Clubroot Symptoms and Resting Spore Production in a Doubled Haploid Population of Oilseed Rape (Brassica napus) Are Controlled by Four Main QTLs

Clubroot, caused by Plasmodiophora brassicae Woronin, is one of the most important diseases of oilseed rape (Brassica napus L.). The rapid erosion of monogenic resistance in clubroot-resistant (CR) varieties underscores the need to diversify resistance sources controlling disease severity and traits related to pathogen fitness, such as resting spore production. The genetic control of disease index (DI) and resting spores per plant (RSP) was evaluated in a doubled haploid (DH) population consisting of 114 winter oilseed rape lines, obtained from the cross ‘Aviso’ × ‘Montego,’ inoculated with P. brassicae isolate “eH.” Linkage analysis allowed the identification of three quantitative trait loci (QTLs) controlling DI (PbBn_di_A02, PbBn_di_A04, and PbBn_di_C03). A significant decrease in DI was observed when combining effects of the three resistance alleles at these QTLs. Only one QTL, PbBn_rsp_C03, was found to control RSP, reducing resting spore production by 40%. PbBn_rsp_C03 partially overlapped with PbBn_di_C03 in a nucleotide-binding leucine-rich repeat (NLR) gene-containing region. Consideration of both DI and RSP in breeding for clubroot resistance is recommended for the long-term management of this disease.

Analysis of strawberry genetic collection (Fragaria L.) for Rca2 and Rpfl genes with molecular markers

Strawberry (Fragaria x ananassa Duch.) varieties are susceptible to many fungal diseases. Identification of forms, carrying resistance genes, is an important stage in breeding programs leading to resistant varieties. The use of molecular markers allows to determine with high reliability the presence of the necessary genes in the genome and to identify promising forms. Some of the common strawberry's diseases, causing significant damage to strawberry plantations, are anthracnose (Colletotrichum acutatum Simmonds) and red stele root rot (Phytophthora fragariae var. fragariae Hickman). Dominant Rca2 gene is involved in monogenic resistance to C. acutatum pathogenicity group 2. Rpf1, Rpf2, Rpf3 genes are determined in monogenic resistance to red stele root rot. The purpose of this study was molecular genetic testing genotypes of genus Fragaria L. to identify carriers of Rca2 allele anthracnose resistance and Rpf1 allele red stele root rot resistance. The objects of study were the wild species of the genus Fragaria L. and strawberry varieties (Fragaria x ananassa Duch.) of different ecological and geographic origin. To assess allelic state Rca2 anthracnose resistance gene the dominant SCAR marker STS-Rca2_240 was used, was linked to the resistance gene Rca2 with a genetic distance of 2.8 cM. Rpf1 gene red stele root rot resistance was identified with the dominant SCAR marker R1A, was linked to the resistance gene Rpf1 with a genetic distance of 3.0 cM. The resistant allele of the marker STS-Rca2_240 was identified in the Laetitia variety (Rca2Rca2 or Rca2rca2 genotype), which allows us to recommend it as a promising source in breeding for anthracnose resistance. The other studied forms have homozygous recessive state of the marker STS-Rca2_240 (putative genotype rca2rca2). The resistant allele of the marker SCAR-R1A in the varieties and wild species of strawberry under study is absent, which presumably indicates their homozygous recessive genotype of Rpf1 gene (rpf1rpf1).

Conventional and Molecular Studies of Brown Planthopper (Nilaparvata lugens Stal) Resistance Genes in Rice: A Basis for Future Study of Natural Insect Resistance Genes Using Molecular Markers in Nepal

