MSc

Rice (Oryza sativa L.) is the most important food crop for more than 60 per cent of world`s population. In India, rice accounts for 43 percent of the nation's overall production of food grains and 46 per cent of its total cereal production. However, the average productivity, at 2.6 t/ha is quite low, with pests and diseases being major constraints. The brown planthopper (BPH), Nilaparvata lugens (Stal.) has evolved to be a key pest of rice across the country, largely due to the injudicious application of insecticides for their management. Use of resistant varieties has been the most favoured strategy for the management of BPH across South Asia. This, in turn, calls for identification of resistant donors and their use in resistance breeding. Local land races or traditional rice varieties have been considered as rich repositories of resistance genes. Hence, the present study was taken up to screen traditional varieties for resistance to BPH and to identify genes conferring the same.

Screening of rice genotypes for BPH resistance through standard seedbox screening technique in glasshouse revealed variability in resistance pattern among the 82 genotypes that were evaluated. Among the genotypes screened, one genotype namely MO 22 (Sreyas) exhibited high resistance to BPH with a damage score below
1.0 while two cultures viz., PTB 33 and MO 23 (Pournami) exhibited resistance to BPH with a damage score below 3.0. Eleven genotypes, namely, Aryankayama, Hraswa, Chenthondi, Adukkan, Karumbaayan, PTB 60 (Vaisakh), Thonnuran, Vellamunda, Orkayama, KAU Manuvarna and PTB 54 (Karuna) were moderately resistant to BPH with a damage score ranging from 3.0 to 5.0 while rest of the genotypes were found susceptible.

Molecular characterization of the genotypes with 25 SSR markers tightly linked to reported BPH resistance genes and QTL`s was carried out. The genotype PTB 60 (Vaisakh) possessed maximum number of R-genes/ QTLs viz., bph4, BPH7, Bph27, Bph31, Bph32 and qBph4.3. All the other genotypes were found to possess at least one R-gene or a QTL. No R-genes/ QTLs were recorded in the susceptible check TN1 except for QTL qBph4.3. The highly resistant genotype MO 22 (Sreyas) was found to possess a major gene Bph32 and a minor QTL qBph4.3.

The genetic diversity was analyzed based on the marker segregation data. The two monomorphic markers viz., RM16766 and RM435 and the 23 polymorphic markers registered 109 alleles. The marker RM2334 revealed maximum number of alleles (11) with highest Polymorphic Information Content (PIC) value of 0.89. The number of alleles per marker ranged from 1 to 11 with an average of 4.36 alleles indicating the presence of average allelic diversity. The PIC values varied from 0 (RM16766 and RM435) to 0.89 (RM2334) indicating the SSR markers used in the study were highly informative.

A dendrogram was constructed using UPGMA employing Jaccard similarity coefficient and the 82 genotypes were categorized into two major clads at 73% level of genetic similarity. The major clad I which included 77 genotypes was divided into two sub-clads IA and IB. Most of the resistant genotypes were grouped under major clad IA. The major clades and subclades clearly represented a wide range of genotypes with different responses to BPH.

Local germplasm, especially adapted landraces, is an important genetic resource for crop improvement. Exploiting the potential of traditional, resistant cultivars like Aryankayama, Adukkan, Thonnuran, Karumbaayan, Vellamunda, and Orkayama with multiple R-genes and QTLs can thus contribute to development of cultivars resistant to BPH. Combining conventional and molecular breeding approaches to stack R-genes / QTLs conferring resistance to BPH into the background of high yielding genotypes would hasten the development of cultivars with improved resistance to the devastating pest.