A transgressive segregation factor (RKN2) in Gossypium barbadense for nematode resistance clusters with gene rkn1 in G. hirsutum

Host plant resistance is an important strategy for managing root-knot nematode (Meloidogyne incognita) in cotton (Gossypium L.). Here we report evidence for enhanced resistance in interspecific crosses resulting from transgressive segregation of clustered gene loci. Recently, a major gene, rkn1, on chromosome 11 for resistance to M. incognita in cv. Acala NemX was identified using an intraspecific G. hirsutum cross with susceptible cv. Acala SJ-2. Using interspecific crosses of Acala NemX × susceptible G. barbadense cv. Pima S-7, F₁, F₂, F_2:3, backcross, and testcross Acala NemX × F₁ (Pima S-7 × SJ-2), parental entries and populations were inoculated in greenhouse tests with M. incognita. Genetic analyses based on nematode-induced root galling and nematode egg production on roots, and molecular marker analysis of the segregating interspecific populations revealed that gene rkn1 interacted with a gene (designated as RKN2) in susceptible Pima S-7 to produce a highly resistant phenotype. RKN2 did not confer resistance in Pima S-7, but when combined with rkn1 (genotype Aa or aa), high levels of resistance were produced in the F₁ and segregating F₂, F₃, and BC₁F₁ populations. One SSR marker MUCS088 was identified tightly linked to RKN2 within 4.4 cM in a NemX × F₁ (Pima S-7 × SJ-2) testcross population. Using mapped SSR markers and interspecific segregating populations, MUCS088 linked to the transgressive gene from the susceptible parent and was located in the vicinity of rkn1 on chromosome 11. Diverse genome analyses among A and D genome diploid and tetraploid cottons revealed that marker MUCS088 (165 and 167 bp) is derived from G. arboreum, A₂ diploid genome. These results demonstrated that a highly susceptible parent contributed to nematode resistance via transgressive segregation. Derived highly resistant lines can be used as improved resistance sources in cotton breeding, and MUCS088 can be used to monitor RKN2 introgression in diverse populations. The close genomic location of the transgressive resistance determinants provides an important model system for studying transgressive segregation and epistasis in plants.     

Mode of Inheritance of Amitraz Resistance in a Brazilian Strain of the Southern Cattle Tick, Boophilus microplus (Acari: Ixodidae)*

The southern cattle tick, Boophilus  microplus (Canestrini), has developed resistance to amitraz in several countries in recent years. A study was conducted at the USDA Cattle Fever Tick Research Laboratory in Texas to investigate the mode of inheritance of amitraz resistance with cross-mating experiments. The Muñoz strain, a laboratory reared acaricide-susceptible reference strain, was used as the susceptible parent and the Santa Luiza strain, originating in Brazil, was used as the resistant parent. A modified Food and Agriculture Organization Larval Packet Test was used to measure the levels of susceptibility of larvae of the parental strains, F₁, backcross, F₂, and F₃ generations. Results of reciprocal crossing experiments suggested that amitraz resistance was inherited as an incomplete recessive trait. There was a strong maternal effect on larval progeny’s susceptibility to amitraz in both the F₁ and the subsequent generations. The values of the degree of dominance were estimated at ?0.156 and ?0.500 for the F₁ larvae with resistant and susceptible female parents, respectively. Results of bioassays on larval progeny of the F₁ backcrossed with the resistant parent strain and that of the F₂ generations suggested that more than one gene was responsible for amitraz resistance in the Santa Luiza strain. Comparisons of biological parameters (engorged female weight, egg mass weight, and female-to-egg weight conversion efficiency index) indicated significant differences between different genotypes. The differences appeared to be heritable, but not related to amitraz resistance. Results from this study may have significant implications for the management of amitraz resistance.     

