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High paraphyly of Swertia L. (Gentianaceae) in the Gentianella -lineage as revealed by nuclear and chloroplast DNA sequence variation
Auteur(s)
Date de parution
2001
In
Plant Systematics and Evolution, Springer, 2001/229/1-2/1-21
Résumé
The genus <i>Swertia</i> L., as currently defined, is polymorphic and mainly distributed in temperate regions of the northern hemisphere. Phylogenetic relationships between <i>Swertia</i> and the other genera of the Swertiinae sensu Struwe et al. (unpubl. data) are discussed based on cladistic analyses of DNA sequence data. The sequences used for this purpose include the <i>trn</i>L (UAA) intron, the intergenic spacers (IGS) between <i>trn</i>L (UAA) and <i>trn</i>F (GAA) exons, and between <i>trn</i>S (UGA) and <i>ycf </i>9 exons of cpDNA, as well as the ITS region of nrDNA. Although moderately resolved, the phylogenies resulting from the separate analyses of nuclear and chloroplast data are congruent, and the incongruence length difference test (Farris et al. 1995) detected no character incongruence. The phylogeny suggested by the analysis of combined data sets defines <i>Swertia</i> as strongly paraphyletic in relation to the other genera. This taxon may have acted as a stem group, giving rise to diverse lineages, some of which are morphologically distinct and have been recognised at the generic level. <i>Latouchea</i> and <i>Obolaria</i> are closely related and occupy the basalmost position in the molecular tree. <i>Swertia</i> species are distributed in 9 different clades, three of which share a basal polytomy with <i>Bartonia</i>, <i>Frasera</i>, <i>Gentianopsis</i>, <i>Halenia</i>, <i>Megacodon</i>, <i>Pterygocalyx</i> and <i>Veratrilla</i>. Two lineages have an intermediate position. The remaining 4 clades occupy a more derived position. Two of the latter clades show a close relation with species of <i>Gentianella</i> s. str., and one is included in a large clade comprising <i>Comastoma</i>, <i>Jaeschkea</i> and <i>Lomatogonium</i>. Selected character states and their proposed polarity, such as number and structure of nectaries, stylar and seedcoat characteristics, pollen morphology, fusion of floral parts and chromosome number are discussed in the context of molecular data. Rugose, spinose, or winged seeds are found mainly in basal lineages, while smooth ones are typical for derived species. Chromosome numbers follow a similar pattern with x=13 restricted to basal lineages, while in more derived clades, x is always smaller than 13. With respect to the molecular phylogeny, taxonomic circumscriptions in the Swertiinae sensu Struwe et al. (unpubl. data) does not seem to reflect phyletic relationships.
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journal article
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