Rust fungi (Basidiomycota, Pucciniales), a major group of plant pathogens, possess very large genomes, much larger than neighbouring taxa. Intriguingly, our analysis suggests that these fungi undergo cycles of diploidisation/haploidisation in life cycle stages that should be strictly haploid, therefore challenging textbook principles stating that karyogamy is immediately followed by meiosis. Moreover, the identification of a rust fungus (Coleosporium tussilaginis f. sp. senecionis-silvatici) containing equal quantities of slightly different (in size) nuclei ('twin peaks' phenomenon) suggests that these could represent each of the nuclei in the dikaryotic cell. (…) With this project we envisage to confirm our initial hypothesis that rust fungi possess a yet undescribed nuclear cycle. This will challenge long-established principles in mycology, particularly if considering that this research deals with diverse fungi at the Order level, and not individual exceptions. We foresee that this project will reveal the phylogenetic limits of this phenomenon and explain its genetic/parasexual principles. We further expect that it may set light on pathogenicity determinants arising from polyploidy and/or supranumerary chromosomes. Under these expected outcomes, we anticipate that this project may be of great impact on fungal biology and on plant pathology.