Citrus taxonomy

Citrus taxonomy refers to the botanical classification of the species, varieties, cultivars, and graft hybrids within the genus Citrus and related genera, found in cultivation and in the wild. Citrus taxonomy refers to the botanical classification of the species, varieties, cultivars, and graft hybrids within the genus Citrus 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. Some differ only in disease resistance. 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. 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 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 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, et al. identified just ten ancestral species of citrus among those studied. Of these, 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. ichangensis), 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 or varieties) or hybrids of these species, 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 micrantha 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 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 calamondin 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 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', which included Poncirus, Citrus, Fortunella, Eremocitrus, Microcitrus, and Clymenia, all but the first now viewed to fall within Citrus. 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 is generally followed globally today with much modification; however, 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 the standard species concept. Tanaka also divides into subgenera, but differently than Swingle did, 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 predate 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 the 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, Swingle's subgenera were polyphyletic, and hence do not represent valid taxonomy. Historical genera are also dubious. 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 has pointed to these groups nesting 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 may likewise belong within this expanded phylogenetic concept of the genus Citrus, including Clymenia, Oxanthera and more controversially Poncirus, along with 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.