Southern China may have provided the initial source of citrus fruits.

An in-depth look at the orange family tree shows the oldest Citrus ancestors arrived in Asia on the Indian tectonic plate over 25 million years ago.

Southern China may have provided the initial source of citrus fruits.

According to a DNA investigation, China is the birthplace of citrus fruits.

According to research published on October 2 in Nature Genetics, a thorough examination of wild members of the citrus family tree suggests that the earliest species of the Citrus genus emerged around 8 million years ago in what is now south central China. Around 25 million years ago, the Indian tectonic plate collided with the Asian plate, possibly bringing the ancestors of the citrus lineage to Asia for the first time.

Today, myriad tart fruits — including human-bred versions of oranges, lemons and limes — stockpile grocery store shelves. But it’s unclear where their oldest ancestors evolved. Previous studies pointed to places like northeastern Australia, southern China or the southeastern foothills of the Himalayas. Piecing together citrus evolution could help researchers make drought- or disease-resilient plant varieties that still bear tasty fruit, says horticulturalist Qiang Xu (SN: 4/14/15).

To pinpoint Citrus’ origins, Xu, of Huazhong Agricultural University in Wuhan, China and colleagues built a family tree using the genetic blueprints of 314 Citrus and Citrus-related plants, which included 15 Citrus species. The researchers also trekked into the field to find where the trees grow in the wild. The oldest citrus fruits — including C. trifoliata, the trifoliate orange — are found in south central China, suggesting that region is the genus’ birthplace.

Citrus evolution didn’t stop there. Some species have roots in the regions that the earlier studies identified, the team found. Pomelo and citron fruits appear to have sprung up in the Himalayan foothills, and some wild limes evolved in Australia.

The researchers also confirmed that a gene called PH4 is linked to the amount of citric acid inside each fruit. PH4 turns on a proton pump that allows citric acid to build up inside oranges and their relatives. The more citric acid, the more tart the taste.