Genetics of resistance to cotton leaf curl disease in Gossypium hirsutum

Twenty-two cotton varieties were screened for resistance to cotton leaf curl disease (CLCuD), a disease of viral origin, using three procedures: field evaluation, whitefly transmission assay and graft inoculation. Viral infection of cotton varieties was determined by visual symptom assessment as well as dot-blot and multiplex PCR diagnostic techniques. Crosses were made between the most susceptible variety (S-12) and highly resistant varieties (CP-15/2, LRA-5166 and CIM-443). All F₁ plants of these crosses were resistant, showing dominant expression of the resistance as well as the absence of extrachromosomal inheritance. The F₂ plants of the crosses CP-15/2 × S12, LRA-5166 × S-12 and CIM-443 × S12 exhibited a ratio of 13 resistant (symptomless) to three susceptible (with symptoms). Screening of the F₂ generation for virus infection by multiplex PCR further subdivided the resistant class into those exhibiting a high level of resistance (HR; PCR-negative) and those exhibiting resistance (R; symptomless, yet showing virus replication by PCR analysis). Hence, the final ratio was 3:10:3 (HR:resistant:susceptible). The F₃ progeny of susceptible F₂ plants segregated for resistance, indicating the probable presence of a suppressor gene ( S ). These findings are consistent with three genes being involved in G. hirsutum resistance to CLCuD, two for resistance ( R _1CLCuDhir and R _2CLCuDhir ) and a suppressor of resistance ( S _CLCuDhir ).     

Inheritance of tolerance to metribuzin in durum wheat

The inheritance of tolerance to the herbicide metribuzin was studied in two durum wheat cultivars, one of which, 'Anton', is resistant and one, 'Nita', susceptible. Parents, F₁, F₂ and F₃ of the crosses 'Anton' × 'Nita' and 'Nita' × 'Anton' were tested for herbicide response. The character evaluated was the increase in weight of plants treated with the herbicide. As there were no significant differences between progenies of reciprocal crosses, cytoplasm was not involved in tolerance. Tolerance was semi-dominant with means values of F₁, F₂ and F₃ progenies in the range of mid-parent. The heritability of this trait estimated by regression of the average of progeny F₃ in their parent F₂ had a value of 0.23 ± 0.063 and a value of 0.52 ± 0.150 estimated by the relation V _G/ V _P for full-sib F₂ families.     

小麦抗条锈病近等基因系Taichung29*6/Strubes Dickkopf抗病主效基因的单体分析

小麦品种Strubes Dickkopf是小麦条锈菌国际鉴别寄主,通过对以其为基因供体与完全感病品种Taichung29杂交转育而成的近等基因品系Taichung29*6/Strubes Dickkopf的单体分析,检测和定位Taichung29*6/Strubes Dickkopf中所含的抗条锈病主效基因,并明确其抗性特点。结果表明,Taichung29*6/Strubes Dickkopf对CY26小种的抗性由1对显性主效抗条锈基因所控制,定位在4B染色体上,暂定名为YrSD。同时说明Strubes Dickkopf中含有与Yr25不同的新的抗条锈病基因YrSD。     

Genetics of avirulences in Erysiphe graminis f.sp. hordei

The genetics of avirulences towards barley mildew resistances were analysed in crosses of the Ervsiphe graminis f.sp. hordei isolate DH14 with CC107 and with CC138. Nine avirulences, Av r_a9, Avr _a10, Avr _a11, Avr _a12, Avr _Ab, Avr _CP, Avr _h, Avr _k and Avr _La, segregated as single genes in one or other cross. However. F₁ segregation data were consistent with avirulence matching the Mla7 resistance gene being controlled by two genes, designated Avr _a7 1 and Avr _a7 2. Infection types of avirulent isolates differed on varieties in which Mla7 had been derived from each of the four known sources of that resistance. Linkage was detected between Avr _a7 1 and Avr _h in the cross CC107 × DH14, and between Avr _a10 and Avr _k, Avr _a11 and Avr _La, and Avr _h and the triadimenol response gene Tdl2 in CC138 × DH14.     

