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133-623: Citrus taxonomy is complex and controversial. Cultivated citrus are derived from various citrus species found in the wild. Some are only selections of the original wild types, many others are hybrids between two or more original species, and some are backcrossed hybrids between a hybrid and one of the hybrid's parent species. Citrus plants hybridize easily between species with completely different morphologies, and similar-looking citrus fruits may have quite different ancestries. Some differ only in disease resistance. Conversely, different-looking varieties may be nearly genetically identical, and differ only by

266-475: A synthetic population . 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. Hybridization can occur in the hybrid zones where the geographical ranges of species, subspecies, or distinct genetic lineages overlap. For example,

399-460: A bactrian camel and a dromedary . There are many examples of felid hybrids , including the liger . The oldest-known animal hybrid bred by humans is the kunga equid hybrid produced as a draft animal and status symbol 4,500 years ago in Umm el-Marra , present-day Syria . The first known instance of hybrid speciation in marine mammals was discovered in 2014. The clymene dolphin ( Stenella clymene )

532-526: A bud mutation . Genomic analysis of wild and domesticated citrus cultivars has suggested that the progenitor of modern citrus species expanded out of the Himalayan foothills in a rapid radiation that has produced at least 11 wild species in South and East Asia and Australia, with more than a half-dozen additional candidates for which either insufficient characterization prevents definitive species designation, or there

665-426: A finger lime and calamansi cross). Initially, many citrus types were identified and named by individual taxonomists, resulting in a large number of identified species: 870 by a 1969 count. Some order was brought to citrus taxonomy by two unified classification schemes, those of Chōzaburō Tanaka and Walter Tennyson Swingle , that can be viewed as extreme alternative visions of the genus. Swingle's system divided

798-557: A species by themselves, especially in folk taxonomy , are: All of these hybrids have in turn been bred back with their parent stocks or with other pure or hybrid citrus to form a broad array of fruits. Naming of these is inconsistent, with some bearing a variant of the name of one of the parents or simply another citrus with superficially-similar fruit, a distinct name, or a portmanteau of ancestral species. The Ponderosa lemon ( Citrus limon × medica ) and Florentine citron ( Citrus × limonimedica ) are both true lemon/citron hybrids,

931-429: A tetraploid . Historically, hybrids with similar characteristics have been placed together in a number of hybrid species, yet relatively recent genomic analysis has revealed some hybrids assigned to the same species to be of quite distinct ancestry. No alternative system of grouping fruit in hybrid species has been adopted. While most citrus hybrids derive from the three core species, hybrids have also been derived from

1064-555: A bear shot by a hunter in the Northwest Territories confirmed the existence of naturally occurring and fertile grizzly–polar bear hybrids . 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,

1197-491: A binomial name, while a unique species name is reserved for each combination of ancestral species, independent of the specific order of crossing or proportional representation of the ancestral species in a given hybrid. Individual hybrids of each type are then distinguished by a variety name. Thus hybrids that are crosses between mandarin ( C. reticulata ) and pomelo ( C. maxima ) would all be C. × aurantium , with specific crosses including: C. × aurantium var. sinensis for

1330-567: A characteristic passed down to the subspecies' hybrid descendants such as hybrid mandarins, oranges, lemons and grapefruit. A distinct class of mandarins are native to the Japanese and neighboring islands. Initial characterization of one of these, the Tachibana orange (Tanaka's Citrus tachibana ), native to Taiwan, the Ryukyu Islands and southern Japan, classified it as a subspecies nesting within

1463-459: A citrus hybrid. His Citrus he likewise subdivided into two subgenera: citrons, pomelos, mandarins, oranges, grapefruits and lemons were placed in subgenus Eucitrus (later called simply subgenus Citrus ), while the hardy but slow-growing trees with relatively unpalatable fruit he placed in subgenus Papeda . His genus Citrus consisted of just 16 species , dividing them further into varieties , and lastly cultivars or hybrids. The Swingle system

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1596-405: A combination of the characteristics of the two contributing species due to the presence of cells from both in that fruit. In an extreme example, on separate branches Bizzarria produces fruit identical to each of the two contributing species, but also fruit that appears to be half one species and half the other, unmixed. In taxonomy, graft hybrids are distinguished from genetic hybrids by designating

1729-407: A consequence of the rapid initial divergence. Taxonomic terminology is not yet settled. Most hybrids express different ancestral traits when planted from seeds ( F2 hybrids ) and can continue a stable lineage only through vegetative propagation . Some hybrids do reproduce true to type via nucellar seeds in a process called apomixis . As such, many hybrid species represent the clonal progeny of

1862-508: 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. 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

1995-504: 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. Polar bears and brown bears are another case of a hybridizing species pairs, and introgression among non-sister species of bears appears to have shaped the Ursidae family tree. Among many other mammal crosses are hybrid camels , crosses between

2128-411: A degree of acceptance, two modern phylogenetic studies obtained results in conflict with Mabberley's, and retained Feronioella as a distinct genus closely related to Luminia , with which Swingle had placed Feroniella in a grouping referred to as 'wood apples'. The pomelo ( Citrus maxima ), a second of the core species from which most citrus hybrids have derived, is native to southeast Asia. Among

2261-501: A degree that none of the originally genetically distinct population remains. 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

2394-419: A different branch of the tribe Citreae and given a new genus by Swingle) and more controversially Poncirus , which is the earliest branching of Swingle's genera, and a genus not previously recognized as a close citrus relative, Feroniella . There remains a lack of consensus as to which wild plants and hybrids merit distinct species status, a phenomenon exacerbated by the prior failure to correctly identify

2527-407: A different organism is called a chimera . Hybrids are not always intermediates between their parents such as in blending inheritance (a now discredited theory in modern genetics by particulate inheritance ), but can show hybrid vigor , 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

2660-602: A donkey as the father. A variety of mechanisms limit the success of hybridization, 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. In plants, some barriers to hybridization include blooming period differences, different pollinator vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and structural differences of

2793-548: A group of about 50 natural hybrids between Australian blacktip shark and the larger common blacktip shark was found by Australia's eastern coast in 2012. Russian sturgeon and American paddlefish were hybridized in captivity when sperm from the paddlefish and eggs from the sturgeon were combined, unexpectedly resulting in viable offspring. This hybrid is called a sturddlefish . The two genera Asymmetron and Branchiostoma are able to produce viable hybrid offspring, even if none have lived into adulthood so far, despite

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2926-508: A group of organisms allows the identification of events in gene evolution such as gene duplication or gene deletion . Often, such events are evolutionarily relevant. For example, multiple duplications of genes encoding degradative enzymes of certain families is a common adaptation in microbes to new nutrient sources. On the contrary, loss of genes is important in reductive evolution , such as in intracellular parasites or symbionts. Whole genome duplication events, which potentially duplicate all

3059-478: A hybrid as such a cross between two species can present challenges. In some cases the parental species that gave rise to a hybrid have yet to be determined, while genotyping reveals some hybrids to descend from three or more ancestral species. In the Ollitrault system, a hybrid will be given a species name corresponding to the ancestral species contributions to it, as well as a distinctive variety name. Hybrid taxonomy

3192-410: A large range of hybrids. Swingle saw three species of mandarin, while Tanaka identified five groups with a total of 36 species. Webber (1948) divided them into four groups, king, satsuma, mandarin, and tangerine, and Hodgson (1967) saw in them four species. Genomic analysis suggests just one mainland-Asian species, Citrus reticulata . In an observation originally made in a study of their hybrid progeny,

3325-439: 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. 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

3458-1239: A mandarin native to the Ryukyu Islands designated C. ryukyuensis , and a rare wild species from Southeast Asia, the mountain citron . A number of further species originally placed in other genera have recently been subsumed into Citrus as a result of phylogenetic analysis, but these have yet to be characterized on a phylogenomic level to confirm their status as unique pure species. Artificial interbreeding seems possible among all citrus plants, though there are certain limitations to natural interbreeding due to plant physiology and differences in natural breeding seasons. This ability to cross-pollinate extends to some related species that some classifications place in distinct genera. The ability of citrus hybrids to self-pollinate and to reproduce sexually also helps create new varieties, as does spontaneous mutation and genome duplication. The three most predominant ancestral citrus taxa are citron ( C. medica ), pomelo ( C. maxima ), and mandarin ( C. reticulata ). These taxa interbreed freely, despite being quite genetically distinct, having arisen through allopatric speciation , with citrons evolving in northern Indochina , pomelos in

3591-494: A mate among the species that raised it, instead of the species of its biological parents. Cagebird breeders sometimes breed bird hybrids known as mules between species of finch , such as goldfinch × canary . 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. Among fish,

3724-432: A phenomenon called heterosis, hybrid vigour, or heterozygote advantage . This is most common with plant hybrids. A transgressive phenotype is a phenotype that displays more extreme characteristics than either of the parent lines. 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

3857-592: A pomelo/citron hybrid, though another lumia variety, the Pomme d'Adam , is a micrantha/citron cross, like the Key lime. The most common and commercially popular 'limes', the Persian limes , are Key lime/lemon hybrids that combine the genetic lineages of four ancestral citrus species: mandarin, pomelo, citron and micrantha. While most other citrus are diploid , many of the Key lime hybrid progeny have unusual chromosome numbers. For example,

3990-492: A recent genomic study by Wu, et al. identified just ten ancestral species of citrus among more than a hundred cultivars studied. Of these ten, seven were native to Asia: pomelo ( Citrus maxima ), the 'pure' mandarins ( C. reticulata – most mandarin cultivars were hybrids of this species with pomelo), citrons ( C. medica ), micranthas ( C. micrantha ), the Ichang papeda ( C. cavaleriei ), the mangshanyegan ( C. mangshanensis ), and

4123-441: A replacement species name for this Yunnan trifoliate orange. Citrus hybrids include many varieties and species that have been selected by plant breeders . This is done not only for the useful characteristics of the fruit, but also for plant size and growth characteristics such as cold-tolerance. Some citrus hybrids occurred naturally, and others have been deliberately created, either by cross pollination and selection among

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4256-468: A second type are the result of hybridization with pomelos followed by subsequent backcrossing with mandarins to retain only a few pomelo traits. The third type arose more recently from the crossing of these hybrids again with pomelos or sweet oranges (which are themselves crosses of hybrid mandarins and pomelos). This produces mandarins with more, longer stretches of pomelo DNA. Some commercial mandarins are hybrids with lemons, while several were found to have

4389-555: A separate genus, Fortunella , named in honor of Robert Fortune . He divided the kumquats into two subgenera, the Protocitrus , containing the primitive Hong Kong kumquat ( F. hindsii ), and Eufortunella , comprising the round ( F. japonica ), oval kumquat ( F. margarita ) and Meiwa kumquats ( F. crassifolia ), to which Tanaka added two others, the Malayan kumquat (which he christened F. swinglei but more commonly called F. polyandra ) and

