Preferential elimination of repeated DNA sequences from the paternal, Nicotiana tomentosiformis genome donor of a synthetic, allotetraploid tobacco
Nicotiana tabacum (tobacco, 2n = 4x = 48) is a natural allotetraploid combining two ancestral genomes closely related to modern Nicotiana sylvestris and Nicotiana tomentosiformis. Here we examine the immediate consequences of allopolyploidy on genome evolution using 20 S-4-generation plants derived from a single synthetic, S-0 plant made by Burk in 1973 (Th37). Using molecular and cytogenetic methods we analysed 14 middle and highly repetitive sequences that together total approximate to 4% of the genome. Two repeats related to endogenous geminiviruses (GRD5) and pararetroviruses (NtoEPRV), and two classes of satellite repeats (NTRS, A1/A2) were partially or completely eliminated at variable frequency (25-60%). These sequences are all from the N. tomentosiformis parent. Genomic in situ hybridization revealed additivity in chromosome numbers in two plants (2n = 48), while a third was aneuploid for an N. tomentosiformis-origin chromosome (2n = 49). Two plants had homozygous translocations between chromosomes of the S- and T-genomes. The data demonstrate that genetic changes in synthetic tobacco were fast, targeted to the paternal N. tomentosiformis-donated genome, and some of the changes showed concordance with changes that presumably occurred during evolution of natural tobacco.