Hybrid (biology)

In biology, a hybrid is the offspring resulting from combining the qualities of two organisms of different breeds, varieties, species or genera through sexual reproduction. Hybrids are not always intermediates between their parents (such as in blending inheritance), but can show hybrid vigour, sometimes growing larger or taller than either parent. The concept of a hybrid is interpreted differently in animal and plant breeding, where there is interest in the individual parentage. In genetics, attention is focused on the numbers of chromosomes. In taxonomy, a key question is how closely related the parent species are.

Species are reproductively isolated by strong barriers to hybridisation, which include morphological differences, differing times of fertility, mating behaviors and cues, and physiological rejection of sperm cells or the developing embryo. Some act before fertilization and others after it. Similar barriers exist in plants, with differences in flowering times, pollen vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and the structure of the chromosomes. A few animal species and many plant species, however, are the result of hybrid speciation, including important crop plants such as wheat, where the number of chromosomes has been doubled.

Human impact on the environment has resulted in an increase in the interbreeding between regional species, and the proliferation of introduced species worldwide has also resulted in an increase in hybridisation. This genetic mixing may threaten many species with extinction, while genetic erosion in crop plants may be damaging the gene pools of many species for future breeding. A form of often intentional human-mediated hybridisation is the crossing of wild and domesticated species. This is common in both traditional horticulture and modern agriculture; many commercially useful fruits, flowers, garden herbs, and trees have been produced by hybridisation. One such flower, Oenothera lamarckiana, was central to early genetics research into mutationism and polyploidy. It is also more occasionally done in the livestock and pet trades; some well-known wild × domestic hybrids are beefalo and wolfdogs. Human selective breeding of domesticated animals and plants has resulted is the development of distinct breeds (usually called cultivars in reference to plants); crossbreeds between them (without any wild stock) are sometimes also imprecisely referred to as "hybrids".

Hybrid humans existed in prehistory. For example, Neanderthals and anatomically modern humans are thought to have interbred as recently as 40,000 years ago.

Mythological hybrids appear in human culture in forms as diverse as the Minotaur, blends of animals, humans and mythical beasts such as centaurs and sphinxes, and the Nephilim of the Biblical apocrypha described as the wicked sons of fallen angels and attractive women.

Juancito
A mule is a sterile hybrid of a male donkey and a female horse. Mules are smaller than horses but stronger than donkeys, making them useful as pack animals.

Etymology

Liger
Liger, a lion/tiger hybrid bred in captivity

The term hybrid is derived from Latin hybrida, used for crosses such as of a tame sow and a wild boar. The term came into popular use in English in the 19th century, though examples of its use have been found from the early 17th century.[1] Conspicuous hybrids are popularly named with portmanteau words, starting in the 1920s with the breeding of tiger–lion hybrids (liger and tigon).[2]

As seen by different disciplines

Animal and plant breeding

From the point of view of animal and plant breeders, there are several kinds of hybrid formed from crosses within a species, such as between different breeds.[3] Single cross hybrids result from the cross between two true-breeding organisms which produces an F1 hybrid (first filial generation). The cross between two different homozygous lines produces an F1 hybrid that is heterozygous; having two alleles, one contributed by each parent and typically one is dominant and the other recessive. Typically, the F1 generation is also phenotypically homogeneous, producing offspring that are all similar to each other.[4] Double cross hybrids result from the cross between two different F1 hybrids (i.e., there are four unrelated grandparents).[5] Three-way cross hybrids result from the cross between an F1 hybrid and an inbred line. Triple cross hybrids result from the crossing of two different three-way cross hybrids.[6] Top cross (or "topcross") hybrids result from the crossing of a top quality or pure-bred male and a lower quality female, intended to improve the quality of the offspring, on average.[7]

Population hybrids result from the crossing of plants or animals in one population with those of another population. These include interspecific hybrids or crosses between different breeds.[8]

In horticulture, the term stable hybrid is used to describe an annual plant that, if grown and bred in a small monoculture free of external pollen (e.g., an air-filtered greenhouse) produces offspring that are "true to type" with respect to phenotype; i.e., a true-breeding organism.[9]

Biogeography

Hybridisation can occur in the hybrid zones where the geographical ranges of species, subspecies, or distinct genetic lineages overlap. For example, the butterfly Limenitis arthemis has two major subspecies in North America, L. a. arthemis (the white admiral) and L. a. astyanax (the red-spotted purple). The white admiral has a bright, white band on its wings, while the red-spotted purple has cooler blue-green shades. Hybridisation occurs between a narrow area across New England, southern Ontario, and the Great Lakes, the "suture region". It is at these regions that the subspecies were formed.[10] Other hybrid zones have formed between described species of plants and animals.

Genetics

Gruppenweise artbildung unter spezieller berücksichtigung der gattung Oenothera (Taf. II) (7164908226)
Oenothera lamarckiana is a permanent natural hybrid, studied intensively by the geneticist Hugo de Vries. Illustration by De Vries, 1913

From the point of view of genetics, several different kinds of hybrid can be distinguished.[11][12] A genetic hybrid carries two different alleles of the same gene, where for instance one allele may code for a lighter coat colour than the other.[11][12] A structural hybrid results from the fusion of gametes that have differing structure in at least one chromosome, as a result of structural abnormalities.[11][12] A numerical hybrid results from the fusion of gametes having different haploid numbers of chromosomes.[11][12] A permanent hybrid results when only the heterozygous genotype occurs, as in Oenothera lamarckiana,[13] because all homozygous combinations are lethal.[11][12] In the early history of genetics, Hugo De Vries supposed these were caused by mutation.[14][15]

Taxonomy

From the point of view of taxonomy, hybrids differ according to their parentage. Hybrids between different subspecies (such as between the Bengal tiger and Siberian tiger) are called intra-specific hybrids.[16] Interspecific hybrids are the offspring from interspecies mating;[17] these sometimes result in hybrid speciation.[18] Intergeneric hybrids result from matings between different genera, such as between sheep and goats.[19] Interfamilial hybrids, such as between chickens and guineafowl or pheasants, are reliably described but extremely rare.[20] Interordinal hybrids (between different orders) are few, but have been made with the sea urchin Strongylocentrotus purpuratus (female) and the sand dollar Dendraster excentricus (male).[21]

Biology

Expression of parental traits

Exotic Pheasant (400938368)
Hybrid between Lady Amherst's pheasant (Chrysolophus amherstiae) and another species, probably golden pheasant (Chrysolophus pictus)

When two distinct types of organisms breed with each other, the resulting hybrids typically have intermediate traits (e.g., one plant parent has red flowers, the other has white, and the hybrid, pink flowers).[22] Commonly, hybrids also combine traits seen only separately in one parent or the other (e.g., a bird hybrid might combine the yellow head of one parent with the orange belly of the other).[22]

Mechanisms of reproductive isolation

Interspecific hybrids are bred by mating individuals from two species, normally from within the same genus. The offspring display traits and characteristics of both parents, but are often sterile, preventing gene flow between the species.[23] Sterility is often attributed to the different number of chromosomes between the two species. For example, donkeys have 62 chromosomes, horses have 64 chromosomes, and mules or hinnies have 63 chromosomes. Mules, hinnies, and other normally sterile interspecific hybrids cannot produce viable gametes, because differences in chromosome structure prevent appropriate pairing and segregation during meiosis, meiosis is disrupted, and viable sperm and eggs are not formed. However, fertility in female mules has been reported with a donkey as the father.[24]

A variety of mechanisms limit the success of hybridisation, including the large genetic difference between most species. Barriers include morphological differences, differing times of fertility, mating behaviors and cues, and physiological rejection of sperm cells or the developing embryo. Some act before fertilization; others after it.[25][26][27][28]

In plants, some barriers to hybridisation include blooming period differences, different pollinator vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and structural differences of the chromosomes.[29]

Speciation

Triticum durum
Durum wheat is tetraploid, derived from wild emmer wheat, which is a hybrid of two diploid wild grasses, Triticum urartu and a wild goatgrass such as Aegilops searsii or Ae. speltoides.[30]

A few animal species are the result of hybridization. The Lonicera fly is a natural hybrid. The American red wolf appears to be a hybrid of the gray wolf and the coyote,[31] although its taxonomic status has been a subject of controversy.[32][33][34] The European edible frog is a semi-permanent hybrid between pool frogs and marsh frogs; its population requires the continued presence of at least one of the parent species.[35] Cave paintings indicate that the European bison is a natural hybrid of the aurochs and the steppe bison.[36][37]

Plant Hybridization is more commonplace compared to animal hybridization. Many crop species are hybrids, including notably the polyploid wheats: some have four sets of chromosomes (tetraploid) or six (hexaploid), while other wheat species have (like most eukaryotic organisms) two sets (diploid), so hybridization events likely involved the doubling of chromosome sets, causing immediate genetic isolation.[38]

Hybridization may be important in speciation in some plant groups. However, homoploid hybrid speciation (not increasing the number of sets of chromosomes) may be rare: by 1997, only 8 natural examples had been fully described. Experimental studies suggest that hybridization offers a rapid route to speciation, a prediction confirmed by the fact that early generation hybrids and ancient hybrid species have matching genomes, meaning that once hybridization has occurred, the new genome can remain stable.[39]

Many hybrid zones are known where the ranges of two species meet, and hybrids are continually produced in great numbers. These hybrid zones are useful as biological model systems for studying the mechanisms of speciation. Recently DNA analysis of a bear shot by a hunter in the North West Territories confirmed the existence of naturally-occurring and fertile grizzly–polar bear hybrids.[40]

Hybrid vigour

Salvia jurisicii x nutans - Flickr - peganum
Hybrid vigour: Salvia jurisicii x nutans hybrids (top centre, with flowers) are taller than their parents Salvia jurisicii (centre tray) or Salvia nutans (top left).

Hybridization between reproductively isolated species often results in hybrid offspring with lower fitness than either parental. However, hybrids are not, as might be expected, always intermediate between their parents (as if there were blending inheritance), but are sometimes stronger or perform better than either parental lineage or variety, a phenomenon called heterosis, hybrid vigour, or heterozygote advantage. This is most common with plant hybrids.[41] A transgressive phenotype is a phenotype that displays more extreme characteristics than either of the parent lines.[42] Plant breeders use several techniques to produce hybrids, including line breeding and the formation of complex hybrids. An economically important example is hybrid maize (corn), which provides a considerable seed yield advantage over open pollinated varieties. Hybrid seed dominates the commercial maize seed market in the United States, Canada and many other major maize-producing countries.[43]

In a hybrid, any trait that falls outside the range of parental variation (and is thus not simply intermediate between its parents) is considered heterotic. Positive heterosis produces more robust hybrids, they might be stronger or bigger; while the term negative heterosis refers to weaker or smaller hybrids.[44] Heterosis is common in both animal and plant hybrids. For example, hybrids between a lion and a tigress ("ligers") are much larger than either of the two progenitors, while "tigons" (lioness × tiger) are smaller. Similarly, the hybrids between the common pheasant (Phasianus colchicus) and domestic fowl (Gallus gallus) are larger than either of their parents, as are those produced between the common pheasant and hen golden pheasant (Chrysolophus pictus).[45] Spurs are absent in hybrids of the former type, although present in both parents.[46]

Human influence

Anthropogenic hybridization

Hybridization is greatly influenced by human impact on the environment,[47] through effects such as habitat fragmentation and species introductions.[48] Such impacts make it difficult to conserve the genetics of populations undergoing introgressive hybridization. Humans have introduced species worldwide to environments for a long time, both intentionally for purposes such as biological control, and unintentionally, as with accidental escapes of individuals. Introductions can drastically affect populations, including through hybridization.[12][49]

