Gnetales

Abstract

The Gnetales (Ephedra, Gnetum and Welwitschia) represent a fascinating group of nonflowering seed plants (gymnosperms). This group of plants has intrigued plant biologists for well over a century due, in part, to their interesting growth habits, medicinal properties, unusual reproductive features, and status a potential link between flowering and other nonflowering seed plants. Indeed, the reproductive biology of Gnetales has increased our understanding of the origin of angiosperms including the process of double fertilisation. The phylogenetic position of Gnetales has been highly debated over the years. Early studies, based largely on structural data, concluded that the Gnetales are monopyletic and represent the sister group to angiosperms (‘anthophyte’ hypothesis). More recent studies provide evidence that the Gnetales represent the sister group to conifers (‘Gnetifer’ hypothesis) or are embedded within conifers and represent the sister group to Pinaceae (‘Gnepine’ hypothesis). Regardless of their phylogenetic position, various structural features of Gnetales have a strong bearing on our understanding of evolutionary trends in seed plants.

Key Concepts:

  • Members of the Gnetales are gymnosperms that possess some angiosperm‐like features.

  • A modified form of double fertilisation occurs in Gnetales.

  • Ephedra is a shrub that produces ephedrine, a stimulant and decongestant found in certain weight‐loss and energy‐boosting products.

  • Welwitschia is endemic to the Namib desert in Africa and is considered by many to be one of the most bizarre plants on Earth.

Keywords: Gnetum; Ephedra; Welwitschia; Gnetophyta; Anthophyte; Gnetifer; Gnetpine

Figure 1.

The three genera of Gnetales. (a) Ephedra grows as a bushy shrub in the southwestern United States and into South America. (b) A species of Gnetum showing the vine‐like growth habit common to this genus. (c) A young specimen of Welwitschia mirabilis grown under greenhouse conditions.

Figure 2.

Female gametophyte development in Ephedra. A diploid megasporocyte undergoes meiosis to produce four haploid megaspores, three of which degenerate. The single surviving megaspore develops into a relatively large female gametophyte. The mature female gametophyte contains binucleate egg cells that are fertilised by binucleate sperm. The fusion of two sperm nuclei with two egg nuclei forms the basis of double fertilisation in Ephedra.

Figure 3.

A comparison of hypothesised phylogenetic relationships between Gnetophytes and other groups of seed plants.

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References

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Further Reading

Chaw SM, Parkinson CL, Cheng Y, Vincent TM and Palmer JD (2000) Seed plant phylogeny inferred from all three plant genomes: monophyly of extant gymnosperms and origin of Gnetales from conifers. Proceedings of the National Academy of Sciences of the USA 97: 4086–4091.

Donoghue MJ (2000) Seed plant phylogeny: demise of the anthophyte hypothesis? Current Biology 10: 106–109.

Friedman WE (1990) Double fertilization in Ephedra, a nonflowering seed plant: its bearing on the origin of angiosperms. Science 247: 951–954.

Gifford EM and Foster AS (1989) Gnetophyta: In: Comparative Morphology of Vascular Plants, pp. 455–483. San Francisco, CA: W.H. Freeman.

Friedman WE (ed.) (1996) Supplement: Biology and Evolution of the Gnetales. International Journal of Plant Sciences 157 (6, supplement): S1–S125.

Mathews S (2009) Phylogenetic relationships among seed plants: persistent questions and the limits of molecular data. American Journal of Botany 96: 228–236.

Price RA (1996) Systematics of the Gnetales: a review of morphological and molecular evidence. International Journal of Plant Sciences 157: S40–S49.

Rydin C and Friis EM (2010) A new Early Cretaceous relative of Gnetales: Siphonospermum simplex gen. et sp. nov. from the Yixian Formation of Northeast China. BMC Evolutionary Biology 10: 183.

Rydin C, Pedersen KR and Friis EM (2004) On the evolutionary history of Ephedra: cretaceous fossils and extant molecules. Proceedings of the National Academy of Sciences 101: 16571–16576.

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How to Cite close
Carmichael, Jeffrey(Apr 2012) Gnetales. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003681.pub2]