Transgenic broccoli expressing aBacillus thuringiensis insecticidal crystal protein: Implications for pest resistance management strategies

We usedAgrobacterium tumefaciens to transform flowering stalk explants of five genotypes of broccoli with a construct containing the neomycin phosphotransferase gene and aBacillus thuringiensis (Bt) gene [CryIA(c) type] optimized for plant expression. Overall transformation efficiency was 6.4%; 181 kanamycin-resistant plants were recovered. Of the 162 kanamycin-resistant plants tested, 112 (69%) caused 100% morality of 1st-instar larvae of aBt-susceptible diamondback moth strain. Southern blots of some resistant transformants confirmed presence of theBt gene. Selected plants that gave 100% mortality of susceptible larvae allowed survival of a strain of diamondback moth that had evolved resistance toBt in the field. F₁ hybrids between resistant and susceptible insects did not survive. Analysis of progeny from 26 resistant transgenic lines showed 16 that gave segregation ratios consistent with a single T-DNA integration. Southern analysis was used to verify those plants possessing a single T-DNA integration. Because these transgenic plants kill susceptible larvae and F₁ larvae, but serve as a suitable host for resistant ones, they provide an excellent model for tests ofBt resistance management strategies.     

The transfer of pyrethroid resistance resulting from crosses between resistant German cockroaches and susceptible Asian cockroaches

Crosses were made between the Asian cockroach,Blattella asahinai Mizukubo, and resistant strains of the German cockroach,B. germanica (L.), to assess the transfer of pyrethroid resistance to the progeny and to study the inheritance mechanism(s) involved. It was shown that the strain of Asian cockroaches studied was susceptible to four pyrethroids. F₁ progeny were essentially susceptible to the same compounds. Tests with F₂ progeny and those from backcrosses to the resistant parent indicated that the data for each pyrethroid fit an hypothesis of simple, autosomal, nearly completely recessive inheritance. The results are discussed from the standpoint of the impact of the Asian genome on the inheritance mechanism(s).     

Inheritance of resistance to the two-spotted spider mite from L. pimpinellifolium (Jusl.) Mill. accession ‘TO-937’

The two-spotted spider mite (Tetranychus urticae Koch) is an important pest of tomato (Lycopersicon esculentum Mill.) crops in temperate regions as this spider mite has a very large capacity for population increase and causes severe tomato yield losses. There is no described tomato cultivar fully resistant to this pest, although resistant accessions have been reported within the green-fruited tomato wild species L. pennellii (Corr.) D’Arcy and L. hirsutum Humb. & Bonpl. We observed a L. pimpinellifolium (Jusl.) Mill. accession, ‘TO-937’, which seemed to be completely resistant to mite attacks and we crossed it with the susceptible L. esculentum cultivar. ‘Moneymaker’ to obtain a family of generations consisting of the two parents, the F₁, the F₂, the BC₁ to L. esculentum, and the BC₁ to L. pimpinellifolium. This family was evaluated for mite resistance in a polyethylene greenhouse using an experimental design in 60 small complete blocks distributed along 12 double rows. Each block consisted of five F₂ plants in one row and one plant of each of the two parents, the F₁, the BC₁ to L. esculentum, and the BC₁ to L. pimpinellifolium in the adjacent row. Plants at the 10–15 leaf stage were artificially infested by putting on them two pieces of French bean leaf heavily infested with T. urticae. After two months, evaluations of infestation were made by visual observation of mite nets and leaf damage. Plants that were free of signs of mite reproduction on the top half were considered as resistant, plants with silky nets only on their basal leaves, intermediate, and plants with mite reproduction on both basal and top canopies were scored as susceptible. Dominance for resistance appeared because all the ‘To-937’, BC₁ to L. pimpinellifolium, and F₁ plants were resistant. Not all ‘Moneymaker’ plants behaved as susceptible because 35% of plants were intermediate. In the BC₁ to L. pimpinellifolium and the F₂, most plants were scored as resistant, only 7 % BC₁ and 3 % F₂ plants were intermediate, and a single F₂ plant (0.3 %) was susceptible. With these figures, resistance seemed to be controlled by either four or two genes according to whether segregation in the BC₁ or in the F₂, respectively, were considered. These results could in part be explained because of appearance of negative interplot interference due to the high frequency of resistant genotypes within most of the generations. Therefore, the family was evaluated again but using a different experimental design. In the new experiment, 16 ‘TO-937’, 17 ‘Moneymaker’, 17 F₁, 37 BC₁ to L. pimpinellifolium, 38 BC₁ to L. esculentum, and 125 F₂ plants were included. Each of these test plants was grown besides a susceptible ‘Moneymaker’ auxilliary plant that served to keep mite population high and homogeneous in the greenhouse. Negative interplot interference was avoided with this design and all the ‘TO-937’, F₁, and BC₁ to L. pimpinellifolium plants were resistant, all ‘Moneymaker’ test plants were susceptible, and 52 % BC₁ to L. esculentum and 25 % F₂ plants were susceptible, which fitted very well with the expected for resistance governed by a single dominant gene. The simple inheritance mode found will favour sucessful introgression of mite resistance into commercial tomatoes from the very close relative L. pimpinellifolium.     

