Difference between revisions of "Gnetophyta"

From Eat Every Plant
Jump to navigation Jump to search
m (1 revision: gymnosperms)
 
(redirect to category)
 
Line 1: Line 1:
−
{{Technical|date=February 2010}}
+
#redirect [[:Category:Gnetophyta]]
−
{{Automatic taxobox
 
−
| image = Welwitschia at Ugab River basin.jpg
 
−
| image_caption = ''[[Welwitschia mirabilis]] female plant with cones''
 
−
| taxon = Gnetopsida
 
−
| subdivision_ranks = Families & Genera
 
−
| subdivision =
 
−
Gnetaceae<br>
 
−
&nbsp;&nbsp;''[[Gnetum]]''<br>
 
−
Welwitschiaceae<br>
 
−
&nbsp;&nbsp;''[[Welwitschia]]''<br>
 
−
Ephedraceae<br>
 
−
&nbsp;&nbsp;''[[Ephedra (genus)|Ephedra]]''
 
−
| range_map = Gnetophyta distribution genera separate.PNG
 
−
| range_map_alt = A distribution map of Gnetophyta colour-coded by genus
 
−
| range_map_caption = Distribution, separated by genus: <br>Green – ''Welwitschia''<br>Blue – ''Gnetum''<br>Red – ''Ephedra''<br>Purple – ''Gnetum'' and ''Ephedra''
 
−
}}
 
−
 
 
−
'''Gnetophyta''' is a division of plants, grouped within the [[gymnosperm]]s (which also includes [[conifer]]s, [[cycad]]s, and [[ginkgo]]s), that consists of some 70 species across the three [[Relict (biology)|relict]] genera: ''[[Gnetum]]'' ([[Family (biology)|family]] Gnetaceae), ''[[Welwitschia]]'' (family Welwitschiaceae), and ''[[Ephedra (genus)|Ephedra]]'' (family Ephedraceae). Fossilized pollen attributed to a close relative of ''Ephedra'' has been dated as far back as the [[Early Cretaceous]].<ref>[http://www.flmnh.ufl.edu/ "Morphology and affinities of an Early Cretaceous Ephedra"].</ref> Though diverse and dominant in the [[Tertiary]],<ref name="Arber and Parkin" /> only three [[Family (biology)|families]], each containing a single [[genus]], are still alive today. The primary difference between gnetophytes and other gymnosperms is the presence of [[vessel element]]s, a system of conduits that transport water within the plant, similar to those found in [[flowering plant]]s. Because of this, gnetophytes were once thought to be the closest gymnosperm relatives to flowering plants, but more recent molecular studies have largely disproven this hypothesis.
 
−
 
 
−
Though it is clear they are all closely related, the exact evolutionary inter-relationships between gnetophytes are unclear. Some classifications hold that all three genera should be placed in a single [[Order (biology)|order]] (Gnetales), while other classifications say they should be distributed among three separate orders, each containing a single family and genus. Most morphological and molecular studies confirm that the genera ''Gnetum'' and ''Welwitschia'' diverged from each other more recently than they did from ''Ephedra''.<ref name="crane">{{cite journal | url = http://www.amjbot.org/cgi/content/full/91/10/1683  | title = Fossils and plant phylogeny |author1=Peter R. Crane |author2=Patrick Herendeen |author3=Else Marie Friis | journal = American Journal of Botany | year = 2004 | volume = 91 | pages = 1683–1699  | doi = 10.3732/ajb.91.10.1683 | issue=10 | pmid = 21652317}}</ref><ref name="Bowe" /><ref name="Gugerli">{{cite journal | url = 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 | title = The evolutionary split of Pinaceae from other conifers: evidence from an intron loss and a multigene phylogeny |author1=Gugerli, F. |author2=Sperisen, C. |author3=Buchler, U. |author4=Brunner, L. |author5=Brodbeck, S. |author6=Palmer, J.D. |author7=Qiu, Y.L. | journal = Molecular Phylogenetics and Evolution | year = 2001 | volume = 21 | pages = 167–175 | doi = 10.1006/mpev.2001.1004 | pmid = 11697913 | issue = 2}}</ref><ref name="Rai">{{cite journal|url=http://columbia.library.ingentaconnect.com/content/nrc/bot/2008/00000086/00000007/art00004 |archive-url=https://archive.is/20140829211047/http://columbia.library.ingentaconnect.com/content/nrc/bot/2008/00000086/00000007/art00004 |dead-url=yes |archive-date=2014-08-29 |title=Inference of higher-order conifer relationships from a multi-locus plastid data set |author1=Rai, H.S. |author2=Reeves, P.A. |author3=Peakall, R. |author4=Olmstead, R.G. |author5=Graham, S.W. |journal=Botany |year=2008 |volume=86 |pages=658–669 |doi=10.1139/B08-062 |issue=7 }}</ref><ref name="Ickert-Bond">{{cite journal | title = 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 | author = Ickert-Bond, S. M., C. Rydin, and S. S. Renner | journal = Journal of Systematics and Evolution  | year = 2009 | volume = 47 | pages = 444–456  | url = http://onlinelibrary.wiley.com/store/10.1111/j.1759-6831.2009.00053.x/asset/j.1759-6831.2009.00053.x.pdf?v=1&t=h3tses6s&s=16e93aab7f364e4fc976943373d12ae7d30d34a7 | doi=10.1111/j.1759-6831.2009.00053.x}}</ref>
 
