Cytological karyotyping and characterization of a 410 kb mini-chromosome in nectria haematococca MPI

Karyotypes of the cucurbit pathogen Nectria haematococca MPI (anamorph Fusarium solani f. sp. cucurbitae race 1) was studied using the two standard strains ATCC18098 and ATCC18099. Complete separation of all chromosomes was difficult with pulsed field gel electrophoresis due to both the large size and co-migration of chromosomes. In contrast, cytological karyotyping was done successfully with fluorescence microscopy combined with the germ tube burst method for sample preparation to visualize mitotic metaphase chromosomes. For each strain the basic chromosome number (CN) was nine, which revises previous chromosome estimates of n = 4. Chromosomes were morphologically characterized by their sizes, intensely fluorescing segments, and protrusion of rDNA. In addition to the basic chromosome complement, ATCC18098 had a mini-chromosome of ~410 kb present as a single copy in somatic nuclei. Chromosome fluorescence in situ hybridization indicated that this mini-chromosome is not a derivative from the other chromosomes in the genome. In addition, crossing experiments suggested that it was transmitted in a Mendelian manner to the ascospore progeny.