Rice productivity is greatly affected by various biotic and abiotic stresses. Insect-pests are one of the major bioticconstraints to cause significant losses in rice production. Brown planthopper (BPH), Nilaparvata lugens Stål, isthe most serious insect-pest of rice in Asia where most of the world rice is produced. Controlling insects usingchemicals is already proven detrimental not only to environment but also to human health. Integrated PestManagement (IPM) is the best approach to control insect pests. Host plant resistance is the principal componentof IPM along with biological, cultural and physical methods. Use of varietal resistance is the best option to controlBPH. Many BPH resistant rice varieties with natural BPH resistance have been developed and widely used againstBPH. However, frequent breakdown of monogenic resistance by new BPH biotypes has been a serious threat tocontrol BPH. To overcome such difficulty in the use of monogenic resistance, development of durable resistanceis needed as the sustainable means to control BPH. To develop durable resistance, pyramiding of BPH resistancegenes and quantitative trait loci (QTLs), through marker-assisted method, is needed. For this, many BPH resistancegenes and QTLs have already been identified and mapped on rice chromosomes. This article reviews identification,mapping and pyramiding toward successful cloning of BPH resistance genes/QTLs and provides the basis/guidelines to work on natural insect resistance genes using molecular markers in Nepal.DOI: http://dx.doi.org/10.3126/njst.v15i1.12032Nepal Journal of Science and TechnologyVol. 15, No.1 (2014) 145-156

Autosomal inheritance of alphamethrin, a synthetic pyrethroid, resistance in Anopheles stephensi – Liston, a malaria mosquito

Anopheles stephensi – Liston (Culicidae: Diptera) is an important urban malarial vector in the Indian sub-continent, accounting for about 15% of the total annual malaria incidence. Chemical control represents a key strategy in the management of this insect vector. However, owing to erratic and continuous application of insecticides, resistance has become a common phenomenon among them and their control has become an uphill task. The genetics of alphamethrin, a synthetic pyrethroid resistance was studied to determine its mode of inheritance. The late third instar larvae were selectively inbred for 27 and ten generations to synthesize homozygous resistant (R) and susceptible (S) stocks, respectively, to the diagnostic dose of 0.12 mg l−1. The log-dosage probit mortality relationships and degree of dominance (D) were calculated. Resistance was observed in both sexes, the dosage-mortality (d-m) line of F1 was towards the resistant parent and the ‘D’ value was found to be 0.8 indicating alphamethrin resistant (amr) gene to be autosomal and incompletely dominant. The d-m lines of F2/backcross exhibited a clear plateau of mortality across a range of doses indicating monogenic resistance. The null hypothesis for monogenic resistance was tested from mortality data of backcross progeny compared with theoretical expectations using the χ2 test and was found to be non-significant. Understanding genetics of insecticide resistance is significant in prediction and management of resistant insects. The amr genes can be used as genetic marker in A. stephensi, which can be used in several applications in conducting basic and applied genetic research.

Segregation of powdery mildew (Podosphaera leucotricha [Ell. et Ev. /Salm./]) resistance within 54 apple progenies

The occurrence of powdery mildew after heavy spontaneous infections was evaluated on 2,500 apple seedlings of 54 progenies that were derived from crosses made in 1993 and 1994. At the same time, the response to mildew was also assessed on 47 cultivars or selections that were used as parents for the crosses. Scores of the strongest infestation were used for final ranking of every seedling or parent. The highest proportions (33 and 28%) of seedlings with complete resistance to mildew were attained in 2 progenies involving parents transmitting monogenic resistance to the pathogen. A few seedlings with this resistance were also found in other 7 progenies. Seedlings with partial resistance to mildew were distributed in the highest proportions (about 30%) among 7 progenies. In 6 out of these progenies, always one parent was derived from partially resistant selections that were selected in the first generation from the cross Spätblühender Taffetapfel × Court Pendu Plat. A very close relationship was found between mid-scores of parents and mean response to powdery mildew infection in progenies. Seedlings on average were significantly more infested than their parents. Progenies determined to have a columnar growth habit were somewhat less infested than progenies possessing a standard growth pattern only. This study revealed considerable improvement of some donors transmitting partial resistance to powdery mildew.  

