Gnetophyta

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Gnetophyta
Welwitchia.jpg
Welwitschia mirabilis female plant with cones
Scientific classification e
Kingdom: Plantae
Division: Gnetophyta
Class: Gnetopsida
Families & Genera

Gnetaceae
  Gnetum
Welwitschiaceae
  Welwitschia
Ephedraceae
  Ephedra

A distribution map of Gnetophyta colour-coded by genus
Distribution, separated by genus:
Green – Welwitschia
Blue – Gnetum
Red – Ephedra
Purple – Gnetum and Ephedra range overlap

The plant division Gnetophyta or gnetophytes consists of three genera of woody plants grouped in the gymnosperms. The living Gnetophyta comprise some 70 species across the three genera Gnetum (family Gnetaceae), Welwitschia (family Welwitschiaceae), and Ephedra (family Ephedraceae).

Welwitschia mirabilis male cones
Ephedra distachya (male flowers)
Ephedra distachya (female plant in bloom)
Gnetum gnemon male
Gnetum gnemon female
Female Ephedra californica cone

The gnetophytes differ from other gymnosperms (i.e. conifers, cycads, and ginkgos) in having vessel elements (which transport water within the plant) as found in flowering plants. The gnetophytes' evolutionary relationships remain unclear: in some classifications, all three genera are placed in a single order (Gnetales), while in other classifications they are distributed among three orders, each containing a single family and genus. Most morphological and molecular studies confirm that Gnetum and Welwitschia diverged from each other more recently than from Ephedra.[1][2][3][4] [5]

Ecology and morphology[edit]

The three genera of the gnetophytes are highly specialized to their respective environments, making it difficult to identify homologous characters.[6] The three extant genera of gnetophytes, a "bizarre and enigmatic" trio,[2] are likely aberrant members of the group, which was diverse and dominant in the Tertiary.[6] Some synapomorphies of the gnetophytes include enveloping bracts around the ovules and microsporangia, and a micropylar projection of the outer membrane of the ovule that produces a pollination droplet.[7]

Gnetum species are mostly woody climbers in tropical forests. However, the most well-known member of this group, Gnetum gnemon, is a tree. Its seeds are used to produce a crispy krupuk snack known as emping or krupuk belinjo.

Welwitschia comprises only one species, Welwitschia mirabilis. It grows only in the deserts of Namibia and Angola. This strange species has only two large strap-like leaves that grow continuously from the base throughout the plant's life.

Plants of the genus Ephedra are known as jointfirs because they have long slender branches which bear tiny scale-like leaves at their nodes. Ephedra has been traditionally used as a stimulant, but is a controlled substance today in many jurisdictions because of the risk of harmful or even fatal overdosing.

Knowledge of fossils of the gnetophytes has increased greatly since the 1980s.[1] There are fossils from the Permian,[8] the Triassic, and the Jurassic which may belong to the gnetophytes, but this is uncertain.[9] The fossil record is richer starting in the early Cretaceous, with fossils of plants as well as seeds and pollen which can be clearly assigned to the gnetophytes.[9]

Classification[edit]

With just three well-defined genera within an entire division, there still is understandable difficulty in establishing an unambiguous interrelationship among them; in earlier times matters were even more difficult and we find for example Pearson in the early 20th century speaking of the class Gnetales, rather than the order.[10] G. H. M. Lawrence referred to them as an order, but remarked that the three families were distinct enough to deserve recognition as separate orders.[11] Foster & Gifford accepted this principle, and placed the three orders together in a common class for convenience, which they called Gnetopsida.[12] In general the evolutionary relationships among the seed plants still are unresolved, and the Gnetophyta have played an important role in the formation of phylogenetic hypotheses. Molecular phylogenies of extant gymnosperms have conflicted with morphological characters with regard to whether the gymnosperms as a whole (including gnetophytes) comprise a monophyletic group or a paraphyletic one that gave rise to angiosperms. At issue is whether the Gnetophyta are the sister group of angiosperms, or whether they are sister to, or nested within, other extant gymnosperms. Numerous fossil gymnosperm clades once existed that are morphologically at least as distinctive as the four living gymnosperm groups, such as Bennettitales, Caytonia and the glossopterids. When these gymnosperm fossils are considered, the question of gnetophyte relationships to other seed plants becomes even more complicated. Several hypotheses, illustrated below, have been presented to explain seed plant evolution.

