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*From Wikipedia, the free encyclopedia*
*Organism cloning*
Organism cloning (also called reproductive cloning) refers to the procedure of creating a new multicellular organism, genetically identical to another. In essence this form of cloning is an asexual method of reproduction, where fertilization or inter-gamete contact does not take place. Asexual reproduction is a naturally occurring phenomenon in many species, including most plants (see vegetative reproduction) and some insects. Scientists have made some major achievements with cloning, including the asexual reproduction of sheep and cows. There is a lot of ethical debate over whether or not cloning should be used. However, cloning, or asexual propagation,[11] has been common practice in the horticultural world for hundreds of years.
*Horticultural*
The term clone is used in horticulture to refer to descendants of a single plant which were produced by vegetative reproduction or apomixis. Many horticultural plant cultivars are clones, having been derived from a single individual, multiplied by some process other than sexual reproduction. As an example, ****some European cultivars of grapes represent clones that have been propagated for over two millennia.**** Other examples are potato and banana. Grafting can be regarded as cloning, since all the shoots and branches coming from the graft are genetically a clone of a single individual, but this particular kind of cloning has not come under ethical scrutiny and is generally treated as an entirely different kind of operation.
Many trees, shrubs, vines, ferns and other herbaceous perennials form clonal colonies naturally. Parts of an individual plant may become detached by fragmentation and grow on to become separate clonal individuals. A common example is in the vegetative reproduction of moss and liverwort gametophyte clones by means of gemmae. Some vascular plants e.g. dandelion and certain viviparous grasses also form seeds asexually, termed apomixis, resulting in clonal populations of genetically identical individuals.
*Vegetative reproduction*
This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (March 200
Production of new individuals along a leaf margin of the air plant, Kalanchoe pinnata. The small plant in front is about 1 cm tall. The concept of "individual" is obviously stretched by this process.
Bryophyllum daigremontianum produces plantlets along the margins of its leaves. When they are mature enough, they drop off and root in any suitable soil beneath.
Vegetative reproduction from a stem cutting less than a week old. Some species are more conducive to this means of propagation than others.
A bulb of Muscari has reproduced vegetatively underground to make two bulbs, each of which produces a flower stem.
Vegetative reproduction (vegetative propagation, vegetative multiplication, vegetative cloning) is a form of asexual reproduction in plants. It is a process by which new individuals arise without production of seeds or spores. It can occur naturally or be induced by horticulturists.
Although most plants normally reproduce sexually, many have the ability for vegetative propagation, or can be vegetatively propagated if small pieces are subjected to chemical (hormonal) treatments. This is because meristematic cells capable of cellular differentiation are present in many plant tissues. Horticulturalists are interested in understanding how meristematic cells can be induced to reproduce an entire plant.
Success rates and difficulty of propagation vary greatly. For example willow and coleus can be propagated merely by inserting a stem in water or moist soil. On the other hand, monocotyledons, unlike dicotyledons, typically lack a vascular cambium and therefore are harder to propagate.
Contents
1 Types
2 Vegetative structures
3 Natural vegetative propagation
4 Artificial vegetative propagation
4.1 Cultivated plants propagated by vegetative methods
5 See also
6 References
Types
In a wide sense, methods of vegetative propagation include cutting, vegetative apomixis, layering, division, budding, grafting and tissue culture. Cutting is the most common artificial vegetative propagation method, where pieces of the "parent" plant are removed and placed in a suitable environment so that they can grow into a whole new plant, the "clone", which is genetically identical to the parent. Cutting exploits the ability of plants to grow adventitious roots (i.e. root material that can generate from a location other than the existing or primary root system, as in from a leaf or cut stem) under certain conditions.
Vegetative propagation is usually considered a cloning method. However, there are several cases where vegetatively propagated plants are not genetically identical. Root cuttings of thornless blackberries will revert to thorny type because the adventitious shoot develops from a cell that is genetically thorny. Thornless blackberry is a chimera, with the epidermal layers genetically thornless but the tissue beneath it genetically thorny.[citation needed] Similarly, leaf cutting propagation of certain chimeral variegated plants, such as snake plant (Sansevieria trifasciata), will produce mainly nonvariegated plants.[citation needed]
Grafting is often not a complete cloning method because sexual seedlings are used as rootstocks. In that case only the top of the plant is clonal. In some crops, particularly apples, the rootstocks are vegetatively propagated so the entire graft can be clonal if the scion and rootstock are both clones.
Apomixis (including apospory and diplospory) is a type of reproduction that does not involve fertilisation. In flowering plants, unfertilized seeds are involved, or plantlets that grow instead of flowers. Hawkweed (Hieracium), dandelion (Taraxacum), some citrus (Citrus) and Kentucky blue grass (Poa pratensis) all use this form of asexual reproduction. Bulbils are sometimes formed instead of the flowers of garlic. These cases would not be vegetative reproduction because normally reproductive parts were involved. They would be considered asexual reproduction however. Vegetative reproduction involves only vegetative structures, i.e. roots, stems or leaves.
