Physical mapping of ribosomal DNA sites and genome size in polyploid series of Urochloa humidicola (Rendle) Morrone & Zuloaga (Poaceae)

Urochloa humidicola (Rendle) Morrone & Zuloaga is a species native to Africa that grows in tropical regions, especially in sites with temporary flooding. Despite its importance as forage crop, the karyotypic characterization of its cytotypes remains mainly unfinished, and its polyploid origin is unknown. Therefore, cytogenetic studies can provide information about the evolutionary history of species and to characterize the core and reserve collection of the germplasm banks. We performed the karyotype analyses, mapped ribosomal sites, and estimated genome size, confirming three ploidy levels (6x, 7x, and 9x) and one aneuploid accession H031 (2n = 6x = 36 + 1). Karyotypes are symmetrical and the number of 35S and 5S rDNA sites varied between the cytotypes. However, all sites were at terminal and interstitial positions in the chromosomes, respectively, except one chromosome of the aneuploid accession with interstitial colocalization. The polymorphism of the number and distribution of rDNA sites in U. humidicola was confirmed by (i) the variation between and within plants and in metaphases of the same meristem in hexaploids and heptaploids; (ii) the occurrence of rDNA sites in hemizygosis in all accessions, except heptaploids; (iii) the size heteromorphism in some ribosomal sites of homologous chromosomes. The karyotype data and the mapping of ribosomal sites corroborate the proposals of allopolyploidy in U. humidicola and the hybrid origin to the H031 accession.