4522-455: A significant contribution (35–65%) from papedas. 'Mangshan wild mandarin' is a name used for all of the similar-looking wild mandarin-like fruit of the Mangshan area, but has been found to include two genetically-distinct groups, one representing pure, wild "true" mandarins (the mangshanyeju subspecies of C. reticulata ), and the other the genetically-distinct and only distantly-related species,

4655-411: A single hybrid ancestor. Novel varieties, and in particular seedless or reduced-seed varieties, have also been generated from these unique hybrid ancestral lines using gamma irradiation of budwood to induce mutations. Mandarin oranges (tangerines, satsumas – Citrus reticulata ) are one of the basic species, but the name mandarin is also used more generally for all small, easily peeled citrus, including

4788-404: A single original F1 cross, though others combine fruit with similar characteristics that have arisen from distinct crosses. All of the wild 'pure' citrus species trace to a common ancestor that lived in the Himalayan foothills, where a late- Miocene citrus fossil, Citrus linczangensis , has been found. At that time, a lessening of the monsoons and resultant drier climate in the region allowed

4921-405: A species, such as between different breeds . 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,

5054-489: A subspecies-level division has been characterized in this mainland-Asian species. Wang, et al. , found that domesticated mandarins fell into two genetic clusters that linked to different branches of the tree of wild mandarins, had different deduced population histories and had distinct patterns of pomelo introgression, suggesting that they derive from separate domestication events. Wu, et al. , would later extend this observation, similarly detecting two divergent subspecies within

5187-455: Is "× Citroncirus ". The group includes the citrange , a hybrid between the trifoliate and sweet oranges , and the citrumelo , a hybrid of trifoliate orange and 'Duncan' grapefruit . As with Citrofortunella, were Poncirus subsumed into Citrus , these hybrids would no longer be intergeneric, and would likewise fall within Citrus , rendering Citroncirus invalid. Due to the sterility of many of

5320-402: Is a cold-hardy plant distinguishable by its compound leaves with three leaflets and its deciduous nature, but is close enough to the genus Citrus to be used as a rootstock. Swingle moved the trifoliate orange from Citrus to its own genus , Poncirus , but Mabberley and Zhang reunited the genera Swingle had separated back into Citrus . Early phylogenetic analysis nested Poncirus within

5453-577: Is a hybrid of two Atlantic species, the spinner and striped dolphins . In 2019, scientists confirmed that a skull found 30 years earlier was a hybrid between the beluga whale and narwhal , dubbed the narluga . Hybridization between species is common in birds. Hybrid birds are purposefully bred by humans, but hybridization is also common in the wild. Waterfowl have a particularly high incidence of hybridization, with at least 60% of species known to produce hybrids with another species. Among ducks , mallards widely hybridize with many other species, and

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5586-407: Is a hybridization test widely used in genetics to determine whether two separately isolated mutants that have the same (or similar) phenotype are defective in the same gene or in different genes (see Complementation (genetics) article). If a hybrid organism containing the genomes of two different mutant parental organisms displays a wild type phenotype, it is ordinarily considered that

5719-444: Is a lack of consensus for their placement within the Citrus genus rather than sister genera. Most commercial cultivars are the product of hybridization among these wild species, with most coming from crosses involving citrons , mandarins and pomelos . Many different phylogenies for the non-hybrid citrus have been proposed, and the phylogeny based on their nuclear genome does not match that derived from their chloroplast DNA, probably

5852-571: Is a particularly common mechanism for speciation in plants, and is now known to be fundamental to the evolutionary history of plants. Plants frequently form polyploids , individuals with more than two copies of each chromosome. Whole genome doubling has occurred repeatedly in plant evolution. When two plant species hybridize, the hybrid may double its chromosome count by incorporating the entire nuclear genome of both parents, resulting in offspring that are reproductively incompatible with either parent because of different chromosome counts. Human impact on

5985-659: 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 also resulted in 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

6118-451: Is always sterile worker ants (and because ants are haplodiploid , unfertilized eggs become males). Without mating with males of the other species, the queens are unable to produce workers, and will fail to establish a colony of their own. 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

6251-547: 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. Conspicuous hybrids are popularly named with portmanteau words , starting in the 1920s with the breeding of tiger–lion hybrids ( liger and tigon ). From the point of view of animal and plant breeders, there are several kinds of hybrid formed from crosses within

6384-512: Is generally followed globally today with much modification; there are still large differences in nomenclature between countries and individual scientists. The 'Tanaka system' (1954) instead provides a separate species name for each cultivar, regardless of whether it is pure or a hybrid of two or more species or varieties, and resulted in 159 identified species. It thus represents an example of taxonomic " splitting ", and in assigning separate species names to horticultural variants does not conform to

6517-441: Is inconsistent. There is disagreement over whether to assign species names to hybrids, and even modern hybrids of known parentage are sold under general common names that give little information about their ancestry, or even imply technically incorrect identity. This can be a problem for those who cannot eat some citrus varieties. Drug interactions with chemicals found in some citrus, including grapefruit and Seville oranges , make

6650-550: Is nested within the citrus phylogenetic tree, most closely related to kumquats, suggesting that all these species should be included in the genus Citrus . Wu, et al. , found that several of the finger lime cultivars were actually hybrids with round lime, and concluded there were just three species among those tested, desert lime ( C. glauca ), round lime ( C. australis ) and the finger lime ( C. australasica ), though their analysis did not include other types previously identified as distinct species. In more limited genomic analysis,

