Development of SCAR markers linked to powdery mildew (<Emphasis Type="Italic">Uncinula necator</Emphasis>) resistance in grapevine (<Emphasis Type="Italic">Vitis vinifera</Emphasis> L. and <Emphasis Type="Italic">Vitis</Emphasis> sp.)

Sequence-characterized amplified regions markers (SCARs) were developed from six randomly amplified polymorphic DNA (RAPD) markers linked to the major QTL region for powdery mildew (Uncinula necator) resistance in a test population derived from the cross of grapevine cultivars “Regent” (resistant) × “Lemberger”(susceptible). RAPD products were cloned and sequenced. Primer pairs with at least 21 nucleotides primer length were designed. All pairs were tested in the F1 progeny of “Regent” × “Lemberger”. The SCAR primers resulted in the amplification of specific bands of expected sizes and were tested in additional genetic resources of resistant and susceptible germplasm. All SCAR primer pairs resulted in the amplification of specific fragments. Two of the SCAR markers named ScORA7-760 and ScORN3-R produced amplification products predominantly in resistant individuals and were found to correlate to disease resistance. ScORA7-760, in particular, is suitable for marker-assisted selection for powdery mildew resistance and to facilitate pyramiding powdery mildew resistance genes from various sources.     

Impact of different pH control agents on biopesticidal activity of<Emphasis Type="Italic"> Bacillus thuringiensis</Emphasis> during the fermentation of starch industry wastewater

Different pH control agents (NaOH/H₂SO₄—SodSulp, NaOH/CH₃COOH—SodAcet, NH₄OH/CH₃COOH—AmmoAcet and NH₄OH/H₂SO₄—AmmoSulp) were used to investigate their effects on growth, enzyme production (alkaline protease and amylase), and entomotoxicity of Bacillus thuringiensis var. kurstaki HD-1 (Btk) against eastern spruce budworm larvae (Choristoneura fumiferana) using starch industry wastewater (SIW) as a raw material in a 15-l fermentor. AmmoSulp and SodSulp were found to be the best pH control agents for alkaline protease and amylase production, respectively; whereas, the fermented broth obtained by using SodAcet as pH control agents recorded the highest delta-endotoxin production of 1043.0 mg/l and entomotoxicity value 18.4 × 10⁹ SBU/l. Entomotoxicity of re-suspended centrifuged pellet in one-tenth of original volume in case of SodAcet as pH control agents was 26.7 × 10⁹ SBU/l and was the highest value compared to three other pH control agents.     

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.     

Development of transgenic pearl millet (Pennisetum glaucum (L.) R. Br.) plants resistant to downy mildew

Transgenic pearl millet lines expressing pin gene—exhibiting high resistance to downy mildew pathogen, Sclerospora graminicola—were produced using particle-inflow-gun (PIG) method. Shoot-tip-derived embryogenic calli were co-bombarded with plasmids containing pin and bar genes driven by CaMV 35S promoter. Bombarded calli were cultured on MS medium with phosphinothricin as a selection agent. Primary transformants 1T₀, 2T₀, and 3T₀ showed the presence of both bar and pin coding sequences as evidenced by PCR and Southern blot analysis, respectively. T₁ progenies of three primary transformants, when evaluated for downy mildew resistance, segregated into resistant and susceptible phenotypes. T₁ plants resistant to downy mildew invariably exhibited tolerance to Basta suggesting co-segregation of pin and bar genes. Further, the downy mildew resistant 1T₁ plants were found positive for pin gene in Southern and Northern analyses thereby confirming stable integration, expression, and transmission of pin gene. 1T₂ progenies of 1T₀ conformed to dihybrid segregation of 15 resistant:1 susceptible plants.     

A region of barley chromosome 6H harbors multiple major genes associated with net type net blotch resistance

