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  • Publication
    Accès libre
    Phylogeny and Biogeography of Exacum (Gentianaceae): A Disjunctive Distribution in the Indian Ocean Basin Resulted from Long Distance Dispersal and Extensive Radiation
    (2005)
    Yuan, Yong-Ming
    ;
    Wohlhauser, Sébastien
    ;
    Möller, Michael
    ;
    Klackenberg, Jens
    ;
    Callmander, Martin W.
    ;
    Disjunctive distributions across paleotropical regions in the Indian Ocean Basin (IOB) often invoke dispersal/vicariance debates. Exacum (Gentianaceae, tribe Exaceae) species are spread around the IOB, in Africa, Madagascar, Socotra, the Arabian peninsula, Sri Lanka, India, the Himalayas, mainland Southeast Asia including southern China and Malaysia, and northern Australia. The distribution of this genus was suggested to be a typical example of vicariance resulting from the breakup of the Gondwanan supercontinent. The molecular phylogeny of Exacum is in principle congruent with morphological conclusions and shows a pattern that resembles a vicariance scenario with rapid divergence among lineages, but our molecular dating analysis demonstrates that the radiation is too recent to be associated with the Gondwanan continental breakup. We used our dating analysis to test the results of DIVA and found that the program predicted impossible vicariance events. Ancestral area reconstruction suggests that Exacum originated in Madagascar, and divergence dating suggests its origin was not before the Eocene. The Madagascan progenitor, the most recent common ancestor of Exacum, colonized Sri Lanka and southern India via long-distance dispersals. This colonizer underwent an extensive range expansion and spread to Socotra-Arabia, northern India, and mainland Southeast Asia in the northern IOB when it was warm and humid in these regions. This widespread common ancestor retreated subsequently from most parts of these regions and survived in isolation in Socotra-Arabia, southern India–Sri Lanka, and perhaps mainland Southeast Asia, possibly as a consequence of drastic climatic changes, particularly the spreading drought during the Neogene. Secondary diversification from these surviving centers and Madagascar resulted in the extant main lineages of the genus. The vicariance-like pattern shown by the phylogeny appears to have resulted from long-distance dispersals followed by extensive range expansion and subsequent fragmentation. The extant African species E. oldenlandioides is confirmed to be recently dispersed from Madagascar.
  • Publication
    Accès libre
    Seedcoat micromorphology of Impatiens (Balsaminaceae) from China
    (2005)
    Yuan, Yong-Ming
    ;
    Song. Yi
    ;
    Seeds of 38 species of Impatiens mostly from south-western China were observed by scanning electron microscopy. High diversity was found in seedcoat micromorphology. Based on the structure and ornamentation of epidermal cells of the seedcoat, particularly the pattern of the arrangement of the cells and the degree of elevation of the anticlinal cell walls, four morphological types can be distinguished, viz, laevigate, granulate, reticulate and protrusive. The laevigate type and granulate type are unique, each occurring in only one species. The other two types are common and can be further divided into subdivisions according to the shape of the epidermal ornamentation of the seedcoat. Descriptions of seed size, shape and seedcoat types are summarized for the genus. Taxonomic and phylogenetic implications of the seedcoat micromorphology are also discussed, in comparison with the available gross morphological and molecular data.
  • Publication
    Métadonnées seulement
    Phylogeny and biogeography of Exacum (Gentianaceae): A disjunctive distribution in the Indian Ocean Basin resulted from long distance dispersal and extensive radiation
    (2005)
    Yuan, Yong-Ming
    ;
    Wohlhauser, Sébastien
    ;
    Moller, Michael
    ;
    Klackenberg, Jens
    ;
    Callmander, Martin
    ;
    Disjunctive distributions across paleotropical regions in the Indian Ocean Basin (IOB) often invoke dispersal/vicariance debates. Exacum (Gentianaceae, tribe Exaceae) species are spread around the IOB, in Africa, Madagascar, Socotra, the Arabian peninsula, Sri Lanka, India, the Himalayas, mainland Southeast Asia including southern China and Malaysia, and northern Australia. The distribution of this genus was suggested to be a typical example of vicariance resulting from the breakup of the Gondwanan supercontinent. The molecular phylogeny of Exacion is in principle congruent with morphological conclusions and shows a pattern that resembles a vicariance scenario with rapid divergence among lineages, but our molecular dating analysis demonstrates that the radiation is too recent to be associated with the Gondwanan continental breakup. We used our dating analysis to test the results of DIVA and found that the program predicted impossible vicariance events. Ancestral area reconstruction suggests that Exacum originated in Madagascar, and divergence dating suggests its origin was not before the Eocene. The Madagascan progenitor, the most recent common ancestor of Exacion, colonized Sri Lanka and southern India via long-distance dispersals. This colonizer underwent an extensive range expansion and spread to Socotra-Arabia, northern India, and mainland Southeast Asia in the northern IOB when it was warm and humid in these regions. This widespread common ancestor retreated subsequently from most parts of these regions and survived in isolation in Socotra-Arabia, southern India-Sri Lanka, and perhaps mainland Southeast Asia, possibly as a consequence of drastic climatic changes, particularly the spreading drought during the Neogene. Secondary diversification from these surviving centers and Madagascar resulted in the extant main lineages of the genus. The vicariance-like pattern shown by the phylogeny appears to have resulted from long-distance dispersals followed by extensive range expansion and subsequent fragmentation. The extant African species E. oldenlandioides is confirmed to be recently dispersed from Madagascar.
