Homologous recombination causes the spontaneous deletion of AVR-Pia in Magnaporthe oryzae

AVR-Pia, an avirulence gene in the genome of the rice blast fungus Magnaporthe oryzae, triggers a hypersensitive reaction in rice cultivars harbouring the resistance gene Pia. The copy number of AVR-Pia was revealed to vary from one to three among M. oryzae isolates avirulent to Pia rice, and three copies of the gene were located on a single chromosome in strain Ina168, from which the gene was originally cloned. The spontaneous avr-Pia mutant originated from Ina168, named Ina168m95-1, which lacks the AVR-Pia gene, and was therefore used to elucidate the molecular mechanism of the deletion of all three copies of AVR-Pia. Screening and analysis of cosmid clones indicated that two copies of the DNA-type transposon Occan (Occan9E12 and Occan3A3) were located on the same chromosome, and three copies of AVR-Pia were located in between the two Occan elements. Ina168m95-1 contains a conserved Occan element, named Occanm95-1, between sequences homologous to the 5'-flanking region of Occan3A3 and the 3'-flanking region of Occan9E12. In addition, sequence polymorphisms indicated a homologous recombination between Occan3A3 and Occan9E12, which resulted in Occanm95-1. Based on these observations, we propose the hypothesis that homologous recombination in the two Occan elements leads to the deletion of AVR-Pia in Ina168m95-1.