Specific resistance to soybean stem canker conferred by the Rdm4 locus

The Rdm4 gene from soybean cv. Hutcheson has been extensively used to incorporate resistance to soybean stem canker (SSC), caused by Diaporthe phaseolorum var . meridionalis (Dpm), into soybean commercial cultivars. The objective of this work was to characterize the inheritance of the Rdm4 locus in different populations derived from the cross: J77-339 ( rdm / rdm , susceptible) × Hutcheson ( Rdm4 / Rdm4 , resistant) in independent interactions with two local isolates of Dpm. Four F₂ populations were obtained and two were advanced to the F₃ generation as separate F_2:3 families to perform progeny tests. Each population was inoculated with the CE109 and/or CE112 isolates of Dpm. Within each plant–pathogen interaction, the resistance gene segregated as completely dominant. However, cross resistance, or opposite disease reactions, to CE109 and CE112 isolates of Dpm were observed in four F_2:3 families, indicating an intergenic recombination event between two nonallelic genes interacting specifically with each isolate of Dpm. The distance between them, estimated as the recombination fraction, was 29%, suggesting that both genes were not tightly linked, but close enough to segregate together in most crosses. Results indicated the existence of a genomic region in cv. Hutcheson composed of race-specific resistance loci with at least two Rdm genes: the previously recognized Rdm4 and a novel gene, tentatively named Rdm5 , conferring specific resistance to Dpm isolates CE109 and CE112.     

Genetic analysis of resistance to Puccinia striiformis in the wheat differential cultivars Heines VII, Heines Peko and Strubes Dickkopf

One major gene for resistance to isolate WYR 85-22 of race 6E0 of Puccinia striiformis was identified by genetic analysis of the differential cultivars Heines Peko, Strubes Dickkopf and Heines VII. This gene was different from Yr2 Yr6 already identified in Heines Peko ( Yr2 Yr6 ) and Heines VII ( Yr2 ), was allelic in the three cultivars and also to a gene expressed in the other differentials Reichersberg 42 ( Yr7 ) and Clement ( Yr9 ). In Heines Peko, Strubes Dickkopf and Heines VII, a minor gene was also postulated, which, it is proposed, gave only a low level of resistance by itself but strengthened the expression of the major gene when the latter was homozygous or heterozygous. The genetics of the resistance was analysed using six resistance classes and applying multidimensional analyses. The number of resistance genes was hypothesized using data from F3 families from crosses between the three differentials and the cultivar Heines Kolben, which is susceptible to this race.     

Complementation between genes for resistance to race 1 of Fusarium oxyspomm f.sp. ciceri in chickpea

The reactions of parents and F₁ and F₂ generations of crosses of chickpea cultivars K-850 with C-104 and JG-62 and F₃ progenies of K-850 × C-104 to race 1 of Fusarium oxysporum f.sp. ciceri were studied. The results indicate that K-850 carries a recessive allele for resistance at a locus different from and independent of that carried by C-104 and recessive alleles at both loci together confer complete resistance. The possible contribution of this recessive gene to late wilting in K-850 is discussed. These observations have important implications in breeding for resistance to wilt in chickpea.     

Transgressive segregation for resistance to Pyrenophora teres in barley

Populations of F₁, F₂, F₁× parent 1 (BC1), F₁× parent 2 (BC2), and F₃ resulting from two crosses between four susceptible varieties of barley used in national and international breeding programmes were tested at the seedling stage for their resistance to an isolate of Pyrenophora teres which is virulent to the parental varieties. Infection type, average lesion size, and number of lesions per unit leaf area were used to assess disease reaction. In the two crosses, F₂ and especially F₃ generations (produced by self pollination of selected resistant F₂ plants) showed more resistance expressed by infection type and average lesion size than either parent. The frequency distributions of F₂ and F₃ generations derived from these crosses were continuous and showed transgressive segregation for resistance. The results indicated that the four cultivars used in this study possessed additive genes for resistance to P. teres. Thus useful resistance could be obtained following crossing of susceptible cultivars and selection in succeeding generations.     

