Genetic studies on resistance to Valsa canker in apple: genetic variance and breeding values estimated from intra- and inter-specific hybrid progeny populations

Malus sieboldii Rehd. exhibits high levels of resistance to Valsa canker caused by Valsa ceratosperma (Tode ex Fr.) Maire while cultivated apples (Malus domestica Borkh.) are susceptible to the disease. In this study, progenies from 23 full-sib families derived from both inter- and intra-specific hybridization among 16 Malus genotypes as parents were assessed for resistance to V. ceratosperma (Vc) for two seasons using an excised shoot assay to determine the pattern of inheritance of the resistance and to also estimate the variance components, narrow-sense heritability, and breeding values of parental genotypes. Generally, M. sieboldii × M. domestica and its reciprocal crosses had more resistant progenies to Vc than intra-specific crosses of M. domestica. Resistance to Vc expressed as the relative lesion length among progenies showed continuous variation irrespective of cross, suggesting the quantitative nature of the resistance to the three virulent isolates of Vc that were tested. Resistance to Vc using the progeny population was analyzed using a mixed linear model based on restricted maximum likelihood. The parental effect (general combining ability (GCA)) was significant while the interaction effect between parents (specific combining ability (SCA)) was relatively small and non-significant. The ratio of SCA/GCA variance was about 32%, suggesting that additive genetic variance had a major contribution to the total genetic variance for resistance to Vc. There was a positive correlation (r = 0.49, p < 0.01) between mid-parental GCA and SCA predictions among 23 full-sib families for the resistance. Narrow-sense heritability estimated by sib analysis was moderate ( [^(h)]² = 0.29 ) \left( {{{\hat{h}}^2} = 0.29} \right) . The predicted breeding values (BV) of the 16 parents indicated that M. sieboldii “Sanashi 63” and “Hayanarisanashi 1” would be useful for breeding for high levels of resistance to Vc.     

Assessment of genes controlling Area under Disease Progress Curve (AUDPC) for stripe rust (P. Striiformis F. Sp. Tritici) in two wheat (Triticum Aestivum L.) crosses

Genetic effects on controlling stripe rust resistance were determined in two wheat crosses, Bakhiawar-92 × Frontana (cross 1) and Inqilab-91 × Fakhre Sarhad (cross 2) using Area under Disease Progress Curve (AUDPC) as a measure of stripe rust resistance. The resistant and susceptible genotypes for crosses were identified by initial assessment of 45 wheat accessions for stripe rust resistance. Mixed inheritance model was applied to the data analysis of six basic populations P ₁, F ₁, P ₂, B ₁, B ₂, and F ₂ in the crosses. The results indicated that AUDPC in cross 1 was controlled by two major genes with additive-dominance epistatic effect plus polygenes with additive-dominance epistatic effects (model E). Whereas in case of cross 2, it was under the control of two major genes with additive-dominance epistatic effect plus additive-dominant polygenes (model E-1). Additive effect was predominant then all other types of genetic effects suggesting the delay in selection for resistance till maximum positive genes are accumulated in the individuals of subsequent generations. Occurrence of transgressive segregants for susceptibility and resistance indicated the presence of resistance as well as some negative genes for resistance in the parents. The major gene heritability was higher than the polygene heritability in B ₁, B ₂ and F ₂ for the crosses. The major gene as well as the polygene heritability was ranging from 48.99 to 87.12% and 2.26 and 36.80% for the two crosses respectively. The highest phenotypic variations in AUDPC (2504.10 to 5833.14) for segregating progenies (BC ₁, BC ₂ and F ₂) represent that the character was highly influenced by the environment. The article is published in the original.     