Net type net blotch (NTNB), caused by Pyrenophora teres f. teres Drechs., is prevalent in barley growing regions worldwide. A population of 118 doubled haploid (DH) lines developed from a cross between barley cultivars ‘Rika’ and ‘Kombar’ were used to evaluate resistance to NTNB due to their differential reaction to various isolates of P. teres f. teres. Rika was resistant to P. teres f. teres isolate 15A and susceptible to isolate 6A. Conversely, Kombar was resistant to 6A, but susceptible to 15A. A progeny isolate of a 15A × 6A cross identified as 15A × 6A#4 was virulent on both parental lines. The Rika/Kombar (RK) DH population was evaluated for disease reactions to the three isolates. Isolate 15A induced a resistant:susceptible ratio of 78:40 (R:S) whereas isolate 6A induced a resistant:susceptible ratio of 40:78. All but two lines had opposite disease reactions indicating two major resistance genes linked in repulsion. Progeny isolate 15A × 6A#4 showed a resistant:susceptible ratio of 1:117 with the one resistant line also being the single line that was resistant to both 15A and 6A. An RK F₂ population segregated in a 1:3 (R:S) ratio for both 15A and 6A indicating that resistance is recessive. Molecular markers were used to identify a region on chromosome 6H that harbors the two NTNB resistance genes. This work shows that multiple NTNB resistance genes exist at the locus on chromosome 6H, and the recombinant DH line harboring the resistance alleles from both parents will be useful for the development of NTNB-resistant barley germplasm.     

Phenotype changes inherited by crossing pyrethroid susceptible and resistant genotypes from the cattle tick <Emphasis Type="Italic">Riphicephalus</Emphasis> (<Emphasis Type="Italic">Boophilus</Emphasis>) <Emphasis Type="Italic">microplus</Emphasis>

Dialelic crosses and backcrosses of pyrethroid resistant (RR) and susceptible (SS) Rhipicephalus (Boophilus) microplus tick strains were carried out and the substitution (Phe-Ile) within the sodium channel gene was monitored in order to analyze the effects of the genotype on the pyrethroid resistance phenotype as measured by the larval packet test (LPT). Parental strains: susceptible (SS) and resistant (RR); dialelic crosses: RS (♂RR × ♀SS), and SR (♂SS × ♀RR); and backcrosses: RS × SS, RS × RR, SR × SS and SR × RR were infested on 280 kg calves. Resistance type (monogenic or polygenic) and effective dominance were determined based on the discriminant concentration (DC) for cipermethrine (0.5%), deltamethrine (0.09%) and flumethrine (0.01%). Allele specific PCR (AS-PCR) was used for genotyping, looking at a sodium channel mutation (Phe-Ile substitution). The mortality rates and allele frequency of susceptible and pyrethroid resistant reference strains were 0% mortality and 90% RR alleles for resistant strain, and 100% mortality and 0% RR alleles as measured by the larval packet test (LPT) and allele specific PCR (AS-PCR) respectively. Backcrossed strain SR × RR showed an effective dominance (D_ML) of 0.605 for cypermethrin, 0.639 for deltamethrin and 0.498 for flumethrin, while survival of backcrosses RS × SS, RS × RR and SR × SS showed a significant tendency to recesivity. Backcrossed strain SR × RR (69.4%) also showed a higher RR genotype frequency with regards to RS × SS (25.5%), RS × RR (36.7%) and SR × SS (32.0%), however, susceptible allele was inherited in general as an incomplete dominant trait. Monogenic inheritance hypothesis was tested and the results showed monogenic inheritance for cypermethrin and flumethrin (P < 0.05) but not for deltamethrin (P > 0.05). However, significant correlation was found between RR genotype and the survival rate for all three pyrethroids used (P < 0.05), suggesting that a single substitution on the sodium channel gene can be responsible for resistance to pyrethroids as a class, due to the high frequency for RR genotypes. Combination with different mutations or metabolic resistance mechanisms cannot be excluded.     

Identification of a STS marker linked to the Aegilops speltoides-derived leaf rust resistance gene Lr28 in wheat

 A sequence-tagged-site (STS) marker is reported linked to Lr28, a leaf rust resistance gene in wheat. RAPD (random amplified polymorphic DNA) analysis of near-isogenic lines (NILs) of Lr28 in eight varietal backgrounds was carried out using random primers. Genomic DNA enriched for low-copy sequences was used for RAPD analysis to overcome the lack of reproducibility due to the highly repetitive DNA sequences present in wheat. Of 80 random primers tested on the enriched DNA, one RAPD marker distinguished the NILs and the donor parent from the susceptible recurrent parents. The additional band present in resistant lines was cloned, sequenced, and STS primers specific for Lr28 were designed. The STS marker (Indian patent pending: 380 Del98) was further confirmed by bulk segregation analysis of F₃ families. It was consistently present in the NILs, the resistant F₃ bulk and the resistant F₃ lines, but was absent in recurrent parents, the susceptible F₃ bulk and the susceptible F₃ lines. Received: 20 February 1998 / Accepted: 4 March 1998     

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.     