A major locus for submergence tolerance mapped on rice chromosome 9

Submergence stress is a widespread problem in rice-growing environments where drainage is impeded. A few cultivars can tolerate more than 10 days of submergence, but the genes conferring this tolerance have not been identified. We used randon-amplified polymorphic DNA (RAPD) and restriction fragment length polymorphism (RFLP) markers to map submergence tolerance in 169 F₂ plants and the resulting F₃ families of a cross between a tolerant indica rice line, IR40931-26, and a susceptible japonica line, PI543851. IR40931-26 inherited strong submergence tolerance from the unimproved cultivar FR13A. Eight-day old F₃ seedlings were submerged for 14–16 days in 55-cm deep tanks, and tolerance was scored after 7 days recovery on a scale of 1 (tolerant) to 9 (susceptible). The tolerant and susceptible parents scored 1.5 and 8.4, respectively, and the F₃ means ranged from 1.6 to 8.9. Two bulks were formed with DNA from F₂ plants corresponding to the nine most tolerant and the nine most susceptible F₃ families. Of 624 RAPD primers used to screen the bulks, five produced bands associated with either tolerance or susceptibility. These markers were mapped to a region of chromosome 9 by linkage to RFLP markers. A submergence tolerance quantitative trait locus (QTL), here designatedSub1, was located ca. 4 cM from the RFLP marker C1232 and accounted for 69% of the phenotypic variance for the trait.     

Genetic analysis of resistance to azinphosmethyl in the pear psylla Cacopsylla pyri

Laboratory selection with azinphosmethyl had little effect on the chemical resistance in Cacopsylla pyri strains, in comparison to the original wild populations. The resistance ratio relative to a susceptible strain varied from 10- to 40-fold depending on the generation studied. Crosses between two resistant strains and the susceptible strain show resistance to be autosomally inherited and semi-dominant in expression. Backcrosses between F1 and the susceptible strain were unable to distinguish unabigously between monogenic and polygenic inheritance. In the majority of experiments, however, the overall dose-response relationship for backcross progeny was consistent with a single gene hypothesis. Additional bioassays showed azinphosmethyl-resistant strain to cross-resistant monocrotophos and phosmet, but not carbamates, pyrethroids, amitraz or the organophosphates chlorpyrifos and mevinphos.     

Inheritance of Bacillus thuringiensis Cry1C resistance in Egyptian cotton leafworm,Spodoptera littoralis (Lepidoptera: Noctuidae)

A field strain of Spodoptera littoralis Biosduval was selected against Cry1C toxin derived from Bacillus thuringiensis entomocidus for 10 subsequent generations under laboratory conditions. Selection pressure resulted in a 29‐fold resistance ratio compared with the susceptible strain. Inheritance of Cry1C resistance was partially dominant and autosomal on the basis of bioassay response to Cry1C toxin in a reciprocal cross between male and/or female F₁. Consistent with earlier findings, resistance was recessive at high concentrations of Cry1C toxin. However, the dominance of resistance increased as the concentration of Cry1C decreased. Analysis of survival and growth of progeny from a backcross (F₁ × resistance strain) suggested that resistance was controlled by either a single or a few loci in cotton leafworm.     