Inheritance of resistance in water yam (Dioscorea alata) to anthracnose (Colletotrichum gloeosporioides)

Colletotrichum gloeosporioides causes anthracnose, the most severe foliar disease of field-grown water yam (Dioscorea alata). The inheritance of resistance to a moderately virulent (FGS) strain of the pathogen was investigated in crosses between tetraploid D. alata genotypes: TDa 95/00328 (resistant)×TDa 95–310 (susceptible) (cross A), and TDa 85/00257 (resistant)×TDa 92–2 (susceptible) (cross B). Segregation of F₁ progeny fitted genetic ratios of 3:1, 5:1 (crosses A and B) and 7:1 (cross A) resistant:susceptible when inoculated with the FGS strain, indicating that resistance is dominantly inherited and suggesting that more than one gene controls the inheritance of resistance to this strain in the accessions studied. When parental and progeny lines of cross A were inoculated with an aggressive (SGG) strain of the pathogen, all plants expressed a susceptible phenotype, indicating strain-specific resistance in TDa 95/00328. Screening of 20 cultivars/landraces confirmed the high susceptibility of D. alata accessions to the SGG strain and revealed the presence of apparent strain non-specific resistance in TDa 85/00257. TDa 85/00257 and TDa 87/01091 which were resistant to the SGG strain, will be useful both as sources of resistance and in the development of a host differential series for D. alata. Received: 15 May 2000 / Accepted: 18 October 2000     

Partial resistance to leaf rust (Hemileia vastatrix) in coffee (Coffea arabica L.): genetic analysis and molecular characterization of putative candidate genes

One of the major production limiting diseases in coffee is the orange leaf rust caused by the fungus Hemileia vastatrix (Berkeley and Broome). Little is known about the inheritance and genetic determinism of partial resistance in coffee (C. arabica L.) to H. vastatrix. This information would be useful to breed durable resistant cultivars efficiently. In this report, a genetic analysis of partial resistance to leaf rust in Coffea arabica was performed using nine segregating progenies from a cross between the susceptible variety Caturra and the resistant introgressed line DI200. Evolution of partial resistance was evaluated under field conditions by measuring rust incidence (RI) and defoliation (DEF) in two separate regions of productive branches per tree and during four successive years (2003–2006). Genetic components of rust resistance were estimated using the Means and Variance Generation Method, under an additive-dominant model. The most important genetic effect was the additive one, while resistance heritability estimates ranged from 73 to 53% for broad and narrow sense heritabilities, respectively. Genetic estimates for the number of segregating genes showed that at least five independent genes or genetic regions are implicated in the partial resistance to rust. We further analyzed the presence of resistance (RGC) and defense (DGC) gene candidates in the resistant and susceptible parents by using a degenerated-primer PCR approach. A total of 40 different genomic coffee sequences were isolated exhibiting strong similarity to known RGC or DGC homologous. Phylogenetic analysis clustered these sequences into nine families. One family exhibited the TIR protein element, representing the first TIR class proteins identified in coffee. While genetic analysis suggest a predictable success in the processes to improve the selection of resistant lines for future varieties with durable resistance, the molecular characterization of candidate genes represent a primary approach towards the identification of mechanisms involved in partial resistance to coffee leaf rust.     

Inheritance of resistance to Yam mosaic virus, genus Potyvirus, in white yam (Dioscorea rotundata)

Yam mosaic virus (YMV) causes the most-widespread and economically important viral disease affecting white yam (Dioscorea rotundata) in West Africa. The genetic basis of resistance in white yam to a Nigerian isolate of YMV was investigated in three tetraploid D. rotundata genotypes: TDr 93–1, TDr 93–2 and TDr 89/01444. F₁ progeny were produced using TDr 87/00571 and TDr 87/00211 as the susceptible parents. Segregation ratios indicated that a single dominant gene in a simplex condition governs the resistance in TDr 89/01444, while the resistance in TDr 93–2 is associated with the presence of a major recessive gene in duplex configuration. Segregation of progeny of the cross TDr 93–1×TDr 87/00211 fitted a genetic ratio of 2.48:1 resistant:susceptible, which can be expected when two simplex heterozygotes are crossed, indicating the possible modifying effect of the susceptible parent. A triple antibody immunosorbent assay (TAS-ELISA) was used for virus detection in inoculated plants. Slight mosaic symptoms appeared on most resistant individuals, while asymptomatic resistant genotypes with high ELISA (A₄₀₅) values were observed in all crosses. Such a heterogeneous response suggests the influence of additional modifier genes that segregate in the progeny. The finding that resistance can be inherited as a dominant or recessive character has important implications for YMV resistance breeding. Received: 15 August 2000 / Accepted: 12 April 2001     

A QTL study on late leaf spot and rust revealed one major QTL for molecular breeding for rust resistance in groundnut (Arachis hypogaea L.)

