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Dwarfing is a process in which a breed of animals or cultivar of plants is changed to become significantly smaller than standard members of their species. The effect can be induced through human intervention or non-human processes, and can include genetic, nutritional or hormonal means. Used most specifically, dwarfing includes pathogenic changes in the structure of an organism (for example, the bulldog, a genetically achondroplastic dog breed), in contrast to non-pathogenic proportional reduction in stature (such as the whippet, a small sighthound dog breed). [1]



In animals, including humans, dwarfism has been described in several ways. Shortened stature can result from growth hormone deficiency, starvation, portal systemic shunts, renal disease, hypothyroidism diabetes mellitus and other conditions. Any of these conditions can be established in a population through genetic engineering, selective breeding, or insular dwarfism, or some combination of the above.

Dwarfing can produce more practical breeds that can fit in small accommodations, or may appeal aesthetically, as well as other associated side effects. Smaller stature may be a deliberate goal of breeding programs, or it may be a side effect of other breeding goals.

Nonpurposeful dwarfing

In some husbandry conditions, humans created dwarf breeds, or allowed them to develop, without specifically selecting for smaller animals. It is likely that the Shetland sheep breed, Shetland collie breed of dogs, and various pony breeds of horses developed in this manner. In the case of the Shetland sheep and collies, it is likely that environmental conditions, such as a lack of abundant fodder, led to farmers selecting smaller animals who continued to reproduce on limited food over larger animals who did not reproduce well on limited diets. In this case, the emphasis was on selecting for survival and reproduction, not size.

Purposeful dwarfing

Humans have encouraged the deliberate development of dwarf breeds of many domestic animals, including horses, cattle, dogs, and chickens. Some have been breeds of smaller animals that were not originally selected for size, but are now held to specific sizes by a breed standard. In many cases, the exact physiological mechanism that alters the growth of individuals in that breed is not well known, and some breeds have multiple mechanisms at play.

As the genetic mutations that cause dwarfing occur in many species, dwarf animals can be the offspring of normal-appearing animals. Even in breeds which have not been selected for dwarfing, some genetic lines may show a tendency to produce dwarfs, which may be encouraged by deliberate breeding. This often takes the form of in-breeding to concentrate recessive genes, and can result in other genetic abnormalities being established in the population.

Some animal breeds that have been formally subject to dwarfing include:


Dwarf Japanese juniper Dwarf Japanese Juniper, 1975-2007.jpg
Dwarf Japanese juniper
Lack of the plant growth factor auxin can cause dwarfing (right) Auxin.jpg
Lack of the plant growth factor auxin can cause dwarfing (right)

As with animals, plants can be dwarfed through genetic engineering and selective breeding, but can also undergo natural, morphological changes to acclimatize to environmental stresses such as soil quality, [2] light, [2] drought, [3] flood, [4] cold, [5] infection, [6] and herbivory [7] resulting in a dwarfed stature. Plants dwarfed due to environmental stress are said to be "stunted." The majority of dwarfing in plants occurs not from the damage environmental stresses inflict on them, but instead by hormones produced in response to the stress. [8] Plant hormones act as a signal to the various tissues of plants inducing one or more responses, the class of plant hormone responsible for dwarfing in plants due to injury are called jasmonates. Such responses include, but are not limited to: less frequent cell divisions [8] and reduction of cell elongation. [9]

Dwarfing trees

In horticulture, dwarfing can be considered a desirable characteristic in modern orchards. This kind of dwarfing can be attained through selective breeding, genetic engineering, or more often, scions are grafted on to dwarfing rootstocks. [10] Almost all modern apples in commercial use are propagated as dwarf or semi-dwarf trees for ease of picking and spraying.

Dwarfing fruit trees acts through a reduction in the nutrients which travel from the roots through the trunk to the leaves and buds. Many commercial orchards of various species use this technique to improve the overall health and productivity of the individual trees. An individual tree may be made up of three or more separate cultivars - one for the root system, which is generally selected for good stability and resistance to soil-borne diseases, one for the trunk, which modifies the overall height of the tree, and one for the productive limbs and buds, which actually produces the fruit. Frequently, the root system stock is the most resistant to cold damage - both by natural selection and by protection from the cold air by the earth. When frost severely damages a tree, the more productive branch and bud cultivar may be killed off, leaving the root to sprout new stalks. In the case of oranges and other citrus, this results in sweet orange trees being frozen back so that the more hardy, cold-tolerant sour orange rootstock puts out new growth.[ citation needed ]

Dwarfing grains

Dwarfing genes are widely used in creating more productive food plants, such as grains. One condition that results in loss of grain crops is called 'lodging', where heavy ears of almost ripe grain bend the stalk until the grain touches the ground, becomes wet, and spoils. During the Green Revolution, research that identified wheat reduced-height genes (Rht) [11] and a rice semidwarf gene (sd1) [12] resulted in crops that yielded significantly more harvestable grain.