Inheritance and mechanism of resistance to herbicides inhibiting acetolactate synthase in Sonchus oleraceus L.

A biotype of Sonchus oleraceus L. (Compositae) has developed resistance to herbicides inhibiting acetolactate synthase (ALS) following field selection with chlorsulfuron for 8 consecutive years. The aim of this study was to determine the inheritance and mechanism of resistance in this biotype. Determination of ALS activity and inhibition kinetics revealed that K_m and V_max did not vary greatly between the resistant and susceptible biotypes. ALS extracted from the resistant biotype was resistant to five ALS-inhibiting herbicides in an in vitro assay. ALS activity from the resistant biotype was 14 19, 2, 3 and 3 times more resistant to inhibition by chlorsulfuron, sulfometuron, imazethapyr, imazapyr and flumetsulam, respectively, than the susceptible biotype. Hybrids between the resistant and a susceptible biotype were produced, and inheritance was followed through the F₁, F₂ and F₃ generations. F₁ hybrids displayed a uniform intermediate level of resistance between resistant and susceptible parents. Three distinct phenotypes, resistant, intermediate and susceptible, were identified in the F₂ generation following chlorsulfuron application. A segregation ratio of 121 was observed, indicative of the action of a single, nuclear, incompletely dominant gene. F₃ families, derived from intermediate F₂ individuals, segregated in a similar manner. Resistance to herbicides inhibiting ALS in this biotype of S. oleraceus is due to the effect of a single gene coding for a resistant form of the target enzyme, ALS.     

Genetical and RFLP studies at the Mla locus conferring powdery mildew resistance in barley

Summary The complex structure of the multigene family at the Mla locus conferring powdery mildew resistance in barley was studied by making diallel crosses between several near-isogenic lines carrying different Mla alleles. The mode of inheritance of the Mla alleles investigated was determined to be dominant for Mla1, Mla6, Mla7 and Mla13 and semidominant for Mla3, Mla12 and Mla20. F₁ plants were backcrossed to the susceptible recurrent parent in order to identify susceptible and double-resistant recombinants in the BC₁F₁ generation. Out of 17605 progenies tested in the BC₁F₁ generation, two susceptible recombinants, one between Mla1 and Mla12 and one between Mla13 and Mla20 were confirmed. The former was also verified by RFLP analysis.     

Characterization and mapping of <Emphasis Type="Italic">Rpi1</Emphasis>, a gene that confers dominant resistance to stalk rot in maize

The maize inbred lines 1145 (resistant) and Y331 (susceptible), and the F₁, F₂ and BC₁F₁ populations derived from them were inoculated with the pathogen Pythium inflatum Matthews, which causes stalk rot in Zea mays. Field data revealed that the ratio of resistant to susceptible plants was 3:1 in the F₂ population, and 1:1 in the BC₁F₁population, indicating that the resistance to P. inflatum Matthews was controlled by a single dominant gene in the 1145×Y331 cross. The gene that confers resistance to P. inflatum Matthews was designated Rpi1 for resistance to P. inflatum) according to the standard nomenclature for plant disease resistance genes. Fifty SSR markers from 10 chromosomes were first screened in the F₂ population to find markers linked to the Rpi1 gene. The results indicated that umc1702 and mmc0371 were both linked to Rpi1, placing the resistance gene on chromosome 4. RAPD (randomly amplified polymorphic DNA) markers were then tested in the F₂population using bulked segregant analysis (BSA). Four RAPD products were found to show linkage to the Rpi1 gene. Then 27 SSR markers and 8 RFLP markers in the region encompassing Rpi1 were used for fine-scale mapping of the resistance gene. Two SSR markers and four RFLP markers were linked to the Rpi1 gene. Finally, the Rpi1 gene was mapped between the SSR markers bnlg1937 and agrr286 on chromosome 4, 1.6 cM away from the former and 4.1 cM distant from the latter. This is the first time that a dominant gene for resistance to maize stalk rot caused by P. inflatum Matthews has been mapped with molecular marker techniques.     

