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
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), 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 (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. 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, being half papeda (type not specified), the other half including kumquat, mandarin and pomelo, with only trace amounts of citron.