Genetic analysis of mechanisms associated with inheritance of resistance to sheath rot of rice

Understanding genetic mechanisms controlling inheritance of disease resistance traits is essential in breeding investigations targeting development of resistant genotypes. Using North Carolina design II, 32 F1 hybrids were generated by crossing eight susceptible to four resistant parents and submitted for field evaluation. The analysis of general and specific combining ability (GCA and SCA) indicated involvement of additive and non-additive gene action controlling inheritance of horizontal resistance to sheath rot of rice. High GCA/SCA ratio and high heritability estimates revealed additive effects and were more predominant than none additive ones. The level of dominance indicated dominant genes was more important than recessive genes. Estimates of GCA and SCA analysis suggested that crop improvement programmes should be directed towards selection of superior parents or good combiners, emphasizing on GCA. As far as source of resistance is concerned, most promising genotypes were Cyicaro, Yunertian and Yunkeng. The predominance of additive genetic effects together with the relevance of dominant genes suggested possibilities of improving the resistance by introgression of resistance genes through recurrent selection coupled with phenotypic selection

Assessment of genetic diversity of rice based on SNP markers for selection of parents for sheath rot (Sarocladium oryzae) resistance breeding

Sheath rot of rice, caused by Sarocladium oryzae, is an important emerging rice disease not only in Rwanda, but also in other rice-growing countries. Given that cultivar resistance is a sustainable management strategy for small-scale farmers, the aim of this study was to identify genetically distant parental materials for sheath rot resistance breeding. Ten resistant and fifteen susceptible accessions were analysed using 94 single nucleotide polymorphism (SNP) markers. The number of alleles amplified per locus ranged from 1 to 4 with a mean of 2.01 and a total of 189 alleles detected from the 25 genotypes. The number of observations per marker locus ranged from 11 to 25 with an average of 23. The mean major allele frequency was 76.2%, whereas the mean polymorphic information content was 0.263, and gene diversity was estimated at 0.325. Consequently, the markers were highly informative and revealed good estimates of genetic diversity among the studied accessions. Genetic distances ranged from 0 to 0.63 and a UPGMA dendrogram distinguished resistant and susceptible genotypes. This study revealed the possibility of improving resistance to sheath rot with minimum risk of genetic depression or reduced variability among progenies through hybridisation of locally adapted germplasm

Molecular Characterization and DNA Fingerprinting of Xanthomonas oryzae pv. oryzae Isolates from Climate Change Prone Areas in East Africa

Genomic DNA fingerprinting is a useful tool for effective and reliable identification and differentiation of Xanthomonas oryzae pv. oryzae (Xoo) pathogen from rice. The study aimed to conduct molecular characterization and DNA fingerprinting of 23 Xoo isolates from East Africa and two Xoo isolates from IRRI (Philippines) as control. PCR analysis was carryout on genomic DNA of 25 Xoo isolates using 6 Xoo specific primer pairs. Cluster analyses of genetic data obtained from 25 Xoo DNA fingerprints revealed two major genotypes (GrpA and GrpB) among the 25 Xoo isolates. GrpA has three subgroups (GrpA1; GrpA2; GrpA3) and GrpB (GrpB1; GrpB2; GrpB3). GrpA genotype consists of 20 Xoo isolates from Uganda, Rwanda and Philippines while GrpB genotype has 5 Xoo isolates from Rwanda. Some Xoo isolates were identical (PX-1, PX-2; UX621, RX2101; RX554, UX623, RX4113; UX211, UX213, UX214, RX4112, UX215). The emergence of subgroup genotypes could possibly be due to mutations and interactions among isolates and strains in host cells. Some Xoo isolates from Rwanda and Uganda were identical suggesting possible pathogen migration between these countries and long-term survival. Durable resistance rice cultivars would need to overcome both GrpA and GrpB Xoo genotypes in order to survive after their deployment into different rice ecologies in East Afric