Inheritance of resistance to the cowpea aphid in cowpea

Summary Inheritance of resistance to cowpea aphid, Aphis craccivora Koch, in three resistant cultivars of cowpea, Vigna unguiculata (L.) Walp, was studied. The parents, F₁ and F₂ population were grown in an insect-proof screenhouse. Each 3-day-old seedling was infested with 10 apterous adult aphids. Seedling reaction was recorded when the susceptible check was killed. The segregation data revealed that the resistance of ICV11 and TVU310 is governed by single dominant genes. All the F₂ seedlings of the cross ICV10xTVU310 were resistant, indicating that they have the same gene for resistance. However, the F₂ populations from the crosses ICV10xICV11 and ICV11xTVU310 segregated in a ratio of 151, indicating that the dominant genes in ICV11 and TVU310 are non-allelic and independent of each other. The resistance gene of ICV10 and TVU310 is designated as Ac₁ and that of ICV11 as Ac₂.     

Inheritance of dicrotophos resistance in greenhouse whitefly

We studied inheritance of resistance to dicrotophos in greenhouse whitefly,Trialeurodes vaporariorum Westwood (Homoptera, Aleyrodidae). Compared with females from a field-collected susceptible strains (S), females from a resistant strain (R) ofT. vaporariorum derived from heavily treated cotton fields had a 28-fold greater LC₅₀ to dicrotophos in laboratory bioassays. Concentration-mortality lines obtained from female progeny of reciprocal F₁ crosses (R_♀ XS_♂ and S_♀ XR_♂) were similar, suggesting that inheritance of dicrotophos resistance was autosomal and not influenced by maternal effects. Responses of F₁ female progeny were similar to those of the parental S strain, indicating that the resistance was partially recessive (degree of dominance, D, was −0.61). Mortality observed in female progeny obtained from a backcross (F_1♀ XR_♂) corresponded more closely to expectations derived from polygenic models than to expectations from a monogenic model. The estimated number of effective factors (sensu Lande, 1981) contributing equally to resistance was three.     

Segregation analysis and RFLP mapping of the R1 and R3 alleles conferring race-specific resistance to Phytophthora infestans in progeny of dihaploid potato parents

Phytophthora infestans (Mont.) de Bary is the most important fungal pathogen of the potato (Solanum tuberosum). The introduction of major genes for resistance from the wild species S. demissum into potato cultivars is the earliest example of breeding for resistance using wild germplasm in this crop. Eleven resistance alleles (R genes) are known, differing in the recognition of corresponding avirulence alleles of the fungus. The number of R loci, their positions on the genetic map and the allelic relationships between different R variants are not known, except that the R1 locus has been mapped to potato chromosome V The objective of this work was the further genetic analysis of different R alleles in potato. Tetraploid potato cultivars carrying R alleles were reduced to the diploid level by inducing haploid parthenogenetic development of 2n female gametes. Of the 157 isolated primary dihaploids, 7 set seeds and carried the resistance alleles R1, R3 and R10 either individually or in combinations. Independent segregation of the dominant R1 and R3 alleles was demonstrated in two F₁ populations of crosses among a dihaploid clone carrying R1 plus R3 and susceptible pollinators. Distorted segregation in favour of susceptibility was found for the R3 allele in 15 of 18 F₁ populations analysed, whereas the RI allele segregated with a 1:1 ratio as expected in five F₁ populations. The mode of inheritance of the R10 allele could not be deduced as only very few F₁ hybrids bearing R10 were obtained. Linkage analysis in two F₁ populations between R1, R3 and RFLP markers of known position on the potato RFLP maps confirmed the position of the R1 locus on chromosome V and localized the second locus, R3, to a distal position on chromdsome XI.     

