Abstract
Meiosis in the haploid plant-pathogenic fungus Mycosphaerella graminicola results in eight ascospores due to a mitotic division following the two meiotic divisions. The transient diploid phase allows for recombination among homologous chromosomes. However, some chromosomes of M. graminicola lack homologs and do not pair during meiosis. Because these chromosomes are not present universally in the genome of the organism they can be considered to be dispensable. Detailed genetic analyses of two high density mapping populations revealed that M. graminicola has 21 chromosomes including up to eight dispensable chromosomes, the highest number reported in filamentous fungi. These chromosomes vary from 0.41 to 0.77 Mb in size, representing 38% of the chromosome number and 11.6% of the genome. Chromosome numbers among progeny isolates varied widely, with some progeny missing up to three chromosomes, while other strains were disomic for one or more chromosomes. Between 15- 20% of the progeny isolates lacked one or more chromosomes that were present in both parents. The two high-density maps showed no recombination of dispensable chromosomes and hence, their meiotic processing may require distributive disjunction, a phenomenon that is rarely observed in fungi. The maps also enabled the identification of individual twin isolates from a single ascus that shared the same missing or doubled chromosomes indicating that the chromosomal polymorphisms were mitotically stable and originated from nondisjunction during the second division and, less frequently, during the first division of fungal meiosis. High genome plasticity could be among the strategies enabling this versatile pathogen to quickly overcome adverse biotic and abiotic conditions in wheat fields. Additionally, we used a Comparative Genomic Hybridization whole-genome array based on the finished genome of M. graminicola (http://genome.jgi-psf.org). This confirmed that chromosomes 14-21 were frequently absent among isolates, without visible effect on viability or virulence, whereas chromosomes 1-13 were invariably present. 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
| Original language | English |
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| Title of host publication | Proceedings of the Fourth Annual DOE Joint Genome Institute User Meeting on Genomics of Energy & Environment, Walnut Creek, California, USA, 25-27 March 2009 |
| Publisher | U.S. Department of Energy, Office of Science |
| Pages | 31 |
| Publication status | Published - 2009 |
| Event | Fourth Annual DOE Joint Genome Institute User Meeting on Genomics of Energy & Environment, Walnut Creek, California, USA - Duration: 25 Mar 2009 → 27 Mar 2009 |
Conference
| Conference | Fourth Annual DOE Joint Genome Institute User Meeting on Genomics of Energy & Environment, Walnut Creek, California, USA |
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| Period | 25/03/09 → 27/03/09 |