Cultivar reactions and the genetic basis of resistance to alternaria stem canker (Alternaria alternata f.sp. lycopersici) in tomato

The reactions of several tomato cultivars and the genetic basis of resistance to an isolate of Alternaria alternata f. sp. lycopersici were examined. Among the 105 cultivars tested only two, Regal and Toper, were susceptible, while the remainder were highly resistant. The resistant cultivar ACE 55 VF (P_;) was crossed with the susceptible cultivar Earlypak No. 7 (P₁)- When F₁, F₂, F₃, BC₁, BC₁ selfed, F₂ X P₂ and BC₁P₂ x P₁ progenies were inoculated at the two-leaf stage with a suspension of spores of the pathogen, the ratios of resistant to susceptible plants indicated that resistance was conferred by a single dominant gene.     

A new blast resistance gene identified in the Indian native rice cultivar Aus373 through allelism and linkage tests

Blast, caused by Pyricularia grisea , is a major constraint on rice production. To widen genetic diversity for disease resistance, the Indian native rice cultivar Aus373 was screened by F₂ segregation analyses to investigate the genetic basis of its high resistance. Aus373 was crossed with a series of Japanese differential cultivars (JDCs) and the Chinese susceptible cultivar Lijiangxintuanheigu (LTH). The resistance ratios of subsequent F₂ progenies were used to determine the number of blast-resistance loci present as well as allelic relationships with known loci. Resistance of Aus373 was governed by dominant alleles at two loci, one at the Pi-k locus and the second apparently at a new locus linked to an isozyme gene Amp-1 with a recombination fraction of 37.9 ± 3.0% on chromosome 2. This putative new locus and allele were designated Pi16 (t).     

Evaluation of carrot resistance to alternaria leaf blight in controlled environments

The objective of this study was to find a technique for plant resistance screening to alternaria leaf blight (ALB), caused by the fungus Alternaria dauci , in controlled environments. Glasshouse and laboratory screening methods were compared using three cultivars and F₂ genotypes segregating for ALB resistance evaluated against self-pollinated F₃ field-grown plants. Plant disease was assessed through a disease index obtained from the size and number of symptoms on carrot leaves. The results indicated the value of glasshouse evaluation and the inadequacy of detached leaf and hypocotyl assays for carrot screening for ALB resistance. Spearman's rank correlation, applied to results obtained with both F₂ plants and their progeny, indicated that the optimal evaluation stage for ALB resistance in carrot is 20 days after inoculation. This test was powerful enough to be used as a prescreening test in breeding programmes.     

Response of interspecific Brassica juncea/Brassica rapa hybrids and their advanced progenies to Albugo candida (white blister)

Transfer of factors for resistance to white blister disease caused by Albugo candida between Brassica species involving two genotypes each of B. juncea and B. rapa was studied in hybrids. More hybrids were obtained by in vivo than in vitro techniques, although an in vitro phase was a prerequisite for the establishment of in vivo hybrids. Hybrids were identified by PCR-based inter-simple sequence repeat (ISSR) markers with both male and female species-specific bands being identified. There was a positive correlation between disease severity and number of days after sowing ( r  > 0·93), the highest being towards pod formation and plant maturity at 110 days after sowing. The plants from F₂ and BC₁ progeny showed higher resistance to A. candida than either of the parents. Plants of B. juncea and B. rapa with high field resistance (disease index < 1·0) were selected from BC₂ and F₂BC₁ generations. The frequency of plants classified as resistant in BC₂ progeny ranged from 4·5 to 39·0% in cross-combinations involving B. juncea genotypes as female parent, compared with 100% in the reciprocal cross involving B. rapa as female parent.     

Quantitative resistance to Plum pox virus in Prunus davidiana P1908 linked to components of the eukaryotic translation initiation complex

A complex, polygenic resistance to Plum pox virus (PPV) was previously described in a wild peach-related species, Prunus davidiana clone P1908. In the current study, an analysis of quantitative trait loci (QTL) was performed on an F₂ population comprising 99 individuals obtained by selfing the F₁ individual #40 of an interspecific cross between susceptible nectarine cv. Summergrand and the resistant P. davidiana clone P1908. Six QTL were identified using both parametric and non-parametric methods of detection, individually explaining 5–28% of the phenotypic variance. The total phenotypic variation explained ranged from 29 to 58%. Alignment of the genetic map of the F₂ cross with the P. davidiana parent map showed consistency of QTL over generations, with three of the six QTL co-localizing at the 1-LOD interval and another one at the 2-LOD interval. Two of the QTL were mapped onto linkage group one, where resistance to PPV was previously mapped in apricot. Development and mapping of new microsatellite markers linked to candidate genes revealed a striking co-localization of three of the detected QTL with gene copies coding for eukaryotic translation initiation factors eIF4E and eIF(iso)4G. As co-localization of one QTL with candidate gene eIF(iso)4E was previously reported in the F₁ population, the results reported here strongly reinforce the idea that components of the eukaryotic translation initiation complex are correlated with resistance to PPV in P. davidiana P1908.     