Mapping of loci involved in quantitatively inherited resistance to the potato cyst-nematode Globodera rostochiensis pathotype Ro1

We report the identification and mapping of two quantitative trait loci (QTLs) of Solanum spegazzinii BGRC, accession 8218-15, involved in resistance to the potato cyst-nematode Globodera rostochiensis pathotype Ro1, by means of restriction fragment length polymorphisms (RFLPs). For this purpose we crossed a susceptible diploid S. tuberosum with the resistant S. spegazzinii, and tested the F₁ population for resistance to the Ro1 pathotype. Since the F₁ segregated for the resistance, the S. spegazzinii parent was concluded to be heterozygous at the nematode resistance loci. For the mapping of the resistance loci we made use of RFLP markers segregating for S. spegazzinii alleles in the F₁. One hundred and seven RFLP markers were tested in combination with four different restriction enzymes; 29 of these displayed a heterozygous RFLP pattern within S. spegazzinii and were used for mapping. Analysis of variance (ANOVA) was applied to test the association of the RFLP patterns of these markers with nematode resistance. Two QTLs involved in disease resistance to Globodera rostochiensis pathotype Ro1 were identified and mapped to chromosomes 10 and 11 respectively.     

Development of Yellow Mosaic Virus (YMV) resistance linked DNA marker in <Emphasis Type="Italic">Vigna mungo</Emphasis> from populations segregating for YMV-reaction

Yellow mosaic virus, YMV, causes one of the most severe of biotic stresses in Vignas, an important group of pulse crops. The viral disease is transmitted through the white fly, Bemicia tabaci, and the yield of the plants is affected drastically. YMV-tolerant lines, generated from a single YMV-tolerant plant identified in the field within a large population of the susceptible cultivar T-9, were crossed with T-9, and F₁, F₂ and F₃ progenies raised. The different generations were phenotyped for YMV-reaction by forced inoculation using viruliferous white flies. A monogenic recessive control of YMV-tolerance was revealed from the F₂ segregation ratio of 3:1 (susceptible: tolerant), which was confirmed by the segregation ratio of the F₃ families. Of 24 pairs of resistance gene analog (RGA) primers screened, only one pair, RGA 1F-CG/RGA 1R, was found to be polymorphic among the parents. Selected F₂ individuals and F₃ families were genotyped with the polymorphic RGA primer pair and the polymorphism was found to be linked with YMV-reaction. This primer pair amplified a 445bp DNA fragment only from homozygous tolerant and the heterozygous lines. The 445bp marker band was sequenced and named 'VMYR1'. The predicted amino acid sequence showed highly significant homology with the NB-ARC domain present in several gene products involved in plant disease resistance, nematode cell death and human apoptotic signaling. To the best of our knowledge, this is the first report of YMV-resistance linked DNA marker development in any crop species using segregating populations. This YMV-resistance linked marker is of potential commercial importance in resistance breeding of plants.     

Development and implementation of a sequence-specific PCR marker linked to a gene conferring resistance to anthracnose disease in narrow-leafed lupin (Lupinus angustifolius L.)

Anthracnose caused by Colletotrichum gloeosporioides is the most serious disease of lupins (Lupinus spp). A cross was made between cultivars Tanjil (resistant) and Unicrop (susceptible) in narrow-leafed lupin (L. angustifolius). Analysis of disease reaction data on the F₂ population and on the resultant F₇ recombinant inbred lines suggested that Tanjil contained a single dominant gene (Lanr1) conferring resistance to anthracnose. The parents and the representative F₂ plants were used to generate molecular markers liked to the Lanr1 gene using the MFLP technique. A co-dominant MFLP polymorphism linked to the Lanr1 gene was identified as a candidate marker. The bands were isolated, re-amplified by PCR, cloned and sequenced. The MFLP polymorphism was converted into a co-dominant, sequence-specific, simple PCR-based marker. Linkage analysis by the computer program MAPMAKER indicated that the marker was 3.5 centiMorgans (cM) from the gene Lanr1. This marker is currently being implemented for marker assisted selection in the Australian National Lupin Breeding Program.     

