Citrus taxonomy is the botanical classification of the

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

, varieties,

cultivars A cultivar is a kind of cultivated plant that people have selected for desired traits and which retains those traits when propagated. Methods used to propagate cultivars include division, root and stem cuttings, offsets, grafting, tissue cult ...

, and graft hybrids within the

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

Citrus ''Citrus'' is a genus of flowering trees and shrubs in the family Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, mandarins, lemons, grapefruits, pomelos, and limes. ''Citrus'' is nativ ...

'' and related genera, found in cultivation and in the wild. 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 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 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

citron The citron (''Citrus medica''), historically cedrate, is a large fragrant citrus fruit with a thick Peel (fruit), rind. It is said to resemble a 'huge, rough lemon'. It is one of the Citrus taxonomy#Citrons, original citrus fruits from which al ...

mandarin Mandarin or The Mandarin may refer to: Language * Mandarin Chinese, branch of Chinese originally spoken in northern parts of the country ** Standard Chinese or Modern Standard Mandarin, the official language of China ** Taiwanese Mandarin, Stand ...

s and

pomelo The pomelo ( ; or pummelo, ''Citrus maxima''), also known as a shaddock, is the largest citrus fruit. It is an ancestor of several cultivated citrus species, including the bitter orange and the grapefruit. It is a natural, non-hybrid citrus fr ...

s. and Supplement and Supplement 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 A stable is a building in which working animals are kept, especially horses or oxen. The building is usually divided into stalls, and may include storage for equipment and feed. Styles There are many different types of stables in use tod ...

lineage only through vegetative propagation. Some hybrids do reproduce true to type via nucellar seeds in a process called

apomixis In botany, apomixis is asexual development of seed or embryo without fertilization. However, other definitions include replacement of the seed by a plantlet or replacement of the flower by bulbils. Apomictically produced offspring are geneti ...

. As such, many hybrid species represent the clonal progeny of a single original F1 cross, though others combine fruit with similar characteristics that have arisen from distinct crosses.

Genetic history

All of the wild 'pure' citrus species trace to a common ancestor that lived in the Himalayan foothills, where a late-

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

citrus fossil, '' Citrus linczangensis'', has been found. At that time, a lessening of the

monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in Atmosphere of Earth, atmospheric circulation and precipitation associated with annu ...

s 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 The Wallace Line or Wallace's Line is a faunal boundary line drawn in 1859 by the British naturalist Alfred Russel Wallace and named by the English biologist Thomas Henry Huxley. It separates the biogeographic realms of Asia and 'Wallacea', a ...

a second radiation took place in the early

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58micranthas (''C. micrantha''), the Ichang papeda (''C. cavaleriei''), the mangshanyegan (''C. mangshanensis''), and 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 In Taxonomy (biology), biological classification, subspecies (: subspecies) is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (Morphology (biology), morpholog ...

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: a mandarin native to the

Ryukyu Islands The , also known as the or the , are a chain of Japanese islands that stretch southwest from Kyushu to Geography of Taiwan, Taiwan: the Ryukyu Islands are divided into the Satsunan Islands (Ōsumi Islands, Ōsumi, Tokara Islands, Tokara and A ...

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

(''C. medica''),

(''C. maxima''), and

(''C. reticulata''). These taxa interbreed freely, despite being quite genetically distinct, having arisen through

allopatric speciation Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...

, with citrons evolving in northern

Indochina Mainland Southeast Asia (historically known as Indochina and the Indochinese Peninsula) is the continental portion of Southeast Asia. It lies east of the Indian subcontinent and south of Mainland China and is bordered by the Indian Ocean to th ...

, pomelos in the

Malay Archipelago The Malay Archipelago is the archipelago between Mainland Southeast Asia and Australia, and is also called Insulindia or the Indo-Australian Archipelago. The name was taken from the 19th-century European concept of a Malay race, later based ...