In spore-bearing plants, apospory is the asexual development of 2n gametophytes from sporophytes without undergoing meiosis or spore formation.[1][2]
Vegetative structures
Virtually all types of shoots and roots are capable of vegetative propagation, including stems, basal shoots, tubers, rhizomes, stolons, corms, bulbs, and buds. In a few species (such as Kalanchoë), leaves are involved in vegetative reproduction.
The rhizome is a modified underground stem serving as an organ of vegetative reproduction, e. g. Polypodium (polypody), Iris, Couch Grass and Nettles.
Prostrate aerial stems, called runners or stolons are important vegetative reproduction organs in some species, such as the strawberry, numerous grasses, and some ferns.
Adventitious buds form on roots near the ground surface, on damaged stems (as on the stumps of cut trees), or on old roots. These develop into above-ground stems and leaves.
A form of budding called suckering is the reproduction or regeneration of a plant by shoots that arise from an existing root system. Species that characteristically produce suckers include Elm (Ulmus), Dandelion (Taraxacum), and members of the Rose Family (Rosa).
Another type of a vegetative reproduction is the production of bulbs. Plants like onion (Allium cepa), hyacinth (Hyacinth), narcissus (Narcissus) and tulips (Tulipa) reproduce by forming bulbs.
Other plants like potatoes (Solanum tuberosum) and dahlia (Dahlia) reproduce by a method similar to bulbs: they produce tubers.
Gladioli and crocuses (Crocus) reproduce by forming a bulb-like structure called a corm.
Some orchids reproduce by the growth of keikis from the stem or cane of the parent plant.
Natural vegetative propagation
Natural vegetative reproduction is mostly a process found in herbaceous and woody perennial plants, and typically involves structural modifications of the stem, although any horizontal, underground part of a plant (whether stem, leaf, or root) can contribute to vegetative reproduction of a plant. Most plant species that survive and significantly expand by vegetative reproduction would be perennial almost by definition, since specialized organs of vegetative reproduction, like seeds of annuals, serve to survive seasonally harsh conditions. A plant that persists in a location through vegetative reproduction of individuals over a long period of time constitutes a clonal colony.
In a sense, this process is not one of "reproduction" but one of survival and expansion of biomass of the individual. When an individual organism increases in size via cell multiplication and remains intact, the process is called "vegetative growth". However, in vegetative reproduction, the new plants that result are new individuals in almost every respect except genetic. Of considerable interest is how this process appears to reset the aging clock.[3]
Artificial vegetative propagation
Vegetative propagation of particular cultivars that have desirable characteristics is very common practice. Reasons for preferring vegetative rather than sexual means of reproduction vary, but commonly include greater ease and speed of propagation of certain plants, such as many perennial root crops and vines. Another major attraction is that the resulting plant amounts to a clone of the parent plant and accordingly is of a more predictable quality than most seedlings. However, as can be seen in many variegated plants, this does not always apply, because many plants actually are chimeras and cuttings might reflect the attributes of only one or some of the parent cell lines. Man-made methods of vegetative reproduction are usually enhancements of natural processes, but they range from rooting cuttings to grafting and artificial propagation by laboratory tissue culture.
In horticulture, a "cutting" is a piece that has been cut off from a mother plant and then caused to grow into a whole plant. Often this involves a piece of stem that is treated with rooting liquid or powder containing hormones. In some species root cuttings can produce shoot growth. When the cutting has become a self-sufficient plant, it is genetically identical to the mother plant except when chimeric tissues or similar complications affect the outcome.
A related form of regeneration is that of grafting. A stem piece or a single bud (the scion) is joined onto the stem of a plant that has roots (the rootstock), or a stem piece can be joined to a root piece. A popular use of grafting is to produce fruit trees, sometimes with more than one variety of the same fruit species growing from the same stem. Rootstocks for fruit trees are either seedlings or propagated by layering.
Cultivated plants propagated by vegetative methods
A number of commonly cultivated plants are usually propagated by vegetative means rather than by seeds. This is a listing of such plants:
African violets leaf cuttings
Apple grafting
Avocado grafting
Banana sucker removal
Blackberries (Rubus occidentalis) stem cuttings
Canna division
Cannabis stem cuttings
Citrus (lemon, orange, grapefruit, Tangerine, dayap) grafting
Date sucker removal
Fig stem cuttings
Garden strawberry runners (stolons)
Grapes stem cuttings, grafting
Hops stem cuttings
Manioc (cassava) stem cuttings
Maple stem cuttings, grafting
Nut crops (walnut, pecan) grafting
Peach grafting
Pear grafting
Pineapple stem cuttings
Plum stem cuttings
Poplar stem cuttings
Potato stem (tuber) cuttings
Sugar cane stem cuttings
Tea stem cuttings
Vanilla stem cuttings
Verbena stem cuttings
Willow stem cuttings
*Apomixis*
Not to be confused with automixis or autogamy.
This article deals almost exclusively with plants. For similar processes in animals and Oomycetes, see Parthenogenesis.