  • Publication
    Accès libre
    Phylogeny and biogeography of Balsaminaceae inferred from ITS sequences
    (2004)
    Yuan, Yong-Ming
    ;
    Song. Y.
    ;
    Geuten, K.
    ;
    Rahelivololona, E.
    ;
    Wohlhauser, Sébastien
    ;
    Fischer, E.
    ;
    Smets, E.
    ;
    Sequences of the internal transcribed spacers (ITS) of nuclear ribosomal DNA were acquired for 112 species of Balsaminaceae worldwide and five species of its closest relatives Marcgraviaceae and Tetrameristaceae. Phylogenetic analyses applying parsimony and distance estimates confirmed the monophyly of Balsaminaceae and suggest the monophyly of Impatiens. Within Impatiens, a few clades are recognized with strong support. Two of the most important clades are the spurless Madagascan endemic group, and the one comprising species with broadly fusiform fruits and the basic chromosome number x = 8, that shows a Southeast Asia, southern India, Africa, and Madagascar connection. Despite recognition of several strongly supported small lineages, ITS data alone could not resolve relationships among most of the lineages with confident support values. ITS phylogenies are therefore of limited taxonomic value for Impatiens. However, ITS phylogenies do reveal that extant Impatiens species are of Southeast Asian origin, from where dispersals to boreal Eurasia and North America, to central Asia and eastern Europe via the Himalayas, and to India and Africa have occurred. The Madagascan Impatiens show an African origin. Molecular phylogenies suggest the ancestral basic chromosome number to be x = 10, and the spurred flowers and elongated linear fruits to be plesiomorphic states in Impatiens. A predominantly descending dysploid chromosome evolution, following dispersal of the clade with broadly fusiform fruits from Southeast Asia to India, Africa, and Madagascar, is also suggested.Sequences of the internal transcribed spacers (ITS) of nuclear ribosomal DNA were acquired for 112 species of Balsaminaceae worldwide and five species of its closest relatives Marcgraviaceae and Tetrameristaceae. Phylogenetic analyses applying parsimony and distance estimates confirmed the monophyly of Balsaminaceae and suggest the monophyly of Impatiens. Within Impatiens, a few clades are recognized with strong support. Two of the most important clades are the spurless Madagascan endemic group, and the one comprising species with broadly fusiform fruits and the basic chromosome number x = 8, that shows a Southeast Asia, southern India, Africa, and Madagascar connection. Despite recognition of several strongly supported small lineages, ITS data alone could not resolve relationships among most of the lineages with confident support values. ITS phylogenies are therefore of limited taxonomic value for Impatiens. However, ITS phylogenies do reveal that extant Impatiens species are of Southeast Asian origin, from where dispersals to boreal Eurasia and North America, to central Asia and eastern Europe via the Himalayas, and to India and Africa have occurred. The Madagascan Impatiens show an African origin. Molecular phylogenies suggest the ancestral basic chromosome number to be x = 10, and the spurred flowers and elongated linear fruits to be plesiomorphic states in Impatiens. A predominantly descending dysploid chromosome evolution, following dispersal of the clade with broadly fusiform fruits from Southeast Asia to India, Africa, and Madagascar, is also suggested.
  • Publication
    Accès libre
    Monophyly and relationships of the tribe Exaceae (Gentianaceae) inferred from nuclear ribosomal and chloroplast DNA sequences
    (2003)
    Yuan, Yong-Ming
    ;
    Wohlhauser, Sébastien
    ;
    Moller, Michael
    ;
    Chassot, Philippe
    ;
    Mansion, Guilhem
    ;
    ; ;
    Klackenberg, Jens
    Both chloroplast trnL (UAA) intron and nuclear ribosomal ITS sequences highly confirmed the monophyly of the tribes of the Gentianaceae defined by the recent classification, and revealed the tribe Exaceae as a basal clade just next to the basal-most lineage, the tribe Saccifolieae. Within the tribe Exaceae, Sebaea (except Sebaea madagascariensis) appeared as the most basal clade as the sister group to the rest of the tribe. The Madagascan endemic genera Gentianothamnus and Tachicrdenus were very closely related to each other, together standing as sister to a clade comprising Sebaea madagascariensis, Ornichia, and Exacum. The saprophytic genus Cotylanthera nested deeply inside Exacum. Sebaea madagascariensis was shown closer to the Madagascan endemic genus Ornichia than to any other sampled Sebaea species. Exacum appeared as the most derived taxon within this tribe. The topology of the phylogenetic trees conform with the Gondwana vicariance hypothesis regarding the biogeography of Exaceae. However, no evidence for matching the older relationships within the family to the tectonic history could be corroborated with various divergence time analyses. Divergence dating estimated a post-Gondwana diverging of the Gentianaceae about 50 million years ago (MYA), and the tribe Exaceae as about 40 MYA. The Mozambique Channel land-bridge could have played an important role in the biogeographic history of the tribe Exaceae. (C) 2003 Elsevier Science (USA). All rights reserved.