, and mandarins in

Vietnam Vietnam, officially the Socialist Republic of Vietnam (SRV), is a country at the eastern edge of mainland Southeast Asia, with an area of about and a population of over 100 million, making it the world's List of countries and depende ...

southern China Northern China () and Southern China () are two approximate regions that display certain differences in terms of their geography, demographics, economy, and culture. Extent The Qinling–Daba Mountains serve as the transition zone between ...

, and

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

. The hybrids of these taxa include familiar citrus fruits like oranges,

grapefruit The grapefruit (''Citrus'' × ''paradisi'') is a subtropical citrus tree known for its relatively large, sour to semi-sweet, somewhat bitter fruit. The flesh of the fruit is segmented and varies in color from pale yellow to dark red. Grapefru ...

lemon The lemon (''Citrus'' × ''limon'') is a species of small evergreen tree in the ''Citrus'' genus of the flowering plant family Rutaceae. A true lemon is a hybrid of the citron and the bitter orange. Its origins are uncertain, but some ...

s, and some limes and

tangerine The tangerine is a type of citrus fruit that is orange in colour, that is considered either a variety of the mandarin orange (''Citrus reticulata''), or a closely related species, under the name ''Citrus tangerina'', or yet as a hybrid (''Citr ...

s. These three have also been hybridized with other citrus taxa, for example, the

Key lime The Key lime or acid lime (''Citrus'' × ''aurantiifolia'' or ''C. aurantifolia'') is a citrus hybrid (''kaffir lime, C. hystrix'' × ''citron, C. medica'') native to tropical Southeast Asia. It has a spherical fruit, in diameter. 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 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 Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

and

Papua New Guinea Papua New Guinea, officially the Independent State of Papua New Guinea, is an island country in Oceania that comprises the eastern half of the island of New Guinea and offshore islands in Melanesia, a region of the southwestern Pacific Ocean n ...

, 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', a finger lime and calamansi cross).

Citrus naming systems

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 , often Romanized as Tyôzaburô Tanaka (November 3, 1885 in Osaka – June 28, 1976), was a Japanese botanist and mycologist. He established one of the two major Biological classification, taxonomic classification systems for citrus and related g ...

and Walter Tennyson Swingle, that can be viewed as extreme alternative visions of the genus. Swingle's system divided the Citrinae subtribe into three groups, the 'primitive citrus' distant relatives, the closer 'near citrus' including citrus-related genera like ''

Atalantia ''Atalantia'' is a genus of flowering plants in the citrus family, the Rutaceae.Swingle, W. T., rev. P. C. ReeceChapter 3: The Botany of ''Citrus'' and its Wild Relatives.In: ''The Citrus Industry'' vol. 1. Webber, H. J. (ed.). Berkeley: Universit ...

'', 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 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

, dividing them further into varieties, and lastly cultivars or hybrids. The Swingle system 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 Splitting may refer to: * Splitting (psychology) * Lumpers and splitters, in classification or taxonomy * Wood splitting * Tongue splitting * Splitting (raylway), Splitting, railway operation Mathematics * Heegaard splitting * Splitting field * S ...

", and in assigning separate species names to horticultural variants does not conform to 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 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 the phylogenetic divisions of pure founder species, Swingle's subgenera were

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

, 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 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 a different branch of the tribe

Citreae Citreae is one of the two tribes of the flowering plant family Rutaceae, subfamily Aurantioideae, the other being Clauseneae. Subtribes and genera Three sub-tribes are included: * Triphasiinae ** '' Luvunga'' Buch.-Ham. ex Wight & Arn. ** '' ...

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 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 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 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 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.

Core species and hybrids

Most commercial varieties are descended from one or more of 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 name In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often con ...

s 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,

s, 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 a single hybrid ancestor. Most of these species are native to Asia. 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.