Late leaf spot (LLS) and rust are two major foliar diseases of groundnut (Arachis hypogaea L.) that often occur together leading to 50–70% yield loss in the crop. A total of 268 recombinant inbred lines of a mapping population TAG 24 × GPBD 4 segregating for LLS and rust were used to undertake quantitative trait locus (QTL) analysis. Phenotyping of the population was carried out under artificial disease epiphytotics. Positive correlations between different stages, high to very high heritability and independent nature of inheritance between both the diseases were observed. Parental genotypes were screened with 1,089 simple sequence repeat (SSR) markers, of which 67 (6.15%) were found polymorphic. Segregation data obtained for these markers facilitated development of partial linkage map (14 linkage groups) with 56 SSR loci. Composite interval mapping (CIM) undertaken on genotyping and phenotyping data yielded 11 QTLs for LLS (explaining 1.70–6.50% phenotypic variation) in three environments and 12 QTLs for rust (explaining 1.70–55.20% phenotypic variation). Interestingly a major QTL associated with rust (QTL_rust01), contributing 6.90–55.20% variation, was identified by both CIM and single marker analysis (SMA). A candidate SSR marker (IPAHM 103) linked with this QTL was validated using a wide range of resistant/susceptible breeding lines as well as progeny lines of another mapping population (TG 26 × GPBD 4). Therefore, this marker should be useful for introgressing the major QTL for rust in desired lines/varieties of groundnut through marker-assisted backcrossing.     

Characterization of microsatellites from cacao–<Emphasis Type="Italic">Moniliophthora perniciosa</Emphasis> interaction expressed sequence tags

Theobroma cacao L.–Moniliophthora perniciosa expressed sequence tags (ESTs) were converted into useful satellite markers for population analysis and genetic mapping. Forty-nine flanking primer pairs from TSH1188 (a resistant genotype) and Catongo (a susceptible genotype) ESTs were designed and screened for polymorphism analysis. Eleven were polymorphic, with an average of 3.81 alleles per locus and a total of 42 alleles. The satellite markers were tested on 21 cacao accessions and two bulked DNAs generated from 6 resistant and 6 susceptible plants from a segregating F₂ (SCA6 × ICS1) population for witches’ broom resistance. These results show that EST-derived microsatellites (short sequence repeats, SSRs) in Theobroma cacao have many potential applications in linkage mapping and the planning of crosses.     

Pollen genotype selection for a simply inherited qualitative factor determining resistance to chlorsulfuron in maize

Pollen genotype selection for genes expressed in both the haploid and diploid phases of the plant life cycle can lead to correlated responses detectable in the sporophyte. A pollen selection was conducted in two genetic backgrounds of maize (Zea mays L.) for chlorsulfuron resistance, conferred by the XA17 allele. Plants of two backcross (BC) lines segregating 1 (heterozygote, resistant) : 1 (homozygote, susceptible) for chlorsulfuron resistance were used as pollen donor. Selection treatment was applied during microspore development, and tassels were cut about 10 days before anthesis and placed into a liquid medium with or without 40 mg l^–1 of chlorsulfuron. Pollen was used to fertilize an unrelated susceptible genotype (tester). The resulting testcrosses (TC) were evaluated in the greenhouse by spraying seedlings with chlorsulfuron at 23 g ha^–1. Non-selected TC progenies derived from heterozygous BC plants showed a proportion of resistant and susceptible plants close to the expected 1:1 ratio, while non-selected TC progenies derived from homozygous BC plants showed susceptible plants only. Selected TC progenies derived from heterozygous BC plants showed a frequency of resistant plants ranging from 89% to 100%. BC plants homozygous for the susceptible allele, subjected to selection treatment, gave poor seed set, and no resistant plants were found in their TC progenies. Resistant TC plants obtained through pollen genotype selection were selfed, and the proportion of resistant plants was close to 3:1 in all selfed families, in accordance with the hypothesis that all of them inherited the XA17 allele through selection. In this study, pollen genotype selection was extremely effective, and its effect persisted in the second sporophytic generation. Received: 19 November 1999 / Accepted: 30 April 2000     

The detection and molecular mapping of a major gene for non-specific adult-plant disease resistance against stripe rust (Puccinia striiformis) in wheat

