleles were recognized. We have already clarified that almost all the japonica-type varieties grown in Japan (except Hokkaido) and Taiwan carry E1 bringing about strong photoperiod sensitivity. Accordingly, it can be concluded that el is the gene essential to Hokkaido varieties: the interchange of E1 with e1 enabled rice cultivation under long-day condition.     

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.     

Inheritance of resistance to rice stripe virus in rice line `BL 1'

Rice stripe is the most serious virus disease in temperate rice-growing countries. The most economical and environmentally safe practice for controlling this disease is virus-resistant cultivars. ‘BL 1’ is an elite germplasm line with the blast resistance gene Pib, and has been used as a differential line for testing the pathogenicity of the blast fungus. We found that certain progenies from BL 1 showed resistance to both blast and rice stripe virus (RSV). The objectives of this study were to evaluate the RSV resistance in the field and under artificial conditions, to assess the reaction to the insect vector(small brown plant hopper, SBPH), and to examine its inheritance and its relationship to blast resistance in BL 1.BL 1 was susceptible to SBPH, but resistant to RSV in field and artificial inoculation tests. The inheritance of RSV resistance in F₃ lines from the cross Nipponbare (NPB)/BL 1 was studied using artificial inoculation with a population of viruliferous SBPH. A serological assay for RSV infection using an enzyme-linked immunosorbent assay (ELISA) was used. RSV resistance in BL 1 was controlled by a single major gene with incomplete dominance. The locus responsible for RSV resistance was genetically independent of the blast resistance gene Pib. The resistance gene for RSV infection in BL 1 was also independent of Stvb-i, a gene widely distributed in resistant Japanese cultivars. Resistance to RSV must be diversified in rice cultivars considering the potential for future emergence of new RSV strains. The new resistance gene identified in BL 1, which has improved plant type and blast resistance, is considered useful for breeding RSV-resistant cultivars in japonica rice. This revised version was published online in August 2006 with corrections to the Cover Date.     

Studies on the amylose content and gelatinisation temperature in certain local cultivars and induced grain shape mutants in rice

Summary Ninety local cultivars and 124 induced grain shape mutants were screened for their amylose content and gelatinisation temperature as indexed by the alkali disintegration patterns. The amylose content was high in most of the local cultivars and ranged from 19.2 to 32.0% and from 12.3 to 33.2% in the mutants. Wide variation was also observed for the alkali disintegration values and most of the varieties had low alkali values. A positive but low correlation between alkali values and amylose content and a negative correlation between alkali values and protein content was observed. The preliminary studies suggest that the amylose content at different milling levels increased with an increase in the level of milling. Some of the local cultivars and induced mutants were found to be superior with high amylose and high gelatinisation temperature and could be utilized in breeding programmes directed towards improving the cooking quality of high yielding varieties.     

Genetics of stem elongation ability in rice (Oryza sativa L.)

Summary The genetics of stem elongation ability in rice was studied in parents, F₁, F₂ and backcross generations of six crosses. Segregation analysis indicated dominance for stem elongation ability. Estimation of genetic parameters under epistatic model indicated more than one locus control stem elongation ability and both additive and nonadditive gene effects were important. Epistatic effects were predominant over additive and dominance effects with an important role of duplicate type of epistasis. The occurrence of significant additive and additive x additive types of genetic variation and the moderately high broad sense heritability indicated the possibility of selection for an increased manifestation of stem elongation ability.