Outstanding: the dispensable chromosomes of Mycosphaerella graminicola

Analysis of two genetic linkage maps of the wheat pathogen Mycosphaerella graminicola identified dispensable chromosomes that were present in both parents but absent in 15-20 % of the progeny. These Copy number Polymorphisms (CNPs) were confirmed with a Comparative Genomic Hybridization whole-genome array based on the finished genome of M. graminicola (http://genome.jgi-psf.org). Chromosomes 14- 21 were frequently absent among isolates, without visible effect on viability or virulence, whereas chromosomes 1-13 were invariably present. Genetic analyses showed that CNPs arises during meiosis, usually from nondisjunction at anaphase II. Overall, M. graminicola has the highest number of dispensable chromosomes reported. Varying from 0.41 to 0.77 Mbp, they comprise 38% of the chromosome number and 11.6% of the genome. The dispensable chromosomes are smaller and have significantly lower gene densities. Most of their genes are duplicated on the essential chromosomes and show a different codon usage. Dispensable chromosomes also contained a higher density of transposons, pseudogenes, and unclassified genes, which could encode novel proteins. Moreover, the dispensable chromosomes show extremely low synteny with other Dothideomycete genomes. We hypothesize that the dispensable chromosomes of M. graminicola are adaptive in some yet unknown way http://www.fgsc.net/25thfgc/fgc25.htm