Electrophoretic karyotypes of Magnaporthe grisea pathogens of diverse grasses

We have analyzed the chromosomes of a wide variety of strains of Magnaporthe grisea using pulsed-field gel electrophoresis (PFGE), in combination with Southern hybridization and genetic crosses. Strains analyzed included rice pathogens, field pathogens of grasses other than rice, and fertile laboratory strains. All M. grisea strains examined contain chromosomes in the size range from 2,000 kilobases (kb) to greater than 10,000 kb. Some strains also contain chromosomes, termed "mini-chromosomes," that range in size from 500 to 2,000 kb. Variation in chromosome numbers seems to be mainly among the minichromosomes. Infertile field isolates, including most rice pathogens, have a relatively high number of chromosome length polymorphisms. In contrast, some M. grisea strains from diverse hosts around the world are interfertile and have a relatively uniform karyotype, although translocations are present even among these fertile strains. There appears to be a correlation between the presence of minichromosomes and low levels of sexual fertility, although it is not yet clear if mini-chromosomes are a result or a cause of the low fertility. A mini-chromosome from a Chinese rice pathogen failed to segregate in crosses and it contained at least one DNA sequence that was found in mini-chromosomes of diverse strains, but not found in the larger standard chromosomes from these strains. These mini-chromosomes appear to be nonessential for growth and pathogenicity. Thus, we conclude that many small M. grisea chromosomes are like B-chromosomes found in plants and animals. However, size alone does not predict whether a small chromosome has B-chromosome-like properties. This is demonstrated by the normal Mendelian segregation of a small chromosome present in one parental strain used in an M. grisea RFLP mapping project that was apparently formed through an unequal translocation between two larger chromosomes.