Genetic Mapping of Magnaporthe grisea Avirulence Gene Corresponding to Leaf and Panicle Blast Resistant QTLs in Jao Hom Nin Rice Cultivar

The avirulence characteristic of Magnaporthe grisea isolate TH16 corresponding to Jao Hom Nin (JHN) rice cultivar was studied by mapping population of 140 random ascospore progenies derived from the cross between B1-2 and TH16 isolates. Segregation analyses of the avirulence characteristic performing on JHN rice at the seedling and flowering stages were performed in this mapping population. We used the reference map of Guy11/2539 to choose microsatellite DNA markers for mapping the avirulence gene. The genetic map of this population was constructed from 39-microsatellite markers. The genetic map was spanned by covering seven chromosomes with an average distance of 11.9 cM per marker. In mapping population the distribution of pathogenic and non-pathogenic progenies on JHN rice were found to be fitted to 1 : 1 ratio for two of the rice stages, seedling and flowering stages. The Quantitative Trait Loci (QTL) analysis for avirulence genes corresponding to two rice stages were located at the same region on chromosome 2 between markers Pyms305 and Pyms435. The LOD score and percentage of phenotypic variance explained (PVE) on two rice stages were 5.01/16.69 and 6.73/20.26, respectively. These loci were designated as Avr-JHN(lb) and Avr-JHN(pb) corresponding to leaf and panicle blast characteristics. The findings of this study can be the initial step for positional cloning and identifying any function of avirulence genes corresponding to leaf and panicle blast characteristics.     

Assessment of Genetic Diversity in Thai Isolates of Pyricularia grisea by Random Amplification of Polymorphic DNA

One hundred and seventy‐four isolates of Pyricularia grisea were collected from various hosts such as barley, rice, weed and wild rice in Thailand. Seven arbitrary decamer primers from the set of University of British Columbia were employed and nine lineages were classified. Lineages B, C and H were predominant, contributing up to 70% of total pathogens in this study. Analysis showed that the distribution of each lineage differs from the predominant lineages across Thailand in such that other lineages were restricted in particular area. For instance, lineage A was limited only in southern Thailand, whereas wide distribution of lineages B and C reflected an influence of both biological and physical effects on pathogen variation. Principal component analysis resulted in a total of four groups of blast pathogen with small distinctions between barley‐, rice‐, weed‐ and wild rice‐infected blast. Bridging relationships occurred among border isolates of weed and rice blast suggesting a chance of migrations between hosts. Higher diversity was observed in northern, north‐eastern and central Thailand while eastern and southern parts were rather low. Genetic diversity indices elucidated an abundance of pathogen lineages existing in northern Thailand suggesting that it should be the centre of diversity.