Gnetophyta consists of only three genera of woody plants grouped in the
gymnosperms. The living gnetophytes are a handful of survivors of a group that
was diverse and dominant in the Tertiary, and fossilized pollen
attributed to a close relative of one of them, Ephedra, has been identified in
the Early Cretaceous. They now comprise some 70 species across the three relict
genera Gnetum, Welwitschia, and Ephedra. The gnetophytes differ from other
gymnosperms in having vessel elements like those found in flowering plants. In
the past, the Gnetophyta were thought to be the gymnosperms' closest relations to
the flowering plants; however, molecular studies have largely contradicted this
hypothesis. Among themselves, the gnetophytes' evolutionary
inter-relationships remain unclear, however: in some classifications, all
three genera are placed in a single order, 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.
Ecology and morphology The three genera of the gnetophytes are
highly specialized to their respective environments, making it difficult to
identify homologous characters. The three extant genera of gnetophytes, a
"bizarre and enigmatic" trio, are likely aberrant members of the group. 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.
Gnetum species are mostly woody climbers in tropical forests. However, the
best-known member of this group, Gnetum gnemon, is a tree. In western Malesia,
where it is native, its seeds are used to produce a crispy cracker snack known
as emping or krupuk belinjo. Welwitschia comprises only one species,
Welwitschia mirabilis. It grows only in the extremely dry deserts of Namibia and
Angola. This strange ground-hugging 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" in the western United States
because they have long slender branches which bear tiny scale-like leaves at
their nodes. Infusions from Ephedra have been traditionally used as a stimulant,
but ephedrine is a controlled substance today in many jurisdictions because of
the risk of harmful or even fatal overdosing.
Fossil Gnetophyta Knowledge of fossils of the gnetophytes
has increased greatly since the 1980s. There are fossils from the Permian, the
Triassic, and the Jurassic which may belong to the gnetophytes, but this is
uncertain. 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. 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
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. 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
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 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. 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.
= 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, lignin chemistry, the layering
of cells in the apical meristem, pollen and megaspore features, short cambial
initials, and lignin syringal groups. However, most genetic studies 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.
= 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 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. It did not gain strong support, however, until the emergence of
molecular data in the late 1990s. 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.
= 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 within the last decade. 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: narrowly triangular
leaves, resin canals, a tiered proembryo, and flat woody ovuliferous
cone scales. 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. = 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. References
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