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Gnetum

Linnaeus 1767

Common names

Gnetophytes; 买麻藤科 mai ma teng ke [Chinese].

Taxonomic notes

There are about 40 species in one genus, Gnetum, which is the sole species in Gnetaceae Blume 1833, the sole family in Gnetales Blume ex von Martius 1835. The genus has been subdivided by Markgraf (1930, in Carlquist 1996b) as follows:

   
Section Subsection Species
GnetumGnetumG. gnemon and one other Indomalesian tree species
MicrognemonesTwo species of African lianas
AraeognemonesNine species of Neotropical lianas
Cylindrostachys About 19 species of Asian and Indomalesian lianas, incl. G. arboreum, G. contractum, G. cuspidatum, G. diminutum, G. gnemonoides, G.hainanense, G. klossii, G. latifolium, G. leptostachyum, G. loerzingii, G.macrostachyum, G. microcarpum, G. montanum, G. neglectum, G. oxycarpum, G.ridleyi, G. ula, and two other species.
   

In this treatment, there are descriptions for six species:

Treatments have not yet been prepared for the following species, discussed in the "Distribution and Ecology" and "Remarks" sections on this page:

Molecular studies continue to improve our understanding of Gnetum. Won and Renner (2006) apply molecular clock analyses to a large data set including the Asian, African and American clades and evaluating genetic material from the chloroplast, mitochondria, and nucleus. They find that the current clades of Gnetum began diverging in late Oligocene to mid-Miocene time, with most species in the large Asian clade appearing in Miocene-Pliocene time. This timing is much later than the break-up of Gondwana and is in fact very late in the history of this ancient group, which may have been around for 250 million years or longer (Wang 2004). The surviving species seem to be a relict group that happens to have done well during the vast climate changes that have swept the globe since the late Oligocene.

Won and Renner (2003) revealed the startling news of horizontal gene transfer of mitochondrial DNA from Petunia to Gnetum. This was the first known instance of horizontal gene transfer between higher plants; it only affects an Asian clade of Gnetum; and molecular clock reasoning suggests that it happened only about 2 to 5 million years ago. The mechanism of such horizontal gene transfer remains unknown, but Won and Renner (2003) state that "agents that have been implied are viruses, bacteria, fungi, and plant cell-piercing insects," all of which have been implicated in other instances of horizontal gene transfer.

Description

Dioecious, evergreen, mostly woody vines, rarely shrubs or trees; stems with swollen nodes. Leaves opposite, petiolate, without stipules, simple, elliptic, with pinnate veins and entire margins; usually with drip tips. Both male and female megastrobili terminal or lateral, sometimes arranged in dense, cauliflorous clusters on old stems. Each megastrobilus consists of a straight axis above a basal pair of opposite, connate bracts; the axis bears usually three to six superposed cupules, each of which contains several to many male or female strobili. The male strobilus consists of a stamen and perianth, the female strobilus of an ovule with 2 integuments and perianth. These structures are usually associated with angiosperms, one of the points that traditionally places Gnetum in an ambiguous state intermediate between the gymnosperms and angiosperms. Seeds drupelike, enclosed in a red, orange, or yellow, fleshy (rarely corky) false seed coat; female gametophyte tissue copious, succulent. Cotyledons 2. Germination epigeal. The wood contain tracheids and is otherwise typical of gymnosperms, but also contains vessels, another angiosperm trait, thought in this case to have arisen independently, without phylogenetic significance (Vidakovic 1991, Carlquist 1996b, Fu et al. 1999).

Distribution and Ecology

Indomalaysia, tropical parts of West Africa, Fiji and the northern regions of South America (Schultes and Raffauf 1990).

The following species are found in Malesia or SE Asia (Carlquist 1996b): Gnetum costatum, G. gnemonoides Brongniart, G. klossii Merrill, G. latifolium Blume, G. leptostachyum Blume, G. loerzingii Markgraf, G. macrostachyum Hooker f., G. microcarpum Blume, Gnetum montanum Markgraf, and Gnetum ula Brongniart.

Gnetum arboreum Foxworthy is restricted to a small portion of Luzon, Philippines (Markgraf 1930, in Carlquist 1996b).

Gnetum contractum Markgraf is found in Travancore, India (Markgraf 1930, in Carlquist 1996b).

Gnetum diminutum Markgraf is restricted to a small portion of Sarawak, Malaysia (Markgraf 1930,in Carlquist 1996b).

Gnetum hainanense Cheng is found in China: Hainan.

Gnetum neglectum Blume is restricted to a small area in E Borneo (Markgraf 1930, in Carlquist 1996b).

Gnetum oxycarpum Ridley is restricted to the Mentawi Islands near Sumatra (Markgraf 1930, in Carlquist 1996b).

