Citrus species are among the most important fruit trees in the world. Citrus has a long history of cultivation, often thought to be more than 4,000 years. Until now, however, the exact genetic origins of cultivated citrus such as sweet orange (Citrus sinensis), lemon (C. limon), and grapefruit (C. paradisi) have been a mystery. A team of researchers from China has published a study in the Journal of the American Society of Horticultural Science that provides genetic evidence of the origins of a variety species of today's cultivated citrus.

The research team, led by Zhiqin Zhou from Southwest University, analyzed amplified fragment length polymorphism (AFLP) fingerprints -- a technique that has been used successfully to assess the origin of potato cultivars -- with chloroplast DNA (cpDNA) sequence analysis and nuclear internal transcribed spacer. "The combination of nuclear DNA and cpDNA data allowed us to identify the exact genetic origin of the cultivated citrus," they wrote.

The results proved that bergamot and lemon were derived from citron and sour orange, and grapefruit was a hybrid that originated from a cross between pummelo and sweet orange. The data demonstrated that sweet orange and sour orange were hybrids of mandarin and pummelo, while rough lemon was a cross between citron and mandarin. The evidence also confirmed that bergamot was a hybrid of sour orange and citron, with sour orange as the maternal parent and citron as the paternal parent.

"Our molecular evidence presented more convincing data than all other previous studies in supporting the origin of lime," noted the scientists. The data confirmed a species of Papeda to be the female parent and C. medica as the male for mexican lime.

The researchers said that a clear understanding of the citrus genetic background is necessary for better characterization and utilization of citrus germplasm, adding that this research will provide important new information for future study on the genetics and breeding of citrus.