Gymnosperm

Gymnosperm
Temporal range: 370–0 Ma

Devonian - Recent

Various gymnosperms.
Scientific classification
Kingdom: Plantae
Subkingdom: Embryophyta
(unranked): Gymnospermae (paraphyletic)
Divisions

Pinophyta (or Coniferophyta) - Conifers
Ginkgophyta - Ginkgo
Cycadophyta - Cycads
Gnetophyta - Gnetum, Ephedra, Welwitschia

Encephalartos sclavoi cone, about 30 cm long

The gymnosperms are a group of seed-producing plants that includes conifers, cycads, Ginkgo, and Gnetales. The term "gymnosperm" comes from the Greek composite word γυμνόσπερμος (γυμνός gymnos, "naked" and σπέρμα sperma, "seed"), meaning "naked seeds", after the unenclosed condition of their seeds (called ovules in their unfertilized state). Their naked condition stands in contrast to the seeds and ovules of flowering plants (angiosperms), which are enclosed within an ovary. Gymnosperm seeds develop either on the surface of scales or leaves, often modified to form cones, or at the end of short stalks as in Ginkgo.

The gymnosperms and angiosperms together compose the spermatophytes or seed plants. By far the largest group of living gymnosperms are the conifers (pines, cypresses, and relatives), followed by cycads, gnetophytes (Gnetum, Ephedra and Welwitschia), and Ginkgo (a single living species).

Classification

Further information: Spermatophyte

In early classification schemes, the gymnosperms (Gymnospermae) were regarded as a "natural" group. There is conflicting evidence on the question of whether the living gymnosperms form a clade.[1][2] The fossil record of gymnosperms includes many distinctive taxa that do not belong to the four modern groups, including seed-bearing trees that have a somewhat fern-like vegetative morphology (the so-called "seed ferns" or pteridosperms.)[3] When fossil gymnosperms such as Bennettitales, Caytonia and the glossopterids are considered, it is clear that angiosperms are nested within a larger gymnosperm clade, although which group of gymnosperms is their closest relative remains unclear.

For the most recent classification on extant gymnosperms see Christenhusz et al. (2011).[4]

Subclass Cycadidae

Subclass Ginkgoidae

Subclass Gnetidae

Subclass Pinidae

Diversity and origin

There are more than 1000[5] extant or currently living species of gymnosperms in 88[5] plant genera belonging to 14 plant families.

It is widely accepted that the gymnosperms originated in the late Carboniferous period, replacing the lycopsid rainforests of the tropical region.[6] [7] This appears to have been the result of a whole genome duplication event around 319 million years ago.[8] Early characteristics of seed plants were evident in fossil progymnosperms of the late Devonian period around 380 million years ago. It has been suggested that during the mid-Mesozoic era, pollination of some extinct groups of gymnosperms was by extinct species of scorpionflies that had specialized proboscis for feeding on pollination drops. The scorpionflies likely engaged in pollination mutualisms with gymnosperms, long before the similar and independent coevolution of nectar-feeding insects on angiosperms.[9][10] Evidence has also been found that mid-Mesozoic gymnosperms were pollinated by Kalligrammatid lacewings, a now-extinct genus with members which (in an example of convergent evolution) resembled the modern butterflies that arose far later. [11]

Conifers are by far the most abundant extant group of gymnosperms with six to eight families, with a total of 65-70 genera and 600-630 species (696 accepted names).[12] Conifers are woody plants and most are evergreens.[13] The leaves of many conifers are long, thin and needle-like, other species, including most Cupressaceae and some Podocarpaceae, have flat, triangular scale-like leaves. Agathis in Araucariaceae and Nageia in Podocarpaceae have broad, flat strap-shaped leaves.

Cycads are the next most abundant group of gymnosperms, with two or three families, 11 genera, and approximately 300 species. The other extant groups are the 75-80 species of Gnetales and one species of Ginkgo.

Uses

Gymnosperms have major economic uses. Pine, fir, spruce, and cedar are all examples of conifers that are used for lumber. Some other common uses for gymnosperms are soap, varnish, nail polish, food, gum, and perfumes.