−
[[File:Welwitschia mirabilis S&J6.jpg|thumb|''Welwitschia mirabilis'' bearing male cones]]
 
−
[[File:Ephedra distachya (male flowers) 1.jpg|thumb|''Ephedra distachya'' (male cones)]]
 
−
[[File:Ephedra distachya (female plant in bloom).jpg|thumb|''Ephedra distachya'' (female plant in bloom)]]
 
−
[[File:Gnetum gnemon male.jpg|thumb|''Gnetum gnemon'' male strobili]]
 
−
[[File:Gnetum gnemon BotGardBln1105C.JPG|thumb|''Gnetum gnemon'' female strobilus]]
 
−
[[File:Female Ephedra californica cone - journal.pone.0053652.g002-A.png|thumb|Female ''[[Ephedra californica]]'' cone]]
 
−
 
 
−
==Ecology and morphology==
 
−
Unlike most biological groupings, it is difficult to find many common characteristics between all of the members of the gnetophytes.<ref name="Arber and Parkin">{{cite journal | url = http://aob.oxfordjournals.org/content/volos-22/issue3/index.dtl  | title = Studies on the evolution of the angiosperms: the relationship of the angiosperms to the Gnetales |author1=Arber, E.A.N.  |author2=Parkin, J. | journal =  Annals of Botany | year = 1908 | volume = 22 | pages = 489–515 }}</ref> The two [[synapomorphy|common characteristics]] most commonly used are the presence of enveloping [[bract]]s around both the [[ovule]]s and [[microsporangia]] as well as a micropylar projection of the outer membrane of the ovule that produces a [[pollination]] droplet,<ref name="Judd">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.</ref> though these are highly specific compared to the similarities between most other plant divisions. L. M. Bowe refers to the gnetophyte genera as a "bizarre and enigmatic" trio<ref name="Bowe">{{cite journal | url = http://www.pnas.org/content/97/8/4092.full | title = Phylogeny of seed plants based on all three genomic compartments: Extant gymnosperms are monophyletic and Gnetales' closest relatives are conifers |author1=Bowe, L.M. |author2=Coat, G. |author3=dePamphilis, C.W. | journal = Proceedings of the National Academy of Sciences | year = 2000 | volume = 97 | pages = 4092–4097 | doi = 10.1073/pnas.97.8.4092 | issue=8 | pmid=10760278 | pmc = 18159 }}</ref> because, the gnetophytes' specialization to their respective environments is so complete that they hardly resemble each other at all. ''Gnetum'' species are mostly [[Woody plant|woody]] vines in tropical forests, though the best-known member of this group, ''[[Gnetum gnemon]]'', is a tree native to western [[Malesia]]. The one remaining species of ''Welwitschia'', ''[[Welwitschia mirabilis]]'', native only to the dry deserts of [[Namibia]] and [[Angola]], is a ground-hugging species with only two large strap-like leaves that grow continuously from the base throughout the plant's life. ''Ephedra'' species, known as "jointfirs" in the United States, have long slender branches which bear tiny scale-like leaves at their nodes. Infusions from these plants have been traditionally used as a [[stimulant]], but [[ephedrine]] is a [[controlled substance]] today in many places because of the risk of harmful or even fatal [[overdose|overdosing]].
 
