Citrus taxonomy(Redirected from Citrus hybrids)
Citrus taxonomy refers to the botanical classification of the species, varieties, cultivars, and graft hybrids within the genus Citrus, subgenus Papeda, and related genera, found in cultivation and in the wild.
Citrus taxonomy is complex. Cultivated citrus are derived from various citrus species found in the wild. Some are only selections of the original wild types, while others are hybrids between two or more ancestors. Citrus plants hybridize easily between species with completely different morphologies, and similar-looking citrus fruits may have quite different ancestries. Conversely, different-looking varieties may be nearly genetically identical, and differ only by a bud mutation.
Detailed 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 10 wild species in South and East Asia and Australia. 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 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. Others do reproduce true to type via nucellar seeds in a process called apomixis. Some differ only in disease resistance. Clear genetic lineages are very important for breeding improved cultivars.
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 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, a recent genomic study by Wu, wt al. identified just ten species of citrus among those studied, seven in Asia and three in Australia. These are the pumello (Citrus maxima), the 'pure' mandarins (C. reticulata - most mandarin cultivars were hybrids of this species with pumello), citrons (C. medica), micranthas (C. micrantha), the Ichang papeda (C. ichangensis), the mangshanyegan (C. mangshanensis), and the oval (Nagami) kumquot (Fortunella margarita or C. japonica var. margarita). In Australia, there were 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 hybrids, though not all cultivars were evaluated.
Interbreeding seems possible between all citrus plants, and between citrus plants and some plants which may or may not be categorized as citrus. The ability of citrus hybrids to self-pollinate and to reproduce sexually also helps create new varieties. The three 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 the Malay Archipelago, and mandarins in Vietnam, southern China, and Japan. The hybrids of these taxa include familiar citrus fruits like oranges, grapefruit, lemons, limes, and some tangerines. Citrons have also been hybridized with other citrus taxa, for example, crossed with micranta to produce the Key lime. In many cases, these crops are propagated asexually, and lose their characteristic traits if bred. However, some of these hybrids have interbred with one another and 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 calamondin) 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 calamondins by modern breeders. Humans have deliberately bred new citrus fruits by propagating wild-found seedlings (e.g. clementines), creating or selecting mutations of hybrids, (e.g. Meyer lemon), and crossing different varieties (e.g. 'Australian Sunrise', a finger lime and calamondin cross).
Citrus naming systemsEdit
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 alternative unified schemes, those of Chōzaburō Tanaka and Walter Tennyson Swingle, which can be viewed as extreme alternative visions of the same genus. Swingle's system, published in 1943, divides citrus and near relatives into three groups, the 'primitive citrus', distant relatives such as Atalantia, the 'true citrus' (subgenus Eucitrus), the cultivated core species and their progeny, and an intermediate group, the 'near citrus' (subgenus Papeda), which included the less-edible members. Within the genus Citrus he identified 16 species, dividing them further into varieties, and lastly divides it again into cultivars or hybrids. The Swingle system is generally followed today with much modification; however, there are still huge differences in nomenclature between countries and even individual scientists. The basis of the 'Tanaka system' (1954) is to provide 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 the standard species concept. Tanaka also divides into subgenera, but differently than Swingle, introducing Archicitrus and Metacitrus. A 1969 analysis by Hodgson came down in between, accepting 36 species. A novel way of viewing the citrus gained traction in the mid-1970s, when several analyses suggested a much smaller number of species, with the majority of cultivars representing hybrids of just three, citron, mandarin and pomelo.
These attempts to systematize the citrus all predate modern genomic analyses, and are to some extent inconsistent with its results. This analysis confirms that the majority of citrus cultivars represent the hybrid progeny of a small number of species, and even Swingle's short list of species includes genetic hybrids. While the subgenera suggested by Tanaka proved similar to the phylogenetic divisions, Swingle's subgenera were polyphyletic, and hence do not represent valid taxonomy. The historical genera are also dubious. Kumquots had been placed in a separate genus Fortunella, while two genera were suggested by Swingle for the Australian limes, Microcitrus and Eremocitrus (Clymenia, kin of the Australian limes, has also been placed in a distinct genus). However, genomic analysis has pointed to these groups nesting within the phylogenetic tree of the other citrus plants. Since their placement in distinct genera would make Citrus a polyphyletic grouping, it has been suggested that these all represent a single genus, that all of the citrus species be placed in Citrus.