Ancestral species

Mandarins

Mandarin orange A mandarin orange (''Citrus reticulata''), often simply called mandarin, is a small, rounded citrus tree fruit. Treated as a distinct species of orange, it is usually eaten plain or in fruit salads. The mandarin is small and oblate, unlike the ...

s (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 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, 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 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 A transposable element (TE), also transposon, or jumping gene, is a type of mobile genetic element, a nucleic acid sequence in DNA that can change its position within a genome. The discovery of mobile genetic elements earned Barbara McClinto ...

adjacent to the ''CitRKD1'' gene led to the ability of these mandarins to reproduce asexually through

, 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

nesting within 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 the Tachibana, proved to be either natural

F1 hybrid F1 hybrid (also known as filial 1 hybrid) is the first filial generation of offspring of distinctly different parental types. F1 hybrids are used in genetics, and in selective breeding, where the term F1 crossbreed may be used. The term is somet ...

s 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, a second type are the result of hybridization with pomelos followed by subsequent

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

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 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, 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 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'.

Pomelos

The

(''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. Some of the more common pomelos are genetically pure, while a number have a single small region of introgressed mandarin DNA on one chromosome, the result of a cross followed by extensive backcrossing with pomelo.

Citrons

Varieties of true (non-hybrid)

(''Citrus medica'') have distinctly different forms. The citron usually propagates by cleistogamy, a

self-pollination Self-pollination is a form of pollination in which pollen arrives at the stigma of a flower (in flowering plants) or at the ovule (in gymnosperms) of the same plant. The term cross-pollination is used for the opposite case, where pollen from ...

within an unopened flower, and this results in the lowest levels of

heterozygosity Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mos ...

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, 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 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 Etrog (, plural: ; Ashkenazi Hebrew: , plural: ) is the yellow citron (''Citrus medica'') used by Jews during the weeklong holiday of Sukkot as one of the four species. Together with the ''lulav'', ''hadass'', and ''Aravah (Sukkot), aravah'', th ...

in the Jewish harvest festival of

Sukkot Sukkot, also known as the Feast of Tabernacles or Feast of Booths, is a Torah-commanded Jewish holiday celebrated for seven days, beginning on the 15th day of the month of Tishrei. It is one of the Three Pilgrimage Festivals on which Israelite ...

. There is also a specific variety of citron called

. The Mountain citron is a complex citrus hybrid that only includes trace amounts of true citron.

Papedas

Swingle coined 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 the citrus phylogenetic tree, the group would be

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

Kumquats were originally classified by

Carl Peter Thunberg Carl Peter Thunberg, also known as Karl Peter von Thunberg, Carl Pehr Thunberg, or Carl Per Thunberg (11 November 1743 – 8 August 1828), was a Sweden, Swedish Natural history, naturalist and an Apostles of Linnaeus, "apostle" of Carl Linnaeus ...

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. 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 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 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 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 species

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 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, 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.

Trifoliate orange

The trifoliate orange 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

, '' Poncirus'', but Mabberley and Zhang reunited the genera Swingle had separated back into ''Citrus''. Early phylogenetic analysis nested ''Poncirus'' within 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 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'', was discovered in

Yunnan Yunnan; is an inland Provinces of China, province in Southwestern China. The province spans approximately and has a population of 47.2 million (as of 2020). The capital of the province is Kunming. The province borders the Chinese provinces ...

(

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

) 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 a replacement species name for this Yunnan trifoliate orange.

Photo gallery

File:Mandarin Oranges (Citrus Reticulata).jpg ,

is a true species (''Citrus reticulata''); it is one of the progenitors of most cultivated

citrus ''Citrus'' is a genus of flowering trees and shrubs in the family Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, mandarins, lemons, grapefruits, pomelos, and limes. ''Citrus'' is nativ ...