−
 
 
−
==Fossil Gnetophyta==
 
−
Knowledge of gnetophyte history through fossil discovery has increased greatly since the 1980s.<ref name="crane"/> Gnetophyte fossils have been found that date from the [[Permian]]<ref>{{cite journal | url = http://aob.oxfordjournals.org/cgi/content/full/94/2/281 | title = A New Permian Gnetalean Cone as Fossil Evidence for Supporting Current Molecular Phylogeny | author = Zi-Qiang Wang | journal = Annals of Botany | year = 2004 | volume = 94 | issue = 2 | pages = 281–288 | doi = 10.1093/aob/mch138 | pmid = 15229124 }}</ref> and the [[Triassic]]. Fossils dating back to the [[Jurassic]] have been found, though whether or not they belong to the gnetophytes is uncertain.<ref name="rydin">{{cite journal | url = http://aob.oxfordjournals.org/cgi/content/full/98/1/123 | journal = Annals of Botany | year = 2006 | volume = 98 | issue = 1 | pages = 123–140 | doi = 10.1093/aob/mcl078 | title = Former Diversity of Ephedra (Gnetales): Evidence from Early Cretaceous Seeds from Portugal and North America |author1=Catarina Rydin |author2=Kaj Raunsgaard Pedersen |author3=Peter R. Crane |author4=Else Marie Friis | pmid = 16675607 | pmc = 2803531}}</ref> Overall, the fossil record is richest in the early [[Cretaceous]], with fossils of plants, [[seed]]s, and [[pollen]] have been found that can clearly be assigned to the gnetophytes.<ref name="rydin"/>
 
−
 
 
−
==Classification==
 
−
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 (biology)|class]] Gnetales, rather than the order.<ref name= "PearsonH">Pearson, H. H. W.  Gnetales. Cambridge University Press 1929. Reissued 2010. {{ISBN|978-1108013987}}</ref> 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.<ref name="GHML">Lawrence, George Hill Mathewson. Taxonomy of vascular plants. Macmillan, 1951</ref> Foster & Gifford accepted this principle, and placed the three orders together in a common class for convenience, which they called Gnetopsida.<ref name="FosGif">Foster, Adriance S., Gifford, Ernest M. Jr. Comparative Morphology of Vascular Plants  Freeman 1974. {{ISBN|0-7167-0712-8}}</ref> In general the evolutionary relationships among the [[spermatophyte|seed plants]] still are unresolved, and the Gnetophyta have played an important role in the formation of [[phylogenetics|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,<ref name="lee">{{cite journal |vauthors=Lee EK, Cibrian-Jaramillo A, Kolokotronis SO, Katari MS, Stamatakis A | year = 2011 | title = A Functional Phylogenomic View of the Seed Plants | url = | journal = PLoS Genet | volume = 7 | issue = 12| page = e1002411 | doi = 10.1371/journal.pgen.1002411 | pmid=22194700 | pmc=3240601|display-authors=etal}}</ref> contradicting the anthophyte hypothesis, which held that gnetophytes were sister to the flowering plants.
 
