Gnetophyta () is a division of plants (alternatively considered the subclass Gnetidae or order Gnetales), grouped within the (which also includes , , and ), that consists of some 70 species across the three relict genus: Gnetum (family Gnetaceae), Welwitschia (family Welwitschiaceae), and Ephedra (family Ephedraceae). The earliest unambiguous records of the group date to the Jurassic, and they achieved their highest diversity during the Early Cretaceous. The primary difference between gnetophytes and other is the presence of , a system of small tubes (xylem) that transport water within the plant, similar to those found in . Because of this, gnetophytes were once thought to be the closest gymnosperm relatives to flowering plants, but more recent molecular studies have brought this hypothesis into question, with many recent phylogenies finding them to be nested within the conifers.
Though it is clear they are all related, the exact evolutionary inter-relationships between gnetophytes are unclear. Some classifications hold that all three genera should be placed in a single 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.
Ecology and morphology
Unlike most biological groupings, it is difficult to find many common characteristics between all of the members of the gnetophytes.
The two
synapomorphy most commonly used are the presence of enveloping
around both the
and
microsporangia as well as a micropylar projection of the outer membrane of the ovule that produces a
pollination droplet,
[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.] 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
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 vines in tropical forests, though the best-known member of this group,
Gnetum gnemon,
[Wan T, Liu Z M, Li L F, et al. A genome for gnetophytes and early evolution of seed plantsJ. Nature plants, 2018, 4(2): 82.
] 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.
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, with Pearson in the early 20th century discussing about the class Gnetales, rather than the order.
[
] 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.
[Lawrence, George Hill Mathewson. Taxonomy of vascular plants. Macmillan, 1951] Foster & Gifford accepted this principle, and placed the three orders together in a common class for convenience, which they called Gnetopsida.
[
] In general the evolutionary relationships among the
spermatophyte still are unresolved, and the Gnetophyta have played an important role in the formation of
phylogenetics 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
Glossopteridales. 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. Some morphological studies have supported a close relationship between Gnetophyta,
Bennettitales and the
Erdtmanithecales.
[
]
Recent research by Lee, Cibrian-Jaramillo, et al. (2011) suggests that the Gnetophyta are a sister group to the rest of the gymnosperms,[
] contradicting the anthophyte hypothesis, which held that gnetophytes were sister to the flowering plants.
Gnetifer hypothesis
In the gnetifer hypothesis, the gnetophytes are sister to the
, and the
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
with circular bordered pits, as in conifers.
[ It however only gained strong support with the emergence of molecular data in the late 1990s.][
]
[
]
[
]
Although the most salient morphological evidence still largely supports the anthophyte hypothesis, some more obscure morphological commonalities between the gnetophytes and conifers 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 of Ephedra and Welwitschia; and reduced .[
]
[
]
[
]
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".[ 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.][
]
[
]
[
]
However, most genetic studies, as well as more recent morphological analyses,[
]
have rejected the anthophyte hypothesis.[
]
[
]
[
]
[
][
]
[
]
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.
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.[ According to this hypothesis, the conifers as currently defined are not a monophyletic group, in contrast with molecular findings that support its monophyly.][ All existing evidence for this hypothesis comes from molecular studies since 1999.][ while a 2021 study placed it earlier, in the Carboniferous.][
]
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 two conifer lineages): narrowly triangular leaves (gnetophytes have diverse leaf shapes), resin canals, a tiered proembryo, and flat woody ovuliferous Conifer cone scales.[ These kinds of major morphological changes are not without precedent in the Pinales, 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.][
]
Gnetophyte-sister hypothesis
Some partitions of the genetic data suggest that the gnetophytes are sister to all of the other extant seed plant groups.[
]
[
]
However, there is no morphological evidence nor examples from the fossil record to support the gnetophyte-sister hypotheses.[
]
Fossil gnetophytes
Knowledge of gnetophyte history through fossil discovery has increased greatly since the 1980s.
Ephedraceae
-
Leongathia V.A. Krassilov, D.L. Dilcher & J.G. Douglas 1998
-
Jianchangia Yang, Wang and Ferguson, 2020
-
Eamesia Yang, Lin and Ferguson, 2018
-
Prognetella Krassilov et Bugdaeva, 1999 Yixian Formation, China, Early Cretaceous (Aptian) (initially interpreted as an angiosperm)
-
Chengia Yang, Lin & Wang, 2013,
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Chaoyangia Duan, 1998 Yixian Formation, China, Early Cretaceous (Aptian)
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Eragrosites Yixian Formation, China, Early Cretaceous (Aptian)
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Gurvanella China, Mongolia, Early Cretaceous
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Alloephedra China, Early Cretaceous
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Amphiephedra China, Early Cretaceous
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Beipiaoa China, Early Cretaceous
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Ephedrispermum Portugal, Early Cretaceous (Aptian-Albian)
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Ephedrites China, Early Cretaceous
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Erenia China, Mongolia, Early Cretaceous
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Liaoxia China, Early Cretaceous
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Dichoephedra China, Early Cretaceous
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Laiyangia P.H. Jin, 2024
Gnetaceae
-
Khitania Guo et al. 2009
Welwitschiaceae
-
Priscowelwitschia Dilcher et al., 2005 Crato Formation, Brazil, Early Cretaceous (Aptian)
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Cratonia Rydin et al., 2003 Crato Formation, Brazil, Early Cretaceous (Aptian)
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Welwitschiostrobus Dilcher et al., 2005 Crato Formation, Brazil, Early Cretaceous (Aptian)
Incertae sedis:
-
Archangelskyoxylon Brea, Gnaedinger & Martínez, 2023 Roca Blanca Formation, Argentina, Sinemurian–Toarcian (closely related to Weltwitschia and Gnetum).
-
Drewria Crane & Upchurch, 1987 Potomac Group, USA, Albian (possible affinities to Welwitschiaceae)
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Bicatia Friis, Pedersen and Crane, 2014
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Liaoningia Yang et al, 2017
-
Protognetum Y. Yang, L. Xie et D.K. Ferguson, 2017
-
Itajuba Ricardi-Branco et al, 2013,
-
Protoephedrites Rothwell et Stockey, 2013
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Siphonospermum Rydin et Friis, 2010
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Welwitschiophyllum Dilcher et al., 2005 Crato Formation, Brazil, Early Cretaceous (Aptian), Akrabou Formation, Morocco, Late Cretaceous (Cenomanian-Turonian) (Initially interpreted as a member of Welwitschiaceae, later considered uncertain).
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Dayvaultia Manchester et al. 2021
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Daohugoucladus Yang et al. 2023
Possible gnetophytes (not confirmed as members of the group)
-
Archaestrobilus Trujillo Formation, Texas, United States, Upper Triassic
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Dechellyia- Masculostrobus Mongolia, Early Cretaceous (Aptian-Albian)
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Dinophyton Chinle Formation, United States, Upper Triassic
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Nataligma Molteno Formation, South Africa, Upper Triassic (Carnian)
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Palaeognetaleana Wang, 2004,
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Sanmiguelia United States, Late Triassic-Early Jurassic
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Eoantha Russia, Early Cretaceous
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Bassitheca Morrison Formation, USA, Late Jurassic (Tithonian)
Other Sources:
