Molecular cytogenetic analyses and phylogenetic studies in the Nicotiana section Tomentosae

Abstract

Phylogenetic schemes based on changing DNA sequence have made a major impact on our understanding of evolutionary relationships and significantly built on knowledge gained by morphological and anatomical studies. Here we present another approach to phylogeny, using fluorescent in situ hybridisation. The phylogenetic scheme presented is likely to be robust since it is derived from the chromosomal distribution of ten repetitive sequences with different functions and evolutionary constraints [GRS, HRS60, NTRS, the Arabidopsis-type telomere repeat (TTTAGGG)n, 18S-5.8S-26S ribosomal DNA (rDNA), 5S rDNA, and four classes of geminiviral-related DNA (GRD)]. The basic karyotypes of all the plant species investigated Nicotiana tomentosiformis, N. kawakamii, N. tomentosa, N. otophora, N. setchellii, N. glutinosa (all section Tomentosae), and N. tabacum (tobacco, section Genuinae) are similar (x=12) but the distribution of genic and non-genic repeats is quite variable, making the karyotypes distinct. We found sequence dispersal, and locus gain, amplification 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 thereafter spread by vertical transmission and speciation into four species. Since GRD is similar to a transgenic construct that was inserted into the N. tabacum genome, its fate over evolutionary time is interesting in the context 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 member of section Tomentosae (T-genome donor). Of section Tomentosae, N. tomentosiformis has the most similar genome to the T genome of tobacco and is therefore the most likely paternal genome donor. It is known for N. tabacum that gene conversion has converted most 18S-5.8S-26S rDNA units of N. sylvestris 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 conversion 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.

Ribosomal DNA evolution and gene conversion in Nicotiana rustica

“…N. rustica has the sum of the loci observed in these putative parents ( Figure 1c). GISH can be used to distinguish the genomic origin of the chromosomes in allopolyploids (Heslop-Harrison, 1992;Gill and Friebe, 1998) and has been used to confirm the ancestry of allopolyploids in a number of species including tobacco (Parokonny and Kenton, 1995;Lim et al, 2000a). Figure 1d shows that the method distinguishes between chromosomes inherited from progenitors of N. undulata and N. paniculata, the U-and P-genomes of N. rustica respectively.…”

“…One of the best characterised genetic events associated with allopolyploidy is gene conversion at the 18-5.8-26S nuclear ribosomal DNA (rDNA) loci in Nicotiana tabacum L. (Kovarik et al, 1996;Volkov et al, 1999;Lim et al, 2000a). N. tabacum (tobacco) is an allotetraploid (2n ¼ 4x ¼ 48) that formed within the last 6 million years (Olmstead and Palmer, 1991) between N. sylvestris Speg.…”

“…& Comes (2n ¼ 2x ¼ 24), the maternal S genome donor, and N. tomentosiformis L. (2n ¼ 2x ¼ 24), the paternal T genome donor (Goodspeed, 1954;Chase et al, 2002;Murad et al, 2002). N. sylvestris has three rDNA loci, whereas N. tomentosiformis has one locus; the sum of these is found in tobacco (Parokonny and Kenton, 1995;Lim et al, 2000a). However, an analysis of the rDNA units shows that over 90% of the units are of an N. tomentosiformis type whereas the remainder are of an N. sylvestris type (Lim et al, 2000b).…”

Characterization of the Arachis (Leguminosae) D genome using fluorescence in situ hybridization (FISH) chromosome markers and total genome DNA hybridization

“…Detection of rRNA genes by fluorescent in situ hybridization (FISH) has proved to be a useful tool in studies of plant genome organization, chromosome homeologies, cytotaxonomy and evolution (Jiang and Gill, 1994;Moscone et al, 1999;Lim et al, 2000;Nakamura et al, 2001). Raina and Mukai (1999) applied this approach to a few Arachis species and demonstrated that the number of ribosomal rDNA loci is variable.…”

Evolution and structure of 5S rDNA loci in allotetraploid Nicotiana tabacum and its putative parental species

“…Nicotiana tabacum (tobacco) is a 5-6 million year old natural amphidiploid derived from ancestors of N. sylvestris, the maternal S genome donor, and N. tomentosiformis the paternal T genome donor (Goodspeed, 1954;Okamuro and Goldberg, 1985;Parokonny and Kenton, 1995;Lim et al, 2000b). Comparisons of the tobacco genome with the genomes of modern diploid relatives have revealed several contrasting evolutionary patterns.…”