−
 
 
−
===Anthophyte hypothesis===
 
−
From the early twentieth century, the anthophyte hypothesis was the prevailing explanation for [[seed plant]] evolution, based on shared [[morphology (biology)|morphological]] characters between the gnetophytes and [[angiosperm]]s. In this hypothesis, the gnetophytes, along with the extinct order [[Bennettitales]], are sister to the angiosperms, forming the "anthophytes".<ref name="Judd" />  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.<ref name="Judd" /><ref name=Donaghue>{{cite journal | url = http://linkinghub.elsevier.com/retrieve/pii/S0960982200003043 | title = Seed plant phylogeny: demise of the anthophyte hypothesis? |author1=Donoghue, M.J.  |author2=Doyle, J.A. | journal = Current Biology | year = 2000 | volume = 10 | pages = R106–R109 | doi = 10.1016/S0960-9822(00)00304-3 | pmid = 10679315 | issue = 3}}</ref><ref name=Loconte>{{cite journal | url = http://www.springerlink.com/content/93518n8n2237k054/ | title = Cladistics of the Spermatophyta |author1=Loconte, H.  |author2=Stevenson, D.W. | journal = Brittonia | year = 1990 | volume = 42 | pages = 197–211 | doi = 10.2307/2807216 | jstor = 2807216 | issue = 3 }}</ref><ref name="Nixon">{{cite journal | title = A reevaluation of seed plant phylogeny |author1=Nixon, K.C. |author2=Crepet, W.L. |author3=Stevenson, D. |author4=Friis, E.M. | journal = Annals of the Missouri Botanical Garden | year = 1994 | volume = 81 | pages = 494–533 | doi = 10.2307/2399901 | issue = 3 | jstor=2399901}}</ref> However, most genetic studies have rejected the anthophyte hypothesis.<ref name="Bowe" /><ref name="Chaw">{{cite journal | url = http://mbe.oxfordjournals.org/cgi/content/abstract/14/1/56 | title = Molecular phylogeny of extant gymnosperms and seed plant evolution: analysis of nuclear 18S rRNA sequences |author1=Chaw, S.M. |author2=Aharkikh, A. |author3=Sung, H.M. |author4=Lau, T.C. |author5=Li, W.H. | journal = Molecular Biology and Evolution | year = 1997 | volume = 14 | pages = 56–68 | pmid = 9000754 | issue = 1 | doi=10.1093/oxfordjournals.molbev.a025702}}</ref><ref name = "Chaw2">{{cite journal | url = http://www.pnas.org/content/97/8/4086.abstract | title = Seed plant phylogeny inferred from all three plant genomes: Monophyly of extant gymnosperms and origin of Gnetales from conifers |author1=Chaw, S.M. |author2=Parkinson, C.L. |author3=Cheng, Y. |author4=Vincent, T.M. |author5=Palmer, J.D. | journal = Proceedings of the National Academy of Sciences USA | year = 2000 | volume = 97 | pages = 4086–4091 | doi = 10.1073/pnas.97.8.4086 | issue=8 | pmid=10760277 | pmc=18157 }}</ref><ref name = "Goremykin">{{cite journal | url = http://mbe.oxfordjournals.org/cgi/content/abstract/13/2/383 | title = Noncoding sequences from the slowly evolving chloroplast inverted repeat in addition to rbcL data do not support gnetalean affinities of angiosperms |author1=Goremykin, V. |author2=Bobrova, V. |author3=Pahnke, J. |author4=Troitsky, A. |author5=Antonov, A. |author6=Martin, W. | journal = Molecular Biology and Evolution | year = 1996 | volume = 13 | pages = 383–396 | pmid = 8587503 | issue = 2 | doi=10.1093/oxfordjournals.molbev.a025597}}</ref><ref name="Hajibabaei">{{cite journal | url = 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 | title = Seed plant phylogeny: Gnetophytes are derived conifers and a sister group to Pinaceae |author1=Hajibabaei, M. |author2=Xia, J. |author3=Drouin, G. | journal = Molecular Phylogenetics and Evolution | year = 2006 | volume = 40 | pages = 208–217 | doi = 10.1016/j.ympev.2006.03.006 | pmid = 16621615 | issue = 1}}</ref><ref name = "Hansen">{{cite journal | url = http://mbe.oxfordjournals.org/cgi/content/citation/16/7/1006 | title = Gnetum and the angiosperms: molecular evidence that their shared morphological characters are convergent rather than homologous |author1=Hansen, A. |author2=Hansmann, S. |author3=Samigullin, T. |author4=Antonov, A. |author5=Martin, W. | journal = Molecular Biology and Evolution | year = 1999 | volume = 16 | issue = 7 | pages = 1006–1009 | doi=10.1093/oxfordjournals.molbev.a026176}}</ref><ref name = "Magallon">{{cite journal | title = Relationships among seed plants inferred from highly conserved genes: sorting conflicting phylogenetic signals among ancient lineages |author1=Magallon, S.  |author2=Sanderson, M.J. | journal = American Journal of Botany | year = 2002 | volume = 89 | pages = 1991–2006 | doi = 10.3732/ajb.89.12.1991 | jstor=4122754 | issue = 12 | pmid = 21665628}}</ref><ref name ="Qui">{{cite journal | url = http://www.nature.com/nature/journal/v402/n6760/full/402404a0.html | title = The earliest angiosperms: evidence from mitochondrial, plastid and nuclear genomes | author = 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. | journal = Nature | year = 1999 | volume = 402 | pages = 404–407 | doi = 10.1038/46536 | pmid = 10586879 | issue = 6760}}</ref><ref name = "Samigullin">{{cite journal | url = http://www.springerlink.com/content/77e79hf2e7vv9u6e/ | title = Molecular data from the chloroplast rpoC1 gene suggest a deep and distinct dichotomy of contemporary spermatophytes into two monophyla: gymnosperms (including Gnetalaes) and angiosperms |author1=Samigullin, T.K. |author2=Martin, W.F. |author3=Troitsky, A.V. |author4=Antonov, A.S. | journal = Journal of Molecular Evolution | year = 1999 | volume = 49 | pages = 310–315 | doi = 10.1007/PL00006553 | pmid = 10473771 | issue = 3}}</ref><ref name = "Sanderson">{{cite journal | url = http://mbe.oxfordjournals.org/cgi/content/abstract/17/5/782 | title = Error, bias, and long-branch attraction in data for two chloroplast photosystem genes in seed plants |author1=Sanderson, M.J. |author2=Wojciechowski, M.F. |author3=Hu, J.M. |author4=Sher Khan, T. |author5=Brady, S.G. | journal = Molecular Biology and Evolution | year = 2000 | volume = 17 | pages = 782–797 | pmid = 10779539 | issue = 5 | doi=10.1093/oxfordjournals.molbev.a026357}}</ref> 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.<ref name="Bowe" /><ref name="Judd" /><ref name="Chaw2" />
 
−
 
 
−
{{clade |
 
−
  1=''[[Ginkgo]]'' |
 
−
  2=[[cycad]]s  |
 
−
  3=[[conifers]] |
 
−
  label4= anthophytes |
 
−
  4={{clade |
 
−
    1=[[angiosperm]]s (flowering plants)  |
 
−
    2=gnetophytes
 
−
  }}
 
−
}}
 
−
 
 
−
===Gnetifer hypothesis===
 
−
In the gnetifer hypothesis, the gnetophytes are sister to the [[conifer]]s, and the [[gymnosperm]]s are a [[monophyletic]] group, sister to the [[angiosperm]]s.  The gnetifer hypothesis first emerged formally in the mid-twentieth century, when vessel elements in the gnetophytes were interpreted as being derived from [[tracheid]]s with circular bordered pits, as in conifers.<ref name="Judd" /> It did not gain strong support, however, until the emergence of molecular data in the late 1990s.<ref name="Chaw" /><ref name ="Qui" /><ref name="Rydin">{{cite journal | title = Seed plant relationships and the systematic position of Gnetales based on nuclear and chloroplast DNA: conflicting data, rooting problems, and the monophyly of conifers |author1=Rydin, C. |author2=Kallersjo, M. |author3=Friist, E.M. | journal = International Journal of Plant Sciences | year = 2002 | volume = 163 | pages = 197–214 | doi = 10.1086/338321 | jstor=3080238 | issue = 2}}</ref><ref name = "Braukmann">{{cite journal | url = http://www.nature.com/nrg/journal/v2/n3/full/nrg0301_186a.html | title = Loss of all plastid nhd genes in Gnetales and conifers: extent and evolutionary significance for the seed plant phylogeny |author1=Braukmann, T.W.A. |author2=Kuzmina, M. |author3=Stefanovic, S. | journal = Current Genetics | year = 2009 | volume = 55 | pages = 323–337 | doi = 10.1007/s00294-009-0249-7 | pmid = 19449185 | issue = 3}}</ref> 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 [[leaf|leaves]] of ''[[Ephedra]]'' and ''[[Welwitschia]]''; and reduced [[sporophyll]]s.<ref name="Magallon" /><ref name="Sanderson" /><ref name = "Burleigh">{{cite journal | url = http://www.amjbot.org/cgi/content/abstract/91/10/1599 | title = Phylogenetic signal in nucleotide data from seed plants: implications for resolving the seed plant tree of life |author1=Burleigh, J.G.  |author2=Mathews, S. | journal = International Journal of Plant Science | year = 2007 | volume = 168 | pages = 125–135 | doi = 10.3732/ajb.91.10.1599 | issue = 10}}</ref>
 
−
 
 
−
{{clade |
 
−
  1=[[angiosperm]]s (flowering plants)|
 
−
  label2=[[gymnosperm]]s  |
 
−
  2={{clade |
 
−
    1=[[cycad]]s  |
 
−
    2=''[[Ginkgo]]'' |
 
−
    3={{clade |
 
−
        1=conifers |
 
−
        2=gnetophytes
 
−
    }}
 
−
  }}
 
−
}}
 
−
 
 
−
===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]].<ref name="Judd" /> According to this hypothesis, the conifers as currently defined are not a monophyletic group, in contrast with molecular findings that support its monophyly.<ref name="Rydin" />  All existing evidence for this hypothesis comes from molecular studies within the last decade.<ref name="Bowe" /><ref name="Gugerli" /><ref name="Chaw2" /><ref name="Hajibabaei" /><ref name="Magallon" /><ref name="Qui" /><ref name="Sanderson" /><ref name="Burleigh" /> 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 [[Conifer cone|cone]] scales.<ref name="Magallon"/> 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.<ref name="Chaw2" />
 
−
 
 
−
{{clade |
 
−
  1=[[angiosperm]]s (flowering plants)|
 
−
  label2=[[gymnosperm]]s  |
 
−
  2={{clade |
 
−
    1=[[cycad]]s  |
 
−
    2=''[[Ginkgo]]'' |
 
−
    label3=[[conifers]] |
 
−
    3={{clade |
 
−
      1={{clade |
 
−
        1=[[Pinaceae]] (the pine family) |
 
−
        2=[[gnetophytes]]
 
−
      }} |
 
−
      2=other [[conifers]]
 
−
    }}
 
−
  }}
 
−
}}
 
−
 
 
−
===Gnetophyte-sister hypothesis===
 
−
Some partitions of the genetic data suggest that the gnetophytes are sister to all of the other extant seed plant groups.<ref name = "Rai" /><ref name="Judd" /><ref name="Magallon" /><ref name="Sanderson" /><ref name="Rydin" /> However, there is no morphological evidence nor examples from the fossil record to support the gnetophyte-sister hypotheses.<ref name="Burleigh" />
 
−
 
 
−
{{clade |
 
−
  1=[[gnetophytes]]|
 
−
  2={{clade |
 
−
    1=[[angiosperms]] (flowering plants) |
 
−
    2={{clade |
 
−
      1=[[cycad]]s  |
 
−
      2=''[[Ginkgo]]'' |
 
−
      3=[[conifer]]s
 
−
    }}
 
−
  }}
 
−
}}
 
−
 
 
−
==References==
 
−
{{commons category|Gnetophyta}}
 
−
{{Reflist}}
 
−
'''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 ([http://www.bioone.org/perlserv/?request=get-abstract&doi=10.3159%2F1095-5674(2006)133%5B119%3APATBOS%5D2.0.CO%3B2&ct=1 abstract])
 
−
 
 
−
{{Plant classification}}
 
−
 
 
−
[[Category:Gnetophyta| ]]
 
−
[[Category:Plant divisions]]
 
−
[[Category:Extant Permian first appearances]]
 

Latest revision as of 14:42, 9 September 2017

Redirect to:

Retrieved from "https://eateveryplant.org/wiki/index.php?title=Gnetophyta&oldid=3966"