Core species and hybridsEdit
Most commercial varieties are descended from one or more of citrons, mandarins, and pomelos. The same common names may be given to different citrus hybrids or mutations. Fruit with similar ancestry may be quite different in name and traits (e.g. grapefruit, common oranges, and ponkans, all pomelo-mandarin hybrids). Note that many traditional citrus groups, such as true sweet oranges and lemons, seem to be bud sports, mutant descendants of a single hybrid ancestor.
Mandarin oranges (tangerines, satsumas - Citrus reticulata) are one of the basic species. Where Swingle sees three species of mandarin and Tanaka identifies 36, genomic analysis suggests just one, Citrus reticulata. Even the divergent Tachibana mandarin (Tanaka's Citrus tachibana), native to Taiwan, the Ryukyu Islands and southern Japan, shows close genomic similarity with mandarins of the East-Asian mainland. Many varieties commercially called mandarins are actually hybrids.. Wu, et al., divided mandarins into three types, based on their degree of hybridization. In addition to the genetically-pure mandarins, a second type have arisen as a result of hybridization with pomelos, followed by subsequent back cross with mandarins to retain only a small selection of pomelo traits. A third type arose from the crossing of this hybrid again with pomelos and also with sweet oranges, which are themselves the product of the back-crossing of hybrid mandarins with pomelos. This produces mandarins with much more extensive pomelo DNA. Some commercial mandarins are hybrids with lemons, while the several were found to have a significant contribution (35-65%) from micranctha.
The mangshanyegan (C. mangshanensis), known colloquially as the 'mangshan mandarin', is a genetically distinct species, the sole representative of the first lineage to diverge from that of the other citrus.
The pomelo (Citrus maxima), a second of the core species from which most citrus hybrids have derived, is native to southeast Asia. Among 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.
Many varieties of true (non-hybrid) Citron (Citron medica) have distinctly different forms. The citron is usually fertilized by self-pollination, and this results in the lowest levels of heterozygosity among the citrus species. This also means that it will generally be the male parent of any hybrid progeny, rather than a female one. Many citron varieties were proven to be non-hybrids despite their morphological differences; however, the florentine citron is probably of hybrid origin.
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 hybrid that only includes trace amounts of citron.
Kumquats are a separate species, with few hybrids. Carl Peter Thunberg originally classified the kumquats as Citrus japonica in his 1784 book, Flora Japonica. In 1915, Swingle reclassified them in a separate genus, Fortunella, named in honor of Robert Fortune. Seven species of Fortunella have generally been recognized—F. japonica, F. margarita, F. crassifolia, F. hindsii, F. obovata and F. polyandra, as well as the recently described 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 lineages revealed such complexity that the genera could not be separated. Consequently, in accordance with the International Code of Nomenclature for algae, fungi, and plants, the correct genus name reverted to Citrus. The Flora of China returns the kumquat to Citrus and combines the species into the single species as Citrus japonica. Such a reclassification is also supported by phylogenetic analysis, which shows Fortunella to be rooted within Citrus.
Swingle coined the Citrus subg. Papeda to separate its members from the more edible citrus. Papedas also differ from other citrus in that its stamens grow separately, not united at the base. Swingle included in this group the Ichang papeda (Citrus ichangensis) as well as the micrantha (Citrus micrantha). Since these species are located on different branches of the citrus phylogenetic tree, the group would be polyphyletic, and not a valid division. The kaffir lime, akin to the Ichang papeda, is distinct from the citron/mandarin/pomelo hybrids more commonly referred to as 'limes'.
