title: Nuclear glutamine synthetase evolution in Nicotiana: phylogenetics and the origins of allotetraploid and homoploid (diploid) hybrids.
abstract: Interspecies relationships in Nicotiana (Solanaceae) are complex because 40 species are diploid (two sets of chromosomes) and 35 species are allotetraploid (four sets of chromosomes, two from each progenitor diploid species). We sequenced a fragment (containing four introns) of the nuclear gene 'chloroplast-expressed glutamine synthetase' (ncpGS) in 65 species of Nicotiana. Here we present the first phylogenetic analysis based on a low-copy nuclear gene for this well studied and important genus. Diploid species have a single-copy of ncpGS, and allotetraploids as expected have two homeologous copies, each derived from their progenitor diploid. Results were particularly useful for determining the paternal lineage of previously enigmatic taxa (for which our previous analyses had revealed only the maternal progenitors). In particular, we were able to shed light on the origins of the two oldest and largest allotetraploid sections, N. sects. Suaveolentes and Repandae. All homeologues have an intact reading frame and apparently similar rates of divergence, suggesting both remain functional. Difficulties in fitting certain diploid species into the sectional classification of Nicotiana on morphological grounds, coupled with discordance between the ncpGS data and previous trees (i.e. plastid, nuclear ribosomal DNA), indicate a number of homoploid (diploid) hybrids in the genus. We have evidence for Nicotiana glutinosa and Nicotiana linearis being of hybrid origin and patterns of intra-allelic recombination also indicate the possibility of reticulate origins for other diploid species.
authors.affiliation: Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS, UK. Electronic address: j.clarkson@kew.org.
authors.forename: James J
authors.initials: JJ
authors.lastname: Clarkson
authors.affiliation: Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS, UK.
authors.forename: Laura J
authors.initials: LJ
authors.lastname: Kelly
authors.affiliation: School of Biological Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, UK.
authors.forename: Andrew R
authors.initials: AR
authors.lastname: Leitch
authors.affiliation: Department of Botany, The Natural History Museum, Cromwell Road, London SW7 5BD, UK.
authors.forename: Sandra
authors.initials: S
authors.lastname: Knapp
authors.affiliation: Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS, UK.
authors.forename: Mark W
authors.initials: MW
authors.lastname: Chase
mesh_terms: D001499:Bayes Theorem; D018744:DNA, Plant / Q000235:genetics; D019143:Evolution, Molecular; D005974:Glutamate-Ammonia Ligase / Q000235:genetics*; D010802:Phylogeny; D011123:Polyploidy; D011995:Recombination, Genetic; D017422:Sequence Analysis, DNA; D014026:Nicotiana / Q000145:classification / Q000201:enzymology / Q000235:genetics*
chemical_list: D018744:DNA, Plant; D005974:Glutamate-Ammonia Ligase