Biochemical and Molecular Mechanisms Involved in Monogenic Resistance Responses to Tomato Powdery Mildew

The monogenic genes Ol-1, ol-2, and Ol-4 confer resistance to tomato powdery mildew Oidium neolycopersici via different mechanisms. The biochemical mechanisms involved in these monogenic resistances were studied by monitoring through time the association of H2O2 and callose accumulation with hypersensitive response (HR) and papilla formation. Our results showed that H2O2 and callose accumulation are coupled with both Ol-1- and Ol-4–mediated HR-associated resistance as well as with the ol-2–mediated papillae-associated resistance. Further, the transcriptomal changes related to these monogenic resistances were studied by using cDNA-amplification fragment length polymorphism. The expression profiling clarified that 81% of DE-TDF (differentially expressed transcript-derived fragments) were up-regulated upon inoculation with O. neolycopersici in both the compatible and Ol-1–mediated incompatible interactions, though with a difference in expression timing. Of these DE-TDF, more than 70% were not detected in the Ol-4–mediated resistance, while 58% were expressed in the ol-2–mediated resistance, generally at later timepoints. Sequence information suggested that most of these DE-TDF are related to genes involved in either basal defense or establishment of compatibility. In addition, DE-TDF (19%) specifically expressed in different incompatible interactions were identified. Expression patterns of some DE-TDF and marker gene GluB suggested that papillae-associated resistance exploits a different defense pathway from that of HR-associated resistance.

Differences in the responses of stem tissues of spring-type Brassica napus cultivars with polygenic resistance and single dominant gene-based resistance to inoculation with Leptosphaeria maculans

Six spring-type Brassica napus L. cultivars, either susceptible or with polygenic or monogenic resistance, were inoculated with Leptosphaeria maculans (Desmaz.) Ces. & De Not. (organism causing phoma stem canker in crucifers) to investigate differences in the responses of host stem tissues to the pathogen. At growth stage 1.06, plants were inoculated with pycnidiospores at the junction of the petiole and stem. The pre-penetration and penetration phases were examined along with the histological, ultrastructural, and histochemical responses. The processes of pycnidiospore attachment, germination, and penetration through the stomata of petioles and stems were found to be similar in all cultivars. Specific post-penetration defense reactions identified were lignification, suberisation, and additional cambium formation in the resistant cultivars. In ‘Surpass 400’, which has monogenic resistance, these responses occurred 4–5 d earlier than in polygenically resistant cultivars, and were more intense (preventing hyphal penetration of the additional cambium layer), and resulted in a hypersensitive reaction without pycnidia formation. Our study clearly emphasizes the variatiability in location, timing, and histochemistry of stem responses between compatible and incompatible interactions and will improve our overall understanding of the role and importance of the mechanisms of resistance in spring-type B. napus to L. maculans.

Mapping of Cotton-Melon Aphid Resistance in Melon

Resistance to cotton-melon aphid (Aphis gossypii Glover) segregated as a single dominant gene in a melon (Cucumis melo L.) mapping population derived from the cross `Top Mark' × PI 414723. Sixty-four F2-derived F3 families were used to map the aphid resistance locus, Vat, with respect to randomly amplified polymorphic DNA (RAPD) and restriction fragment length polymorphism (RFLP) markers. RFLP markers NBS-2 and AC-39 flanked Vat at distances of 3.1 cM and 6.4 cM, respectively. NBS-2 is homologous to the nucleotide binding site-leucine-rich repeat (NBS-LRR) superfamily of plant resistance genes. Another homolog of this superfamily, NBS-5, was positioned ≈16.8 cM from Vat, raising the possibility that Vat resides in a cluster of NBS-LRR paralogs. RFLP marker AC-8, which has similarity to plant lipoxygenases, was positioned at ≈5.5 cM from Vat. Monogenic resistance to A. gossypii has been identified in two sources of melon germplasm, Indian accession PI 371795 (progenitor of PI 414723) and Korean accession PI 161375. To test for an allelic relation between the genes controlling aphid resistance in these two distinct germplasm sources, melon plants of a backcross population from a cross between two resistant lines having Indian- or Korean-derived resistance were infested with aphids. At least 90 out of 92 segregating progeny were aphid resistant, suggesting that the same resistance gene, Vat, is present in both sources of melon germplasm.