Identification and mapping of a rice blast resistance gene Pi-g(t) in the cultivar Guangchangzhan

A single dominant blast resistance gene was identified in Chinese indica rice ( Oryza sativa ) cv. Guangchangzhan (GCZ), which shows complete resistance to Japanese isolate Ken53-33 of Magnaporthe grisea . Genetic analysis of the backcross (BC₁) and second-generation (F₂) populations from a cross between susceptible cv. Lijiangxintuanheigu (LTH) and GCZ indicated that the resistance was conferred by one dominant gene. This gene was mapped on the long arm of chromosome 2 and flanked by RM166 and RM208 at distances of 4·00 ± 4·90 and 6·30 ± 4·89 cM (centiMorgans), respectively. It was designated tentatively as Pi-g(t) .     

Host–parasite relationships of Meloidogyne incognita on spinach

Host–parasite relationships in root-knot disease of spinach caused by Meloidogyne incognita race 1 were studied under glasshouse conditions. Nematode-induced mature galls were large and usually contained one or more females and egg masses with eggs. Feeding sites were characterized by the development of giant cells containing granular cytoplasm and many hypertrophied nuclei. The cytoplasm in these giant cells was aggregated alongside the thickened cell walls. Stelar tissues within galls appeared disorganized. The relationship between initial nematode population density ( P _i) in a series from 0–128 eggs and second-stage juveniles per cm³ soil and growth of spinach cv. Symphony F₁ seedlings was tested under glasshouse conditions. A Seinhorst model [ y = m  + (1 −  m ) z ^P–T ] was fitted to fresh top- and total plant-weight data for inoculated and control plants. Tolerance limits ( T ) of spinach cv. Symphony F₁ to M. incognita race 1 for fresh top and total plant weights were 0·25 and 0·5 eggs and second-stage juveniles per cm³ soil, respectively. The minimum relative values for fresh top and total plant weights were zero in both cases at P _i ≥ 32 eggs and second-stage juveniles per cm³ soil. Root galling was least at low initial population densities and greatest at 16 eggs and second-stage juveniles per cm³ soil. Maximum nematode reproduction rate was 33·1-fold at the lowest P _i.     

Inheritance of partial resistance to race 2 of Albugo candida in canola-quality mustard (Brassica juncea) and its role in resistance breeding

The inheritance of partial resistance to race 2 of Albugo candida was studied in a canola-quality line of Brassica juncea . This partially resistant line was crossed with the susceptible B. juncea cultivar Commercial Brown. F₁, F₁(reciprocal), F₂, BC and doubled haploid generations from the cross were inoculated with a zoospore suspension of race 2 to study segregation of partial resistance. The partially resistant phenotype appeared to be controlled by a single dominant gene that has variable expression. This partial resistance can have implications in breeding for disease resistance against white rust, as adult plants did not develop hypertrophic growth or stagheads under greenhouse and field conditions.     

Occurrence and inheritance of resistance to powdery mildew (Oidium lycopersici) in Lycopersicon species

Among 146 accessions of Lycopersicon pimpinellifolium , 132 of L. esculentum var. cerasiforme and 53 of L. peruvianum screened for resistance to powdery mildew, caused by Oidium lycopersici , a wide variation in reactions was found. Two plants of L. esculentum var. cerasiforme accession LA-1230 were resistant. One resistant symptomless plant of accession LA-1230, designated LC-95, produced homozygous resistant progenies. LC-95 was crossed with cv. Marmande (susceptible parent) and F₁, F₂ and backcrosses to the resistant and the susceptible parents were derived. These genotypes were grown in glasshouses at 23°C and 95–100% RH and inoculated with O. lycopersici . The F₁ plants were susceptible. F₂ and backcross segregations fitted the hypothesis of a single recessive gene which is here designated ol-2 .     

