Gnetophyta

Gnetophyta
Welwitschia mirabilis
Scientific classification
Kingdom:	Plantae
Division:	Gnetophyta
Class:	Gnetopsida
Families & Genera
Gnetaceae   Gnetum Welwitschiaceae   Welwitschia Ephedraceae   Ephedra

The plant division Gnetophyta or gnetophytes consists of three genera of woody plants grouped in the gymnosperms. The living Gnetophyta are in the genera Gnetum (family Gnetaceae), Welwitschia (family Welwitschiaceae), and Ephedra (family Ephedraceae). The gnetophytes differ from other gymnosperms (conifers, cycads, and ginkgos) in having vessel elements as in the flowering plants. In some classifications, all three genera are placed in a single order (Gnetales) but in others 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]

Ecology and Morphology

The three genera of the gnetophytes are highly specialized to their respective environments, making it difficult to identify homologous characters.^[5] 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.^[5] 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.^[6]

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,^[7] the Triassic, and the Jurassic which may belong to the gnetophytes, but this is uncertain.^[8] 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.^[8]

Classification

The evolutionary relationships among the seed plants are highly unresolved, and the gnetophytes 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.

Anthophyte hypothesis

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”.^[6] 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.^{[6][9][10][11]} However, most genetic studies have rejected the anthophyte hypothesis.^{[2][12][13][14][15][16][17][18][19][20]} 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][13][6]

Gnetifer hypothesis

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 formally emerged 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.^[6] It did not gain strong support, however, until the emergence of molecular data in the late 1990s.^{[12][18][21][22]} Although the morphological evidence still largely supports the anthophyte hypothesis, there are some 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.^[9][17][23]

Gnepine hypothesis

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.^[6] According to this hypothesis, the conifers as currently defined are not a monophyletic group, in contrast with molecular findings that support its monophyly.^[21]. All existing evidence for this hypothesis comes from molecular studies within the last decade.^{[2][3][13][20][17][15][18][23]} 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.^[12][15].

Gnetophyte-sister hypothesis

Some partitions of the genetic data suggest that the gnetophytes are sister to all of the other extant seed plant groups.^{[6][4][20][17][21]} However, there is no morphological evidence nor evidence in the fossil record to support the gnetophyte-sister hypotheses.^[23]