Recent research by Lee EK, Cibrian-Jaramillo A, et al. (2011) suggests that the Gnetophyta are a sister group to the rest of the gymnosperms,[13] contradicting the anthophyte hypothesis, which held that gnetophytes were sister to the flowering plants.

Anthophyte hypothesis[edit]

From the early twentieth century, the anthophyte hypothesis was the prevailing explanation for seed plant evolution, based on shared morphological characters between the gnetophytes and angiosperms. In this hypothesis, the gnetophytes, along with the extinct order Bennettitales, are sister to the angiosperms, forming the "anthophytes".[7] Some morphological characters that were suggested to unite the anthophytes include vessels in wood, net-veined leaves (in Gnetum only), lignin chemistry, the layering of cells in the apical meristem, pollen and megaspore features (including thin megaspore wall), short cambial initials, and lignin syringal groups.[7][14][15][16] However, most genetic studies have rejected the anthophyte hypothesis.[2][17][18][19][20][21][22][23][24][25] Several of these studies have suggested that the gnetophytes and angiosperms have independently derived characters, including flower-like reproductive structures and tracheid vessel elements, that appear shared but are actually the result of parallel evolution.[2][7][18]


Ginkgo



cycads



conifers


anthophytes

angiosperms (flowering plants)



gnetophytes



Gnetifer hypothesis[edit]

In the gnetifer hypothesis, the gnetophytes are sister to the conifers, and the gymnosperms are a monophyletic group, sister to the angiosperms. The gnetifer hypothesis first emerged formally in the mid-twentieth century, when vessel elements in the gnetophytes were interpreted as being derived from tracheids with circular bordered pits, as in conifers.[7] It did not gain strong support, however, until the emergence of molecular data in the late 1990s.[17][23][26][27] Although the most salient morphological evidence still largely supports the anthophyte hypothesis, there are some more obscure morphological commonalities between the gnetophytes and conifers that lend support to the gnetifer hypothesis. These shared traits include: tracheids with scalariform pits with tori interspersed with annular thickenings, absence of scalariform pitting in primary xylem, scale-like and strap-shaped leaves of Ephedra and Welwitschia; and reduced sporophylls.[22][25][28]


angiosperms (flowering plants)


gymnosperms

cycads



Ginkgo




conifers



gnetophytes




Gnepine hypothesis[edit]

The gnepine hypothesis is a modification of the gnetifer hypothesis, and suggests that the gnetophytes belong within the conifers as a sister group to the Pinaceae.[7] According to this hypothesis, the conifers as currently defined are not a monophyletic group, in contrast with molecular findings that support its monophyly.[26] All existing evidence for this hypothesis comes from molecular studies within the last decade.[2][3][18][20][22][23][25][28] However, the morphological evidence remains difficult to reconcile with the gnepine hypothesis. If the gnetophytes are nested within conifers, they must have lost several shared derived characters of the conifers (or these characters must have evolved in parallel in the other conifer lineages): narrowly triangular leaves (gnetophytes have diverse leaf shapes), resin canals, a tiered proembryo, and flat woody ovuliferous cone scales.[22] These kinds of major morphological changes are not without precedent in the Pinaceae, however: the Taxaceae, for example, have lost the classical cone of the conifers in favor of a single-terminal ovule surrounded by a fleshy aril.[18]


angiosperms (flowering plants)


gymnosperms

cycads



Ginkgo


conifers


Pinaceae (the pine family)



gnetophytes




other conifers




Gnetophyte-sister hypothesis[edit]

Some partitions of the genetic data suggest that the gnetophytes are sister to all of the other extant seed plant groups.[4][7][22][25][26] However, there is no morphological evidence nor examples from the fossil record to support the gnetophyte-sister hypotheses.[28]


gnetophytes




angiosperms (flowering plants)




cycads



Ginkgo



conifers




References[edit]