Management

Aquilegia pubescens-formosa hybrid-swarm flowers close
Examples of hybrid flowers from hybrid swarms of Aquilegia pubescens and Aquilegia formosa

There is a kind of continuum with three semi-distinct categories dealing with anthropogenic hybridization: hybridization without introgression, hybridization with widespread introgression (backcrossing with one of the parent species), and hybrid swarms (highly variable populations with much interbreeding as well as backcrossing with the parent species). Depending on where a population falls along this continuum, the management plans for that population will change. Hybridization is currently an area of great discussion within wildlife management and habitat management. Global climate change is creating other changes such as difference in population distributions which are indirect causes for an increase in anthropogenic hybridization.[47]

Conservationists disagree on when is the proper time to give up on a population that is becoming a hybrid swarm, or to try and save the still existing pure individuals. Once a population becomes a complete mixture, the goal becomes to conserve those hybrids to avoid their loss. Conservationists treat each case on its merits, depending on detecting hybrids within the population. It is nearly impossible to formulate a uniform hybridization policy, because hybridization can occur beneficially when it occurs "naturally", and when hybrid swarms are the only remaining evidence of prior species, they need to be conserved as well.[47]

Genetic mixing and extinction

Regionally developed ecotypes can be threatened with extinction when new alleles or genes are introduced that alter that ecotype. This is sometimes called genetic mixing.[50] Hybridization and introgression, which can happen in natural and hybrid populations, of new genetic material can lead to the replacement of local genotypes if the hybrids are more fit and have breeding advantages over the indigenous ecotype or species. These hybridization events can result from the introduction of non-native genotypes by humans or through habitat modification, bringing previously isolated species into contact. Genetic mixing can be especially detrimental for rare species in isolated habitats, ultimately affecting the population to such a degree that none of the originally genetically distinct population remains.[51][52]

Effect on biodiversity and food security

Irrigation1
The Green Revolution of the 20th century relied on hybridization to create high-yielding varieties, along with increased reliance on inputs of fertilizers, pesticides, and irrigation.[53]

In agriculture and animal husbandry, the Green Revolution's use of conventional hybridization increased yields by breeding "high-yielding varieties". The replacement of locally indigenous breeds, compounded with unintentional cross-pollination and crossbreeding (genetic mixing), has reduced the gene pools of various wild and indigenous breeds resulting in the loss of genetic diversity.[54] Since the indigenous breeds are often well-adapted to local extremes in climate and have immunity to local pathogens, this can be a significant genetic erosion of the gene pool for future breeding. Therefore, commercial plant geneticists strive to breed "widely adapted" cultivars to counteract this tendency.[55]

In different taxa

In animals

Familiar examples of equid hybrids are the mule, a cross between a female horse and a male donkey, and the hinny, a cross between a female donkey and a male horse. Pairs of complementary types like the mule and hinny are called reciprocal hybrids.[56] Among many other mammal crosses are hybrid camels, crosses between a bactrian camel and a dromedary.[57] The first known instance of hybrid speciation in marine mammals was discovered in 2014. The clymene dolphin (Stenella clymene) is a hybrid of two Atlantic species, the spinner and striped dolphins.[58]

Cagebird breeders sometimes breed bird hybrids known as mules between species of finch, such as goldfinch × canary.[59]

Among amphibians, Japanese giant salamanders and Chinese giant salamanders have created hybrids that threaten the survival of Japanese giant salamanders because of competition for similar resources in Japan.[60]

Among fish, a group of about fifty natural hybrids between Australian blacktip shark and the larger common blacktip shark was found by Australia's eastern coast in 2012.[61]

Among insects, so-called killer bees were accidentally created during an attempt to breed a strain of bees that would both produce more honey and be better adapted to tropical conditions. It was done by crossing a European honey bee and an African bee.[62]

The Colias eurytheme and C. philodice butterflies have retained enough genetic compatibility to produce viable hybrid offspring.[63] Hybrid speciation may have produced the diverse Heliconius butterflies,[64] but that is disputed.[65]

Zeedonk 800

A "zonkey", a zebra/donkey hybrid

Jaglion

A "jaglion", a jaguar/lion hybrid

In plants

Platanus xhispanica in Lucenec9
The London plane, Platanus × acerifolia is a natural hybrid, popular for street planting.