Mapping of SMV resistance gene Rsc-7 by SSR markers in soybean

Soybean mosaic virus (SMV) is one of the most prevalent pathogens that limit soybean production. In this study, segregation ratios of resistant plants to susceptible plants in P₁, P₂, F₁, F₂ populations of Kefeng No. 1 (P₁)×Nannong 1138-2 (P₂) and derived RIL populations, were used to study the inheritance of resistance to the SMV strain SC-7. Populations Kefeng No. 1 and F₁ were found to be completely resistant to this SMV strain while Nannong 1138-2 was susceptible to it. The F₂ and RIL populations segregated to fit a ratio of 3:1 and 1:1for resistant plants to susceptible ones, respectively. These results indicated that a single dominant gene, designated as Rsc-7, controlled resistance to the SMV strain SC-7 in Kefeng No.1. SSR markers were used to analyze the RIL population and MAPMAKER/EXP 3.0b was employed to establish linkage between markers and this resistance gene. Combining the data of SSRs and resistance identification, a soybean genetic map was constructed. This map, covering 2625.9 cM of the genome, converged into 24 linkage groups, consisted of 221 SSR markers and the resistance gene Rsc-7. The Rsc-7 gene was mapped to the molecular linkage group G8-D1b+W. SSR markers Satt266, Satt634, Satt558, Satt157, and Satt698 were found linked to Rsc-7 with distances of 43.7, 18.1, 26.6, 36.4 and 37.9 cM, respectively.     

Inheritance of powdery mildew (Erysiphe polygoni DC) resistance in mungbean (Vigna radiata L. Wilczek)

Detached mungbean (Vigna radiata L.Wilczek) leaves were inoculated with a conidial suspension of a local isolate (TI-1) of the powdery mildew pathogen (Erysiphe polygoni DC) under controlled environment conditions. Based on the latent period and severity of the infection, a rating scale of 0–5 was used to classify the host pathogen interactions. Reactions 0, 1 and 2 were considered resistant and referred to as R0, R1 and R2 while 3, 4 and 5 were classified as susceptible (S). RUM lines (resistant to powdery mildew) and their derivatives are crossed with several susceptible (reaction types 3–5) genotypes and the inheritance of the resistance was studied in the F₁, F₂ and F₃ generations. The results showed that powdery mildew resistance in mungbean is governed by two dominant genes designated as Pm-1 and Pm-2. When both Pm-1 and Pm-2 were present, an R0 reaction was observed after inoculation with TI-1. The resistant reaction was R1 when only Pm-1 was present and R2 in the presence of Pm-2. In the absence of both Pm-1 and Pm-2, susceptible reactions 3, 4 and 5 were observed.     

Cytogenetic studies on an intergeneric hybrid betweenAgropyron tsukushiense andElymus mollis

Elymus mollis is distributed widely from Korea to Japan, Kamchatka and Alaska, the northern part of U.S.S.R., and Northern and Eastern Canada, Greenland and Iceland. This species is tetraploid (2n=28). A strain of this species collected in Hokkaido was crossed withAgropyron tsukushiense var.transiens collected in Mishima. From this cross, 22 F₁ plants were produced. Crossability calculated from the number of hybrid plants produced and the number of floret pollinated was 30.6%. The shape of the F₁ spikes was of theAgropyron type but the glumes were hairy as were those of theElymus parent. One of the characteristics distinguishingElymus fromAgropyron is the production of two spikelets at almost all nodes of the rachis. This character was not expressed in the F₁ plants. All pollen grains of the F₁ plants were completely abortive. The average chromosome pairing at the MI of the PMCs of the F₁ amounted to 2.03 bivalents and 30.95 univalents. Almost all bivalents ranging from one to seven were rod-shaped connected with interstitial or terminal chiasma. These results indicate a lack of genomic homology between the three genomes ofA. tsukushiense and the two genomes ofE. mollis. Contribution No. 37 from the Plant Germ-plasm Institute, Faculty of Agriculture, Kyoto University, Kyoto, Japan.