  • Publication
    Accès libre
    High paraphyly of Swertia L. (Gentianaceae) in the Gentianella -lineage as revealed by nuclear and chloroplast DNA sequence variation
    (2001)
    Chassot, P.
    ;
    Nemomissa, S.
    ;
    Yuan, Yong-Ming
    ;
    The genus Swertia L., as currently defined, is polymorphic and mainly distributed in temperate regions of the northern hemisphere. Phylogenetic relationships between Swertia 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 trnL (UAA) intron, the intergenic spacers (IGS) between trnL (UAA) and trnF (GAA) exons, and between trnS (UGA) and ycf 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 Swertia 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. Latouchea and Obolaria are closely related and occupy the basalmost position in the molecular tree. Swertia species are distributed in 9 different clades, three of which share a basal polytomy with Bartonia, Frasera, Gentianopsis, Halenia, Megacodon, Pterygocalyx and Veratrilla. 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 Gentianella s. str., and one is included in a large clade comprising Comastoma, Jaeschkea and Lomatogonium. 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.
  • Publication
    Accès libre
    Chromosomal evolution of Gentiana and Jaeschkea (Gentianaceae), with further documentation of chromosome data for 35 species from western China
    (1998)
    Yuan, Yong-Ming
    ;
    ;
    Zeltner, Louis
    Chromosome numbers were recorded for 63 populations of 34 species belonging to the genus Gentiana from the high altitude regions of western China. Counts for 22 species were reported for the first time and new numbers were found for G. heleonastes (2n = 36), G. prattii (2n = 20) and G. pseudoaquatica (2n = 40). Incorporating previous data, a complete series of gametic chromosome numbers from n = 6 to 24 and 26 was established for the genus, suggesting rapid karyotypic evolution by a combination of dysploidy and polyploidy. The cytotype 2n = 20 is proposed as the ancestral type in sect. Chondrophyllae s. l. The chromosome number 2n = 16 was found for Jaeschkea microsperma for the first time which, with previous reports of 2n = 18, 20 and 22, indicates that Jaeschkea is a typically dysploid genus.
  • Publication
    Accès libre
    The monophyly and rapid evolution of Gentiana sect. Chondrophyllae Bunge s.l. (Gentianaceae): evidence from the nucleotide sequences of the internal transcribed spacers of nuclear ribosomal DNA
    (1997)
    Yuan, Yong-Ming
    ;
    The nucleotide sequences of the internal transcribed spacers (ITS) of nuclear ribosomal DNA of 24 representative species of sect. Chondrophyllae s.l. have been determined and analysed phylogenetically, together with some species of other sections of the genus Gentiana. The ITS sequences strongly support the monophyly of the sect. Chondrophyllae s.l. as a whole complex including various different dysploid cytotypes. Species, such as G. boryi and G. pyrenaica, that had been split into distinct genera by some cytotaxonomists have been proven to be closely related. However, the ITS sequences do not provide sufficient information to make a robust estimation of the phylogenetic relationships among the closely related species and dysploid cytotypes of the complex, beyond recognizing their monophyly and rapid evolution.
  • Publication
    Accès libre
    Monophyly and relationships of the tribe Exaceae (Gentianaceae) inferred from nuclear ribosomal and chloroplast DNA sequences
    Yuan, Yong-Ming
    ;
    Wohlhauser, Sébastien
    ;
    Möller, Michael
    ;
    Chassot, Philippe
    ;
    Mansion, Guilhem
    ;
    ; ;
    Klackenberg, Jens
    Both chloroplast trnL (UAA) intron and nuclear ribosomal ITSsequences highly confirmed the monophyly of the tribes of the Gentianaceae defined by the recent classification, and revealed the tribe Exaceae as a basal clade just next to the basal-most lineage, the tribe Saccifolieae. Within the tribe Exaceae, Sebaea (except Sebaea madagascariensis) appeared as the most basal clade as the sister group to the rest of the tribe. The Madagascan endemic genera Gentianothamnus and Tachiadenus were very closely related to each other, together standing as sister to a clade comprising Sebaea madagascariensis, Ornichia, and Exacum. The saprophytic genus Cotylanthera nested deeply inside Exacum. Sebaea madagascariensis was shown closer to the Madagascan endemic genus Ornichia than to any other sampled Sebaea species. Exacum appeared as the most derived taxon within this tribe. The topology of the phylogenetic trees conform with the Gondwana vicariance hypothesis regarding the biogeography of Exaceae. However, no evidence for matching the older relationships within the family to the tectonic history could be corroborated with various divergence time analyses. Divergence dating estimated a post-Gondwana diverging of the Gentianaceae about 50 million years ago (MYA), and the tribe Exaceae as about 40 MYA. The Mozambique Channel land-bridge could have played an important role in the biogeographic history of the tribe Exaceae.