File:Citrus grandis - Honey White.jpg , The

(''Citrus maxima'') File:Citron (8448513463).jpg , These varieties of

(''Citrus medica''),

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

and fingered, have distinctly different appearances File:3 etrog.JPG , Three varieties of ''

im'' (''Citrus medica'' acceptable for Jewish ritual use) that are all true non-hybrid citrons File:Fort-hindsii.jpg , Hong Kong kumquat File:Citrus glauca fruit.jpg , The Australian desert lime, '' Citrus glauca'', hangs from a branch File:Poncirusfruits2001HD.jpg , Trifoliate orange

Hybrids

Citrus hybrids include many varieties and

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 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 Hybrid speciation is a form of speciation where hybridization between two different species leads to a new species, reproductively isolated from the parent species. Previously, reproductive isolation between two species and their parents was tho ...

and 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 The multiplication sign (), also known as the times sign or the dimension sign, is a mathematical symbol used to denote the operation of multiplication, which results in a product. The symbol is also used in botany, in botanical hybrid nam ...

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 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.

Labelling of hybrids

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 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

and Seville oranges, make the ancestry of citrus fruit of interest: many commonly sold citrus varieties are grapefruit hybrids or

-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

furanocoumarin The furanocoumarins, or furocoumarins, are a class of organic chemical compounds produced by a variety of plants. Most of the plant species found to contain furanocoumarins belong to a handful of plant families. The families Apiaceae and Rutacea ...

s. Citrus allergies can also be specific to only some fruit or some parts of some fruit.

Major citrus hybrids

The most common citrus hybrids that are sometimes treated as a

by themselves, especially in

folk taxonomy A folk taxonomy is a vernacular name, naming system, as distinct from Taxonomy (general), scientific taxonomy. Folk biological classification is the way people traditionally describe and organize the world around them, typically making generous us ...

, are: * Orange: a name used for several distinct crosses between

and a

mandarin orange A mandarin orange (''Citrus reticulata''), often simply called mandarin, is a small, rounded citrus tree fruit. Treated as a distinct species of orange, it is usually eaten plain or in fruit salads. The mandarin is small and oblate, unlike the ...

. 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

bitter orange The bitter orange, sour orange, Seville orange, bigarade orange, or marmalade orange is the hybrid citrus tree species ''Citrus'' × ''aurantium'', and its fruit. It is native to Southeast Asia and has been spread by humans to many parts of th ...

and sweet orange both arose from mandarin-pomelo crosses, the former involving a pure mandarin, the latter with a mandarin already containing small amounts of pomelo. *

Grapefruit The grapefruit (''Citrus'' × ''paradisi'') is a subtropical citrus tree known for its relatively large, sour to semi-sweet, somewhat bitter fruit. The flesh of the fruit is segmented and varies in color from pale yellow to dark red. Grapefru ...

: grapefruits, like oranges, include genetic contributions from both mandarin and pomelo, but more of the latter, arising from a natural backcross of a sweet orange with a pomelo. The 'cocktail grapefruit', or

mandelo A mandelo (or ''Mandalo'', also known as a "cocktail grapefruit") is a citrus fruit that is smaller than a grapefruit, has yellow or yellow-green coloured skin and bright yellow or yellow-orange flesh, but is sweeter than a grapefruit. Origin Th ...

, is distinct, instead the product of a low-acid pomelo variety hybridized with a mandarin that itself was a cross between two distinct mandarin stocks. *

Lemon The lemon (''Citrus'' × ''limon'') is a species of small evergreen tree in the ''Citrus'' genus of the flowering plant family Rutaceae. A true lemon is a hybrid of the citron and the bitter orange. Its origins are uncertain, but some ...