6783-427: Is that hybrid individuals can form a "bridge" transmitting potentially helpful genes from one species to another when the hybrid backcrosses with one of its parent species, a process called introgression . Hybrids can also cause speciation , either because the hybrids are genetically incompatible with their parents and not each other, or because the hybrids occupy a different niche than either parent. Hybridization

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6916-515: 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

7049-531: The Citrus subgenus Papeda to separate its members from the more edible citrus that also differ from other citrus in having stamens that grow separately, not united at their base. He included in this group the kaffir lime ( Citrus hystrix ), as well as its likely taxonomic synonym the micrantha ( Citrus micrantha ), and the Ichang papeda ( Citrus cavaleriei ). Since the latter two species locate to different branches of

7182-446: The mangshanyegan ( C. mangshanensis ), akin to another local fruit known as the yuanju , and found to be the most distant branch of all the citrus. In a limited genomic analysis, Feroniella was found to cluster with C. reticulata deep within Citrus , leading botanist David Mabberley to propose that the sole member of this genus, F. lucida , be moved to the genus Citrus and rechristened C. lucida . Though this has received

7315-501: The Bergamot orange is a sweet orange/lemon hybrid and the Oroblanco is a grapefruit/pomelo mix, while tangelos are tangerine (mandarin)/pomelo or mandarin/grapefruit hybrids, orangelos result from grapefruit backcrossed with sweet orange, and a sweet orange backcrossed with a tangerine gives the tangor . One lumia , a member of the sweet lemons , is the product of crossing a lemon with

7448-457: The Indian wild orange , once suggested as a possible ancestor of today's cultivated citrus fruits, yielded conflicting phylogenetic placements in more limited genetic analysis, but study of nuclear markers and chloroplast DNA showed it to be of maternal citron lineage, with further genetic contributions from mandarin and papeda. A large group of commercial hybrids involve the kumquat , Fortunella in

7581-616: The Malay Archipelago , and mandarins in Vietnam , southern China , and Japan . The hybrids of these taxa include familiar citrus fruits like oranges , grapefruit , lemons , and some limes and tangerines . These three have also been hybridized with other citrus taxa, for example, the Key lime arose from a citron crossing with a micrantha. In many cases, the varieties are propagated asexually , and lose their characteristic traits if bred. Some of

7714-638: 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. Hybridization between species plays an important role in evolution, though there is much debate about its significance. Roughly 25% of plants and 10% of animals are known to form hybrids with at least one other species. One example of an adaptive benefit to hybridization

7847-527: The New Guinea wild lime , Clymenia and Oxanthera (false orange) all cluster with the Australian limes as members of Citrus . The outback lime is a desert lime agriculturally selected for more commercial traits, while some commercial varieties of the Australian lime are hybrids with mandarins, lemons, and/or sweet oranges. Clymenia , will hybridize with kumquats and some limes . The trifoliate orange

7980-455: The florentine citron is probably of hybrid origin. Genetic analysis of citrons has shown that they divide into three groups. One cluster consists of wild citrons that originated in China and produce non-fingered fruit with pulp and seeds. A second cluster, also native to China, consist of the fingered citrons, most of which are seedless and must be propagated artificially. The third cluster represents

8113-464: The sea urchin Strongylocentrotus purpuratus (female) and the sand dollar Dendraster excentricus (male). 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). Commonly, hybrids also combine traits seen only separately in one parent or

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8246-461: The trifoliate orange does not naturally interbreed with core taxa due to different flowering times, but hybrids have been produced artificially between Poncirus and members of the genus Citrus . Genomic analysis of several such hybrids showed all to have involved P. trifoliata and not P. polyandra . In the Swingle system the name coined for these intra-generic crosses, represented as a hybrid genus,

8379-438: The "suture region". It is at these regions that the subspecies were formed. Other hybrid zones have formed between described species of plants and animals. From the point of view of genetics, several different kinds of hybrid can be distinguished. 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. A structural hybrid results from

8512-938: The 'core species', citrons , mandarins , and pomelos , which share in common a complex floral anatomy that gives rise to more complex fruit. These core species, and to a lesser extent other citrus, have given rise to a wide variety of hybrids for which the naming is inconsistent. The same common names may be given to different species, citrus hybrids or mutations. For example, citrus with green fruit tend to be called 'limes' independent of their origin: Australian limes, musk limes , Key limes , kaffir limes , Rangpur limes , sweet limes and wild limes are all genetically distinct. Fruit with similar ancestry may be quite different in name and traits (e.g. grapefruit, common oranges, and ponkans are all mandarin–pomelo hybrids). Many traditional citrus groups, such as true sweet oranges and lemons, seem to be bud sports , clonal families of cultivars that have arisen from distinct spontaneous mutations of

8645-540: The Citrinae subtribe into three groups, the 'primitive citrus' distant relatives, the closer 'near citrus' including citrus-related genera like Atalantia , and the "true citrus", for the species that had historically been placed in Citrus but many of which he elevated to separate genera: Poncirus (trifoliate orange), Fortunella (kumquat), Eremocitrus (desert limes), Microcitrus (finger and round limes), as well as an additional genus, Clymenia , formerly thought to be

8778-503: The F1 generation is also phenotypically homogeneous, producing offspring that are all similar to each other. Double cross hybrids result from the cross between two different F1 hybrids (i.e., there are four unrelated grandparents). 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. Top cross (or "topcross") hybrids result from

