The gymnosperms (/ˈdʒɪmnəˌspɜːrmz,-noʊ-/ⓘJIM-nə-spurmz, -noh-;
lit.'revealed seeds') are a group of
seed-producing plants that includes
conifers,
cycads, Ginkgo, and
gnetophytes, forming the clade Gymnospermae. The term gymnosperm comes from the composite word in
Greek: γυμνόσπερμος (γυμνός, gymnos, 'naked' and σπέρμα, sperma, 'seed'), literally meaning 'naked seeds'. The name is based on the unenclosed condition of their seeds (called
ovules in their unfertilized state). The non-encased condition of their seeds contrasts with 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, which are often modified to form
cones, or on their own as in
yew, Torreya, Ginkgo.[2] Gymnosperm lifecycles involve
alternation of generations. They have a dominant
diploidsporophyte phase and a reduced
haploidgametophyte phase which is dependent on the sporophytic phase. The term "gymnosperm" is often used in
paleobotany to refer to (the
paraphyletic group of) all non-angiosperm seed plants. In that case, to specify the modern
monophyletic group of gymnosperms, the term Acrogymnospermae is sometimes used.
The gymnosperms and
angiosperms together constitute the
spermatophytes or seed plants. The spermatophytes are subdivided into five
divisions, the angiosperms and four divisions of gymnosperms: the
Cycadophyta,
Ginkgophyta,
Gnetophyta, and
Pinophyta (also known as Coniferophyta). Newer classification place the gnetophytes among the conifers.[3] Numerous extinct seed plant groups are recognised including those considered
pteridosperms/seed ferns, as well other groups like the
Bennettitales.[4]
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 biloba (a single living species). About 65% of gymnosperms are
dioecious,[5] but conifers are almost all
monoecious.[6]
Some genera have
mycorrhiza, fungal associations with roots (Pinus), while in some others (Cycas) small specialised roots called coralloid roots are associated with nitrogen-fixing
cyanobacteria.
Diversity and origin
Over 1,000 living species of gymnosperm exist.[2] It was previously widely accepted that the gymnosperms originated in the
Late Carboniferous period, replacing the
lycopsid rainforests of the tropical region, but more recent phylogenetic evidence indicates that they diverged from the ancestors of
angiosperms during the
Early Carboniferous.[7][8] The radiation of gymnosperms during the late Carboniferous appears to have resulted from a whole
genome duplication event around 319 million years ago.[9] Early characteristics of seed plants are evident in fossil
progymnosperms of the late
Devonian period around 383 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.[10][11] Evidence has also been found that mid-Mesozoic gymnosperms were pollinated by
Kalligrammatid lacewings, a now-extinct family with members which (in an example of
convergent evolution) resembled the modern butterflies that arose far later.[12]
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).[18] Most conifers are
evergreens.[19] The
leaves of many conifers are long, thin and needle-like, while 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 338 species. A majority of cycads are native to tropical climates and are most abundantly found in regions near the equator. The other extant groups are the 95–100 species of
Gnetales and one species of Ginkgo.[4]
Today gymnosperms are the most threatened of all plant groups.[20]
A formal classification of the living gymnosperms is the "Acrogymnospermae", which form a
monophyletic group within the
spermatophytes.[25][26] The wider "Gymnospermae" group includes extinct gymnosperms and is thought to be
paraphyletic. 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).[27] When fossil gymnosperms such as these and the
Bennettitales,
glossopterids, and Caytonia are considered, it is clear that angiosperms are nested within a larger gymnospermae clade, although which group of gymnosperms is their closest relative remains unclear.
The extant gymnosperms include 12 main families and 83 genera which contain more than 1000 known species.[2][26][28]
Gymnosperms, like all
vascular plants, have a sporophyte-dominant life cycle, which means they spend most of their life cycle with diploid cells, while the
gametophyte (gamete-bearing phase) is relatively short-lived. Like all
seed plants, they are
heterosporous, having two spore types,
microspores (male) and
megaspores (female) that are typically produced in pollen cones or ovulate cones, respectively.[29] The exception is the females in the cycad genus Cycas, which form a loose structure called megasporophylls instead of cones.[30] As with all heterosporous plants, the gametophytes develop within the spore wall. Pollen grains (microgametophytes) mature from microspores, and ultimately produce sperm cells.[29] Megagametophytes develop from megaspores and are retained within the ovule. Gymnosperms produce multiple
archegonia, which produce the female gamete.
During pollination, pollen grains are physically transferred between plants from the pollen cone to the ovule. Pollen is usually moved 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
flagellated motile sperm[31] that swim directly to the egg inside the ovule, whereas conifers and
gnetophytes have sperm with no flagella that are moved along a
pollen tube to the egg. 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.[32]
Gymnosperms ordinarily reproduce by
sexual reproduction, and only rarely express parthenogenesis[33]. Sexual reproduction in gymnosperms appears to be required for maintaining long-term
genomic integrity [33].
Meiosis in sexual land plants provides a direct mechanism for
repairing DNA in reproductive tissues[33]. The likely primary benefit of cross-pollination in gymnosperms, as in other eukaryotes, is that it allows the avoidance of inbreeding depression caused by the presence of recessive deleterious mutations[34].
Genetics
The first published sequenced genome for any gymnosperm was the genome of Picea abies in 2013.[35]
Uses
Gymnosperms have major economic uses. Pine, fir, spruce, and cedar are all examples of conifers that are used for
lumber, paper production, and resin. Some other common uses for gymnosperms are
soap,
varnish,
nail polish, food, gum, and
perfumes.[36]
^Bond, W. J. (March 1989). "The tortoise and the hare: ecology of angiosperm dominance and gymnosperm persistence". Biological Journal of the Linnean Society. 36 (3): 227–249.
doi:
10.1111/j.1095-8312.1989.tb00492.x.
Cantino, Philip D.; Doyle, James A.; Graham, Sean W.; Judd, Walter S.; Olmstead, Richard G.; Soltis, Douglas E.; Soltis, Pamela S.; Donoghue, Michael J. (August 2007). "Towards a phylogenetic nomenclature of Tracheophyta". Taxon. 56 (3): 822–846.
doi:
10.2307/25065864.
JSTOR25065864.
External links
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