See also

Related Research Articles

<span class="mw-page-title-main">Fruit tree propagation</span> Usually carried out vegetatively by grafting or budding a desired variety onto a suitable rootstock

Fruit tree propagation is usually carried out vegetatively (non-sexually) by grafting or budding a desired variety onto a suitable rootstock.

<span class="mw-page-title-main">Dwarfism</span> Small size of an organism, caused by growth deficiency or genetic mutations

Dwarfism is a condition wherein an organism is exceptionally small, and mostly occurs in the animal kingdom. In humans, it is sometimes defined as an adult height of less than 147 centimetres, regardless of sex; the average adult height among people with dwarfism is 122 centimetres, although some individuals with dwarfism are slightly taller. Disproportionate dwarfism is characterized by either short limbs or a short torso. In cases of proportionate dwarfism, both the limbs and torso are unusually small. Intelligence is usually normal, and most have a nearly normal life expectancy. People with dwarfism can usually bear children, though there are additional risks to the mother and child dependent upon the underlying condition.

<span class="mw-page-title-main">Shetland Sheepdog</span> Dog breed

The Shetland Sheepdog, often known as the Sheltie, is a breed of herding dog that originated in the Shetland Islands of Scotland. The original name was Shetland Collie, but this caused controversy amongst Rough Collie breeders of the time, so the breed's name was formally changed. This diligent small dog is clever, vocal, excitable and willing to please. They are incredibly trustworthy to their owners to the point where they are often referred to as "shadows" due to their attachment to family. This breed was formally recognized by The Kennel Club (UK) in 1909.

<span class="mw-page-title-main">Selective breeding</span> Breeding used to develop desired characteristics

Selective breeding is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant males and females will sexually reproduce and have offspring together. Domesticated animals are known as breeds, normally bred by a professional breeder, while domesticated plants are known as varieties, cultigens, cultivars, or breeds. Two purebred animals of different breeds produce a crossbreed, and crossbred plants are called hybrids. Flowers, vegetables and fruit-trees may be bred by amateurs and commercial or non-commercial professionals: major crops are usually the provenance of the professionals.

<span class="mw-page-title-main">Plant hormone</span> Chemical compounds that regulate plant growth and development

Plant hormone are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pathogen defense, stress tolerance and through to reproductive development. Unlike in animals each plant cell is capable of producing hormones. Went and Thimann coined the term "phytohormone" and used it in the title of their 1937 book.

<span class="mw-page-title-main">Breed</span> Specific group of domestic animals

A breed is a specific group of domestic animals having homogeneous appearance (phenotype), homogeneous behavior, and/or other characteristics that distinguish it from other organisms of the same species. In literature, there exist several slightly deviating definitions. Breeds are formed through genetic isolation and either natural adaptation to the environment or selective breeding, or a combination of the two. Despite the centrality of the idea of "breeds" to animal husbandry and agriculture, no single, scientifically accepted definition of the term exists. A breed is therefore not an objective or biologically verifiable classification but is instead a term of art amongst groups of breeders who share a consensus around what qualities make some members of a given species members of a nameable subset.

<span class="mw-page-title-main">Jasmonate</span>

Jasmonate (JA) and its derivatives are lipid-based plant hormones that regulate a wide range of processes in plants, ranging from growth and photosynthesis to reproductive development. In particular, JAs are critical for plant defense against herbivory and plant responses to poor environmental conditions and other kinds of abiotic and biotic challenges. Some JAs can also be released as volatile organic compounds (VOCs) to permit communication between plants in anticipation of mutual dangers.

Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, to achieve offspring with a genetic identity closer to that of the parent. It is used in horticulture, animal breeding, and production of gene knockout organisms.

<span class="mw-page-title-main">Landrace</span> Regionally-adapted agricultural variety

A landrace is a domesticated, locally adapted, often traditional variety of a species of animal or plant that has developed over time, through adaptation to its natural and cultural environment of agriculture and pastoralism, and due to isolation from other populations of the species. Landraces are distinct from cultivars and from standard breeds.

<span class="mw-page-title-main">Plant senescence</span> Process of aging in plants

Plant senescence is the process of aging in plants. Plants have both stress-induced and age-related developmental aging. Chlorophyll degradation during leaf senescence reveals the carotenoids, such as anthocyanin and xanthophylls, which are the cause of autumn leaf color in deciduous trees. Leaf senescence has the important function of recycling nutrients, mostly nitrogen, to growing and storage organs of the plant. Unlike animals, plants continually form new organs and older organs undergo a highly regulated senescence program to maximize nutrient export.

