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The Gnetales: fossils and phylogenies
Stockholm University, Faculty of Science, Department of Botany.
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The phylogeny of seed plants has been debated for more than a hundred years and is still not fully understood. Morphological analyses have consistently resulted in a phylogeny in which cycads are the earliest diverging seed plants, and Gnetales and angiosperms are sisters. Molecular data has, however, rarely supported this result. Mitochondrial data resolves Gnetales as sister to the conifer family Pinaceae, but results from the nuclear and chloroplast genomes are ambiguous. My studies showed a conflict between transitions and transversions within the chloroplast genes rbcL and atpB, and consequently, different topologies are obtained from different substitution models and character codings. Transversion parsimony as well as maximum likelihood using GTR-model results in an association between Gnetales and conifers, whereas equally weighted parsimony and a simpler model (F81) resolves Gnetales as sister to all other seed plants. Taxon sampling error may be another problem. I could easily obtain seemingly well-supported, but conflicting, topologies simply by substituting a few terminals in bayesian and parsimony analyses of rbcL. These problems do not only concern the systematic position of the Gnetales, but other fundamental issues such as the monophyly of conifers and gymnosperms, the sister group of Ginkgo etc. The most recent articles on the subject argue that monophyletic gymnosperms and a sister relationship between Gnetales and Pinaceae is the most probable scenario because this topology is supported by mitochondrial data and by chloroplast data from which the fastest evolving sites (i.e. transitions or third codon positions) have been removed or down-weighted.

The Gnetales have often been considered a small remnant of a former much greater diversity due to the pronounced morphological and ecological differences between the genera. But until recently, megafossils, which could support this were rare. We have described a seedling from the Early Cretaceous of Brazil that shares several synapomorphies with Welwitschia and dates the split between Gnetum and Welwitschia to before 110 Myr. Ephedra, the sister group of the Gnetum-Welwitschia clade, comprises a homogenous group of species; very similar in gross morphology and with few informative characters in the investigated gene regions. The similarities made it difficult to resolve relationships within the group and to assign fossils to subgroups within Ephedra. We have studied Early Cretaceous fossils from Portugal, Virginia and China and they share several unique features with modern Ephedra, e.g. the peculiar naked male gametophyte. They differ in detailed morphology, however, indicating that a diversity of Ephedra species was present in the Early Cretaceous flora. We have compared the fossils to extant species and to the extinct Erdtmanispermum, and discuss character evolution within the ephedroid lineage. The paucity of information in most gene regions has indicated a very young age (8-32 Myr) for modern species in molecular dating analyses. The fossils document that the characteristic Ephedra features were present and widespread in the Early Cretaceous and they indicate that the crown group may be of Mesozoic origin (110-127 Myr). A recently published fossil from China, which possesses a character only present in a few extant species, further supports this idea. Ephedra might once again demonstrate the absence of a linear correlation between the amount of substitutions and time.

