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
The large citrus fruit of today evolved originally from small, edible berries over millions of years. Citrus plants diverged from a common ancestor about 15 million years ago, which was about when it diverged from the closely related severinia, for example the Chinese box orange. About 7 million years ago, citrus plants diverged into two groups, the main citrus genus and the ancestors of the trifoliate orange (poncirus), which is closely enough related that it can still be hybridized with all other citrus. These estimates are made using genetic mapping of plant chloroplasts. A DNA study published in "Nature" in 2018 concludes that citrus trees originated in the foothills of the Himalayas, in the area of Assam (India), western Yunnan (China), and northern Myanmar.