A major gene determining non-specific adult-plant disease resistance against stripe rust (Puccinia striiformis) designated Yrns-B1 was mapped by using a cross between ’Lgst.79–74’ (resistant) and ’Winzi’ (susceptible). Analyzing F₃ lines of two consecutive experimental years contrary modes of inheritance were observed due to the intermediate character of the gene and the difference in the disease pressure during the seasons. Using the disease scoring data of both experimental years independently two maps were constructed detecting Yrns-B1 20.5 and 21.7 cM, respectively, proximal to the wheat microsatellite (WMS) marker Xgwm493 on the short arm of chromosome 3BS. The genetic relationships to other major genes or to quantitative trait loci controlling adult plant disease resistance against rusts in wheat are discussed. Received: 27 May 1999 / Accepted: 28 September 1999     

Genetics of resistance to the rust fungus Coleosporium ipomoeae in three species of morning glory (Ipomoea)

We examined the genetic basis of resistance to the rust pathogen Coleosporium ipomoea in three host species: Ipomoea purpurea, I. hederacea, and I. coccinea (Convolvulaceae). In crosses between resistant and susceptible individuals, second-generation selfed offspring segregated in ratios that did not differ statistically from the 3:1 ratio indicative of single-gene resistance with the resistant allele dominant. One out of three crosses between resistant individuals from two different populations revealed that resistance loci differed in the two populations, as evidenced by the production of susceptible individuals among the S(2) generation. These results suggest that gene-for-gene interactions contribute substantially to the dynamics of coevolution in this natural pathosystem. They also suggest that evolution of resistance to the same pathogen strain may involve different loci in different Ipomoea populations.     

Heritable,De Novo Resistance to Leaf Rust and Other Novel Traits in Selfed Descendants of Wheat Responding to Inoculation with Wheat Streak Mosaic Virus

Stable resistance to infection with Wheat streak mosaic virus (WSMV) can be evolved de novo in selfing bread wheat lines subjected to cycles of WSMV inoculation and selection of best-performing plants or tillers. To learn whether this phenomenon might be applied to evolve resistance de novo to pathogens unrelated to WSMV, we examined the responses to leaf rust of succeeding generations of the rust- and WSMV-susceptible cultivar ‘Lakin’ following WSMV inoculation and derived rust-resistant sublines. After three cycles of the iterative protocol five plants, in contrast to all others, expressed resistance to leaf and stripe rust. A subset of descendant sublines of one of these, ‘R1’, heritably and uniformly expressed the new trait of resistance to leaf rust. Such sublines, into which no genes from a known source of resistance had been introgressed, conferred resistance to progeny of crosses with susceptible parents. The F₁ populations produced from crosses between, respectively, susceptible and resistant ‘Lakin’ sublines 4-3-3 and 4-12-3 were not all uniform in their response to seedling inoculation with race TDBG. In seedling tests against TDBG and MKPS races the F₂s from F₁ populations that were uniformly resistant had 3∶1 ratios of resistant to susceptible individuals but the F₂s from susceptible F₁ progenitors were uniformly susceptible. True-breeding lines derived from resistant individuals in F₂ populations were resistant to natural stripe and leaf rust inoculum in the field, while the ‘Lakin’ progenitor was susceptible. The next generation of six of the ‘Lakin’-derived lines exhibited moderate to strong de novo resistance to stem rust races TPMK, QFCS and RKQQ in seedling tests while the ‘Lakin’ progenitor was susceptible. These apparently epigenetic effects in response to virus infection may help researchers fashion a new tool that expands the range of genetic resources already available in adapted germplasm.     