Potential of doubled-haploid lines and localization of quantitative trait loci (QTL) for partial resistance to bacterial leaf streak (Xanthomonas campestris pv. hordei) in barley

 Genetic variability for partial resistance to bacterial leaf streak in barley, caused by Xanthomonas campestris pv. hordei, was investigated in 119 doubled-haploid lines (DH) developed by the Hordeum bulbosum method from the F₁ progeny of the cross between two cultivars, ‘Morex’ (resistant) and ‘Steptoe’ (susceptible). Two experiments were undertaken in a randomized complete block design with three replicates, in a controlled growth chamber. Twenty seeds per replicate were planted in plastic containers (60×40×8 cm) containing moistened vermiculite. At the two-leaf stage seedlings were inoculated with an Iranian strain of the pathogen. Genetic variability was observed among the 119 DH lines for partial resistance to the disease. Some DH lines were significantly more resistant than ‘Morex’ (resistant parent) to bacterial leaf streak. Genetic gain in percentage of resistant parent for 5% of the selected DH lines was significant (47.70% and 33.72% in the first and the second experiment, respectively). A QTL analysis of bacterial leaf streak resistance showed that three QTLs were detected on chromosomes 3 and 7. Multilocus allelic effects of the three QTLs account for almost 54% of the mean difference between the parents and nearly 30% of the phenotypic variation of the trait in the mean experiment. The resistance locus on chromosome 3, near ABG377, apprears to be a major gene. Received: 15 July 1997 / Accepted: 4 August 1997     

Occurrence and characterization of a nucleopolyhedrovirus from <Emphasis Type="Italic">Maruca vitrata</Emphasis> (Lepidoptera,Pyralidae) isolated in Taiwan

A nucleopolyhedrovirus (MaviMNPV) was isolated from diseased larvae of legume pod borer (LPB), Maruca vitrata, at Tainan in Taiwan. Electron microscopical studies on the ultrastructure of MaviMNPV occlusion bodies (OBs) showed several virions (up to 19) with multiple nucleocapsids (up to 6) packaged within a single viral envelope. The diameter of OBs was 0.9 to 1.3 μm with a mean of 1.152±0.116 μm. The complete sequence of the MaviMNPV polyhedrin (Polh) gene contained 735 nucleotides (GenBank accession number DQ399596). Phylogenetic analyses using the complete sequence of the Polh gene of MaviMNPV indicated that this virus clusters with Group I NPVs. The genome size of MaviMNPV estimated with restriction enzymes viz., HindIII, EcoRI, BglII and PstI was 113.41 ± 1.50 kbp. First instar LPB larvae were the most susceptible stage (LC₅₀ 2.053 × 10² OBs/ml) followed by second, third and fourth instars with the median lethal concentrations (LC₅₀s) 1.410 × 10³, 2.390 × 10³ and 2.636 × 10³ OBs/ml, respectively. This is the first record of this virus from this region. The first and second authors have equal contributions in this paper     

Use of in vitro method to evaluate some grapevine varieties for tolerance and susceptibility to sodium bicarbonate-induced chlorosis

Nodal shoot segments of four grapevine genotypes well known for their Fe-chlorosis characteristic [Vitis vinifera × Vitis berlandieri Fercal, resistant; V. berlandieri × Vitis rupestris 1103 P, mid-resistant; Solonis (Vitis riparia × V. rupestris × Vitis candicans) × Othello (Vitis labrusca × V. riparia × V. vinifera) 1613 C, susceptible; V. vinifera L. cv. Perlette, resistant] were cultured in vitro. The effects of three levels of iron sodium ethylene-diaminotetraacetate (FeNaEDTA; 9, 18, 36 mg l⁻¹) and three mixtures of iron and 840 mg l⁻¹ NaHCO₃ (sodium bicarbonate) in the Murashige and Skoog (MS) medium supplemented 4.9 μM indole-3-butyric acid (IBA) were compared. We assayed the chlorosis rating of leaves, total chlorophyll of leaves, dry shoot weights of the plantlets, and active and total Fe content of the leaves. The most suitable concentration in determining the reaction of genotypes to iron chlorosis was 9 mg l⁻¹ FeNaEDTA. Bicarbonate addition had negative effects on the iron intake and development of plants. While all genotypes were affected by non-ferrous conditions, Fercal and Perlette were found to be the most resistant genotypes and 1613 C rootstock as the most susceptible. The chlorosis rating of the tested genotypes ranked according to their known degree of tolerance and susceptibility to lime-induced chlorosis. The results of this study showed that the in vitro technique could successfully be used in viticulture to get results in shorter times in the studies, aiming at breeding new rootstocks and varieties suitable to calcareous soil conditions and determining the reactions of existing genotypes to Fe chlorosis.     

