Tart citrus fruits vary from olive-sized kumquats to four-pound pomelos. Most are round, but some, such as the finger lime, are elongated like sausages. Others, such as the Buddha’s hand, grow in weirdly gnarled segments.
“Citrus is fascinating,” says Gayle Volk, a plant physiologist at the U.S. Department of Agriculture, who studies the genetic preservation of citrus and other fruits. “The number of diverse crops produced through hybridization among different citrus species is much higher than that of apples or grapes.”
Trying to pinpoint exactly where this diverse, heavily cultivated group of fruits originated—previously hypothesized as anywhere from the Himalayan foothills to the balmy jungles of northeastern Australia—has soured the topic for many researchers. But a new paper takes an in-depth genetic approach to fleshing out oranges’ origins, along with those of their citrus kin. The study, published last week in Nature Genetics, analyzed the genomes of hundreds of species across the orange subfamily Aurantioideae—and revealed that citrus-related fruits likely originated on the ancient Indian subcontinent before further diversifying their sharp taste in south-central China.
Aurantioideae is a titanic taxonomic group encompassing more than 33 genera of fruit-bearing plants found throughout Asia, Africa and Polynesia. This includes the Citrus genus, whose members (such as oranges, grapefruit, lemons and limes) are cultivated worldwide.
Horticulturist Qiang Xu of Huazhong Agricultural University in China and his colleagues recently set out to map the evolutionary journey of the orange subfamily. They assembled the genomes of 12 species and compared those with 314 existing genetic records for members of Aurantioideae. They then organized this genetic database into a phylogenetic tree, which is akin to an evolutionary family tree. Using this, the researchers could determine how different varieties and groups are related. This in turn provides clues to when and where certain species originated.
The team found that the precursors to citrus plants originated more than 25 million years ago on the Indian subcontinent as it was ramming into continental Asia (creating the Himalayas in the process). As the continents collided, these ancestral citrus plants spread into Asia, as is evident from citruslike plant fossils discovered in southern China. The researchers posit that true Citrus species, such as mandarins and trifoliate oranges, first evolved in south-central China around eight million years ago. They speculate that other early Citrus species, including the pomelo and citron, emerged slightly later in the Himalayan foothills.
Location appears to have been crucial for the success of these early fruits. Xu thinks south-central China provided “a complex situation for citrus.” He speculates that several million years ago drastic local climate change, which transformed the area from relatively dry tropical conditions to a wetter climate dominated by monsoons, provided ideal growing conditions. He thinks the region’s budding citrus diversity exploded when local human populations began cultivating the plants thousands of years ago for things ranging from food to medicine.
By building such a thorough genetic database from across the orange subfamily, the researchers also discovered that citrus plants differed greatly from their relatives in the expression of the PH4 gene, which plays a major role in determining the amount of citric acid—a key component of flavor—in a given fruit. Noncitrus fruits had barely any citric acid. Citrus fruits, with their higher expression of PH4 genes, had much greater concentrations.
“The PH4 gene is important for citric acid accumulation of fruits for both Citrus and Citrus relatives,” Xu says. When his team experimentally overexpressed or decreased the gene’s activity, they found that the citric acid concentrations responded accordingly. This has a big impact on a given fruit’s taste—small concentrations of citric acid provide a sweet tartness to oranges; larger amounts give lemons and limes their mouth-puckering tartness.
Volk, who was not involved in the new study, thinks learning more about citrus fruits’ past could inform their future preservation. “Refining genetic origins of citrus and related genera is critical for effective conservation of these plants,” she says. The ability to determine where certain species originated could help researchers protect habitats that are rich in wild species. This work also informs which varieties should be preserved in genetic banks to capture the greatest amount of diversity. In the face of climate change, pests and disease, these genetic stockpiles could help prevent a bitter future for the sweetest citrus.