Inheritance of resistance to southern corn rust in tropical-by-corn-belt maize populations

 The inheritance of resistance to southern rust (caused by Puccinia polysora Underw.) was investigated in two F_2:3 populations derived from crossing two temperate-adapted, 100% tropical maize (Zea mays L.) inbred lines (1416-1 and 1497-2) to a susceptible Corn Belt Dent hybrid, B73Ht×Mo17Ht. The inbred lines possess high levels of resistance to southern rust and may be unique sources of resistance genes. Heritability for resistance was estimated as 30% and 50% in the two populations from regression of F_2:3 family mean scores on F₂ parent scores, and as 65% and 75% from variances among F_2:3 families on a single-plot basis. RFLP loci on three chromosomal regions previously known to possess genes for resistance to either southern rust or common rust (P. sorghi Schw.) were used to localize genes affecting resistance to southern rust in selected genotypes of both populations, and to estimate their genetic effects. A single locus on 10S, bnl3.04, was associated with 82–83% of the variation among field resistance scores of selected F_2:3 families in the two populations. Loci on chromosomes 3 (umc26) and 4 (umc31) were significantly associated with resistance in the 1497-2 population, each accounting for 13–15% of the phenotypic variation for F_2:3 field scores. Multiple-marker locus models, including loci from chromosomes 3, 4, and 10 and their epistatic interactions, accounted for 96–99% of the variation in F_2:3 field scores. Similar results were obtained for resistance measured by counting pustules on juvenile plants in the greenhouse. An attempt was made to determine if the major gene for resistance from 1416-1 was allelic to Rpp9, which is also located on 10S. Testcross families from the cross (1416-1×B37Rpp9)×B14AHt were evaluated for resistance to southern rust in Mexico. Neither source of resistance was completely effective in this environment, preventing determination of allelism of the two genes; however, both sources of resistance had better partial resistance to southern rust than did B14AHt. Received: 6 May 1997/Accepted: 19 September 1997     

Host resistance and pathogen virulence across a plant hybrid zone

Hybridization between locally adapted plant populations has been postulated to have significant evolutionary consequences, and, in particular, may influence host-pathogen interactions with respect to resistance and virulence structure. This study investigated patterns of resistance and virulence in a hybrid zone between ”bog” and ”hill” ecotypes of the native Australian flax, Linum marginale, where the host is subject to attack by the rust pathogen, Melampsora lini. Analysis of the resistance structure of adjoining bog, hill and hybrid populations found that bog plants were generally susceptible to pathogen isolates taken from all these sites, but that hybrids exhibited resistance levels similar to the more resistant hill plants. Similarly, the virulence structure of rust isolates collected from the hybrid population was more similar to that of the hill isolates than the bog. Controlled crosses between bog and hill plants showed that crosses in one direction (bog females×hill males) were much more successful than the other. A multi-year reciprocal transplant study further indicated that bog plants had significantly higher survivorship than hill plants, regardless of site. It is suggested that likelihood of differential gene flow and survivorship for bog and hill plants may at least partially explain the maintenance of a relatively narrow hybrid zone. Received: 5 October 1999 / Accepted: 1 July 1999     

Genetic studies of leaf and stem rust resistance in six accessions of Triticum turgidum var. dicoccoides.

Six accessions of Triticum turgidum var. dicoccoides L. (4x, AABB) of diverse origin were tested with 10 races of leaf rust (Puccinia recondita f.sp. tritici Rob. ex Desm.) and 10 races of stem rust (P. graminis f.sp. tritici Eriks. &Henn.). Their infection type patterns were all different from those of lines carrying the Lr or Sr genes on the A or B genome chromosomes with the same races. The unique reaction patterns are probably controlled by genes for leaf rust or stem rust resistance that have not been previously identified. The six dicoccoides accessions were crossed with leaf rust susceptible RL6089 durum wheat and stem rust susceptible 'Kubanka' durum wheat to determine the inheritance of resistance. They were also crossed in diallel to see whether they carried common genes. Seedlings of F1, F2, and BC1F2 generations from the crosses of the dicoccoides accessions with RL6089 were tested with leaf rust race 15 and those from the crosses with 'Kubanka' were tested with stem rust race 15B-1. The F2 populations from the diallel crosses were tested with both races. The data from the crosses with the susceptible durum wheats showed that resistance to leaf rust race 15 and stem rust race 15B-1 in each of the six dicoccoides accessions is conferred by a single dominant or partially dominant gene. In the diallel crosses, the dominance of resistance appeared to be affected by different genetic backgrounds. With one exception, the accessions carry different resistance genes: CI7181 and PI 197483 carry a common gene for resistance to leaf rust race 15. Thus, wild emmer wheat has considerable genetic diversity for rust resistance and is a promising source of new rust resistance genes for cultivated wheats.     