8911-501: The Jiangsu kumquat ( F. obovata ), and Huang added another, F. bawangica . Since the kumquat is a cold-hardy species , there are many hybrids between common citrus members and the kumquat. Swingle coined a separate hybrid genus for these, which he called × Citrofortunella . Subsequent study of the many commercial citrus lineages revealed such complexity that the genera could not be separated, and genomic analysis rooted Fortunella within

9044-534: The Mediterranean citrons, thought to have originally been introduced there from India. Some fingered citron varieties are used in buddhist offerings , and some more common varieties are used as the etrog in the Jewish harvest festival of Sukkot . There is also a specific variety of citron called etrog . The Mountain citron is a complex citrus hybrid that only includes trace amounts of true citron. Swingle coined

9177-671: The MutS family revealed that the gene found in H. pylori was not in the same subfamily as those known to be involved in mismatch repair. Furthermore, he suggested that this "phylogenomic" approach could be used as a general method for prediction functions of genes. This approach was formally described in 1998. For reviews of this aspect of phylogenomics see Brown D, Sjölander K. Functional classification using phylogenomic inference. Traditional phylogenetic techniques have difficulty establishing differences between genes that are similar because of lateral gene transfer and those that are similar because

9310-535: The Persian lime is triploid , deriving from a diploid Key lime gamete and a haploid lemon ovule . A second group of Key lime hybrids, including the Tanepao lime and Madagascar lemon, are also triploid but instead seem to have arisen from a backcross of a diploid Key lime ovule with a citron haploid gamete. The "Giant Key lime" owes its increased size to a spontaneous duplication of the entire diploid Key lime genome to produce

9443-452: The Swingle system. Citrofortunella was coined as a genus containing intergeneric hybrids between members of the Citrus and the Fortunella , and is named after its parent genera. Such hybrids often combine the cold hardiness of the kumquat with some edibility properties of the other Citrus species. As members of a hybrid genus, these crosses were marked with the multiplication sign before

9576-533: The Tachibana, proved to be either natural F1 hybrids between this native Ryukyu mandarin and mainland mandarin species that had recolonized the islands after a period of isolation, or else later agricultural hybrids with introduced Asian cultivars. All characterized commercial varieties called mandarins are actually inter-species hybrids . Wu, et al. , divided mandarins into three types, based on their degree of hybridization. In addition to genetically pure mandarins,

9709-455: The ancestry of citrus fruit of interest: many commonly sold citrus varieties are grapefruit hybrids or pomelo -descended grapefruit relatives . One medical review has advised patients on medication to avoid all citrus juice, although some citrus fruits contain none of the problematic furanocoumarins . Citrus allergies can also be specific to only some fruit or some parts of some fruit. The most common citrus hybrids that are sometimes treated as

9842-471: The anomalies created from these factors are overwhelmed by the pattern of evolution indicated by the majority of the data. Through phylogenomics , it has been discovered that most of the photosynthetic eukaryotes are linked and possibly share a single ancestor. Researchers compared 135 genes from 65 different species of photosynthetic organisms. These included plants , alveolates , rhizarians , haptophytes and cryptomonads . This has been referred to as

9975-511: 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. Hybridization occurs between a narrow area across New England, southern Ontario, and the Great Lakes,

10108-460: The cells of the two intermix at the graft site, and can produce shoots from the same tree that bear different fruit. For example, the 'Faris' lemon, has some branches with purple immature leaves and flowers with a purple blush that give rise to sour fruit, while other branches produce genetically distinct sweet lemons coming from white flowers, with leaves that are never purple. Graft hybrids can also give rise to an intermixed shoot that bears fruit with

10241-426: The chromosomes. 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 , although its taxonomic status has been a subject of controversy. 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

10374-539: The chromosomes. Chromosome duplication allows orderly meiosis and so viable seed can be produced. Plant hybrids are generally given names that include an "×" (not in italics), such as Platanus × hispanica for the London plane, a natural hybrid of P. orientalis (oriental plane) and P. occidentalis (American sycamore). The parent's names may be kept in their entirety, as seen in Prunus persica × Prunus americana , with

10507-452: The citrus ancestor to expand across south and east Asia in a rapid genetic radiation. After the plant crossed the Wallace line a second radiation took place in the early Pliocene (about 4 million years ago) to give rise to the Australian species. Most modern cultivars are actually hybrids derived from a small number of 'pure' original species. Though hundreds of species names have been assigned,

10640-520: The citrus phylogenetic tree, the group would be polyphyletic and not a valid division. Both the micrantha and the Ichang papeda have also given rise to hybrids with other citrus. Sometimes included among the papedas was the mountain citron, not affiliated with the true citrons, and subsequently found to be a pure species most closely related to kumquats. Kumquats were originally classified by Carl Peter Thunberg as Citrus japonica in his 1784 book Flora Japonica . In 1915, Swingle reclassified them in

10773-427: The citrus, consistent with a single genus, but the genomic sequencing of Wu, et al. , placed it outside the cluster representing Citrus , and the authors retained a separate genus Poncirus . Ollitrault, Curk and Krueger indicate that the majority of data are consistent with the enlarged Citrus that includes the trifoliate orange, though they recognize that many botanists still follow Swingle. A further complication to

10906-481: The common pheasant and hen golden pheasant ( Chrysolophus pictus ). Spurs are absent in hybrids of the former type, although present in both parents. Hybridization is greatly influenced by human impact on the environment, through effects such as habitat fragmentation and species introductions. Such impacts make it difficult to conserve the genetics of populations undergoing introgressive hybridization . Humans have introduced species worldwide to environments for

