title: Long-term genome diploidization in allopolyploid Nicotiana section Repandae (Solanaceae).
abstract: Here, we analyze long-term evolution in Nicotiana allopolyploid section Repandae (the closest living diploids are N. sylvestris, the maternal parent, and N. obtusifolia, the paternal parent). We compare data with other more recently formed Nicotiana allopolyploids. We investigated 35S and 5S nuclear ribosomal DNA (rDNA) chromosomal location and unit divergence. A molecular clock was applied to the Nicotiana phylogenetic tree to determine allopolyploid ages. N. tabacum and species of Repandae were c. 0.2 and 4.5 Myr old, respectively. In all Repandae species, the numbers of both 35S and 5S rDNA loci were less than the sum of those of the diploid progenitors. Trees based on 5S rDNA spacer sequences indicated units of only the paternal parent. In recent Nicotiana allopolyploids, the numbers of rDNA loci equal the sum of those of their progenitors. In the Repandae genomes, diploidization is associated with locus loss. Sequence analysis indicates that 35S and 5S units most closely resemble maternal and paternal progenitors, respectively. In Nicotiana, 4.5 Myr of allopolyploid evolution renders genomic in situ hybridization (GISH) unsuitable for the complete resolution of parental genomes.
authors.affiliation: Jodrell Laboratory, Royal Botanic Gasrdens, Kew, Richmond, Surrey, TW9 3DS, UK. j.clarkson@rbgkew.org.uk
authors.forename: James J
authors.initials: JJ
authors.lastname: Clarkson
authors.forename: K Yoong
authors.initials: KY
authors.lastname: Lim
authors.forename: Ales
authors.initials: A
authors.lastname: Kovarik
authors.forename: Mark W
authors.initials: MW
authors.lastname: Chase
authors.forename: Sandra
authors.initials: S
authors.lastname: Knapp
authors.forename: Andrew R
authors.initials: AR
authors.lastname: Leitch
mesh_terms: D005075:Biological Evolution; D018744:DNA, Plant; D004275:DNA, Ribosomal / Q000235:genetics*; D018745:Genome, Plant; D010802:Phylogeny; D011003:Ploidies; D013045:Species Specificity; D014026:Nicotiana / Q000235:genetics*
chemical_list: D018744:DNA, Plant; D004275:DNA, Ribosomal