Citrus taxonomy
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 x limonimedica") are both true lemon/citron hybrids, 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 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.
Citrus taxonomy
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, for which he used "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.