DIVERSITY OF LIFE

DESCRIPTION OF THE PHYLUM GNETOPHYTA (BESSEY 1907)

EUKARYA>ARCHAEPLASTIDA>VIRIDIPLANTAE>STREPTOBIONTA>EMBRYOPHYTA>TRACHEOPHYTA>SPERMOPHYTA>GNETOPHYTA

Gnetophyta (ne-TA-fa-ta) is derived from the Moluccan Malay word Gamemu and plant (phyto -φυτο).  The reference is to Gamemu (the Moluccan word for Gnetum).

 

INTRODUCTION TO THE GNETOPHYTA

The gnetophytes are reduced to three far-flung and remarkably different genera (each genus in its own order).  They have very few vegetative synapomorphies except decussate leaves.  However, the similarities in reproductive structures serve to unite the taxa (see the description below).  All are dioecious and both the ovulate and staminate strobili are compound.  They have distinctive ovules with elongate tubular micropyles and extra integuments.

Ephedra (Figure 1), also known as Mormon Tea, is a native to desert areas of the North American West.  These plants are highly branched, green-stemmed shrubs with small decussate leaves, which appear to occur in whorls. The compound strobili usually emerge from the axils of leaves.  Staminate plants have strobili that have been reduced to single stamens together in a compound strobilus.  The development of pollen is similar to that of Pinus.  Pollination is effected in a way similar to that of Pinus as well (it exudes a droplet that catches the pollen and pulls them back through the elongate micropyle (Figure 2).  The two sperm may participate in the life history differently.  Friedman (1990a, 1990b, 1992, 1994) reported that while one sperm fertilizes an egg, the other fuses with gametophyte tissue, a process called double fertilization.

Gnetum (Figure 3) species usually grow as vines (rarely shrubs or trees).  Their leaves are opposite and pinnately net-veined, which gives them the vegetative appearance of a flowering plant. Their compound strobili are spike-like, and they emerge from the axils of united scale-like bracts.

Welwitschia (Figure 4) is one of the strangest plants on earth.  [One of my former Botany professors said, with his tongue planted squarely in his cheek, that it appeared to have evolved to be pollinated by elephants.]  It occurs only in the Namibian desert, one of the driest places on earth.  It has a large tap root and a short stem from which emerge two broad, continuously growing, strap-like, opposite leaves with parallel veins. Compound strobili are cone-like and grow from the axils of bracts.  The staminate strobilus seems to have evolved as a reduction from a bisexual strobilus.  Each simple strobilus within the compound structure has a rudimentary ovule.

The similarities with flowering plants include the occurrence of vessels in the xylem, double fertilization, net-veining in the leaves of Gnetum, and dicotyledonous embryos.  Despite this, other compelling evidence, mainly molecular, places the Gnetophyta as a sister group to the Pinales within the Coniferophyta (e.g. Doyle 2006).


FIGURE 1


FIGURE 3


FIGURE 4

 

FIGURE 2. The life history of Ephedra.  The axis on the right has a compound microstrobilus, whose microsporangia release pollen.  The axis on the left is a compound megastrobilus that illustrates the development of the megagametophyte, each with more than one archegonium.  Pollen are caught at the tip of the elongate micropyle and pulled into the pollen chamber where the microgametophyte germinates and grows the pollen tube into the archegonial chamber, where, in Ephedra as in the flowering plants, double fertilization occurs.  M= meiosis.  S= syngamy.

From Cocucci and Hunziker (1976)

 

 

SYSTEMATICS OF THE GNETOPHYTA

Like the flowering plants, they have vessels in the wood.  Also, they have dicot-like leaves that are decussate.  These features seem to point to a connection with the flowering plants.  

The nature of the ovule and the apparent occurrence of double fertilization in Ephedra caused Friedman (1990a, 1990b, 1992, 1994) to consider the gnetophytes as a sister group to the flowering plants.  Systems based on anatomy and fossil evidence (e.g. Doyle 2006, Hilton and Bateman 2006, and Tomescu 2008) follow this view which is termed the anthophyte hypothesis.  However, molecular taxonomies of the seed-bearing plants paint a very different picture.  Chaw et al (2000) and Bowe et al. (2000) both compared sequences from the nucleus, the mitochondrion, and the chloroplast.  They consistently point to the Pinaceae as the sister group to the gnetophytes.  If interpreted strictly, the gnetophytes would be a group of conifers.  This Gne-Pine hypothesis has been supported by other molecular studies (e.g. Soltis et al. 2002, Matthews 2009).  Friedman and Floyd (2001) discuss the problem of the two phylogenies and the relative importance of morphological and molecular evidence.   The situation with the gnetophytes has been further confused by other molecular studies which point to an association with the other conifers (Cupressopsida), which is called the Gne-Cup hypothesis (e.g. Zhong et al. 2010, Zhong et al. 2011).  Other molecular studies have the gnetophytes emerge as sisters to the conifers (e.g. Ran et al. 2010) and sisters to all other living seed-bearing plants (Rai et al. 2008).  

It appears to us that the most conservative position would be to accept that the gnetophytes are sisters to the conifers as shown in Figure 5.  Because of the uncertainty, we follow the classical view of Bold et al. (1987) with 1 class which has 3 orders.