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.     

Localization in the fourth linkage group of a genedi affecting diabetes insipidus in rats

The diabetes insipidus gene has been localized in the fourth linkage group at a distance of 23.1±1.9 cM from the nonagouti loci and 29.5±2.1 cM from theSvp-1 loci based on the backcross progeny analysis of ratsRattus norvegicus mating (August×Brattleboro)F₁×Brattleboro. The proposed gene order isSvp-1—a—di.     

Mapping of resistance to spot blotch disease caused by Bipolaris sorokiniana in spring wheat

Spot blotch caused by Bipolaris sorokiniana is a destructive disease of wheat in warm and humid wheat growing regions of the world. The development of disease resistant cultivars is considered as the most effective control strategy for spot blotch. An intervarietal mapping population in the form of recombinant inbred lines (RILs) was developed from a cross ‘Yangmai 6’ (a Chinese source of resistance) × ‘Sonalika’ (a spot blotch susceptible cultivar). The 139 single seed descent (SSD) derived F₆, F₇, F₈ lines of ‘Yangmai 6’ × ‘Sonalika’ were evaluated for resistance to spot blotch in three blocks in each of the 3 years. Joint and/or single year analysis by composite interval mapping (CIM) and likelihood of odd ratio (LOD) >2.2, identified four quantitative trait loci (QTL) on the chromosomes 2AL, 2BS, 5BL and 6DL. These QTLs were designated as QSb.bhu-2A, QSb.bhu-2B, QSb.bhu-5B and QSb.bhu-6D, respectively. A total of 63.10% of phenotypic variation was explained by these QTLs based on the mean over years. Two QTLs on chromosomes 2B and 5B with major effects were consistent over 3 years. All QTL alleles for resistance were derived from the resistant parent ‘Yangmai 6’.     

Deletion of a large genomic segment in tobacco varieties that are resistant to potato virus Y (PVY)

Recessive alleles (va, va ¹ , va ² , etc) of the tobacco Va locus confer resistance to potato virus Y (PVY). To elucidate the mechanism underlying this resistance, we attempted to identify randomly amplified polymorphic (RAPD) markers that reveal polymorphism between two nearly isogenic lines (NILs) that differ in their susceptibility to PVY. Using each of 500 primers and 800 pairs of primers, we identified over 100 RAPD fragments that differed between the NILs. We applied these RAPD primers or primer combinations to an F₂ population obtained from a cross between the susceptible line BY4 and the resistant va ² -bearing NIL, F55. It was found that only 10 RAPD markers were polymorphic between resistant and susceptible plants. Unexpectedly, these markers were all linked to Va. All 10 RAPD markers were present in all 8 susceptible varieties tested. At least one RAPD marker was not detected in 8 out of 10 resistant varieties. Southern analysis revealed that the sequences of markers were not present in the genomes of resistant varieties, and the markers were found in individually distinct positions on the chromosomes of susceptible tobacco varieties. These results strongly suggest that the resistance conferred by va is due to deletions at the Va locus governing susceptibility to PVY. Received: 20 May 1999 / Accepted: 17 August 1999     

Decrease in tolerance to fenvalerate, in resistantHelicoverpa armigera after pupal diapause

Pyrethroid resistant and susceptible adults ofHelicoverpa armigera (Lepidoptera: Noctuidae) were screened for tolerance to pyrethroids after 6 wk or 12 wk pupal diapuse. Resistant larvae and F₂ larvae from a cross between resistant and susceptible parents (two replicates), were reared under conditions to induce pupal diapause. After eclosion, adults were tested in glass vials coated with the pyrethroid fenvalerate, at a dose (DD) that is known to discriminate between susceptible and heterozygous resistant individuals. In all diapause experiments, the frequency of resistance was lower in the test groups that had experienced diapause compared with the non-diapausing control group. The underlying cause of the decline is not certain but selective mortality of resistant versus susceptible individuals could not account for all the difference in two of the three experiments — tolerance to the pyrethroid, fenvalerate, is most likely to have declined either as a consequence of diapause or from the extended time of development associated with diapause. These results indicate that monitoring programs could underestimate pyrethroid resistance frequencies when usingH. armigera adults emerging from diapause.     

