The 1.6 Mb chromosome carrying the avirulence gene AvrPik in Magnaporthe oryzae isolate 84R-62B is a chimera containing chromosome 1 sequences

A genetic map was constructed previously from a cross between Magnaporthe oryzae isolates 84R-62B and Y93-245c-2, and genetic markers closely linked to the cultivar-specific avirulence (Avr) gene, AvrPik, were assigned to a 1.6 Mb small chromosome of 84R-62B that is absent from Y93-245c-2. In the present study, the 1.6 Mb chromosome was characterized by using contour-clamped homogeneous electric fields (CHEF) electrophoresis and hybridization analysis. CHEF electrophoresis analysis showed that the 1.6 Mb chromosome was inherited in Mendelian fashion, and co-segregated with AvrPik. Southern hybridization analysis revealed that the 1.6 Mb chromosome carried sequences only distributed to the supernumerary chromosome in M. oryzae isolates, as well as sequences corresponding to those in the supercontig 17 of chromosome 1 in the M. grisea database. Thus, we conclude that the Mendelian 1.6 Mb chromosome is a chimera containing sequences from chromosome 1 and from supernumerary chromosomes in M. oryzae.