DNA markers linked to eastern filbert blight resistance in ��Ratoli�� hazelnut (Corylus avellana L.)

Eastern filbert blight (EFB), caused by the pyrenomycete Anisogramma anomala (Peck) E. Müller, is a major disease problem and production constraint in orchards of European hazelnut (Corylus avellana L.) in Oregon’s Willamette Valley. Host genetic resistance is viewed as the most economical means of controlling this disease. A dominant resistance gene from “Gasaway” has been used extensively in the hazelnut breeding program at Oregon State University, but concern about the durability of a single resistance gene stimulated a search for new sources of resistance. “Ratoli,” a minor cultivar from Spain, showed no signs or symptoms of the fungus following a series of inoculations. The objective of this study was to study segregation for disease response in two progenies from crosses of Ratoli with susceptible selections and identify linked DNA markers. About half of the seedlings were resistant, suggesting control by a dominant allele at a single locus. A total of 900 random amplified polymorphic DNA (RAPD) primers and 64 amplified fragment length polymorphism (AFLP) primer combinations were screened. Four RAPD markers and two ALFP markers were identified and a linkage map constructed. On this map, disease resistance was flanked by AFLP marker C4-255 and RAPD marker G17-800 at distances of 0.4 cM and 2.8 cM, respectively. Based on co-segregation with SSR markers, Ratoli resistance was assigned to linkage group 7 while Gasaway resistance is on linkage group 6. Ratoli provides a novel source of EFB resistance, and robust RAPD marker G17-800 is useful for marker-assisted selection.     

Identification of molecular markers linked to the Yr15 stripe rust resistance gene of wheat originated in wild emmer wheat, Triticum dicoccoides

 The Yr15 gene of wheat confers resistance to the stripe rust pathogen Puccinia striiformis West., which is one of the most devastating diseases of wheat throughout the world. In the present study, molecular markers flanking the Yr15 gene of wheat have been identified using the near-isogenic-lines approach. RFLP screening of 76 probe-enzyme combinations revealed one polymorphic marker (Nor/TaqI) between the susceptible and the resistant lines. In addition, out of 340 RAPD primers tested, six produced polymorphic RAPD bands between the susceptible and the resistant lines. The genetic linkage of the polymorphic markers was tested on segregating F₂ population (123 plants) derived from crosses between stripe rust-susceptible Triticum durum wheat, cv D447, and a BC₃F₉ resistant line carrying Yr15 in a D447 background. A 2.8-kb fragment produced by the Nor RFLP probe and a 1420-bp PCR product generated by the RAPD primer OPB13 showed linkage, in coupling, with the Yr15 gene. Employing the standard maximum-likelihood technique it was found that the order OPB13 ₁₄₂₀ –Yr15–Nor1 on chromosome 1B appeared to be no less than 1000-times more probable than the closest alternative. The map distances between OPB13 ₁₄₂₀ –Yr15–Nor1 are 27.1 cM and 11.0 cM for the first and second intervals, respectively. The application of marker-assisted selection for the breeding of new wheat cultivars with the stripe rust resistance gene is discussed. Received: 27 February 1997/Accepted: 7 March 1997     

Partial resistance to late blight (Phytophthora infestans) in hybrid progenies of four South American Solanum species crossed with diploid S. tuberosum

Resistant genotypes of the diploid tuber-bearing South American species Solarium arnezii x hondelmannii, S. berthaultii, S. leptophyes and S. microdontum were crossed with three diploid genotypes of S. tuberosum that varied in resistance and maturity type. The progenies were field tested for 2 years for resistance to a complex race of Phytophthora infestans. A wealth of genetic variation for resistance was found in most of the progenies. At least two susceptibility groups could be distinguished in some progenies of S. microdontum. This could be explained by the presence of several major resistance genes in the wild parent and, unexpectedly, in the susceptible parent SH 82-44-111. In most of the wild parents and in the susceptible parent SH 77-114-2988 there appeared to be minor resistance genes. General combining ability effects were predominant; small specific combining ability effects were detected in some crosses of S. microdontum. Gene action appeared dominant in some crosses.     

