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Title: DISTRIBUTION OF MEIOTIC CROSSOVER POINTS SURROUNDING THE AVR1-CO39 AVIRULENCE GENE LOCUS OF MAGNAPORTHE GRISEA

Author
item FARMAN, MARK - UNIVERSITY OF WISCONSIN
item Leong, Sally

Submitted to: Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/9/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: Rice blast disease is the most important fungal disease of rice worldwide. Rice is the staple food for two thirds of the human population. The most effective method of disease control is to grow disease resistant plants. Unfortunately, the causal fungus is able to overcome this resistance within 2-3 years after these plants are cultivated widely. We are trying to understand the details fo how the rice blast fungus is recognized by rice plants that are resistant to blast and how the fungus changes in order to overcome this recognition. In order to improve our understanding of this host-parasite interaction, we have characterized and cloned a pathogen gene AVR1-C039 that is involved in recognition of the pathogen by the resistant plant. This is the second AVR gene cloned from the rice blast fungus. We employed a combination of physical and genetic methods to obtain the cloned gene. These methods may hve broad appllication to other organisms in which map-based cloning approaches are being used to clone a gene.

Technical Abstract: The avrCO39 gene conferring avirulence toward rice cultivar CO39 was previously mapped to chromosome 1 of Magnaporthe grisea between cosegregating markers CH5-120H and 1.2H, and marker5-10-F (SMITH and LEONG, 1994). In the present study, this region of the chromosome was physically mapped using RecA-mediated Achilles# cleavage (RecA- AC). Cleavage of genomic DNA sequences within CH5-120H and 5-10-F liberated a 610 kb restriction fragment, representing the physical distance between these markers. Chromosome walking was initiated from both markers but was curtailed by the presence of repetitive DNA and absence of overlapping clones in several cosmid libraries. These obstacles were overcome by directly subcloning the target region after release by Achilles cleavage. Four cosmid contigs were thus assembled and the avrCO39 gene was cloned. Twelve meiotic crossover points were monitored throughout the walking process and an additional eight exchanges were analyzed subsequently. Recombination points were unevenly distributed leading to clustering of chromosome exchanges immediately around the avrCO39 locus. The relationship between genetic and physical distance varied by up to fourteen-fold over the chromosomal region examined. Thus the poor correlation of genetic to physical distance previously observed in M. grisea (FARMAN AND LEONG, 1995) appears to be manifested even over relatively short distances.