Gnetum ridleyi Gamble is restricted to a small area in Pahang, Malaysia (Markgraf 1930, in Carlquist 1996b).

Big tree

Oldest

Dendrochronology

There is no record of any work being done with G. gnemon, and as the remaining species of the genus are all woody vines, they are presumably not usable in dendrochronology.

Ethnobotany

Ethnopharmacological studies have found C-glycosyl-flavones; and a group of complex stilbenes and stilbene-substituted benzofurans. Gnetol, a hydroxylated stilbene, occurs in G. ula. "Lignans are of the guaiacyl-syringyl-type; the antiasthmatic properties for which G. parviflorum is used in Chinese medicine have been ascribed to the presence of demethylcoclaurine hydrochloride" (Schultes and Raffauf 1990).

Observations

Remarks

As with Ephedra and Welwitschia, Gnetum is pollinated by insects. Several features suggest this: pollen is not produced in large volumes; both male and female strobili are aromatic (the odor varying with species from sweetish to putrid); and strobili typically open in the morning, or in some species in the evening. The female strobilus bears a droplet of sugary, sticky fluid that in time retracts into the strobilus, carrying any captured pollen grains to the nucellus. This unusual mechanism works best in high humidity settings and has been suggested as one reason why Gnetum is restricted to rainforest habitats. Moths (Pyralidae and Geometridae) and small flies (Lauxaniidae) have been observed visiting evening-opening species and are likely among the agents of pollination (Biswas and Johri 1997).

"[L]ocating and collecting tropical lianoid Gnetum species is difficult because the fertile portions necessary to identify them as species of Gnetum often occur in the high forest canopy, whereas the more easily accessible stems visible at ground level cannot be identified for certain as stems of Gnetum rather than as any of numerous other lianas" (Carlquist 1996b).

Gnetum leptostachyum Blume and Gnetum montanum Markgraf are listed as threatened in Viet Nam by the World Conservation Monitoring Centre.

Gnetum africanum Welwitsch: See Shiembo et al. 1996.

Gnetum ula Brongniart: See Augustine and D'Souza (1997).

"At Mt. Bosavi a protected area is proposed, occupying and sharing areas of the Southern [Highlands], Western and Gulf Provinces [of Papua New Guinea]. The mountain is an isolated volcano south of the Central Divide. The area has locally endemic plant species, e.g. the only known site of an endemic Gnetum species. There are also many bird species. The area should extend down to lowland forest to preserve areas dominated by Vatica massak and other lowland species typical for this region" (Filer 1991).

For the following species, I know nothing but their name: Gnetum buchholzianum, Gnetum cuspidatum Blume, Gnetum oblongum, Gnetum schwackeanum, Gnetum tenuifolium, Gnetum venosum.

Citations

Augustine, A. C. and L. D'Souza. 1997. Somatic embryogenesis in Gnetum ula Brongn. (Gnetum edule) (Willd) Blume. Plant cell reports 16(5): 354.

Biswas, C. and B. M. Johri. 1997. The Gymnosperms. New Delhi: Narosa.

Blume, C.L. 1833. Nov. Pl. Expos., p. 23.

Linnaeus, C. 1767. Systema naturae, Tom. XII. Pars II. Editio duodecima, reformata. Holmiae. (Laurentii Salvii). P. 637.

Martius, C.F.P. von. 1835. Conspectus Regni Vegetabilis: Secundum Characteres Morphologicos Praesertim Carpicos in Classes Ordines et Famillas Digesti, p. 11.

Shiembo, P.N., A.C. Newton and R.R.B. Leakey. 1996. Vegetative propagation of Gnetum africanum Welw., a leafy vegetable from West Africa. The journal of horticultural science 71(1):149.

Wang, Z.-Q. 2004. A new Permian gnetalean cone as fossil evidence for supporting current molecular phylogeny. Ann. Bot. 94: 281-288.

Won, Hyosig, and S.S. Renner. 2003. Horizontal gene transfer from flowering plant to Gnetum. Proceedings of the National Academy of Science 100(19):10824-10829. Available www.umsl.edu/~renners/Won%20%26%20RennerLGT.PNAS2003.pdf, accessed 2010.02.11.

Won, Hyosig, and S.S. Renner. 2006. Dating dispersal and radiation in the gymnosperm Gnetum (Gnetales) – clock calibration when outgroup relationships are uncertain. Systematic Biology 55(4):610-622.

See also

The Gnetales page of Sherwin Carlquist. Interesting text and several photos of Gnetum cymes.

Carlquist 1996a.

Friedman, William E. and Jeffrey S. Carmichael. 1998. Heterochrony anddevelopmental innovation: evolution of female gametophyte ontogeny in Gnetum, a highly apomorphic seed plant. Evolution 52(4):1016.

Maheshwari and Vasil 1961.

Last Modified 2012-11-23