In botany, apomixis was defined by Hans Winkler as replacement of the normal sexual reproduction by asexual reproduction, without fertilization.[1] This definition notably does not mention meiosis. Thus "normal asexual reproduction" of plants, such as propagation from cuttings or leaves, has never been considered to be apomixis, but replacement of the seed by a plantlet, or replacement of the flower by bulbils are types of apomixis. Apomictically produced offspring are genetically identical to the parent plant.
In flowering plants, the term "apomixis" is commonly used in a restricted sense to mean agamospermy, i.e. asexual reproduction through seeds.
Apogamy is a related term that has had various meanings over time. In plants with independent gametophytes (notably ferns), the term is still used interchangeably with "apomixis", and both refer to the formation of sporophytes by parthenogenesis of gametophyte cells.
Male apomixis (paternal apomixis) involves replacement of the genetic material of the egg cell by that from the pollen.
Contents
1 Apomixis and evolution
2 Apomixis in non-flowering plants
2.1 Apospory in ferns
3 Apomixis in flowering plants (angiosperms)
3.1 Types of apomixis in flowering plants
3.1.1 Types of gametophytic apomixis
3.2 Incidence of apomixis in flowering plants
4 Some terms related to apomixis
5 See also
6 References
Apomixis and evolution
Because apomictic plants are genetically identical from one generation to the next, each lineage has some of the characters of a true species, maintaining distinctions from other congeneric apomicts, while having much smaller differences than is normal between species of most genera. They are therefore often called microspecies. In some genera, it is possible to identify and name hundreds or even thousands of microspecies, which may be grouped together as aggregate species, typically listed in floras with the convention "Genus species agg." (e.g., the bramble, Rubus fruticosus agg.). In some plant families, genera with apomixis are quite common, e.g. in Asteraceae, Poaceae, and Rosaceae. Examples of apomixis can be found in the genera Crataegus (hawthorns), Amelanchier (shadbush), Sorbus (rowans and whitebeams), Rubus (brambles or blackberries), Poa (meadow grasses), Hieracium (hawkweeds) and Taraxacum (dandelions).
Although the evolutionary advantages of sexual reproduction are lost, apomixis can pass along traits fortuitous for evolutionary fitness. As Clausen eloquently put it[2] (page 470)
The apomicts actually have discovered the effectiveness of mass production long before Mr Henry Ford applied it to the production of the automobile. ... Facultative apomixis ... does not prevent variation; rather, it multiplies certain varietal products.
Facultative apomixis means that apomixis does not always occur, i.e. sexual reproduction can also happen. It appears likely[3] that all apomixis in plants is facultative, i.e. that "obligate apomixis" is an artifact of the observation methods, an illusion.
Apomixis in non-flowering plants
[icon] This section requires expansion. (July 2011)
Apospory in ferns
Apomixis in flowering plants (angiosperms)
Agamospermy, asexual reproduction through seeds, occurs in flowering plants through many different mechanisms[3] and a simple hierarchical classification of the different types is not possible. Consequently there are almost as many different usages of terminology for apomixis in angiosperms as there are authors on the subject. For English speakers, Maheshwari 1950[4] is very influential. German speakers might prefer to consult Rutishauser 1967.[5] Some older text books[6] on the basis of misinformation (that the egg cell in a meiotically unreduced gametophyte can never be fertilized) attempted to reform the terminology to match parthenogenesis as it is used in zoology, and this continues to cause much confusion.
Agamospermy occurs mainly in two forms: In gametophytic apomixis, the embryo arises from an unfertilized egg cell (i.e. by parthenogenesis) in a gametophyte that was produced from a cell that did not complete meiosis. In adventitious embryony (sporophytic apomixis), an embryo is formed directly (not from a gametophyte) from nucellus or integument tissue (see nucellar embryony).
Types of apomixis in flowering plants
Maheshwari[4] used the following simple classification of types of apomixis in flowering plants:
Nonrecurrent apomixis: In this type "the megaspore mother cell undergoes the usual meiotic divisions and a haploid embryo sac [megagametophyte] is formed. The new embryo may then arise either from the egg (haploid parthenogenesis) or from some other cell of the gametophyte (haploid apogamy)." The haploid plants have half as many chromosomes as the mother plant, and "the process is not repeated from one generation to another" (which is why it is called nonrecurrent). See also parthenogenesis and apogamy below.
Recurrent apomixis, is now more often called gametophytic apomixis: In this type, the megagametophyte has the same number of chromosomes as the mother plant because meiosis was not completed. It generally arises either from an archesporial cell or from some other part of the nucellus.
Adventive embryony, also called sporophytic apomixis, sporophytic budding, or nucellar embryony: Here there may be a megagametophyte in the ovule, but the embryos do not arise from the cells of the gametophyte; they arise from cells of nucellus or the integument. Adventive embryony is important in several species of Citrus, in Garcinia, Euphorbia dulcis, Mangifera indica etc.
Vegetative apomixis: In this type "the flowers are replaced by bulbils or other vegetative propagules which frequently germinate while still on the plant". Vegetative apomixis is important in Allium, Fragaria, some grasses, etc.