Life cycle

Gymnosperms, like all vascular plants, have a sporophyte-dominant life cycle. The gametophyte (gamete-bearing phase) is relatively short-lived. Two spore types, microspores and megaspores, are typically produced in pollen cones or ovulate cones, respectively. Gametophytes, as with all heterosporous plants, develop within the spore wall. Pollen grains (microgametophytes) mature from microspores, and ultimately produce sperm cells. Megagametophytes develop from megaspores and are retained within the ovule. They typically produce multiple archegonia. During pollination, pollen grains are physically transferred between plants, from pollen cone to the ovule, being transferred by wind or insects. Whole grains enter each ovule through a microscopic gap in the ovule coat (integument) called the micropyle. The pollen grains mature further inside the ovule and produce sperm cells. Two main modes of fertilization are found in gymnosperms. Cycads and Ginkgo have motile sperm that swim directly to the egg inside the ovule, whereas conifers and gnetophytes have sperm with no flagella that are conveyed to the egg along a pollen tube. After syngamy (joining of the sperm and egg cell), the zygote develops into an embryo (young sporophyte). More than one embryo is usually initiated in each gymnosperm seed. The mature seed comprises the embryo and the remains of the female gametophyte, which serves as a food supply, and the seed coat (integument).[14]

Genetics

The first published sequenced genome for any gymnosperm was the genome of Picea abies in 2013.[15]

References

  1. ↑ Jeffrey D. Palmer, Douglas E. Soltis and Mark W. Chase (2004). "The plant tree of life: an overview and some points of view". American Journal of Botany 91 (10): 1437–1445. doi:10.3732/ajb.91.10.1437. PMID 21652302.
  2. ↑ Stevens, P. F. "Angiosperm Phylogeny Website - Seed Plant Evolution".
  3. ↑ Hilton, Jason, and Richard M. Bateman. 2006. Pteridosperms are the backbone of seed-plant phylogeny. Journal of the Torrey Botanical Society 133: 119-168 (abstract)
  4. ↑ Christenhusz, M.J.M., J.L. Reveal, A. Farjon, M.F. Gardner, R.R. Mill, and M.W. Chase (2011). A new classification and linear sequence of extant gymnosperms. Phytotaxa 19:55-70. http://www.mapress.com/phytotaxa/content/2011/f/pt00019p070.pdf
  5. 1 2 "Gymnosperms on The Plant List". Theplantlist.org. Retrieved 2013-07-24.
  6. ↑ Sahney, S., Benton, M.J. & Falcon-Lang, H.J. (2010). "Rainforest collapse triggered Pennsylvanian tetrapod diversification in Euramerica" (PDF). Geology 38 (12): 1079–1082. doi:10.1130/G31182.1.
  7. ↑ Campbell and Reece; Biology, Eighth edition
  8. ↑ Jiao Y, Wickett NJ, Ayyampalayam S, Chanderbali AS, Landherr L, Ralph PE, Tomsho LP, Hu Y, Liang H, Soltis PS, Soltis DE, Clifton SW, Schlarbaum SE, Schuster SC, Ma H, Leebens-Mack J, Depamphilis CW (2011) Ancestral polyploidy in seed plants and angiosperms. Nature
  9. ↑ Ollerton, J.; Coulthard, E. (2009). "Evolution of Animal Pollination". Science 326 (5954): 808–809. doi:10.1126/science.1181154. PMID 19892970.
  10. ↑ Ren, D; Labandeira, CC; Santiago-Blay, JA; Rasnitsyn, A; et al. (2009). "A Probable Pollination Mode Before Angiosperms: Eurasian, Long-Proboscid Scorpionflies". Science 326 (5954): 840–847. doi:10.1126/science.1178338. PMC 2944650. PMID 19892981.
  11. ↑ Labandeira, Conrad C.; Yang, Qiang; Santiago-Blay, Jorge A.; Hotton, Carol L.; Monteiro, Antónia; Wang, Yong-Jie; Goreva, Yulia; Shih, ChungKun; Siljeström, Sandra; Rose, Tim R.; Dilcher, David L.; Ren, Dong. "The evolutionary convergence of mid-Mesozoic lacewings and Cenozoic butterflies". Proceedings of the Royal Society B: Biological Sciences 283 (1824). doi:10.1098/rspb.2015.2893.
  12. ↑ Catalogue of Life: 2007 Annual checklist - Conifer database Archived January 15, 2009 at the Wayback Machine
  13. ↑ Campbell, Reece, "Phylum Coniferophyta."Biology. 7th. 2005. Print. P.595
  14. ↑ Walters, Dirk R Walters Bonnie By (1996). Vascular plant taxonomy. Dubuque, Iowa: Kendall/Hunt Pub. Co. p. 124. ISBN 978-0-7872-2108-9.
  15. ↑ The Norway spruce genome sequence and conifer genome evolution Nature 497, 579–584 doi:10.1038/nature12211

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