Molecular cytogenetic analyses and phylogenetic studies in the Nicotiana section Tomentosae

Phylogenetic schemes based on changing DNA sequence have made a major impac t on our understanding of evolutionary relationships and significantly buil t on knowledge gained by morphological and anatomical studies. Here we pres ent another approach to phylogeny, using fluorescent in situ hybridisation. The phylogenetic scheme presented is likely to be robust since it is deriv ed from the chromosomal distribution of ten repetitive sequences with diffe rent functions evolutionary constraints [GRS, HRS60, NTRS, the Arabidopsis- type telomere repeat (TTTAGGG)(n), 18S-5.8S-26S ribosomal DNA (rDNA), 5S rD NA, and four classes of geminiviral-related DNA (GRD)]. The basic karyotype s of all the plant species investigated Nicotiana tomentosiformis, N, kawak amii, N, tomentosa, N. otophora N, setchellii, N. glutinosa tall section To mentosae), and N, tabacum (tobacco, section Genuinae) are similar (x = 12) but the distribution of genic and non-genic repeats is quite variable, maki ng the karyotypes distinct. We round sequence dispersal, and locus gain, am plification and loss, all within the regular framework of the basic genomic structure. We predict that the GRD classes of sequence integrated into an ancestral genome only once in the evolution of section Tomentosae and there after spl ead by vertical transmission and speciation into four species. Si nce GRD is similar to a transgenic construct that was inserted into the N. tabacum genome, its rate over evolutionary time is interesting in the conte xt of the debate on genetically modified organisms and the escape of genes into the wild. Nicotiana tabacum is thought to be an allotetraploid between presumed progenitors of N. sylvestris (maternal, S-genome donor) and a mem ber of section Tomentosae (T-genome donor). Of section Tomentosae, N. tomen tosiformis has the most similar genome to the T genome of tobacco and is th erefore the most likely paternal genome donor. It is known for N. tabacum t hat gene conversion has converted most 18S-5.8S-26S rDNA units of N. sylves tris is origin into units of an N. tomentosiformis type. Clearly if such a phenomenon were widespread across the genome, genomic in situ hybridisation (GISH) to distinguish the S and T genomes would probably not work since co nversion would tend to homogenise the genomes. The fact that GISH does work suggests a limited role for gene conversion in the evolution of N, tabacum .