Place, publisher, year, edition, pages
Stockholm: Botaniska institutionen , 2005. , 39 p.
Keyword [en]
seed plants, Gnetales, Ephedra, fossils, DNA, phylogeny, morphology
Identifiers
URN: urn:nbn:se:su:diva-488ISBN: 91-7155-058-5 (print)OAI: oai:DiVA.org:su-488DiVA: diva2:194543
Public defence
2005-05-27, föreläsningssalen, Botanicum, Lilla Frescativägen 5, Stockholm, 10:00
Opponent
Supervisors
Available from: 2005-05-04 Created: 2005-05-04Bibliographically approved
List of papers
1. Seed plant relationships and the systematic position of Gnetales based on nuclear and chloroplast DNA: conflicting data, rooting problems and the monophyly of conifers
Open this publication in new window or tab >>Seed plant relationships and the systematic position of Gnetales based on nuclear and chloroplast DNA: conflicting data, rooting problems and the monophyly of conifers
2002 In: International Journal of Plant Sciences, ISSN 1058-5893, Vol. 163, no 2, 197-214 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-23789 (URN)
Note
Part of urn:nbn:se:su:diva-488Available from: 2005-05-04 Created: 2005-05-04Bibliographically approved
2. Taxon sampling and seed plant phylogeny
Open this publication in new window or tab >>Taxon sampling and seed plant phylogeny
2002 In: Cladistics, ISSN 0748-3007, Vol. 18, no 5, 485-513 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-23790 (URN)
Note
Part of urn:nbn:se:su:diva-488Available from: 2005-05-04 Created: 2005-05-04Bibliographically approved
3. Cratonia cotyledon gen. et sp. nov.: a unique seedling related to Welwitschia
Open this publication in new window or tab >>Cratonia cotyledon gen. et sp. nov.: a unique seedling related to Welwitschia
2003 In: Biology Letters, Royal Society of London, ISSN 0962-8436, Vol. 270, 29-32 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-23791 (URN)
Note
Part of urn:nbn:se:su:diva-488Available from: 2005-05-04 Created: 2005-05-04Bibliographically approved
4. On the evolutionary history of Ephedra: Cretaceous fossils and extant molecules
Open this publication in new window or tab >>On the evolutionary history of Ephedra: Cretaceous fossils and extant molecules
2004 In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, Vol. 101, no 47, 16571-16576 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-23792 (URN)
Note
Part of urn:nbn:se:su:diva-488Available from: 2005-05-04 Created: 2005-05-04Bibliographically approved
5. Pollen germination in Welwitschia mirabilis Hook. f.: comparing differences between the polyplicate pollen producing genera of the Gnetales
Open this publication in new window or tab >>Pollen germination in Welwitschia mirabilis Hook. f.: comparing differences between the polyplicate pollen producing genera of the Gnetales
2005 (English)In: Grana, ISSN 0017-3134, E-ISSN 1651-2049, Vol. 44, no 3, 137-141 p.Article in journal (Refereed) Published
Abstract [en]

 

 

 

 

 

 

 

Pollen grains of the seed plant genera

 

Ephedra L. and Welwitschia

Hook. f. (Gnetales) are of similar

size, shape, and have a polyplicate exine with alternating thicker and thinner regions.

 

Ephedra

pollen is

considered inaperturate and the exine is shed during germination, leaving the male gametophyte naked.

The shed exine curls up and forms a characteristic structure with transverse striations. Such upcurled

exines have been found in situ in Early Cretaceous seeds with affinities to

 

Ephedra

. The purpose of this

study was to document the germination of

 

Welwitschia

pollen and investigate whether they also discard

their exine during this process.

The pollen grains of

 

Welwitschia

are monoaperturate with a distinct, distal sulcus. During

germination, the sulcus splits open and the gametophyte expands to a spherical form that extends out

of the exine. The pollen tube starts to grow one or two hours later and as in

 

Ephedra

, it is displaced

towards one side. The exine is not shed but remains as a ‘‘cap’’ that partly covers the male

gametophyte. Thus, in this respect the germination process is distinctly different from that in

 

Ephedra

and this study demonstrates that discharging the exine during pollen germination is unique to

 

Ephedra

,

among the polyplicate pollen producing genera in the Gnetales.

Place, publisher, year, edition, pages
Taylor & Francis, 2005
National Category
Botany
Identifiers
urn:nbn:se:su:diva-23793 (URN)10.1080/00173130500230459 (DOI)
Note
Part of urn:nbn:se:su:diva-488Available from: 2005-05-04 Created: 2005-05-04 Last updated: 2017-12-13Bibliographically approved
6. Former diversity of Ephedra (Gnetales): evidence from Early Cretaceous seeds from Portugal and North America.
Open this publication in new window or tab >>Former diversity of Ephedra (Gnetales): evidence from Early Cretaceous seeds from Portugal and North America.
Manuscript (Other academic)
Identifiers
urn:nbn:se:su:diva-23794 (URN)
Note
Part of urn:nbn:se:su:diva-488Available from: 2005-05-04 Created: 2005-05-04 Last updated: 2010-01-13Bibliographically approved

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