Citrus taxonomy

Most commercial cultivars are the product of hybridization among these wild species, with most coming from crosses involving citrons, mandarins and pomelos.

[11] 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.

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 the oval (Nagami) kumquat (Fortunella margarita or C. japonica var.

The ability of citrus hybrids to self-pollinate and to reproduce sexually also helps create new varieties, as does spontaneous mutation and genome duplication.

[7] 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.

[14] The hybrids of these taxa include familiar citrus fruits like oranges, grapefruit, lemons, and some limes and tangerines.

Kumquats do not naturally interbreed with core taxa due to different flowering times,[17] 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.

[18] 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.

The Swingle system is generally followed globally today with much modification; there are still large differences in nomenclature between countries and individual scientists.

[18][21] 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.

[24][21][25] Phylogenetic analysis confirms this hybrid origin of most citrus cultivars, indicating a small number of founder species.

Swingle had elevated kumquats into a separate genus Fortunella, while two genera were suggested by him for the Australian limes, Microcitrus and Eremocitrus.

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.

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.

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.

[27][28] 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'.

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.

Since the latter two species locate to different branches of the citrus phylogenetic tree, the group would be polyphyletic and not a valid division.

[45] 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.

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.

[7] 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.

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.

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.

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.

For example, the Indian wild orange, once suggested as a possible ancestor of today's cultivated citrus fruits,[63] yielded conflicting phylogenetic placements in more limited genetic analysis,[24][64] 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 was coined as a genus containing intergeneric hybrids between members of the Citrus and the Fortunella, and is named after its parent genera.

[43] Examples of the Citrofortunella include the calamansi, limequat, and yuzuquat, crossing kumquat with tangerine, Key lime, and yuzu respectively.

As with kumquats, the trifoliate orange does not naturally interbreed with core taxa due to different flowering times,[17] but hybrids have been produced artificially between Poncirus and members of the genus Citrus.