Citron
The citron ("Citrus medica") is a large fragrant citrus fruit with a thick rind. It is one of the original citrus fruits from which all other citrus types developed through natural hybrid speciation or artificial hybridization. Though citron cultivars take on a wide variety of physical forms, they are all closely related genetically. It is used widely in Asian cuisine, and also in traditional medicines, perfume, and for religious rituals and offerings. Hybrids of citrons with other citrus are commercially prominent, notably lemons and many limes.
Citrus taxonomy
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"). In Australia three 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.