Shared Pivotal Genome of European and South-American Polyploid Fescues ( Festuca sp.) Uncovered by Comparative Cytogenetics

BACKGROUND AND AIMS: Polyploidization is one of the major evolutionary forces in grasses (Poaceae), yet the genomic relationships between polyploid species across different continents remain poorly understood. Here, we aim to determine whether phylogenetically distinct South American allohexaploid fescues, Festuca ulochaeta Steud. (subg. Subulatae) and F. fimbriata Ness. (subg. Mallopetalon) retain a shared ancestral (pivotal) genome with the broad-leaved Eurasian fescues of subg. Schedonorus. METHODS: We used comparative cytogenetics (genomic in situ hybridization) coupled with flow cytometry. KEY RESULTS: Flow cytometry confirmed that both species are hexaploid (2n = 6x = 42) with genome sizes of 10.84 and 11.59 pg/2C, respectively. In situ hybridization demonstrated that F. ulochaeta and F. fimbriata are closely related to each other, and that the genome of F. ulochaeta is closely related to one of the diploid genomes of the tetraploid F. glaucescens, the tetraploid progenitor of hexaploid tall fescue (F. arundinacea). The F. ulochaeta DNA probe consistently labelled 14 chromosomes in tetraploids F. mairei, F. apennina, and F. glaucescens, in both (Continental and Mediterranean) morphotypes of hexaploid F. arundinacea and in octoploid F. atlantigena, indicating a shared pivotal diploid genome linking South American and Eurasian lineages. This trans-continental genomic affinity supports the role of South America as a secondary centre of Festuca diversification and implies that an ancestral diploid lineage colonized the continent prior to, or concurrent with, allopolyploidization. CONCLUSIONS: We revealed, for the first time, the genomic connection between polyploid South American and European fescues. This has direct implications for understanding the evolution of the haplo-insufficient meiotic pairing regulator present in polyploid fescues and for exploiting South American germplasm in grass breeding.