FROM oranges and lemons to limes and grapefruit, the wealth of citrus fruits available today belies the fact that there are only three true citrus species. So says a researcher in Florida, who insists all the rest are mongrels resulting from different combinations of pomelos, citrons and mandarins.
Records of citrus cultivation stretch back to 2100 BC, but scientists have always disagreed about how many species there are in this plant genus. Some researchers argue that they all belong to a single species, while others have suggested there are as many as 147.
Several factors make classification difficult. First, it has been tricky to work out where citrus fruits originated, though it鈥檚 now generally thought they came from eastern India, northern Burma and south-west China. Exactly how the fruit dispersed is also unclear. Then there鈥檚 the rather complicated reproductive biology of the Citrus genus. Many citrus trees reproduce asexually through a process known as 鈥渘ucellar embryony鈥�. Often these nucellar embryos-essentially clones-win out over sexually generated embryos.
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Until the 1970s, taxonomists had to rely on the appearance of the fruit, together with geographical and historical data, to reach their conclusions. Now Gloria Moore, a geneticist at the University of Florida in Gainesville, has put the old clues together with new evidence from DNA. In a future issue of Trends in Genetics, she presents her synthesis of the research, concluding that there are only three citrus species: the pomelo, a popular dessert fruit in south-east Asia; the citron, now grown mainly as the raw material for candied peel; and the more familiar mandarin.
The range of other citrus fruits we enjoy are the result of accidental hybridisation and spontaneous mutations, picked out and cultivated by farmers through the ages. 鈥淎ll of the oranges that we have are just selections we have found growing,鈥� says Moore.
Limes, for instance, seem to be the offspring of citrons and pomelos, while lemons are a complex hybrid of citrons and Seville oranges. Seville oranges are mainly mandarin, but have incorporated some genes from the pomelo.
