Genome size variation in the pine fusiform rust pathogen Cronartium quercuum f.sp. fusiforme as determined by flow cytometry

The genome size of the pine fusiform rust pathogen Cronartium quercuum f.sp. fusiforme (Cqf) was determined by flow cytometric analysis of propidium iodide-stained, intact haploid pycniospores with haploid spores of two genetically well characterized fungal species, Sclerotinia sclerotiorum and Puccinia graminis f.sp. tritici, as size standards. The Cqf haploid genome was estimated at ~90 Mb, similar to other Pucciniales species for which reference genome sequences are available. Twenty-three Cqf pycniospore samples were compared that comprised three samples obtained from naturally occurring pine galls and 20 samples obtained after artificial inoculation with parental isolates and their progeny. Significant variation in genome size (>10% of mean) was detected among unrelated as well as sibling Cqf samples. The unexpected plasticity in Cqf genome size observed among sibling samples is likely to be driven by meiosis between parental genomes that differ in size.