: "true" lemons derive from one common hybrid ancestor, having diverged by mutation. The original lemon was a hybrid between a male citron and a female sour orange, itself a pomelo/pure-mandarin hybrid; citrons contribute half of the genome, while the other half is divided between pomelo and mandarin. There are other hybrids also known as 'lemons'. Rough lemons arose from a cross between citron and mandarin, without the pomelo contribution found in true lemons, while the Meyer lemon comes from a citron crossed with a sweet (as opposed to sour) orange. * Limes: A highly diverse group of hybrids go by this name. Rangpur limes, like rough lemons, arose from crosses between citron and mandarin. The sweet limes, so-called due to their low acid pulp and juice, come from crosses of citron with either sweet or sour oranges, while the Key lime arose from a cross between a citron and a micrantha. 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 In linguistics, a blend—also known as a blend word, lexical blend, or portmanteau—is a word formed by combining the meanings, and parts of the sounds, of two or more words together.

of ancestral species. The Ponderosa lemon (''Citrus limon'' × ''medica'') and Florentine citron (''Citrus'' × ''limonimedica'') are both true lemon/citron hybrids, the

Bergamot orange ''Citrus bergamia'', the bergamot orange (pronounced ), is a fragrant citrus fruit the size of an orange, with a yellow or green colour similar to a lime, depending on ripeness. Genetic research into the ancestral origins of extant citrus cul ...

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 The tangor (''C. reticulata'' × ''C. sinensis'') is a citrus fruit hybrid of the mandarin orange (''Citrus reticulata'') and the sweet orange (''Citrus sinensis''). The name "tangor" is a formation from the "tang" of tangerine and the "or" of " ...

. One lumia, a member of the sweet lemons, is the product of crossing a lemon with 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 lime Persian lime (''Citrus'' × ''latifolia''), also known by other common names such as seedless lime, Bearss lime and Tahiti lime, is a citrus fruit species of hybrid origin, known only in cultivation. The Persian lime is a triploid cross betwe ...

s, 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 Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Here ''sets of chromosomes'' refers to the number of maternal and paternal chromosome copies, ...

, many of the Key lime hybrid progeny have unusual chromosome numbers. For example, the Persian lime is triploid, deriving from a diploid Key lime

gamete A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...

and a haploid lemon

ovule In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the ''integument'', forming its outer layer, the ''nucellus'' (or remnant of the sporangium, megasporangium), ...

. 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 a

tetraploid Polyploidy is a condition in which the cells of an organism have more than two paired sets of ( homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two complete sets of chromosomes, one fro ...

. 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.

Hybrids from other citrus species

While most citrus hybrids derive from the three core species, hybrids have also been derived from the micrantha, Ichang papeda, kumquat, Australian limes, and trifoliate orange. The best known hybrid from micrantha is the

(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 Yuzu (''Citrus'' × ''junos'', from Japanese language, Japanese or ; ) is a citrus fruit and plant in the family Rutaceae of China, Chinese origin. Yuzu has been cultivated mainly in East Asia, though it has also recently been grown in New Z ...

(which also includes smaller contributions from pomelo and kumquat). Other more exotic citrus have likewise proved hybrids that include papeda. 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 papeda.

''Citrofortunella''

A large group of commercial hybrids involve the kumquat, ''Fortunella'' in 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 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

, and

yuzu Yuzu (''Citrus'' × ''junos'', from Japanese language, Japanese or ; ) is a citrus fruit and plant in the family Rutaceae of China, Chinese origin. Yuzu has been cultivated mainly in East Asia, though it has also recently been grown in New Z ...

respectively.

''Citroncirus''

As with kumquats, 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, 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'

. As with Citrofortunella, were ''Poncirus'' subsumed into ''Citrus'', these hybrids would no longer be intergeneric, and would likewise fall within ''Citrus'', rendering ''Citroncirus'' invalid.

Graft hybrids

Due to the sterility of many of 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 Grafting or graftage is a horticulture, horticultural technique whereby tissues of plants are joined so as to continue their growth together. The upper part of the combined plant is called the scion () while the lower part is called the roots ...

, the cells from the scion and rootstock are not somatically fused, but rather 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 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 the two contributing species with a plus sign between the individual names (''Citrus medica'' + ''C. aurantium'').

References

External links

USDA Citrus Genome Database

USDA Classification

a comprehensive article on citrus taxonomy

GRIN database for Species of Citrus
{{Taxonomy of... Citrus

* Botanical nomenclature