Types of gametophytic apomixis
Gametophytic apomixis in flowering plants develops in several different ways.[7] A megagametophyte develops with an egg cell within it that develops into an embryo through parthenogenesis. The central cell of the megagametophyte may require fertilization to form the endosperm, pseudogamous gametophytic apomixis, or in autonomous gametophytic apomixis fertilization is not required.
In diplospory (also called generative apospory), the megagametophyte arises from a cell of the archesporium.
In apospory (also called somatic apospory), the megagametophyte arises from some other nucellus cell.
Considerable confusion has resulted because diplospory is often defined to involve the megaspore mother cell only, but a number of plant families have a multicellular archesporium and the megagametophyte could originate from another archesporium cell.
Diplospory is further subdivided according to how the megagametophyte forms:
Allium odorumA. nutans type. The chromosomes double (endomitosis) and then meiosis proceeds in an unusual way, with the chromosome copies pairing up (rather than the original maternal and paternal copies pairing up).
Taraxacum type: Meiosis I fails to complete, meiosis II creates two cells, one of which degenerates; three mitotic divisions form the megagametophyte.
Ixeris type: Meiosis I fails to complete; three rounds of nuclear division occur without cell-wall formation; wall formation then occurs.
BlumeaElymus types: A mitotic division is followed by degeneration of one cell; three mitotic divisions form the megagametophyte.
AntennariaHieracium types: three mitotic divisions form the megagametophyte.
EragrostisPanicum types: Two mitotic division give a 4-nucleate megagametophyte, with cell walls to form either three or four cells.
Incidence of apomixis in flowering plants
Apomixis occurs in at least 33 families of flowering plants, and has evolved multiple times from sexual relatives.[8][9] Apomictic species or individual plants often have a hybrid origin, and are usually polyploid.[9]
In plants with both apomictic and meiotic embryology, the proportion of the different types can differ at different times of year,[7] and photoperiod can also change the proportion.[7] It appears unlikely that there are any truly completely apomictic plants, as low rates of sexual reproduction have been found in several species that were previously thought to be entirely apomictic.[7]
The genetic control of apomixis can involve a single genetic change that affects all the major developmental components, formation of the megagametophyte, parthenogenesis of the egg cell, and endosperm development.[10] However, the timing of the various developmental processes is critical to successful development of an apomictic seed, and the timing can be affected by multiple genetic factors.[10]
Some terms related to apomixis
Parthenogenesis: Development of an embryo directly from an egg cell without fertilization is called parthenogenesis. It is of two types:
Haploid parthenogenesis: Parthenogenesis of a normal haploid egg (a meiotically reduced egg) into an embryo is termed haploid parthenogenesis. If the mother plant was diploid, then the haploid embryo that results is monoploid, and the plant that grows from the embryo is sterile. If they are not sterile, they are sometimes useful to plant breeders (especially in potato breeding, see dihaploidy). This type of apomixis has been recorded in Solanum nigram, Lilium spp., Orchis maculate, Nicotiana tabacum etc.
Diploid parthenogenesis: When the megagametophyte develops without completing meiosis, so that the megagametophyte and all cells within it are meiotically unreduced (aka diploid, but diploid is an ambiguous term), this is called diploid parthenogenesis, and the plant that develops from the embryo will have the same number of chromosomes as the mother plant. Diploid parthenogenesis is a component process of gametophytic apomixis (see above).
Androgenesis and androclinesis are synonyms. These terms are used for two different processes that both have the effect of producing an embryo that has "male inheritance".
The first process is a natural one. It is may also be referred to as male apomixis or paternal apomixis It involves fusion of the male and female gametes and replacement of the female nucleus by the male nucleus. This has been noted as a rare phenomenon in many plants (e.g. Nicotiana and Crepis), and occurs as the regular reproductive method in the Saharan Cypress, Cupressus dupreziana.[11][12]
The second process that is referred to as androgenesis or androclinesis involves (artificial) culture of haploid plants from anther tissue or microspores.[13]
Apogamy: Although this term was (before 190 used for other types of apomixis, and then discarded as too confusing, it is still sometimes used when an embryo develops from a cell of the megagametophyte other than the egg cell. In flowering plants, the cells involved in apogamy would be synergids or antipodal cells.
Addition hybrids, called BIII hybrids by Rutishauser:[5] An embryo is formed after a meiotically unreduced egg cell is fertilized. The ploidy level of the embryo is therefore higher than that of the mother plant. This process occurs in some plants that are otherwise apomictic, and may play a significant role in producing tetraploid plants from triploid apomictic mother plants (if they receive pollen from diploids). Because fertilization is involved, this process does not fit the definition of apomixis.
