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Department of Plant Pathology, The Pennsylvania State University, University Park 16802, USA(Received on September 27, 2010; Accepted on November 2, 2010)Genetic instability of the rice blast fungus Magnaportheoryzae has been suggested as a major factor underlyingthe rapid breakdown of host resistance in the field.However, little information is available on the mech-anism of genetic instability. In this study, we assessedthe stability of repetitive DNA elements and several keyphenotypic traits important for pathogenesis afterserially transferring two isolates though rice plants andan artificial medium. Using isolate 70-15, we obtained atotal of 176 single-spore isolates from 10 successiverounds of culturing on artificial medium. Another 20isolates were obtained from germ tubes formed at thebasal and apical cells of 10 three-celled conidia. Addi-tionally, 60 isolates were obtained from isolate KJ201after serial transfers through rice plants and an arti-ficial medium. No apparent differences in phenotypes,including mycelial growth, conidial morphologies, coni-diation, conidial germination, appressorium formation,and virulence, or in DNA fingerprints using MGR586,MAGGY, Pot2, LINE, MG-SINE and PWL2 as probeswere observed among isolates from the same parent iso-late. Southern hybridization and sequence analysis oftwo avirulence genes, AVR-Pita1 and AVR-Pikm, show-ed that both genes were also maintained stably during10 successive generations on medium and plants. How-ever, one reversible loss of restriction fragments wasfound in the telomere-linked helicase gene ( TLH1) family,suggesting some telomere regions may be more unstablethan the rest of the genome. Taken together, our resultssuggest that phenotype and genotype of M. oryzae iso-lates do not noticeably change, at least up to 10 successivegenerations on a cultural medium and in host plants.Keywords :genetic stability, genotypes, Magnaporthe grisea,phenotypes, repeat sequencesMagnaporthe oryzae is a heterothallic ascomycete fungusthat causes rice blast, one of the most devastating diseasesin rice throughout the world. Since there is no evidence forsexual reproduction in the field in most parts of the world,asexual spores (conidia) are the most important source ofinoculum. Through the production of conidia, M. oryzaenormally undergoes 8-11 disease cycles in rice plants underfield conditions during one growing season (Han et al.,1997). Although asexual reproduction is expected to pro-duce genetically identical clones, the high rate of appear-ance of new races with the ability to infect previouslyresistant varieties of rice, suggests that M. oryzae may havehigh mutation rates in avirulence genes, which determinerace (also called pathotype in M. oryzae) (Kang et al., 2001;Kang and Lee, 2000; Zhou et al., 2007).Although the most effective control strategy for rice blastis cultivating resistant varieties, it has often achieved onlyshort-term success due to frequent breakdown of newlyintroduced resistance. This resistance breakdown has beenattributed to genetic variability in M. oryzae (Bonman et al.,1986; Correa-Victoria and Zeigler, 1993; Correa-Victoriaand Zeigler, 1993). Various mechanisms have been sug-gested to explain the frequent generation of race variants,including heterokaryosis, parasexuality, and mutations(Genovesi and Magill, 1976; Wu and Magill, 1995; Xia andCorrell, 1995; Zeigler et al., 1997). Parasexual recombina-tion of genetic markers, resulting new genetic variants, hasbeen demonstrated in the laboratory for M. oryzae, sug-gesting the potential for parasexual recombination togenerate new races (Noguchi et al., 2006; Zeigler et al.,1997). However, no evidence for parasexuality has beenfound in nature for M. oryzae. The activity of transposableDNA elements is another possible mechanism that has beensuggested (Kang et al., 2001; Nishimura et al., 2000; Zhouet al., 2007). For example, mobilization of Pot2, Pot3, andLINE elements has been shown to cause variation by insert-ing into promoter region or coding sequences of avirulence

Genetic Stability of Magnaporthe oryzae during Successive Passages through Rice Plants and on Artificial Medium