 

 

FIGURE 5. The relationships between spermophytes (seed plants) is an integration of molecular studies (Chaw et al. 2000, Soltis et al. 2002, Matthews 2009, Zhong et al. 2010, Zhong et al. 2011,  Ran et al. 2010, Rai et al. 2008), anatomy and fossil evidence (Doyle 2006, Hilton and Bateman 2006, and Tomescu 2008).  In this cladogram, the gnetophytes are sisters to the conifers.

 

 
FURTHER READING:

INTRODUCTION TO THE DOMAIN EUKARYA

 

LITERATURE CITED

Banks, H. P. 1975. Reclassification of Psilophyta. Taxon. 24: 401-413.

Bierhorst, D. W. 1971. Morphology of Vascular Plants. In: N. H. Giles and J. G. Torrey. The MacMillan Biology Series. The MacMillan Co. New York.

Bold, H. C., C. J. Alexopoulos, and T. Delevoryas. 1987. Morphology of Plants and Fungi. 5th Edition. HarperCollins Publishers, Inc. New York. 

Cantino, P., J. A. Doyle, S. W. Graham, W. S. Judd, R. G. Olmstead, D. E. Soltis, P. S. Soltis, and M. J. Donoghue. 2007. Towards a phylogenetic nomenclature of Tracheophyta. Taxon 56(3): E1-E44.  

Chaw S.-M., C. L. Parkinson, Y. Cheng, T. M. Vincent, and J. D. Palmer. 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 (USA) 97:4086-4086. 

Cocucci, A. and A. Hunziker. 1976. Los ciclos biologicos en el Reino Vegetal. Ed. Academia Nacional de Ciencias. Cordoba.Doyle, J. A. 2005. Seed ferns and the origin of angiosperms. Journal of the Torrey Botanical society. 133:  169-209. 

Crane, P.. 1996. Spermatopsida. Seed Plants. Version 01 January 1996 (temporary). http://tolweb.org/Spermatopsida/20622/1996.01.01 in The Tree of Life Web Project, http://tolweb.org/

Dittmer, H. J. 1964. Phylogeny and Form in the Plant Kingdom.  Van Norstrand Company, Inc. New York.

Doyle, J. A. 1998b. Phylogeny of vascular plants. Annual Review of Ecology and Systematics. 29:567-599.

Doyle, J. A. 2006. Seed ferns and the origin of angiosperms. Journal of the Torrey Botanical Society. 133(1):  169-209. [C]

Friedman, W. E. 1990a. Double fertilization in Ephedra, a nonflowering seed plant: its bearing on the origin of angiosperms. Science 247:951-954. 

Friedman, W. E. 1990b. Sexual reproduction in Ephedra nevadensis (Ephedraceae): further evidence of double fertilization in a nonflowering seed plant. American Journal of Botany. 77:1582-1598.

Friedman, W. E. 1992. Evidence of a pre-angiosperm origin of endosperm: implications for the evolution of flowering plants. Science 225:336-339.

Friedman, W. E., R. C. Moore, and M. D. Purugganan. 2004. The evolution of plant development. American Journal of Botany 91: 1726-1741.

Friedman, W. E. 1994. The evolution of embryogeny in seed plants and the developmenal origin and early history of endosperm. American Journal of Botany. 81:109:153-226.

Friedman, W. E. and S. K. Floyd. 2001. Perspective: The origin of flowering plants and their reproductive biology - a tale of two phylogenies. Evolution 55:217-231.

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Kenrick, P. and P. R. Crane. 1997b. The Origin and Early Diversification of Land Plants: A Cladistic Study. Smithsonian Institute Press. Washington, DC.

Matthews, S. 2009. Phylogenetic relationships among seed plants: persistent questions and the limits of molecular data. American Journal of Botany. 96(1): 228-236.

Northington, D. K. and J. R. Goodin. 1984. The Botanical World. Times Mirror/Mosby College Publishing, St. Louis.

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Rai, H. S., P. A. Reeves, R. Peakall, R. G. Olmstead, and S. W. Graham. 2008. Inference of higher-order conifer relationships from multi-locus plastid data set. Botany. 86:658-669.

Ran, J-H., H. Gao, X-Q. Wang. 2010. Fast evolution of the retroprocessed mitochondrial rps3 gene in Conifer II and further evidence for the phylogeny of gymnosperms. Molecular Phylogenetics and Evolution. 54: 136-149.

Soltis, D. E., P. S. Soltis, and M. J. Zanis. 2002. Phylogeny of seed plants based on evidence from eight genes. American Journal of Botany. 89:1670-1681.

Tomescu, A. M. F. 2008. Megaphylls, microphylls and the evolution of leaf development. Trends in Plant Science. 14(1): 5-12

Zgurski, J. M., H. S. Rai, Q. M. Fai, D. J. Bogler, and J. Francisco-Ortega. 2008. How well do we understand the overall backbone of cycad phylogeny? New insights from a large, multigene plastid data set. Molecular Phylogenetics and Evolution. 47: 1232-1237.

Zhong, B., T. Yonezawa, Y. Zhong, and M. Hasegawa. 2010. The position of Gnetales among seed plants: overcoming pitfalls of chloroplast phylogenomics. Molecular Biology and Evolution. 27(12): 2855-2863.

Zhong, B., O. Deusch, V. V. Goremykin, D. Penny, P. J. Biggs, R. A. Atherton, S. V. Nikiforova, and P. J. Lockhart. 2011. Systematic error in seed plant phylogenomics. Genome Biology and Evolution. 3: 1340-1348.

 

By Jack R. Holt.  Last revised: 11/20/2020