Pseudogamy refers to any reproductive process that requires pollination but does not involve male inheritance. It is sometimes used in a restrictive sense to refer to types of apomixis in which the endosperm is fertilized but the embryo is not. A better term for the restrictive sense is centrogamy.[13]
Agamospecies, the concept introduced by Göte Turesson: "an apomict population the constituents of which, for morphological, cytological or other reasons, are to be considered as having a common origin," i.e., basically synonymous with "microspecies.[14]
*Organism cloning*
Organism cloning (also called reproductive cloning) refers to the procedure of creating a new multicellular organism, genetically identical to another. In essence this form of cloning is an asexual method of reproduction, where fertilization or inter-gamete contact does not take place. Asexual reproduction is a naturally occurring phenomenon in many species, including most plants (see vegetative reproduction) and some insects. Scientists have made some major achievements with cloning, including the asexual reproduction of sheep and cows. There is a lot of ethical debate over whether or not cloning should be used. However, cloning, or asexual propagation,[11] has been common practice in the horticultural world for hundreds of years.
*Horticultural*
The term clone is used in horticulture to refer to descendants of a single plant which were produced by vegetative reproduction or apomixis. Many horticultural plant cultivars are clones, having been derived from a single individual, multiplied by some process other than sexual reproduction. As an example, ****some European cultivars of grapes represent clones that have been propagated for over two millennia.**** Other examples are potato and banana. Grafting can be regarded as cloning, since all the shoots and branches coming from the graft are genetically a clone of a single individual, but this particular kind of cloning has not come under ethical scrutiny and is generally treated as an entirely different kind of operation.
Many trees, shrubs, vines, ferns and other herbaceous perennials form clonal colonies naturally. Parts of an individual plant may become detached by fragmentation and grow on to become separate clonal individuals. A common example is in the vegetative reproduction of moss and liverwort gametophyte clones by means of gemmae. Some vascular plants e.g. dandelion and certain viviparous grasses also form seeds asexually, termed apomixis, resulting in clonal populations of genetically identical individuals.
*Vegetative reproduction*
This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (March 200
Production of new individuals along a leaf margin of the air plant, Kalanchoe pinnata. The small plant in front is about 1 cm tall. The concept of "individual" is obviously stretched by this process.
Bryophyllum daigremontianum produces plantlets along the margins of its leaves. When they are mature enough, they drop off and root in any suitable soil beneath.
Vegetative reproduction from a stem cutting less than a week old. Some species are more conducive to this means of propagation than others.
A bulb of Muscari has reproduced vegetatively underground to make two bulbs, each of which produces a flower stem.
Vegetative reproduction (vegetative propagation, vegetative multiplication, vegetative cloning) is a form of asexual reproduction in plants. It is a process by which new individuals arise without production of seeds or spores. It can occur naturally or be induced by horticulturists.
Although most plants normally reproduce sexually, many have the ability for vegetative propagation, or can be vegetatively propagated if small pieces are subjected to chemical (hormonal) treatments. This is because meristematic cells capable of cellular differentiation are present in many plant tissues. Horticulturalists are interested in understanding how meristematic cells can be induced to reproduce an entire plant.
Success rates and difficulty of propagation vary greatly. For example willow and coleus can be propagated merely by inserting a stem in water or moist soil. On the other hand, monocotyledons, unlike dicotyledons, typically lack a vascular cambium and therefore are harder to propagate.
Contents
1 Types
2 Vegetative structures
3 Natural vegetative propagation
4 Artificial vegetative propagation
4.1 Cultivated plants propagated by vegetative methods
5 See also
6 References
Types
In a wide sense, methods of vegetative propagation include cutting, vegetative apomixis, layering, division, budding, grafting and tissue culture. Cutting is the most common artificial vegetative propagation method, where pieces of the "parent" plant are removed and placed in a suitable environment so that they can grow into a whole new plant, the "clone", which is genetically identical to the parent. Cutting exploits the ability of plants to grow adventitious roots (i.e. root material that can generate from a location other than the existing or primary root system, as in from a leaf or cut stem) under certain conditions.
Vegetative propagation is usually considered a cloning method. However, there are several cases where vegetatively propagated plants are not genetically identical. Root cuttings of thornless blackberries will revert to thorny type because the adventitious shoot develops from a cell that is genetically thorny. Thornless blackberry is a chimera, with the epidermal layers genetically thornless but the tissue beneath it genetically thorny.[citation needed] Similarly, leaf cutting propagation of certain chimeral variegated plants, such as snake plant (Sansevieria trifasciata), will produce mainly nonvariegated plants.[citation needed]
Grafting is often not a complete cloning method because sexual seedlings are used as rootstocks. In that case only the top of the plant is clonal. In some crops, particularly apples, the rootstocks are vegetatively propagated so the entire graft can be clonal if the scion and rootstock are both clones.
Apomixis (including apospory and diplospory) is a type of reproduction that does not involve fertilisation. In flowering plants, unfertilized seeds are involved, or plantlets that grow instead of flowers. Hawkweed (Hieracium), dandelion (Taraxacum), some citrus (Citrus) and Kentucky blue grass (Poa pratensis) all use this form of asexual reproduction. Bulbils are sometimes formed instead of the flowers of garlic. These cases would not be vegetative reproduction because normally reproductive parts were involved. They would be considered asexual reproduction however. Vegetative reproduction involves only vegetative structures, i.e. roots, stems or leaves.