11039-400: 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. 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. In biology, the result of crossing of two populations is called

11172-444: 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

11305-407: The diverse Heliconius butterflies , but that is disputed. The two closely related harvester ant species Pogonomyrmex barbatus and Pogonomyrmex rugosus have evolved to depend on hybridization. When a queen fertilizes her eggs with sperm from males of her own species, the offspring is always new queens. And when she fertilizes the eggs with sperm from males of the other species, the offspring

11438-413: The doubling of chromosome sets, causing immediate genetic isolation. 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 eight natural examples had been fully described. Experimental studies suggest that hybridization offers a rapid route to speciation, a prediction confirmed by

11571-611: 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 hybridization. This has been referred to as genetic pollution out of concern that it may threaten many species with extinction. Similarly, genetic erosion from monoculture in crop plants may be damaging the gene pools of many species for future breeding. The conservation impacts of hybridization between species are highly debated. While hybridization could potentially threaten rare species or lineages by "swamping"

11704-429: The fact that early generation hybrids and ancient hybrid species have matching genomes, meaning that once hybridization has occurred, the new hybrid genome can remain stable. 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

11837-550: The female parent's name given first, or if not known, the parent's names given alphabetically. Phylogenomics Phylogenomics is the intersection of the fields of evolution and genomics . The term has been used in multiple ways to refer to analysis that involves genome data and evolutionary reconstructions. It is a group of techniques within the larger fields of phylogenetics and genomics. Phylogenomics draws information by comparing entire genomes, or at least large portions of genomes. Phylogenetics compares and analyzes

11970-478: The four species ancestral to most commercial hybrids, and did not include similar species designations for more exotic hybrids involving other citrus species, such as the Ichang papeda, kumquat, or trifoliate orange. Likewise, Ollitrault, Curk and Krueger accepted that the whole-genome characterization necessary to unambiguously assign a hybrid species name under their system is not available for many varieties. Most commercial varieties are descended from one or more of

12103-560: The fusion of gametes that have differing structure in at least one chromosome, as a result of structural abnormalities . A numerical hybrid results from the fusion of gametes having different haploid numbers of chromosomes . A permanent hybrid results when only the heterozygous genotype occurs, as in Oenothera lamarckiana , because all homozygous combinations are lethal. In the early history of genetics, Hugo de Vries supposed these were caused by mutation . Genetic complementation

12236-533: The genes in a genome at once, are drastic evolutionary events with great relevance in the evolution of many clades, and whose signal can be traced with phylogenomic methods. Traditional single-gene studies are effective in establishing phylogenetic trees among closely related organisms, but have drawbacks when comparing more distantly related organisms or microorganisms. This is because of lateral gene transfer , convergence , and varying rates of evolution for different genes. By using entire genomes in these comparisons,

12369-431: The genetic hybrids as well as disease- or temperature-sensitivity of some Citrus trees, domesticated citrus cultivars are usually propagated via grafting to the rootstock of other, often hardier though less palatable citrus or close relatives. As a result, graft hybrids, also called graft-chimaeras , can occur in Citrus . After grafting , the cells from the scion and rootstock are not somatically fused , but rather

12502-507: The genetic relationships between ducks are further complicated by the widespread gene flow between wild and domestic mallards. One of the most common interspecific hybrids in geese occurs between Greylag and Canada geese ( Anser anser x Branta canadensis ). One potential mechanism for the occurrence of hybrids in these geese is interspecific nest parasitism , where an egg is laid in the nest of another species to be raised by non-biological parents. The chick imprints upon and eventually seeks

12635-630: The genetically "pure" individuals with hybrids, hybridization could also save a rare lineage from extinction by introducing genetic diversity. It has been proposed that hybridization could be a useful tool to conserve biodiversity by allowing organisms to adapt, and that efforts to preserve the separateness of a "pure" lineage could harm conservation by lowering the organisms' genetic diversity and adaptive potential, particularly in species with low populations. While endangered species are often protected by law, hybrids are often excluded from protection, resulting in challenges to conservation. The term hybrid

12768-448: The genetically pure citrus strains and distinguish them from hybrids. In 2020, a new taxonomic system was proposed by Ollitrault, Curk and Krueger, with the goal of harmonizing traditional naming systems with the new genomic data that have both allowed the pure ancestral species to be distinguished from hybrids, and enabled the ancestry of those hybrids to be identified among the ancestral species. In their system, each ancestral species has

12901-529: The genus name, for example × Citrofortunella microcarpa . With the return of kumquats to within Citrus , Citrofortunella are no longer viewed as being intergeneric hybrids and thus likewise belong in Citrus , while Citrofortunella as a distinct genus name would no longer be valid . Examples of the Citrofortunella include the calamansi , limequat , and yuzuquat , crossing kumquat with tangerine , Key lime , and yuzu respectively. As with kumquats,

13034-401: 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

13167-457: The hybrids deriving from mandarin/pomelo crosses, there is a direct correlation between the proportion of pomelo DNA in the hybrid and fruit size, while the more palatable mandarins are those that have received specific genes from pomelos that alter their acidity. Some of the more common pomelos are genetically pure, while a number have a single small region of introgressed mandarin DNA on one chromosome,