Australian and New Guinean speciesEdit
Australian and New Guinean citrus species had been viewed as belonging to a separate genus by Swingle, who placed them in genera Microcitrus and Eremocitrus. However, genomic analysis shows that though they form a distinct clade from other citrus, this is nested within the citrus phylogenetic tree, most closely related to kumquots, suggesting that they too should be included in the genus Citrus. Wu, et al., found that several of the finger lime cultivars they evaluated were actually hybrids with round lime, and concluded there were just three species, desert lime (C. glauca), round lime (C. australis) and the finger lime (C. australasica), though their analysis did not include other previously-identified species, the New Guinea wild lime and Clymenia. The latter grouped with the Australian limes in a more limited phylogenetic analysis. Some commercial varieties are hybrids with mandarins, lemons, and/or sweet oranges.
Citrus hybrids include many varieties and species that have been selected by plant breeders, usually for the useful characteristics of the fruit. Some citrus hybrids occurred naturally, and others have been deliberately created, either by cross pollination and selection among 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 intermediate traits, or the most desirable traits of the parents. In some cases, particularly with the natural hybrids, it has been viewed as hybrid speciation and the new plants are considered a different species from any of their parents. Citrus hybrid names are usually marked with a multiplication sign after the word "Citrus", for example the key lime is Citrus × aurantifolia, or joining the names of the crossed species when a unique species name is not assigned, as with Yuzu – Citrus ichangensis × reticulata.
Labelling of hybridsEdit
Hybrid taxonomy is inconsistent. There is disagreement over whether to assign species names to hybrids, and even modern hybrids of known parentage are sold under general names that give little information about their ancestry, or technically incorrect information. This can be a problem for those who can eat only some citrus varieties. Drug interactions with chemicals found in some citrus, including grapefruit and Seville oranges, make 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 no furanocoumarins.
Major citrus hybridsEdit
- Orange: a name used for several distinct crosses between pommelo and a mandarin orange. They have the orange color of the mandarin in their outer peels and segments, and are easier to peel than the pomelos. Oranges are all intermediate between the two ancestors in size, flavor and shape. The sour orange derives from an early cross between a male pure mandarin and a female pomelo, while the common sweet orange came from the cross of a mandarin/pomelo hybrid again with pomelo.
- Grapefruit: Grapefruits are more akin to the pommelo ancestor, resulting from a cross between a sweet orange and a pomelo.
- Lemon and Lime: Most lemons are from one common ancestor, and diverged by mutation. The ancestor was a hybrid between a male citron and a female sour orange, itself a pummelo/mandarin hybrid; citrons contribute half of the genome, while the other half is divided between pomelo and mandarin.
These hybrids have in turn been bred back with their parent stocks to form a broader array of citrus fruits, for example the Ponderosa lemon (Citrus limon × medica) and Florentine citron (Citrus x limonimedica) are both lemon/citron hybrids, and Oroblanco is a grapefruit/pomelo mix, while tangelos are mandarin/pomelo or mandarin/grapefruit hybrids. The lumia, a member of the sweet lemons named due to their low acid pulp and juice, is the product of crossing a lemon with a pomelo/citron hybrid.
Other hybrids have been produced or have naturally arisen from the core species, and these confusingly are referred to by the common names of better-known hybrids. Both the Rangpur lime and the rough lemon arose from crosses between citron and mandarin, with no contribution from pomelo found in true lemons and limes, with half of their genomes representing each contributing species. The 'cocktail grapefruit', or Mandelo, is the product of a low-acid pomelo variety hybridized with a mandarin that itself was a cross between two distinct mandarin stocks. Unlike the true grapefruit, it does not derive from sweet orange.
