Microsatellite markers for population studies of the rice blast fungus, Magnaporthe grisea

International audienceWe developed nine new microsatellite markers for rice blast (Magnaporthe grisea) population studies. These markers were used in addition to nine microsatellite markers previously developed by our group for mapping purpose. Altogether, the 18 markers were used in multiplex PCR (polymerase chain reaction) to characterize six populations from different geographical origins. The average number of alleles per locus across populations ranged from 1.2 to 7 and the total number of alleles detected from 2 to 19. Based on this large range of polymorphism, this set of markers is expected to be useful for different kind of population studies at different geographical scales

World population structure of the rice blast fungus, Magnaporthe oryzae

National audienceMagnaporthe oryzae is a heterothallic fungus reproducing clonally in the field. The diversity and structure of populations of this pathogen of rice was described in many countries during the last 20 years. The expected clonal structure of the populations has been illustrated in various studies. But, relationships between the different populations was hardly testable mainly due to the choice of molecular markers. We recently developed a set of 18 microsatellite markers for population studies (Adreit et al. 2007). We used these markers to genotype more than 1,700 isolates from 40 countries. This sample included reference isolates from previous studies and real populations (1 site, 1 year). The world population structure show evidence for some geographic structuration. However, local adaptation and intercontinental migration are (or were) also common. A structure in three major genetic groups is observed. Two groups correspond to isolates of mating type Mat1.1 and Mat1.2 respectively. The third group gathers isolates of both mating types. Diversity is higher in Asia and more precisely in the countries of the Himalayan foothills. This area is the only one where female fertile isolates (able to produce perithecia and ascospores in vitro) were identified. Both mating types are also present in these populations. Whether sexual reproduction is (was) taking place in this region will be tested. The area of the Himalayan foothills is a center of diversity and is a good candidate for the center of origin of the rice blast fungus

World population structure and migration of the rice blast fungus, Magnaporthe oryzae

International audienceThe clonal structure of Magnaporthe oryzae populations was observed in various countries. This information was used to propose new resistance deployment strategies. However, our understanding of how new virulent races appear and spread remains limited. Population genetic analyses, with neutral and selected markers, provide tools to evaluate such events. We used microsatellite markers to study populations at the worldwide scale. We observed three major genetic groups. Two groups include isolates of only one mating each. The third group includes isolates of both mating types. During natural epidemics spores disperse over short distances (1-5 m). This is likely to cause the partial geographic structuring we observed. However, long distance migrations are possible through the transportation of infected seeds. We observed that, in some cases, one population is more closely related to populations from other continents than from a population from the same area. Long distance migration was also confirmed by studying the distribution of the genotypes of the cloned avirulence gene ACE1. Two major virulent genotypes were identified. These genotypes appeared by a complex duplication/deletion event. These two genotypes are widely distributed over different continents. Altogether, these results suggest a unique apparition event followed by long distance migrations of virulent genotypes

Structure of African Populations of Pyricularia oryzae from Rice

International audienceRice blast, caused by the filamentous ascomycete Pyricularia oryzae, is one of the most devastating diseases of rice. Four genetic clusters were previously identified, and three have a large geographic distribution. Asia is the center of diversity and the origin of most migrations to other continents, and sexual reproduction persisted only in the South China-Laos-North Thailand region, which was identified as the putative center of origin of all P. oryzae populations on rice. Despite the importance of rice blast disease, little is known about the diversity and the population structure of the pathogen in Africa (including Madagascar). The present study was intended to describe the structure of African populations of P. oryzae and identify the relationship between African and worldwide genetic clusters. A set of 2,057 strains (937 African and 1,120 Madagascan strains) were genotyped with 12 simple sequence repeat markers to assess the diversity and the population structure of P. oryzae. Four genetic clusters were identified in Africa and Madagascar. All four clusters previously identified are present in Africa. Populations from West Africa, East Africa, and Madagascar are highly differentiated. The geographic structure is consistent with limited dispersion and with some migration events between neighboring countries. The two mating types are present in Africa with a dominance of Mat1.2, but no female-fertile strain was detected, supporting the absence of sexual reproduction on this continent. This study showed an unsuspected high level of genetic diversity of P. oryzae in Africa and suggested several independent introductions