13300-727: The hybrids have in turn interbred with one another hybrid or with the original taxa, making the citrus family tree a complicated network. Kumquats do not naturally interbreed with core taxa due to different flowering times, but hybrids (such as the calamansi ) exist. Australian limes are native to Australia and Papua New Guinea , so they did not naturally interbreed with the core taxa, but they have been crossbred with mandarins and calamansis by modern breeders. Humans have deliberately bred new citrus fruits by propagating seedlings of spontaneous crosses (e.g. clementines ), creating or selecting mutations of hybrids, (e.g. Meyer lemon ), and crossing different varieties (e.g. 'Australian Sunrise',

13433-407: 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 hybridization, which include genetic and morphological differences, differing times of fertility, mating behaviors and cues, and physiological rejection of sperm cells or

13566-400: The loss of genetic diversity . 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. Familiar examples of equid hybrids are the mule,

13699-518: The micrantha, Ichang papeda, kumquat , Australian limes, and trifoliate orange. The best known hybrid from micrantha is the Key lime (or Mexican lime), derived from the breeding of a male citron and a female micrantha. Several citrus varieties are Ichang papeda/mandarin crosses (for which Swingle coined the term ichandarin), including Sudachi and Yuzu (which also includes smaller contributions from pomelo and kumquat). Other more exotic citrus have likewise proved hybrids that include papeda. For example,

13832-460: The new plants have been viewed as different species from any of their parents. In older taxonomic systems, citrus hybrids have often been given unique hybrid names , marked with a multiplication sign after the word Citrus (or abbreviation C. ); for example, the Key lime is Citrus × aurantifolia , and also are referred to by joining the names of the crossed species or hybrids that produced them, as with sunquat – C. limon × japonica . Styling

13965-452: The number of chromosomes has been doubled. A form of often intentional human-mediated hybridization 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 hybridization. One such flower, Oenothera lamarckiana , was central to early genetics research into mutationism and polyploidy. It

14098-420: The only remaining evidence of prior species, they need to be conserved as well. 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. Hybridization and introgression, which can happen in natural and hybrid populations, of new genetic material can lead to the replacement of local genotypes if

14231-451: The organisms shared an ancestor. By comparing large numbers of genes or entire genomes among many species, it is possible to identify transferred genes, since these sequences behave differently from what is expected given the taxonomy of the organism. Using these methods, researchers were able to identify over 2,000 metabolic enzymes obtained by various eukaryotic parasites from lateral gene transfer. The comparison of complete gene sets for

14364-438: The other (e.g., a bird hybrid might combine the yellow head of one parent with the orange belly of the other). 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. Sterility is often attributed to the different number of chromosomes between

14497-510: The oval (Nagami) kumquat ( Fortunella margarita or C. japonica var. margarita ). Three from Australia were identified: the desert lime ( C. glauca ), round lime ( C. australis ) and the finger lime ( C. australasica ). Many other cultivars previously identified as species were found to be closely related variants ( subspecies or varieties ) or hybrids of these species, though not all cultivars were evaluated. Subsequent studies have added two additional species to this list of pure species:

14630-573: 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. Conservationists disagree on when

14763-532: The parent species. Cave paintings indicate that the European bison is a natural hybrid of the aurochs and the steppe bison . 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

14896-506: The parents' common ancestor living tens of millions of years ago. 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 . The Colias eurytheme and C. philodice butterflies have retained enough genetic compatibility to produce viable hybrid offspring. Hybrid speciation may have produced

15029-423: The phylogenetic divisions of pure founder species, Swingle's subgenera were polyphyletic , and hence do not represent valid taxonomy. His novel genera also fail to withstand phylogenetic analysis. Swingle had elevated kumquats into a separate genus Fortunella , while two genera were suggested by him for the Australian limes, Microcitrus and Eremocitrus . However, genomic analysis shows these groups nested within

15162-460: The phylogenetic tree of Citrus . Since their placement in distinct genera would make Citrus a paraphyletic grouping, it has been suggested that all of these are correctly members of the genus Citrus . Similarly, genomic analysis has suggested that other genera previously split off from Citrus likewise belong within this expanded phylogenetic concept of the genus Citrus , including Clymenia , Oxanthera (false oranges, moved out of Citrus to

15295-535: The placement of Poncirus is the conflicting phylogenetic data: its nuclear genome places Poncirus as an outgroup to other citrus, while its chloroplast DNA (cpDNA) nests within a Citrus subclade. This led Talon, et al. to conclude that the trifoliate orange likely is either the progeny of an ancient hybridization between a core citrus and an unidentified more distant relative, or at some time in its history acquired an introgressed cpDNA genome from another species. A second trifoliate orange, Poncirus polyandra ,

15428-637: The point of view of taxonomy , hybrids differ according to their parentage. Hybrids between different subspecies (such as between the dog and Eurasian wolf ) are called intra-specific hybrids. Interspecific hybrids are the offspring from interspecies mating ; these sometimes result in hybrid speciation. Intergeneric hybrids result from matings between different genera, such as between sheep and goats . Interfamilial hybrids, such as between chickens and guineafowl or pheasants , are reliably described but extremely rare. Interordinal hybrids (between different orders) are few, but have been engineered between

15561-453: The polyphyletic tree of Citrus . As a result there is growing acceptance for the restoration of kumquats to Citrus , though the assignment of individual species among the kumquats remains controversial due in part to insufficient genomic data on the variants. The Flora of China unites all kumquats as the single species, Citrus japonica . Based on chromosomal analysis, Yasuda, et al., identified Jiangsu and Malayan kumquats as hybrids and see