In spore-bearing plants, apospory is the asexual development of 2n gametophytes from sporophytes without undergoing meiosis or spore formation.[1][2]
Vegetative structures
Virtually all types of shoots and roots are capable of vegetative propagation, including stems, basal shoots, tubers, rhizomes, stolons, corms, bulbs, and buds. In a few species (such as Kalanchoë), leaves are involved in vegetative reproduction.
The rhizome is a modified underground stem serving as an organ of vegetative reproduction, e. g. Polypodium (polypody), Iris, Couch Grass and Nettles.
Prostrate aerial stems, called runners or stolons are important vegetative reproduction organs in some species, such as the strawberry, numerous grasses, and some ferns.
Adventitious buds form on roots near the ground surface, on damaged stems (as on the stumps of cut trees), or on old roots. These develop into above-ground stems and leaves.
A form of budding called suckering is the reproduction or regeneration of a plant by shoots that arise from an existing root system. Species that characteristically produce suckers include Elm (Ulmus), Dandelion (Taraxacum), and members of the Rose Family (Rosa).
Another type of a vegetative reproduction is the production of bulbs. Plants like onion (Allium cepa), hyacinth (Hyacinth), narcissus (Narcissus) and tulips (Tulipa) reproduce by forming bulbs.
Other plants like potatoes (Solanum tuberosum) and dahlia (Dahlia) reproduce by a method similar to bulbs: they produce tubers.
Gladioli and crocuses (Crocus) reproduce by forming a bulb-like structure called a corm.
Some orchids reproduce by the growth of keikis from the stem or cane of the parent plant.
Natural vegetative propagation
Natural vegetative reproduction is mostly a process found in herbaceous and woody perennial plants, and typically involves structural modifications of the stem, although any horizontal, underground part of a plant (whether stem, leaf, or root) can contribute to vegetative reproduction of a plant. Most plant species that survive and significantly expand by vegetative reproduction would be perennial almost by definition, since specialized organs of vegetative reproduction, like seeds of annuals, serve to survive seasonally harsh conditions. A plant that persists in a location through vegetative reproduction of individuals over a long period of time constitutes a clonal colony.
In a sense, this process is not one of "reproduction" but one of survival and expansion of biomass of the individual. When an individual organism increases in size via cell multiplication and remains intact, the process is called "vegetative growth". However, in vegetative reproduction, the new plants that result are new individuals in almost every respect except genetic. Of considerable interest is how this process appears to reset the aging clock.[3]
Artificial vegetative propagation
Vegetative propagation of particular cultivars that have desirable characteristics is very common practice. Reasons for preferring vegetative rather than sexual means of reproduction vary, but commonly include greater ease and speed of propagation of certain plants, such as many perennial root crops and vines. Another major attraction is that the resulting plant amounts to a clone of the parent plant and accordingly is of a more predictable quality than most seedlings. However, as can be seen in many variegated plants, this does not always apply, because many plants actually are chimeras and cuttings might reflect the attributes of only one or some of the parent cell lines. Man-made methods of vegetative reproduction are usually enhancements of natural processes, but they range from rooting cuttings to grafting and artificial propagation by laboratory tissue culture.
In horticulture, a "cutting" is a piece that has been cut off from a mother plant and then caused to grow into a whole plant. Often this involves a piece of stem that is treated with rooting liquid or powder containing hormones. In some species root cuttings can produce shoot growth. When the cutting has become a self-sufficient plant, it is genetically identical to the mother plant except when chimeric tissues or similar complications affect the outcome.
A related form of regeneration is that of grafting. A stem piece or a single bud (the scion) is joined onto the stem of a plant that has roots (the rootstock), or a stem piece can be joined to a root piece. A popular use of grafting is to produce fruit trees, sometimes with more than one variety of the same fruit species growing from the same stem. Rootstocks for fruit trees are either seedlings or propagated by layering.
Cultivated plants propagated by vegetative methods
A number of commonly cultivated plants are usually propagated by vegetative means rather than by seeds. This is a listing of such plants:
African violets leaf cuttings
Apple grafting
Avocado grafting
Banana sucker removal
Blackberries (Rubus occidentalis) stem cuttings
Canna division
Cannabis stem cuttings
Citrus (lemon, orange, grapefruit, Tangerine, dayap) grafting
Date sucker removal
Fig stem cuttings
Garden strawberry runners (stolons)
Grapes stem cuttings, grafting
Hops stem cuttings
Manioc (cassava) stem cuttings
Maple stem cuttings, grafting
Nut crops (walnut, pecan) grafting
Peach grafting
Pear grafting
Pineapple stem cuttings
Plum stem cuttings
Poplar stem cuttings
Potato stem (tuber) cuttings
Sugar cane stem cuttings
Tea stem cuttings
Vanilla stem cuttings
Verbena stem cuttings
Willow stem cuttings
*Apomixis*
Not to be confused with automixis or autogamy.
This article deals almost exclusively with plants. For similar processes in animals and Oomycetes, see Parthenogenesis.