Hybrids from other citrus speciesEdit
While most citrus hybrids derive from the three core species, hybrids have also been derived from the micrantha, the Ichang papeda and the kumquat. The best known hybrid from micrantha is the Key lime (or Mexican lime), which is not closely related to the other 'limes'. It represents the product of a breeding between a male citron and a female micrantha. The Shikwasa is a micrantha/mandarin hybrid. Several common varieties are Ichang papeda/mandarin crosses, including Sudachi and Yuzu. Other more exotic citrus have likewise proved hybrids that include micrantha. For example, 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 micrantha. The wild Mountain citron, native to the Malay peninsula and called Citrus halimii at the time of its discovery, has been found to be a complex mix of micrantha, kumquat, mandarin and pomelo with only trace amounts of citron.
A large group of commercial hybrids involve the kumquat, Fortunella. Citrofortunella according to the Swingle system is a hybrid genus containing intergeneric hybrids between members of the Citrus and the closely-related Fortunella, and is named after its parent genera. Such hybrids often combine the cold hardiness of the Fortunella, with some edibility properties of the Citrus species. Citrofortunellas, which are all hybrids, are marked with the multiplication sign before the word "Citrofortunella", for example × Citrofortunella microcarpa. As with its parent, Fortunella, Citrofortunella are now viewed as regular citrus, and giving Citrofortunella a distinct genus name would no longer be valid.
These plants are hardier and more compact than most citrus plants, often referred to as cold hardy citrus. They produce small acidic fruit and make good ornamental plants. Citrofortunella hybrids include:
- Calamondin – (tangerine crossed with kumquat)
- Citrangequat – (citrange crossed with kumquat)
- Limequat – (Citrofortunella floridana) – (Key lime crossed with kumquat)
- Orangequat – Citrofortunella nippon – (satsuma mandarin crossed with kumquat)
- Procimequat – (Citrofortunella floridana) – (limequat crossed with kumquat)
- Sunquat – Citrus limon × japonica – (lemon crossed with kumquat)
- Yuzuquat – (yuzu, Citrus ichangensis × reticulata, crossed with kumquat)
Graft-chimaeras, also called graft hybrids, can occur in Citrus. The cells are not somatically fused but rather mix the tissues from scion and rootstock after grafting, a popular example the Bizzaria orange. In formal usage, these are marked with a plus sign "+" instead with an "x".
The trifoliate orange is a cold hardy plant distinguishable by its leaves with three leaflets. It is the only member of the Poncirus genus, but is close enough to the Citrus genus to be used as a rootstock. Wu, et al. found it not to cluster with the other citrus, and considered it to be a separate genus. As with kumquats, the trifolate orange does not naturally interbreed with core taxa due to different flowering times, but hybrids have been produced artificially, notably the citrange.
Citrocirus also according to the Swingle system, is a hybrid genus, containing hybrids between members of the genus Citrus and the closely related Poncirus, which includes the trifoliate orange, a cold hardy plant that is commonly used as a citrus rootstock. Citrocirus commonly refers to the citranges which are hybrids between the trifoliate and sweet oranges. However a molecular investigation suggested that Fortunella, Citrofortunella, Poncirus and Citrocirus should all be equivocally included in the genus Citrus.
According to the Swingle system, the trifoliate orange is not included in the genus Citrus, but in a related genus, Poncirus. Therefore, the citrange, which is a hybrid between the trifoliate and the sweet orange, is placed into a hybrid genus called Citrocirus (not a valid botanical name).
- Citrange – Citrus sinensis × Poncirus trifoliata – three cultivars: 'Troyer', 'Rusk' and 'Carrizo'.
- Citrumelo – Citrus paradisi × Poncirus trifoliata
Triphasia and ClymeniaEdit
Two additional genera, Triphasia and Clymenia, are likewise very closely related to the Citrus genus and bear hesperidium fruits, but they are not usually considered part of it. At least one, Clymenia, will hybridize with kumquats and some limes.
The wild lime—native to southern Florida and Texas in the United States, Mexico, Central America, the Caribbean, and South America as far south as Paraguay—does not belong in the Citrus genus. It is part of another genus in the Rutaceae family, Zanthoxylum (which also includes Sichuan pepper), and was classified as Zanthoxylum fagara by Charles Sprague Sargent.
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