15694-402: The progeny, or (rarely, and only recently) as somatic hybrids . The aim of plant breeding of hybrids is to use two or more different citrus varieties or species, in order to get traits intermediate between those of the parents, or to transfer individual desirable traits of one parent into the other. In some cases, particularly with the natural hybrids, it has been viewed as hybrid speciation and

15827-590: The remainder of the Eufortunella subgenus as a single species, while retaining a distinct species designation for the Hong Kong kumquat. Australian and New Guinean citrus species had been viewed as belonging to separate genera by Swingle, who placed in Microcitrus all but the desert lime, which he assigned to Eremocitrus . However, genomic analysis shows that though they form a distinct clade from other citrus, this

15960-545: The result of a cross followed by extensive backcrossing with pomelo. Varieties of true (non-hybrid) citron ( Citrus medica ) have distinctly different forms. The citron usually propagates by cleistogamy , a self-pollination within an unopened flower, and this results in the lowest levels of heterozygosity among the citrus species. Because of this, it will generally serve as the male parent of any hybrid progeny. Many citron varieties were proven to be non-hybrids despite their rather dramatic morphological differences; however,

16093-563: The sequences of single genes, or a small number of genes, as well as many other types of data. Four major areas fall under phylogenomics: The ultimate goal of phylogenomics is to reconstruct the evolutionary history of species through their genomes. This history is usually inferred from a series of genomes by using a genome evolution model and standard statistical inference methods (e.g. Bayesian inference or maximum likelihood estimation ). When Jonathan Eisen originally coined phylogenomics , it applied to prediction of gene function. Before

16226-460: The standard species concept. Tanaka also divided into subgenera, but different than in Swingle's system, introducing Archicitrus (which he subdivided into five sections, Papeda, Limonellus, Aruntium, Citrophorum and Cephalocitrus) and Metacitrus (divided into Osmocitrus, Acrumen and Pseudofortunella). This system is commonly used in Tanaka's native Japan. A 1969 analysis by Hodgson intended to harmonize

16359-495: The sweet orange, C. × aurantium var. paradisi for grapefruit, and C. × aurantium var. clementina for the clementine. Likewise, hybrids combining mandarins and citrons would all be varieties of C. × limonia , those of pomelo and citron, C. x lumia , while tri-species hybrids of citrons, pomelos and mandarins would be C. × limon , and a tetra-species cross involving these three species along with C. micrantha would be C. × latifolia . This naming system focused on

16492-470: The term negative heterosis refers to weaker or smaller hybrids. 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

16625-475: The two contributing species with a plus sign between the individual names ( Citrus medica + C. aurantium ). Hybrid (biology) In biology , a hybrid is the offspring resulting from combining the qualities of two organisms of different varieties , subspecies , species or genera through sexual reproduction . Generally, it means that each cell has genetic material from two different organisms, whereas an individual where some cells are derived from

16758-400: The two parental mutant organisms are defective in different genes. If the hybrid organism displays a distinctly mutant phenotype, the two mutant parental organisms are considered to be defective in the same gene. However, in some cases the hybrid organism may display a phenotype that is only weakly (or partially) wild-type, and this may reflect intragenic (interallelic) complementation. From

16891-448: The two schemes accepted 36 species. These initial attempts at Citrus systematization all predated the recognition, which began to gain traction in the mid-1970s, that the majority of cultivars represent hybrids of just three species, citron, mandarin and pomelo. Phylogenetic analysis confirms this hybrid origin of most citrus cultivars, indicating a small number of founder species. While the subgenera suggested by Tanaka proved similar to

17024-462: 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

17157-537: The use of phylogenomic techniques, predicting gene function was done primarily by comparing the gene sequence with the sequences of genes with known functions. When several genes with similar sequences but differing functions are involved, this method alone is ineffective in determining function. A specific example is presented in the paper "Gastronomic Delights: A movable feast". Gene predictions based on sequence similarity alone had been used to predict that Helicobacter pylori can repair mismatched DNA . This prediction

17290-519: The wild mandarins of the East-Asian mainland. However, a directed study of these island cultivars revealed the existence of a second mandarin true-species that diverged from the mainland species between 2.2 and 2.8 million years ago, following the geographical isolation of the islands through rising sea levels. Unlike the mainland species, this Ryukyu mandarin, named C. ryukyuensis , reproduces sexually. The previously-characterized island cultivars, including

17423-450: The wild populations that gave rise to Wang's northern and southern domesticate classes, which they described as 'common mandarins' and mangshanyeju ( Mangshan wild mandarins). It was specifically in the latter that a genetic mutation caused by the insertion of a transposable element adjacent to the CitRKD1 gene led to the ability of these mandarins to reproduce asexually through apomixis ,

17556-485: Was based on the fact that this organism has a gene for which the sequence is highly similar to genes from other species in the "MutS" gene family which included many known to be involved in mismatch repair. However, Eisen noted that H. pylori lacks other genes thought to be essential for this function (specifically, members of the MutL family). Eisen suggested a solution to this apparent discrepancy – phylogenetic trees of genes in

17689-464: Was discovered in Yunnan ( China ) in the 1980s. Zhang and Mabberley later concluded this was likely a hybrid between the trifoliate orange and some other Citrus . However, recent genomic analysis of P. polyandra showed low heterozygosity, the opposite of what one would expect for a hybrid. Were Poncirus subsumed into Citrus , C. polyandra would be unavailable, so C. polytrifolia has been suggested as

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