Citron
There are also a number of citron hybrids; for example, ponderosa lemon, the lumia and rhobs el Arsa are known citron hybrids, some are claiming that even the Florentine citron is not pure citron, but a citron hybrid.
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.