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New insights into the evolutionary history of Gnetum (Gnetales)
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. Imperial College London, England.
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
Number of Authors: 4
2015 (English)In: Taxon, ISSN 0040-0262, E-ISSN 1996-8175, Vol. 64, no 2, 239-253 p.Article in journal (Refereed) Published
Abstract [en]

Gnetum (Gnetales: Gnetaceae) constitutes an evolutionarily isolated gymnosperm clade, comprising about 40 species that inhabit tropical areas of the world. While its closest living relative, the monotypic Welwitschia, has a well-documented fossil record from the Early Cretaceous, Gnetum-like fossils are rare and poorly understood. The phylogeny of Gnetum has been studied previously but the distant relationship to outgroups and the difficulty of obtaining plant material mean it is not yet fully resolved. Most species are tropical lianas with an angiospermous vegetative habit that are difficult to find and identify. Here a new phylogeny is presented based on nuclear and chloroplast data from 58 Gnetum accessions, representing 27 putative species, and outgroup information from other seed plants. The results provide support for South American species being sister to the remaining species. The two African species constitute a monophyletic group, sister to an Asian clade, within which the two arborescent species of the genus are the earliest diverging. Estimated divergence times indicate, in contrast with previous results, that the major lineages of Gnetum diverged in the Late Cretaceous. This result is obtained regardless of tree prior used in the BEAST analyses (Yule or birth-death). Together these findings suggest a correlation between early divergence events in extant Gnetum and the breakup of Gondwana in the Cretaceous. Compared to the old stem ages of major subclades of Gnetum, crown nodes date to the Cenozoic: the Asian crown group dates to the Cretaceous-Paleogene (K-Pg) boundary, the African crown group to the mid-Paleogene, and the South American crown group to the Paleogene-Neogene boundary. Although dispersal must have contributed to the current distribution of Gnetum, e.g., within South America and from Southeast Asian islands to the East Asian mainland, dispersal has apparently not occurred across major oceans, at least not during the Cenozoic.

Place, publisher, year, edition, pages
2015. Vol. 64, no 2, 239-253 p.
Keyword [en]
Bayesian inference, birth-death prior, classification, node ages, phylogeny, Yule prior
National Category
Biological Sciences
Research subject
Plant Systematics
Identifiers
URN: urn:nbn:se:su:diva-118277ISI: 000354372300004OAI: oai:DiVA.org:su-118277DiVA: diva2:824400
Available from: 2015-06-22 Created: 2015-06-15 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Evolutionary studies of the Gnetales
Open this publication in new window or tab >>Evolutionary studies of the Gnetales
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Gnetales consist of three distinct genera, Ephedra, Gnetum and Welwitschia with considerable divergence among them regarding morphological, ecological and molecular characters. A longstanding debate of the similarity between the Gnetales and angiosperms and the unresolved seed plant phylogeny intrigues plant scientists to further investigate the evolutionary history of the Gnetales. The presented projects deal with interdisciplinary questions on proteomics, chloroplast genomes, phylogenetic relationships, gross morphology and taxonomy. The thesis aims to summarize general problems encountered in previous studies, and to provide new insights and future perspectives based on the results of completed and ongoing projects.   

In Ephedra, the Mediterranean species E. foeminea has been shown to be entomophilous and it possesses an important phylogenetic status as the sister of the remaining genus. Therefore, the chloroplast genome of E. foeminea was assembled and compared to that previously presented (of the anemophilous Asian species E. equisetina, nested in the core clade of Ephedra). The genome has a quadripartite structure and comprises 118 genes and 109,584 base pairs. A pairwise genome comparison was conducted between E. foeminea and E. equisetina, resulting in the detection of 2,352 variable sites, the obtained data can be used for prospective phylogenetic studies. A proteomic study was also conducted on E. foeminea along with three anemophilous Ephedra species, in order to investigate the biochemistry of the pollination drops. The results show that detected proteins in the pollination drops of Ephedra vary dramatically among species but always occur in very low amounts. The majority of the detected proteins are degradome proteins, i.e., waste products from degrading cells of the nucellus. Some secretome proteins were also found, which are putatively functional, but also these proteins occur in very low amounts. The repeatability of the proteomic studies can, however, be questioned. The sampling methods and proteomic analyses are probably problematic although some suggestions for improvement are provided. Thus I chose to continue with other projects.

In Gnetum, reconstruction of the genus phylogeny and assessments of divergence times of clades were performed using an extensive sampling of ingroup and outgroup accessions. The results show that the South American lineage separated from the remaining genus in the Late Cretaceous. The continued diversification event gave rise to an African lineage and an Asian lineage. The crown age of the Asian clade, which comprises two arborescent species sister to the remaining liaonid species, was estimated to the Cretaceous-Paleogene (K-Pg) boundary. In light of the genus phylogeny and estimated node ages, we suggest that the breakup of Gondwana influenced diversification patterns in Gnetum. Later dispersal events also contributed to the current distribution of Gnetum, and to the phylogenetic patterns within each of the major clades. From my results, it is however clear that taxonomy and species delimitations are poorly defined, and needs to be further studied for all subclades of Gnetum. I have initiated this task by studying the Chinese lianoid clade of Gnetum more in depth. Eleven chloroplast genomes were generated, aligned and compared. Based on the information, four chloroplast markers were designed and applied to further resolve the species relationships with an extensive sampling. The results show, with strong support, that G. parvifolium is sister to all the remaining species of the Chinese linaoid clade. Another five lianoid species are confirmed using both morphological and molecular data, but several names are represented by type material that cannot be considered separate species. Modified keys for identification of male and female plants are presented, based on vegetative and reproductive structures. A subsequent dating analysis indicates that diversification in the Chinese lianoid Gnetum clade took place mainly in the Neogene, during which environmental changes probably facilitated diversification in the lineage.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University, 2016. 44 p.
National Category
Botany
Research subject
Plant Systematics
Identifiers
urn:nbn:se:su:diva-128439 (URN)978-91-7649-371-7 (ISBN)
Public defence
2016-06-03, Vivi Täckholmsalen (Q-salen), NPQ-huset, Frescativägen 20 A, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Manuscript. Paper 5: Manuscript.

Available from: 2016-05-11 Created: 2016-03-28 Last updated: 2017-02-17Bibliographically approved

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