References

1. ^ ^{a b} Peter R. Crane, Patrick Herendeen and Else Marie Friis (2004). "Fossils and plant phylogeny". American Journal of Botany 91: 1683–1699. doi:10.3732/ajb.91.10.1683. http://www.amjbot.org/cgi/content/full/91/10/1683. 
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 Science 97: 4092-4097. doi:10.1073/pnas.97.8.4092. http://www.pnas.org/content/97/8/4092.full. 
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: 167-175. doi:10.1006/mpev.2001.1004. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WNH-456JS33-H&_user=18704&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1138931837&_rerunOrigin=google&_acct=C000002018&_version=1&_urlVersion=0&_userid=18704&md5=0a3650492a24edd77c6acd1ffef1ae87. 
4. ^ ^{a b} Rai, H.S.; Reeves, P.A.; Peakall, R.; Olmstead, R.G.; and Graham, S.W. (2008). "Inference of higher-order conifer relationships from a multi-locus plastid data set". Botany 86: 658-669. doi:10.1139/B08-062. http://columbia.library.ingentaconnect.com/content/nrc/bot/2008/00000086/00000007/art00004. 
5. ^ ^{a b} Arber, E.A.N. and Parkin, J. (1908). "Studies on the evolution of the angiosperms: the relationship of the angiosperms to the Gnetales". Annals of Botany 22: 489-515. http://aob.oxfordjournals.org/content/volos-22/issue3/index.dtl. 
6. ^ ^{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.
7. ^ 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. http://aob.oxfordjournals.org/cgi/content/full/94/2/281. 
8. ^ ^{a b} Catarina Rydin, Kaj Raunsgaard Pedersen, Peter R. Crane and Else Marie Friis (2006). "Former Diversity of Ephedra (Gnetales): Evidence from Early Cretaceous Seeds from Portugal and North America". Annals of Botany 98 (1): 123–140. doi:10.1093/aob/mcl078. PMID 16675607. http://aob.oxfordjournals.org/cgi/content/full/98/1/123. 
9. ^ ^{a b} Donaghue, M.J. and Doyle, J.A. (2000). "Seed plant phylogeny: demise of the anthophyte hypothesis?". Current Biology 10: R106-R109. doi:10.1016/S0960-9822(00)00304-3. http://linkinghub.elsevier.com/retrieve/pii/S0960982200003043. 
10. ^ Loconte, H. and Stevenson, D.W. (1990). "Cladistics of the Spermatophyta". Brittonia 42: 197-211. doi:10.2307/2807216. http://www.springerlink.com/content/93518n8n2237k054/. 
11. ^ Nixon, K.C.; Crepet, W.L.; Stevenson, D.; and Friis, E.M. (1994). "A reevaluation of seed plant phylogeny". Annals of the Missouri Botanical Garden 81: 494-533. doi:110.2307/2399901. http://www.jstor.org/stable/2399901. 
12. ^ ^{a b c} Chaw, S.M.; Aharkikh, A.; Sung, H.M.; Lau, T.C.; and Li, W.H. (1997). "Molecular phylogeny of extant gymnosperms and seed plant evolution: analysis of nuclear 18S rRNA sequences". Molecular Biology and Evolution 14: 56-68. http://mbe.oxfordjournals.org/cgi/content/abstract/14/1/56. 
13. ^ ^{a b c} 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 the origin of Gnetales from conifers". Proceedings of the National Academy of Science USA 97: 4086-4091. doi:10.1073/pnas.97.8.4086. http://www.pnas.org/content/97/8/4086.abstract. 
14. ^ Goremykin, V.; Bobrova, V.; Pahnke, J.; Troitsky, A.; Antonov, A.; and Martin, W. (1996). "Noncoding sequences from the slowly evolving chloroplast inverted repeat in addition to rbcL data do not support gnetalean affinities of angiosperms". Molecular Biology and Evolution 13: 383-396. http://mbe.oxfordjournals.org/cgi/content/abstract/13/2/383. 
15. ^ ^{a b c} Hajibabaei, M.; Xia, J.; and Drouin, G. (2006). "Seed plant phylogeny: Gnetophytes are derived conifers and a sister group to Pinaceae". Molecular Phylogenetics and Evolution 40: 208-217. doi:10.1016/j.ympev.2006.03.006. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WNH-4JRVDYS-B&_user=18704&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1138965526&_rerunOrigin=google&_acct=C000002018&_version=1&_urlVersion=0&_userid=18704&md5=699cd25dedc2e55d5b819401b1972968. 
16. ^ 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: 1006-1009. http://mbe.oxfordjournals.org/cgi/content/citation/16/7/1006. 
17. ^ ^{a b c d} 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: 1991-2006. doi:10.3732/ajb.89.12.1991. http://www.jstor.org/stable/4122754. 
18. ^ ^{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: 404-407. http://www.nature.com/nature/journal/v402/n6760/full/402404a0.html. 
19. ^ 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: 310-315. doi:10.1007/PL00006553. http://www.springerlink.com/content/77e79hf2e7vv9u6e/. 
20. ^ ^{a b c} Sanderson, M.J.; Wojciechowski, M.F.; Hu, J.M.; Sher Khan, T.; and Brady, S.G. (2000). "Error, bias, and long-branch attraction in data for two chloroplast photosystem genes in seed plants". Molecular Biology and Evolution 17: 782-797. http://mbe.oxfordjournals.org/cgi/content/abstract/17/5/782. 
21. ^ ^{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: 197-214. doi:10.1086/338321. http://www.jstor.org/stable/3080238. 
22. ^ Braukmann, T.W.A.; Kuzmina, M.; and Stefanovic, S. (2009). "Loss of all plastid nhd genes in Gnetales and conifers: extent and evolutionary significance for the seed plant phylogeny". Current Genetics 55: 323-337. http://www.nature.com/nrg/journal/v2/n3/full/nrg0301_186a.html. 
23. ^ ^{a b c} Burleigh, J.G. and Mathews, S. (2007). "Phylogenetic signal in nucleotide data from seed plants: implications for resolving the seed plant tree of life". International Journal of Plant Science 168: 125-135. http://www.amjbot.org/cgi/content/abstract/91/10/1599. 

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)