Undescribed wheat gene for partial leaf rust and stripe rust resistance from Thatcher derivatives RL6058 and 90RN249 carryingLr34

Inheritance of partial leaf rust and stripe rust resistance of a Thatcher wheat 90RN2491, earlier reported to carry two doses of the gene pairLr34-Yr18 and the reference line RL6058 (6*Thatcher/PI58548) for theLr34-Yr18 gene pair was studied against predominant and highly virulent Indian races. Thatcher derivatives 90RN2491 and RL6058 were intercrossed as well as crossed with the leaf rust and stripe rust susceptible Indian cultivar WL711. The F₁, F₂ and F₃ generations from these crosses were assessed for rust severity against leaf rust race 77-5 and stripe rust race 46S119. The F₂ and F₃ generations from the crosses of RL6058 and 90RN2491 with WL711, segregated 15 resistant : 1 susceptible (F₂) and 7 homozygous resistant : 8 segregating : 1 homozygous susceptible (F₃) ratios, respectively, both for leaf rust and stripe rust severity. Therefore, partial resistance against each of the leaf rust and stripe rust races in both RL6058 and 90RN2491 is ascribed to two independently inherited dominant genes. One of the two genes for leaf rust and stripe rust resistance in 90RN2491 and RL6058 isLr34 and the linked geneYr18, respectively. The second leaf rust resistance gene in both the Thatcher lines segregated independently of stripe rust resistance. Therefore, it is notLr34 and it remains unidentified.     

A cereal cyst nematode (Heterodera avenae Woll.) resistance gene transferred from Aegilops triuncialis to hexaploid wheat

 The cereal cyst nematode (Heterodera avenae) is an important root parasite of common wheat. A high level of resistance was transferred to wheat from Aegilops triuncialis (TR lines) using the cross [(T. turgidum×Ae. triuncialis)×T. aestivum]. Low fertility (3–5 viable kernels per plant) was observed during the process but the surviving hybrid plants were highly vigorous. To obtain stable resistant lines further crosses to T. aestivum were performed. The resistance in TR lines seems to be transferred from the C genome of Ae. triuncialis (genomes CCUU). Ae. triuncialis was highly resistant to the two Spanish populations of H. avenae tested, as well as to four French races and two Swedish populations. The histological analysis showed a hypersensitive reaction in the roots of a resistant TR line inoculated with the Ha71 pathotype of H. avenae, whereas well-formed syncytia were observed in the roots of the susceptible control. Resistance to the H. avenae Ha71 pathotype seemed to be inherited as determined by a single dominant factor in the crosses between resistant TR lines and susceptible cultivars. Received: 11 November 1997 / Accepted: 9 December 1997     

Inheritance of Cry1Ac resistance and associated biological traits in the cotton bollworm, Helicoverpa armigera (Lepidoptera: Noctuidae)

The analysis of reciprocal genetic crosses between resistant Helicoverpa armigera strain (BH-R) (227.9-fold) with susceptible Vadodara (VA-S) strain showed dominance (h) of 0.65-0.89 and degree of dominance (D) of 0.299-0.782 suggesting Cry1Ac resistance as a semi-dominant trait. The D and h values of F₁ hybrids of female resistant parent were higher than female susceptible parent, showing maternally enhanced dominance of Cry1Ac resistance. The progeny of F₂ crosses, backcrosses of F₁ hybrid with resistant BH-R parent did not differ significantly in respect of mortality response with resistant parent except for backcross with female BH-R and male of F₁ (BH-R × VA-S) cross, suggesting dominant inheritance of Cry1Ac resistance. Evaluation of some biological attributes showed that larval and pupal periods of progenies of reciprocal F₁ crosses, backcrosses and F₂ crosses were either at par with resistant parent or lower than susceptible parent on treated diet (0.01 μg/g). The susceptible strain performed better in terms of pupation and adult formation than the resistant strain on untreated diet. In many backcrosses and F₂ crosses, Cry1Ac resistance favored emergence of more females than males on untreated diet. The normal larval period and the body weight (normal larval growth) were the dominant traits associated with susceptible strain as contrast to longer larval period and the lower body weight (slow growth) associated with resistance trait. Further, inheritance of larval period in F₂ and backcross progeny suggested existence of a major resistant gene or a set of tightly linked loci associated with Cry1Ac sensitivity.