Plant species hybridize more readily than animal species, and the resulting hybrids are fertile more often. Many plant species are the result of hybridization, combined with polyploidy, which duplicates the chromosomes. Chromosome duplication allows orderly meiosis and so viable seed can be produced.[66]

Plant hybrids are generally given names that include an "×" (not in italics), such as Platanus × acerifolia for the London plane, a natural hybrid of P. orientalis (oriental plane) and P. occidentalis (American sycamore).[67][68] The parent's names may be kept in their entirety, as seen in Prunus persica × Prunus americana, with the female parent's name given first, or if not known, the parent's names given alphabetically.[69]

Plant species that are genetically compatible may not hybridize in nature for various reasons, including geographical isolation, differences in flowering period, or differences in pollinators. Species that are brought together by humans in gardens may hybridize naturally, or hybridization can be facilitated by human efforts, such as altered flowering period or artificial pollination. Hybrids are sometimes created by humans to produce improved plants that have some of the characteristics of each of the parent species. Much work is now being done with hybrids between crops and their wild relatives to improve disease-resistance or climate resilience for both agricultural and horticultural crops.[70]

Some crop plants are hybrids from different genera (intergeneric hybrids), such as Triticale, × Triticosecale, a wheat–rye hybrid.[71] Most modern and ancient wheat breeds are themselves hybrids; bread wheat, Triticum aestivum, is a hexaploid hybrid of three wild grasses.[30] Several commercial fruits including loganberry (Rubus × loganobaccus)[72] and grapefruit (Citrus × paradisi)[73] are hybrids, as are garden herbs such as peppermint (Mentha × piperita),[74] and trees such as the London plane (Platanus × acerifolia).[75][76] Among many natural plant hybrids is Iris albicans, a sterile hybrid that spreads by rhizome division,[77] and Oenothera lamarckiana, a flower that was the subject of important experiments by Hugo de Vries that produced an understanding of polyploidy.[13]

Trilliumhybrid2

A sterile hybrid between Trillium cernuum and T. grandiflorum

Lily Lilium 'Citronella' Flower

An ornamental lily hybrid known as Lilium 'Citronella'[78]

Sterility in a non-polyploid hybrid is often a result of chromosome number; if parents are of differing chromosome pair number, the offspring will have an odd number of chromosomes, which leaves them unable to produce chromosomally-balanced gametes.[79] While that is undesirable in a crop such as wheat, for which growing a crop that produces no seeds would be pointless, it is an attractive attribute in some fruits. Triploid bananas and watermelons are intentionally bred because they produce no seeds and are also parthenocarpic.[80]

Oase 2-Homo Sapiens
Oase 2 skull may be a human-Neanderthal hybrid.

In humans

There is evidence of hybridisation between modern humans and other species of the genus Homo. In 2010, the Neanderthal genome project showed that 1–4% of DNA from all people living today, apart from most Sub-Saharan Africans, is of Neanderthal heritage. Analyzing the genomes of 600 Europeans and East Asians found that combining them covered 20% of the Neanderthal genome that is in the modern human population.[81] Ancient human populations lived and interbred with Neanderthals, Denisovans, and at least one other extinct Homo species.[82] Thus, Neanderthal and Denisovan DNA has been incorporated into human DNA by introgression.[83]