<span class="mw-page-title-main">Methyl jasmonate</span> Chemical compound

Methyl jasmonate is a volatile organic compound used in plant defense and many diverse developmental pathways such as seed germination, root growth, flowering, fruit ripening, and senescence. Methyl jasmonate is derived from jasmonic acid and the reaction is catalyzed by S-adenosyl-L-methionine:jasmonic acid carboxyl methyltransferase.

<span class="mw-page-title-main">Dwarf rabbit</span> Breed of rabbit

Dwarf rabbit refers either (formally) to a rabbit with the dwarfing gene, or (informally) to any small breed of domestic rabbit or specimen thereof, or (colloquially) to any small rabbit. Dwarfism is a genetic condition that may occur in humans and in many animals, including rabbits. True dwarfism is often associated with a cluster of physical abnormalities, including pituitary dwarfism. The process of dwarfing is used to selectively breed for smaller stature with each generation. Small stature is a characteristic of neoteny, which may account for the attraction of dwarf animals.

<i>Malus sieversii</i> Species of plant

Malus sieversii is a wild apple native to the mountains of Central Asia in southern Kazakhstan. It has recently been shown to be the primary ancestor of most cultivars of the domesticated apple. It was first described as Pyrus sieversii due to its similarities with pears in 1833 by Carl Friedrich von Ledebour, a German naturalist who saw them growing in the Altai Mountains.

Plant reproduction is the production of new offspring in plants, which can be accomplished by sexual or asexual reproduction. Sexual reproduction produces offspring by the fusion of gametes, resulting in offspring genetically different from either parent. Asexual reproduction produces new individuals without the fusion of gametes, resulting in clonal plants that are genetically identical to the parent plant and each other, unless mutations occur.

Important structures in plant development are buds, shoots, roots, leaves, and flowers; plants produce these tissues and structures throughout their life from meristems located at the tips of organs, or between mature tissues. Thus, a living plant always has embryonic tissues. By contrast, an animal embryo will very early produce all of the body parts that it will ever have in its life. When the animal is born, it has all its body parts and from that point will only grow larger and more mature. However, both plants and animals pass through a phylotypic stage that evolved independently and that causes a developmental constraint limiting morphological diversification.

A doubled haploid (DH) is a genotype formed when haploid cells undergo chromosome doubling. Artificial production of doubled haploids is important in plant breeding.

<span class="mw-page-title-main">Paclobutrazol</span> Chemical compound

Paclobutrazol (PBZ) is the ISO common name for an organic compound that is used as a plant growth retardant and triazole fungicide. It is a known antagonist of the plant hormone gibberellin, acting by inhibiting gibberellin biosynthesis, reducing internodal growth to give stouter stems, increasing root growth, causing early fruitset and increasing seedset in plants such as tomato and pepper. PBZ has also been shown to reduce frost sensitivity in plants. Moreover, paclobutrazol can be used as a chemical approach for reducing the risk of lodging in cereal crops. PBZ has been used by arborists to reduce shoot growth and shown to have additional positive effects on trees and shrubs. Among those are improved resistance to drought stress, darker green leaves, higher resistance against fungi and bacteria, and enhanced development of roots. Cambial growth, as well as shoot growth, has been shown to be reduced in some tree species.

<i>Thinopyrum intermedium</i> Species of flowering plant

Thinopyrum intermedium, known commonly as intermediate wheatgrass, is a sod-forming perennial grass in the Triticeae tribe of Pooideae native to Europe and Western Asia. It is part of a group of plants commonly called wheatgrasses because of the similarity of their seed heads or ears to common wheat. However, wheatgrasses generally are perennial, while wheat is an annual. It has gained the Royal Horticultural Society's Award of Garden Merit as an ornamental.

Elicitors in plant biology are extrinsic or foreign molecules often associated with plant pests, diseases or synergistic organisms. Elicitor molecules can attach to special receptor proteins located on plant cell membranes. These receptors are able to recognize the molecular pattern of elicitors and trigger intracellular defence signalling via the Octadecanoid pathway. This response results in the enhanced synthesis of metabolites which reduce damage and increase resistance to pest, disease or environmental stress. This is an immune response known as pattern triggered immunity or PTI. PTI is effective against necrotrophic microorganisms.

<span class="mw-page-title-main">Dwarfism in chickens</span>

Dwarfism in chickens is an inherited condition found in chickens consisting of a significant delayed growth, resulting in adult individuals with a distinctive small size in comparison with normal specimens of the same breed or population.


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