| Summary: | <i>Festuca</i> and <i>Lolium</i> grass species are used for <i>Festulolium</i> hybrid variety production where they display trait complementarities. However, at the genome level, they show antagonisms and a broad scale of rearrangements. A rare case of an unstable hybrid, a donor plant manifesting pronounced variability of its clonal parts, was discovered in the F2 group of 682 plants of <i>Lolium multiflorum</i> × <i>Festuca arundinacea</i> (2n = 6x = 42). Five phenotypically distinct clonal plants were determined to be diploids, having only 14 chromosomes out of the 42 in the donor. GISH defined the diploids as having the basic genome from <i>F. pratensis</i> (2n = 2x = 14), one of the progenitors of <i>F. arundinacea</i> (2n = 6x = 42), with minor components from <i>L. multiflorum</i> and another subgenome, <i>F. glaucescens</i>. The 45S rDNA position on two chromosomes also corresponded to the variant of <i>F. pratensis</i> in the <i>F. arundinacea</i> parent. In the highly unbalanced donor genome, <i>F. pratensis</i> was the least represented, but the most involved in numerous recombinant chromosomes. Specifically, FISH highlighted 45S rDNA-containing clusters involved in the formation of unusual chromosomal associations in the donor plant, suggesting their active role in karyotype realignment. The results of this study show that <i>F. pratensis</i> chromosomes have a particular fundamental drive for restructuring, which prompts the disassembly/reassembly processes. The finding of <i>F. pratensis</i> “escaping” and rebuilding itself from the chaotic “chromosomal cocktail” of the donor plant points to a rare chromoanagenesis event and extends the view of plant genome plasticity.
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