Identification and location of Stb9, a gene for resistance to septoria tritici blotch in wheat cultivars Courtot and Tonic

This study reports the discovery of a gene for resistance to septoria tritici blotch (STB) in two spring wheat cultivars, Courtot and Tonic. The gene, named Stb9 , confers resistance to Mycosphaerella graminicola isolate IPO89011. It was mapped by quantitative trait loci (QTL) analysis using an existing map of Courtot × Chinese Spring and was located between markers Xfbb226 (3·6 cM) and XksuF1b (9 cM) on the long arm of chromosome 2B. Markers linked to Stb9 in Courtot were then shown to be linked to resistance to IPO89011 in F₃ families of Tonic × Longbow. Allelism tests in which Tonic was crossed with Courtot confirmed that Tonic has a gene for resistance to IPO89011 at or very close to the Stb9 locus. SSR markers flanking Stb9 may be used in marker-assisted selection to introgress this gene into winter cultivars or in spring wheat breeding programmes outside Europe.     

A genetic study of the relationship between height, time of ear emergence and resistance to Septoria nodorum in wheat

Winter wheat cultivars of varying height were intercrossed and the progeny were studied in field trials to determine whether the greater resistance to S. nodorum of the taller cultivars was fortuitous or the result of genetic association.
Variation in height, in resistance and in time of ear emergence was continuous, providing no evidence of the existence of individual genes of major effect. In F₂ and backcross generations, the inheritance of all three characters was principally additive. Among random F₃ and F₄ families there was a clear tendency for tall straw and resistance to segregate together, proving that the association between these characters was not fortuitous and providing evidence of linkage or pleiotropy in their genetic control. There was a similar though less consistent tendency for late ear emergence and resistance to segregate together. There was substantial residual variation in resistance, not associated with variation in height or time of ear emergence, but its heritability was difficult to prove by comparisons between F₃ and F₄ generations, perhaps partly because of genotype x environment interaction.
It is proposed that resistance is under polygenic control and pleiotropy rather than linkage is suggested as the explanation of its genetic association with height and with time of ear emergence. The existence of genes affecting resistance independently of height and time of ear emergence is also inferred. It is suggested that they permit the breeding of resistant cultivars of any height, though with increasing difficulty as height is reduced.     

Studies on the inheritance of resistance to metalaxyl in Phytophthora infestans

Single-oospore progeny from matings between field isolates of Phytophthora infestans either highly resistant or sensitive to metalaxyl were analysed for fungicide sensitivity in vitro, mating type and, in some cases, allozyme variation at the locus for glucosephosphate isomerase ( GPI-1. ) In each cross the majority of first-generation (F₁) progeny showed intermediate sensitivity to metalaxyl. Frequency distributions were skewed towards sensitivity and a few progeny were either wholly sensitive or resistant phenotypes. Allozyme analysis of F₁ progeny from a cross between parents of Dutch and Mexican origin showed that c. 10% were selfs of both parents and of parental phenotype for metalaxyl sensitivity. The selfs of the A2 but not the Al parent segregated for mating type. Two backcross generations to the metalaxyl-resistant Dutch parent gave unexpected phenotypic frequency distributions and aberrant ratios for genotypes at the GPI-I locus.
Progeny of another backcross between an F₁ isolate of intermediate sensitivity and its sensitive Egyptian parent gave a 1:1 ratio for sensitive: intermediate phenotypes. F₂ progeny from a related sibmating between intermediate phenotypes segregated in a ratio close to 1:2:1 for sensitive: intermediate: resistant phenotypes. Segregation was also observed among sexual progeny of an intermediate self-fertile isolate from the backcross generation but not among progeny of a resistant self-fertile phenotype. Mating type segregated in both cases.
These data are consistent with resistance to metalaxyl in P. infestans being governed by a single nuclear locus exhibiting incomplete dominance. This hypothesis is discussed in relation to the incidence of resistance to metalaxyl in natural populations of P. infestans.