Rice Genetics IV - IRRI books - International Rice Research Institute

facilitate breeding durable resistance in rice (Levy et al 1993, Zeigler et al 1994). Inany case, this simplified view of the rice pathogen population as sets of discrete lineagesprovides a genealogical framework for evaluating pathogen evolution and potentialfor variation.The first examples of population analysis of AVR-Pita gene structure validatelineage structure as defined by neutral DNA markers such as MGR586. DNA gel blotanalysis using the AVR-Pita gene as a probe determined that restriction fragmentpatterns were conserved within single genetic lineages of the fungus in Colombia(Montenegro-Chamorro 1997) and in the Philippines (Zeigler et al 1995). AVR-Pitarestriction fragment patterns generally varied between lineages. Particular AVR-Pitarestriction fragments correlated with avirulence and others correlated with virulence.A few lineages lacked homology to AVR-Pita.Montenegro-Chamorro (1997) sequenced AVR-Pita alleles from Colombian lineagesand demonstrated striking conservation of the AVR-Pita coding sequence relativeto the gene from the Chinese rice pathogen O-137. Uniformly avirulent lineagesSRL-1 and SRL-2 contain genes encoding AVR-Pita proteins that differ by only twoor three amino acids from the O-137 protein. Virulent lineages SRL-3 and ALT-7contain avr-pita – genes encoding proteins that uniformly have seven amino acid differencesrelative to AVR-Pita proteins that confer avirulence. Again, these avr-pita –proteins differ from each other by only one or two amino acids. The avr-pita – genesalso contain a putative protease motif that differs by a single allowed amino acidsubstitution within the motif region (Montenegro-Chamorro 1997, G.T. Bryan, S.Kang, and B. Valent, unpublished results). Thus, it is possible that evolution of AVR-Pita alleles may involve modifications of a protein resulting in preservation of metabolicfunction while altering its recognition as an avirulence factor.An opportunity to test the lineage exclusion hypothesisMGR-DNA fingerprinting of blast fungus in southern Brazil, Argentina, and Uruguayhas identified sister lineages, BZ-A, ARG-1, and URU-1, to Colombian lineagesSRL-1 and SRL-2, which contain an avirulent allele of AVR-Pita (M. Levy, A.Livore, S. Avila, and B. Valent, unpublished results). The Southern Cone lineages arestatistically indistinguishable and more than 85% similar to SRL-1 and SRL-2 byMGR-DNA fingerprinting, and they retain the AVR-Pita gene structure (commonRFLPs) and avirulence to Pi-ta characteristic of SRL-1 and SRL-2. Moreover, theseSouthern Cone lineages are the only components of the diverse blast population ineach country that agressively infect tropical indica varieties, such as El Paso 144,constituting the major rice germplasm grown in this region. The lineage exclusionhypothesis predicts that introgression of Pi-ta into El Paso 144 and related varietieswill protect these varieties from the only genetic families currently known to infectthem. Transgenic strategies to test this hypothesis are under way.Enhancing deployment of genes for blast resistance: . . . 317