In 1998, a complete prehistorical skeleton found in Portugal, the Lapedo child, had features of both anatomically modern humans and Neanderthals.[84] Some ancient human skulls with especially large nasal cavities and unusually shaped braincases represent human-Neanderthal hybrids. A 37,000- to 42,000-year-old human jawbone found in Romania's Oase cave contains traces of Neanderthal ancestry[a] from only four to six generations earlier.[86] All genes from Neanderthals in the current human population are descended from Neanderthal fathers and human mothers.[87] A Neanderthal skull unearthed in Italy in 1957 reveals Neanderthal mitochondrial DNA, which is passed on through only the maternal lineage, but the skull has a chin shape similar to modern humans. It is proposed that it was the offspring of a Neanderthal mother and a human father.[88]

In mythology

Minotauros Myron NAMA 1664 n1
The Minotaur of ancient Greek mythology was (in one version of the myth) supposedly the offspring of Pasiphaë and a white bull.

Folk tales and myths sometimes contain mythological hybrids; the Minotaur was the offspring of a human, Pasiphaë, and a white bull.[89] More often, they are composites of the physical attributes of two or more kinds of animals, mythical beasts, and humans, with no suggestion that they are the result of interbreeding, as in the centaur (man/horse), chimera (goat/lion/snake), hippocamp (fish/horse), and sphinx (woman/lion).[90] The Old Testament mentions a first generation of half-human hybrid giants, the Nephilim,[91][92] while the apocryphal Book of Enoch describes the Nephilim as the wicked sons of fallen angels and attractive women.[93]

See also

Notes

  1. ^ Signs of Neanderthal lineage include a wide jaw and large teeth that get bigger toward the back of the mouth.[85]

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External links

Animal Crossing (disambiguation)

Animal Crossing is a series of video games produced by Nintendo.

Animal Crossing may also refer to:

Animal Crossing series:

Animal Crossing (video game), for GameCube

Animal Crossing: Wild World, for Nintendo DS

Animal Crossing: City Folk, for Wii

Animal Crossing: New Leaf, for Nintendo 3DS

Animal Crossing: Pocket Camp, for iOS/Android devices

Dōbutsu no Mori (film), the animated film based on the series

Wildlife crossing, a structure letting wildlife cross a man-made barrier

Hybrid (biology) the result of crossing two animals or plants

Breeding

Breeding is sexual reproduction that produces offspring, usually animals or plants.

Breeding may refer to:

Breeding in the wild, the natural process of reproduction in the animal kingdom

Animal husbandry, through selected specimens such as dogs, horses, and rabbits

Sexual reproduction of plants

Plant breeding, through specimens selected by humans for desirable traits

Comparison of Standard Malay and Indonesian

Malaysian and Indonesian are two standardised registers of the Malay language, used in Malaysia and Indonesia, respectively. Both varieties are generally mutually intelligible, yet there are noticeable differences in spelling, grammar, pronunciation and vocabulary, as well as the predominant source of loanwords. The differences can range from those mutually unintelligible with one another, to those having a closer familial resemblance. The regionalised and localised varieties of Malay can become a catalyst for intercultural conflict, especially in higher education.

