The origin of oranges
All trees in an orange orchard are genetically almost identical. This makes them more susceptible for diseases. Now a team of Chinese researchers show in Nature Genetics the origin of oranges, and that breeders can use this to breed resistant varieties.
The oranges that we know find their origin about 4000 years ago in China. It was a product of the mating of two citrus species, but which ones exactly, about that are the researchers not completely sure. The biggest contenders are mandarins and pummelos.
After the birth of oranges, farmers used cuttings to propagate them. In this way they made sure they kept the preferred flavour. But this has one big drawback. Genetically all orange trees are more or less identical. This makes them extra susceptible to pathogens.
Sweet oranges are most closely related to sour oranges
Through freshly crossing the parents of a crop, breeders can get back a lot of the original genetic diversity. Which they can subsequently use to develop disease resistant crops. Only the problem is, for oranges it is not really clear who its parents are.
To find out the researchers sequenced the genomes of 226 citrus species. Those species came from orchards, but also from the wild. In addition to sweet oranges, the researchers analysed sour oranges, mandarins, pummelo’s and hybrid species.
Al together showed that the sweet oranges were closed related to sour oranges. The genomes of both species are made up half from mandarin chromosomes and half from pummelo chromosomes. However, sweet oranges are no direct descendants from pummelo’s. Instead, they are the result of a crossing between sour oranges and mandarins.
Crossing sour oranges with mandarins gives you some sweet oranges
To confirm this the researchers crossed disease resistant sour oranges with mandarins. This resulted in a wide range of offspring, often with sour oranges. But three were very similar to commercially available sweet oranges, both in flavour as in shape and colour.
The hope was that these oranges were also disease resistant. To investigate this the researchers analysed what it was that caused the sour orange to be disease resistant. This turns out to be a mix of antibacterial compounds. Some of the offspring from the cross with mandarins also accumulated those compounds, including one with sweet flavoured oranges.
Breeders can by crossing disease resistant sour oranges with mandarins obtain new resistant sweet oranges. Although they need to be patient. Oranges take their time growing up.
Literature
Liu, S., Xu, Y., Yang, K. et al. Origin and de novo domestication of sweet orange. Nat Genet 57, 754–762 (2025). https://doi.org/10.1038/s41588-025-02122-4
Plant en Zo 