  1. ^ a b Peter R. Crane, Patrick Herendeen and Else Marie Friis (2004). "Fossils and plant phylogeny". American Journal of Botany 91 (10): 1683–1699. doi:10.3732/ajb.91.10.1683. PMID 21652317. 
  2. ^ a b c d e Bowe, L.M.; Coat, G.; and dePamphilis, C.W. (2000). "Phylogeny of seed plants based on all three genomic compartments: Extant gymnosperms are monophyletic and Gnetales' closest relatives are conifers". Proceedings of the National Academy of Sciences 97 (8): 4092–4097. doi:10.1073/pnas.97.8.4092. PMC 18159. PMID 10760278. 
  3. ^ a b Gugerli, F.; Sperisen, C.; Buchler, U.; Brunner, L.; Brodbeck, S.; Palmer, J.D.; and Qiu, Y.L. (2001). "The evolutionary split of Pinaceae from other conifers: evidence from an intron loss and a multigene phylogeny". Molecular Phylogeny and Evolution 21 (2): 167–175. doi:10.1006/mpev.2001.1004. PMID 11697913. 
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  5. ^ Ickert-Bond, S. M., C. Rydin, and S. S. Renner (2009). "A fossil-calibrated relaxed clock for Ephedra indicates an Oligocene age for the divergence of Asian and New World clades, and Miocene dispersal into South America". Journal of Systematics and Evolution 47: 444–456. doi:10.1111/j.1759-6831.2009.00053.x. 
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  7. ^ a b c d e f g Judd, W.S.; Campbell, C.S.; Kellogg, E.A.; Stevens, P.F.; and Donoghue, M.J. (2008) Plant Systematics: A Phylogenetics Approach. 3rd ed. Sunderland, Massachusetts, USA: Sinauer Associates, Inc.
  8. ^ Zi-Qiang Wang (2004). "A New Permian Gnetalean Cone as Fossil Evidence for Supporting Current Molecular Phylogeny". Annals of Botany 94 (2): 281–288. doi:10.1093/aob/mch138. PMID 15229124. 
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  18. ^ a b c d Chaw, S.M.; Parkinson, C.L.; Cheng, Y.; Vincent, T.M.; Palmer, J.D. (2000). "Seed plant phylogeny inferred from all three plant genomes: Monophyly of extant gymnosperms and origin of Gnetales from conifers". Proceedings of the National Academy of Science USA 97 (8): 4086–4091. doi:10.1073/pnas.97.8.4086. PMC 18157. PMID 10760277. 
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  21. ^ Hansen, A.; Hansmann, S.; Samigullin, T.; Antonov, A.; and Martin, W. (1999). "Gnetum and the angiosperms: molecular evidence that their shared morphological characters are convergent rather than homologous". Molecular Biology and Evolution 16 (7): 1006–1009. doi:10.1093/oxfordjournals.molbev.a026176. 
  22. ^ a b c d e Magallon, S. and Sanderson, M.J. (2002). "Relationships among seed plants inferred from highly conserved genes: sorting conflicting phylogenetic signals among ancient lineages". American Journal of Botany 89 (12): 1991–2006. doi:10.3732/ajb.89.12.1991. JSTOR 4122754. PMID 21665628. 
  23. ^ a b c Qiu, Y.L.; Lee, J.; Bernasconi-Quadroni, F.; Soltis, D.E.,; Soltis, P.S.; Zanis, M.; Zimmer, E.A.; Chen, Z.; Savalainen, V.; and Chase, M.W. (1999). "The earliest angiosperms: evidence from mitochondrial, plastid and nuclear genomes". Nature 402 (6760): 404–407. doi:10.1038/46536. PMID 10586879. 
  24. ^ Samigullin, T.K.; Martin, W.F.; Troitsky, A.V.; and Antonov, A.S. (1999). "Molecular data from the chloroplast rpoC1 gene suggest a deep and distinct dichotomy of contemporary spermatophytes into two monophyla: gymnosperms (including Gnetalaes) and angiosperms". Journal of Molecular Evolution 49 (3): 310–315. doi:10.1007/PL00006553. PMID 10473771. 
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  26. ^ a b c Rydin, C.; Kallersjo, M.; and Friist, E.M. (2002). "Seed plant relationships and the systematic position of Gnetales based on nuclear and chloroplast DNA: conflicting data, rooting problems, and the monophyly of conifers". International Journal of Plant Sciences 163 (2): 197–214. doi:10.1086/338321. JSTOR 3080238. 
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Other Sources:

  • Gifford, Ernest M., Adriance S. Foster. 1989. Morphology and Evolution of Vascular Plants. Third edition. WH Freeman and Company, New York.
  • Hilton, Jason, and Richard M. Bateman. 2006. Pteridosperms are the backbone of seed-plant phylogeny. Journal of the Torrey Botanical Society 133: 119-168 (abstract)