Domestication of animals

The domestication of animals is the mutual relationship between animals and the humans who have influence on their care and reproduction. Charles Darwin recognized a small number of traits that made domesticated species different from their wild ancestors. He was also the first to recognize the difference between conscious selective breeding in which humans directly select for desirable traits, and unconscious selection where traits evolve as a by-product of natural selection or from selection on other traits. There is a genetic difference between domestic and wild populations. There is also such a difference between the domestication traits that researchers believe to have been essential at the early stages of domestication, and the improvement traits that have appeared since the split between wild and domestic populations. Domestication traits are generally fixed within all domesticates, and were selected during the initial episode of domestication of that animal or plant, whereas improvement traits are present only in a proportion of domesticates, though they may be fixed in individual breeds or regional populations.Domestication should not be confused with taming. Taming is the conditioned behavioral modification of a wild-born animal when its natural avoidance of humans is reduced and it accepts the presence of humans, but domestication is the permanent genetic modification of a bred lineage that leads to an inherited predisposition toward humans. Certain animal species, and certain individuals within those species, make better candidates for domestication than others because they exhibit certain behavioral characteristics: (1) the size and organization of their social structure; (2) the availability and the degree of selectivity in their choice of mates; (3) the ease and speed with which the parents bond with their young, and the maturity and mobility of the young at birth; (4) the degree of flexibility in diet and habitat tolerance; and (5) responses to humans and new environments, including flight responses and reactivity to external stimuli.It is proposed that there were three major pathways that most animal domesticates followed into domestication: (1) commensals, adapted to a human niche (e.g., dogs, cats, fowl, possibly pigs); (2) prey animals sought for food (e.g., sheep, goats, cattle, water buffalo, yak, pig, reindeer, llama, alpaca, and turkey); and (3) targeted animals for draft and nonfood resources (e.g., horse, donkey, camel). The dog was the first to be domesticated, and was established across Eurasia before the end of the Late Pleistocene era, well before cultivation and before the domestication of other animals. Unlike other domestic species which were primarily selected for production-related traits, dogs were initially selected for their behaviors. The archaeological and genetic data suggest that long-term bidirectional gene flow between wild and domestic stocks – including donkeys, horses, New and Old World camelids, goats, sheep, and pigs – was common. One study has concluded that human selection for domestic traits likely counteracted the homogenizing effect of gene flow from wild boars into pigs and created domestication islands in the genome. The same process may also apply to other domesticated animals.

Equid hybrid

Equid hybrids, also called hybrid equines, are created from the crossing of members from the horse family such as a horse, donkey and zebra.

Form (botany)

In botanical nomenclature, a form (forma, plural formae) is one of the "secondary" taxonomic ranks, below that of variety, which in turn is below that of species; it is an infraspecific taxon. If more than three ranks are listed in describing a taxon, the "classification" is being specified, but only three parts make up the "name" of the taxon: a genus name, a specific epithet, and an infraspecific epithet.

The abbreviation "f." or the full "forma" should be put before the infraspecific epithet to indicate the rank. It is not italicised.

For example:

Acanthocalycium spiniflorum f. klimpelianum or

Acanthocalycium spiniflorum forma klimpelianum (Weidlich & Werderm.) Donald

Crataegus aestivalis (Walter) Torr. & A.Gray var. cerasoides Sarg. f. luculenta Sarg. is a classification of a plant whose name is:

Crataegus aestivalis (Walter) Torr. & A.Gray f. luculenta Sarg.A form usually designates a group with a noticeable morphological deviation. The usual taxonomic practice is that the individuals classified within the form are not necessarily known to be closely related (they may not form a clade). For instance, white-flowered plants of species that usually have coloured flowers can be grouped and named (e.g., as "f. alba"). Formae apomicticae are sometimes named among plants that reproduce asexually, by apomixis. There are theoretically countless numbers of forms based on minor genetic differences, and only a few that have particular significance are likely to be named.

Gene map

Gene maps help describe the spatial arrangement of genes on a chromosome. Genes are designated to a specific location on a chromosome known as the locus and can be used as molecular markers to find the distance between other genes on a chromosome. Maps provide researchers with the opportunity to predict the inheritance patterns of specific traits, which can eventually lead to a better understanding of disease-linked traits.

History of plant breeding

Plant breeding started with sedentary agriculture, particularly the domestication of the first agricultural plants, a practice which is estimated to date back 9,000 to 11,000 years. Initially, early human farmers selected food plants with particular desirable characteristics and used these as a seed source for subsequent generations, resulting in an accumulation of characteristics over time. In time however, experiments began with deliberate hybridization, the science and understanding of which was greatly enhanced by the work of Gregor Mendel. Mendel's work ultimately led to the new science of genetics. Modern plant breeding is applied genetics, but its scientific basis is broader, covering molecular biology, cytology, systematics, physiology, pathology, entomology, chemistry, and statistics (biometrics). It has also developed its own technology. Plant breeding efforts are divided into a number of different historical landmarks.