In botany, apomixis was defined by Hans Winkler as replacement of the normal sexual reproduction by asexual reproduction, without fertilization.[1] This definition notably does not mention meiosis. Thus "normal asexual reproduction" of plants, such as propagation from cuttings or leaves, has never been considered to be apomixis, but replacement of the seed by a plantlet, or replacement of the flower by bulbils are types of apomixis. Apomictically produced offspring are genetically identical to the parent plant.
In flowering plants, the term "apomixis" is commonly used in a restricted sense to mean agamospermy, i.e. asexual reproduction through seeds.
Apogamy is a related term that has had various meanings over time. In plants with independent gametophytes (notably ferns), the term is still used interchangeably with "apomixis", and both refer to the formation of sporophytes by parthenogenesis of gametophyte cells.
Male apomixis (paternal apomixis) involves replacement of the genetic material of the egg cell by that from the pollen.
Contents
1 Apomixis and evolution
2 Apomixis in non-flowering plants
2.1 Apospory in ferns
3 Apomixis in flowering plants (angiosperms)
3.1 Types of apomixis in flowering plants
3.1.1 Types of gametophytic apomixis
3.2 Incidence of apomixis in flowering plants
4 Some terms related to apomixis
5 See also
6 References
Apomixis and evolution
Because apomictic plants are genetically identical from one generation to the next, each lineage has some of the characters of a true species, maintaining distinctions from other congeneric apomicts, while having much smaller differences than is normal between species of most genera. They are therefore often called microspecies. In some genera, it is possible to identify and name hundreds or even thousands of microspecies, which may be grouped together as aggregate species, typically listed in floras with the convention "Genus species agg." (e.g., the bramble, Rubus fruticosus agg.). In some plant families, genera with apomixis are quite common, e.g. in Asteraceae, Poaceae, and Rosaceae. Examples of apomixis can be found in the genera Crataegus (hawthorns), Amelanchier (shadbush), Sorbus (rowans and whitebeams), Rubus (brambles or blackberries), Poa (meadow grasses), Hieracium (hawkweeds) and Taraxacum (dandelions).
Although the evolutionary advantages of sexual reproduction are lost, apomixis can pass along traits fortuitous for evolutionary fitness. As Clausen eloquently put it[2] (page 470)
The apomicts actually have discovered the effectiveness of mass production long before Mr Henry Ford applied it to the production of the automobile. ... Facultative apomixis ... does not prevent variation; rather, it multiplies certain varietal products.
Facultative apomixis means that apomixis does not always occur, i.e. sexual reproduction can also happen. It appears likely[3] that all apomixis in plants is facultative, i.e. that "obligate apomixis" is an artifact of the observation methods, an illusion.
Apomixis in non-flowering plants
[icon] This section requires expansion. (July 2011)
Apospory in ferns
Apomixis in flowering plants (angiosperms)
Agamospermy, asexual reproduction through seeds, occurs in flowering plants through many different mechanisms[3] and a simple hierarchical classification of the different types is not possible. Consequently there are almost as many different usages of terminology for apomixis in angiosperms as there are authors on the subject. For English speakers, Maheshwari 1950[4] is very influential. German speakers might prefer to consult Rutishauser 1967.[5] Some older text books[6] on the basis of misinformation (that the egg cell in a meiotically unreduced gametophyte can never be fertilized) attempted to reform the terminology to match parthenogenesis as it is used in zoology, and this continues to cause much confusion.
Agamospermy occurs mainly in two forms: In gametophytic apomixis, the embryo arises from an unfertilized egg cell (i.e. by parthenogenesis) in a gametophyte that was produced from a cell that did not complete meiosis. In adventitious embryony (sporophytic apomixis), an embryo is formed directly (not from a gametophyte) from nucellus or integument tissue (see nucellar embryony).
Types of apomixis in flowering plants
Maheshwari[4] used the following simple classification of types of apomixis in flowering plants:
Nonrecurrent apomixis: In this type "the megaspore mother cell undergoes the usual meiotic divisions and a haploid embryo sac [megagametophyte] is formed. The new embryo may then arise either from the egg (haploid parthenogenesis) or from some other cell of the gametophyte (haploid apogamy)." The haploid plants have half as many chromosomes as the mother plant, and "the process is not repeated from one generation to another" (which is why it is called nonrecurrent). See also parthenogenesis and apogamy below.
Recurrent apomixis, is now more often called gametophytic apomixis: In this type, the megagametophyte has the same number of chromosomes as the mother plant because meiosis was not completed. It generally arises either from an archesporial cell or from some other part of the nucellus.
Adventive embryony, also called sporophytic apomixis, sporophytic budding, or nucellar embryony: Here there may be a megagametophyte in the ovule, but the embryos do not arise from the cells of the gametophyte; they arise from cells of nucellus or the integument. Adventive embryony is important in several species of Citrus, in Garcinia, Euphorbia dulcis, Mangifera indica etc.
Vegetative apomixis: In this type "the flowers are replaced by bulbils or other vegetative propagules which frequently germinate while still on the plant". Vegetative apomixis is important in Allium, Fragaria, some grasses, etc.
