Allopolyploidy and extensive rDNA site variation underlie rapid karyotype evolution in Nothoscordum section Nothoscordum (Amaryllidaceae)

Nothoscordum bonariense (2n = 26; 22M + 4A) is a hexaploid species morphologically related to other species of section Nothoscordum. Its meiotic behaviour and that of its hybrids suggests an allopolyploid origin. Here, we investigate the origin of N. bonariense and its relationships with most closely related species based on cytological and molecular data. Cytogenetic analyses included chromosome number and morphology, distribution of ribosomal DNA (rDNA) sites, genomic in situ hybridization (GISH) and nuclear genome size. Molecular phylogenetic analyses were based on nuclear ITS and the plastid trnG intron and the atpB-rbcL and trnL-trnF spacer sequences. Nothoscordum bonariense and South American samples of N. bivalve formed a clade sister to N. gaudichaudianum, whereas N. montevidense was recovered in a different clade. Molecular phylogenetic relationships, congruent genome size variation, 35S and 5S rDNA sites distribution and GISH suggest that N. bonariense is a hybrid. Different types of reduced or unreduced gametes (2x or 3x) or somatic doubling in a hybrid may have led to the formation of N. bonariense karyotype. Genomic DNA of N. gaudichaudianum hybridized the x = 4 subgenome of N. bonariense more strongly than that of N. montevidense, suggesting that the former species is one of N. bonariense progenitors, instead of the latter. Genomic DNA of N. gaudichaudianum also revealed a x = 4 subgenome in one N. bivalve (2n = 18, 14M + 4A) accession, revealing that this species may have an allopolyploid origin. However, the non-monophyly of its accessions and the variability of chromosome number and rDNA composition reported here suggest that this species needs an extensive taxonomic, biogeographic and phylogenetic revision. Allopolyploidization involving progenitors with two different basic chromosome numbers (x = 4 and x = 5) and the remarkable 5S and 35S rDNA variation described here provide further evidence of rapid karyotype evolution in section Nothoscordum.