Hybrid seed

In agriculture and gardening, hybrid seed is produced by cross-pollinated plants. Hybrid seed production is predominant in modern agriculture and home gardening. It is one of the main contributors to the dramatic rise in agricultural output during the last half of the 20th century. The alternatives to hybridization are open pollination and clonal propagation.All of the hybrid seeds planted by the farmer will produce similar plants, while the seeds of the next generation from those hybrids will not consistently have the desired characteristics. Controlled hybrids provide very uniform characteristics because they are produced by crossing two inbred strains. Elite inbred strains are used that express well-documented and consistent phenotypes (such as high crop yield) that are relatively good for inbred plants.Hybrids are chosen to improve the characteristics of the resulting plants, such as better yield, greater uniformity, improved color, disease resistance.

An important factor is the heterosis or combining ability of the parent plants. Crossing any particular pair of inbred strains may or may not result in superior offspring. The parent strains used are therefore carefully chosen so as to achieve the uniformity that comes from the uniformity of the parents, and the superior performance that comes from heterosis.

Index of biology articles

Biology is the study of life and its processes. Biologists study all aspects of living things, including all of the many life forms on earth and the processes in them that enable life. These basic processes include the harnessing of energy, the synthesis and duplication of the materials that make up the body, the reproduction of the organism and many other functions. Biology, along with chemistry and physics is one of the major disciplines of natural science.

List of hybrid creatures in folklore

The following is a list of hybrid entities from the folklore record grouped morphologically based on their constituent species. Hybrids not found in classical mythology but developed in the context of modern pop culture are listed in a separate section. For actual hybridization in zoology, see Hybrid (biology)#In different taxa.

Perennial rice

Perennial rice are varieties of long-lived rice that are capable of regrowing season after season without reseeding; they are being developed by plant geneticists at several institutions. Although these varieties are genetically distinct and will be adapted for different climates and cropping systems, their lifespan is so different from other kinds of rice that they are collectively called perennial rice. Perennial rice—like many other perennial plants—can spread by horizontal stems below or just above the surface of the soil but they also reproduce sexually by producing flowers, pollen and seeds. As with any other grain crop, it is the seeds that are harvested and eaten by humans.

Perennial rice is one of several perennial grains that have been proposed, researched or are being developed, including perennial wheat, sunflower, and sorghum. Agronomists have argued that increasing the amount of agricultural landscapes covered at any given time with perennial crops is an excellent way to stabilize and improve the soil, and provide wildlife habitat.Perennial rice breeding was initiated at the International Rice Research Institute, Philippines and are currently being developed at the Yunnan Academy of Agricultural Sciences, People's Republic of China, and other institutions, but are not yet available for distribution.

Plant variety

Plant variety may refer to

Variety (botany), a taxonomic nomenclature rank in botany, below subspecies, but above subvariety and form

Plant variety (law), a non-taxonomic, exclusively legal term

An informal and ambiguous substitute for form (botany) (a taxonomic nomenclature rank in botany, below variety (botany))

An older substitute for cultivar or hybrid (biology), now discouraged by the International Code of Nomenclature for Cultivated Plants. It is still used especially with regard to grapes and rice.

Variety (botany)

In botanical nomenclature, variety (abbreviated var.; in Latin: varietas) is a taxonomic rank below that of species and subspecies, but above that of form. As such, it gets a three-part infraspecific name. It is sometimes recommended that the subspecies rank should be used to recognize geographic distinctiveness, whereas the variety rank is appropriate if the taxon is seen throughout the geographic range of the species.

Evolution
Population genetics
Development
Of taxa
Of organs
Of processes
Tempo and modes
Speciation
History
Philosophy
Related
Key concepts
Selection
Effects of selection
on genomic variation
Genetic drift
Founders
Related topics
Bovidae
Camelidae
Canidae
Delphinidae
Elephantidae
Equus
Felidae
Hominidae
Macropodinae
Sus
Mustela
Ursus
Lysenkoists
Dissidents
Concepts
Soviet policies
Basic concepts
Geographic modes
Isolating factors
Hybridization
Speciation in taxa

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