Characterisation of acidic sites of Pseudomonas biomass capable of binding protons and cadmium and removal of cadmium via biosorption

Summary The ability of Pseudomonas aeruginosa to accumulate Cd(II) ions from wastewater industries was experimentally investigated and mathematically modelled. From the potentiometric titration and non-ideal competitive analysis (NICA) model, it was found that the biomass contains three acidic sites. The values of proton binding (pK _i =1.66±3.26×10⁻³, 1.92±1.63×10⁻⁴ and 2.16±3.79×10⁻⁴) and binding constant of cadmium metal ions (pK _M1=1.99±2.45×10⁻³ and pK _M2=1.67±4.08×10⁻³) on the whole surface of biomass showed that protonated functional groups and biosorption of Cd(II) ions could be attributed to a monodentate binding to one acidic site, mainly the carboxylic group. From the isothermal sorption experimental data and Langmuir model, it was also found that the value of Langmuir equilibrium (pK _f) constant is 2.04±2.1×10⁻⁵ suggesting that the carboxyl group is the main active binding site. In addition, results showed that the maximum cadmium capacity (q _max) and affinity of biomass towards cadmium metal ions (b) at pH 5.1 and 20 min were 96.5±0.06 mg/g and 3.40×10⁻³± 2.10×10⁻³, respectively. Finally, interfering metal ions such as Pb(II), Cu(II), Cr(III), Zn(II), Fe(II), Mn(II), Ca(II) and Mg(II) inhibited Cd(II) uptake. Comparing the biosorption of Cd(II) by various Pseudomonas isolates from contaminated environment samples (soil and sewage treatment plant) showed that maximum capacities and equilibrium times were different, indicating that there was a discrepancy in the chemical composition between biomasses of different strains.     

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     

Genes tagging and molecular diversity of red rot susceptible/tolerant sugarcane hybrids using c-DNA and unigene derived markers

Sugarcane is an important international commodity as a valuable agricultural crop especially in tropical and subtropical countries. Two bulked DNA used to screen polymorphic primers from commercial hybrids (varieties) with moderately resistant and highly susceptible to red rot disease. Among 145 simple sequence repeat and unigene primers screened, 37 (25%) were found to be highly robust and polymorphic with Polymorphism Information Content values ranging from 0.50 to 1.00 with the mean value of 0.82. Among these microsatellites, twenty one were used in the study of genetic relationships and marker identification in sugarcane varieties for red rot resistance. A total of 105 polymorphic DNA bands were identified, with their fragment size ranging from 54 to 1,280 bp. Jaccard’s similarity coefficient value recorded between closely related hybrids was 0.986 while lowest coefficient value of 0.341 was detected with distantly related hybrids. The average similarity coefficient among these hybrids was 0.663. Cluster analysis resulted in a dendrogram with two major clusters separating the moderately resistant varieties from highly susceptible varieties. Three group specific fragments amplified by unigene Saccharum microsatellite primers viz; two markers UGSM316₈₅₀ and UGSM316₆₀ were closely associated with moderately resistant varieties by appearing bands in this region but the bands were absent in highly susceptible varieties. Similarly UGSM316₄₀₀ marker was tightly linked with highly susceptible varieties by amplifying uniformly in sugarcane varieties showing highly susceptible reaction to red rot but it was absent in moderately resistant varietal groups. Validation of red rot resistance/susceptibility associated markers on a group of different mapping populations for red rot resistant/susceptible traits is in progress.     

Chromosomal localization and molecular-marker tagging of the powdery mildew resistance gene (Lv) in tomato

We report the tagging of a powdery mildew [Leveillula taurica (Lév.) Arnaud.] resistance gene (Lv) in tomato using RAPD and RFLP markers. DNA from a resistant (cv Laurica) and a susceptible cultivar were screened with 300 random primers that were used to amplify DNA of resistant and susceptible plants. Four primers yielded fragments that were unique to the resistant line and linked to the resistance gene in an F₂ population. One of these amplified fragments, OP248, with a molecular weight of 0.7 kb, was subsequently mapped to chromosome 12, 1 cM away from CT134. Using RFLP markers located on chromosome 12, it was shown that approximately one half of chromosome 12 (about 42 cM), in the resistant variety is comprised of foreign DNA, presumably introgressed with the resistance gene from the wild species L. chilense. Further analysis of a backcross population revealed that the Lv gene lies in the 5.5-cM interval between RFLP markers, CT211 and CT219. As a prelude to map-based cloning of the Lv gene, we are currently enriching the density of markers in this region by a combination of RAPD primers and other techniques.