Types of gametophytic apomixis
Gametophytic apomixis in flowering plants develops in several different ways.[7] A megagametophyte develops with an egg cell within it that develops into an embryo through parthenogenesis. The central cell of the megagametophyte may require fertilization to form the endosperm, pseudogamous gametophytic apomixis, or in autonomous gametophytic apomixis fertilization is not required.
In diplospory (also called generative apospory), the megagametophyte arises from a cell of the archesporium.
In apospory (also called somatic apospory), the megagametophyte arises from some other nucellus cell.
Considerable confusion has resulted because diplospory is often defined to involve the megaspore mother cell only, but a number of plant families have a multicellular archesporium and the megagametophyte could originate from another archesporium cell.
Diplospory is further subdivided according to how the megagametophyte forms:
Allium odorumA. nutans type. The chromosomes double (endomitosis) and then meiosis proceeds in an unusual way, with the chromosome copies pairing up (rather than the original maternal and paternal copies pairing up).
Taraxacum type: Meiosis I fails to complete, meiosis II creates two cells, one of which degenerates; three mitotic divisions form the megagametophyte.
Ixeris type: Meiosis I fails to complete; three rounds of nuclear division occur without cell-wall formation; wall formation then occurs.
BlumeaElymus types: A mitotic division is followed by degeneration of one cell; three mitotic divisions form the megagametophyte.
AntennariaHieracium types: three mitotic divisions form the megagametophyte.
EragrostisPanicum types: Two mitotic division give a 4-nucleate megagametophyte, with cell walls to form either three or four cells.
Incidence of apomixis in flowering plants
Apomixis occurs in at least 33 families of flowering plants, and has evolved multiple times from sexual relatives.[8][9] Apomictic species or individual plants often have a hybrid origin, and are usually polyploid.[9]
In plants with both apomictic and meiotic embryology, the proportion of the different types can differ at different times of year,[7] and photoperiod can also change the proportion.[7] It appears unlikely that there are any truly completely apomictic plants, as low rates of sexual reproduction have been found in several species that were previously thought to be entirely apomictic.[7]
The genetic control of apomixis can involve a single genetic change that affects all the major developmental components, formation of the megagametophyte, parthenogenesis of the egg cell, and endosperm development.[10] However, the timing of the various developmental processes is critical to successful development of an apomictic seed, and the timing can be affected by multiple genetic factors.[10]
Some terms related to apomixis
Parthenogenesis: Development of an embryo directly from an egg cell without fertilization is called parthenogenesis. It is of two types:
Haploid parthenogenesis: Parthenogenesis of a normal haploid egg (a meiotically reduced egg) into an embryo is termed haploid parthenogenesis. If the mother plant was diploid, then the haploid embryo that results is monoploid, and the plant that grows from the embryo is sterile. If they are not sterile, they are sometimes useful to plant breeders (especially in potato breeding, see dihaploidy). This type of apomixis has been recorded in Solanum nigram, Lilium spp., Orchis maculate, Nicotiana tabacum etc.
Diploid parthenogenesis: When the megagametophyte develops without completing meiosis, so that the megagametophyte and all cells within it are meiotically unreduced (aka diploid, but diploid is an ambiguous term), this is called diploid parthenogenesis, and the plant that develops from the embryo will have the same number of chromosomes as the mother plant. Diploid parthenogenesis is a component process of gametophytic apomixis (see above).
Androgenesis and androclinesis are synonyms. These terms are used for two different processes that both have the effect of producing an embryo that has "male inheritance".
The first process is a natural one. It is may also be referred to as male apomixis or paternal apomixis It involves fusion of the male and female gametes and replacement of the female nucleus by the male nucleus. This has been noted as a rare phenomenon in many plants (e.g. Nicotiana and Crepis), and occurs as the regular reproductive method in the Saharan Cypress, Cupressus dupreziana.[11][12]
The second process that is referred to as androgenesis or androclinesis involves (artificial) culture of haploid plants from anther tissue or microspores.[13]
Apogamy: Although this term was (before 190
used for other types of apomixis, and then discarded as too confusing, it is still sometimes used when an embryo develops from a cell of the megagametophyte other than the egg cell. In flowering plants, the cells involved in apogamy would be synergids or antipodal cells.Addition hybrids, called BIII hybrids by Rutishauser:[5] An embryo is formed after a meiotically unreduced egg cell is fertilized. The ploidy level of the embryo is therefore higher than that of the mother plant. This process occurs in some plants that are otherwise apomictic, and may play a significant role in producing tetraploid plants from triploid apomictic mother plants (if they receive pollen from diploids). Because fertilization is involved, this process does not fit the definition of apomixis.
Pseudogamy refers to any reproductive process that requires pollination but does not involve male inheritance. It is sometimes used in a restrictive sense to refer to types of apomixis in which the endosperm is fertilized but the embryo is not. A better term for the restrictive sense is centrogamy.[13]
Agamospecies, the concept introduced by Göte Turesson: "an apomict population the constituents of which, for morphological, cytological or other reasons, are to be considered as having a common origin," i.e., basically synonymous with "microspecies.[14]
