Molecular Systematics, GISH and the Origin of Hybrid Taxa in Nicotiana (Solanaceae)

Abstract

Phylogenetic relationships in the genus Nicotiana were investigated using parsimony analyses of the internal transcribed spacer (ITS) regions of nuclear ribosomal DNA (nrDNA). In addition, origins of some amphidiploid taxa in Nicotiana were investigated using the techniques of genomic in situ hybridization (GISH), and the results of both sets of analyses were used to evaluate previous hypotheses about the origins of these taxa. Phylogenetic analyses of the ITS nrDNA data were performed on the entire genus (66 of 77 naturally occurring species, plus three artificial hybrids), comprising both diploid and polyploid taxa, and on the diploid taxa only (35 species) to examine the effects of amphidiploids on estimates of relationships. All taxa, regardless of ploidy, produced clean, single copies of the ITS region, even though some taxa are hybrids. Results are compared with a published plastid (matK) phylogeny using fewer, but many of the same, taxa. The patterns of relationships in Nicotiana, as seen in both analyses, are largely congruent with each other and previous evolutionary ideas based on morphology and cytology, but some important differences are apparent. None of the currently recognized subgenera of Nicotiana is monophyletic and, although most of the currently recognized sections are coherent, others are clearly polyphyletic. Relying solely upon ITS nrDNA analysis to reveal phylogenetic patterns in a complex genus such as Nicotiana is insufficient, and it is clear that conventional analysis of single data sets, such as ITS, is likely to be misleading in at least some respects about evolutionary history. ITS sequences of natural and well-documented amphidiploids are similar or identical to one of their two parents-usually, but not always, the maternal parent-and are not in any sense themselves 'hybrid'. Knowing how ITS evolves in artificial amphidiploids gives insight into what ITS analysis might reveal about naturally occurring amphidiploids of unknown origin, and it is in this perspective that analysis of ITS sequences is highly informative.

A taxonomic review of the genus Phaeocystis

“…These species complexes would appear still to be able to interbreed with one another because multiple diVerent haplotypes from diVerent clades can be found within a single strain. Similar Wndings of multiple haplotypes being present in known hybrids of Xowering plant species are quite common (Chase et al 2003) and it is known that in some hybrids of Xowering plants, the hybrid can loose one of the parental haplotypes in as few as 12 generations (Bateman, pers. com.)…”

Ribosomal DNA evolution and gene conversion in Nicotiana rustica

“…Phylogenetic studies of the chloroplast sequence suggest that the ancestor of N. paniculata and/or N. knightiana (section Paniculatae) may be the maternal genome donor of N. rustica (Aoki and Ito, 2000;Chase et al, 2003). The closest relative of N. undulata (section Undulatae) is N. glutinosa although Goodspeed (1954) placed N. glutinosa in section Tomentosae.…”

Reconstructing patterns of reticulate evolution in angiosperms: what can we do?

“…However, it would be a waste of potentially important information if the hybrid sequences were not used in such analyses. In addition, inclusion of the hybrid might not have a disruptive effect on the trees after all (see Chase & al., 2003;Guo & al., 2004;Koontz & al., 2004). As outlined above, one solution to this problem could be to conduct analyses that both include and exclude putative hybrids and present both results (as in Sun & al., 2002;Aguilar & Feliner, 2003;Chase & al., 2003;Guo & al., 2004;Koontz & al., 2004;Wieringa & Guhl, in press).…”

“…(2004); Hippophae (Asteraceae) by Sun & al. (2002); and Nicotiana (Solanaceae) by Chase & al. (2003).…”

“…Morphology or patterns of geographical distribution can provide valuable additional evidence for a hybrid origin (Hughes & Harris, 1998;Bateman & Hollingsworth, 2004;Peterson & al., 2004;Shan & al., 2005). Additionally, karyological evidence, such as chromosome counts, C-values, and GISH or FISH patterns can discriminate between parental genome donors and the hybrid relationships (Hodkinson & al., 2002;Borgen & al., 2003;Chase & al., 2003;Bures & al., 2004;Harper & al. 2004;Pires & al., 2004;Tel-Zur & al., 2004).…”