Almost all citruses - the , , , , etc. - are hybrids of three original citrus species: the , , and . Most often, we represent genealogy as a tree, but the complex hybridization process that led to modern citrus would render more of a complex, incestual web. It does, however, lend itself to another representation: a ternary plot. Each corner represents one of the original species, and the closer the fruit is to that corner, the higher percentage it is of that species - press on any individual node to see its exact genetic distribution.
The original species diverged five or more million years ago, when a sudden climatic event caused the genus’ evolutionary radiation into these core species - , , and - among others, such as kumquats and kaffir limes, though these have not been hybridized to the same extent.
Historical movement and cultivation amplified that blending. After early domestication in Asia, citrus spread through Austronesian movement and later westward trade routes into the Mediterranean, where additional selection and crossing produced familiar market groups. That is why , , , and are better understood as overlapping recombination clusters than as isolated branches. Over time, human breeding tended toward sweeter, more mandarin-rich market varieties - as can be seen in the , which is largely unidirectional.
The ternary framing is helpful, but not entirely complete. Another original citrus variety, the samuyao (), entered the genetic citrus lottery through , a citron and samuyao hybrid. Then, other varieties like the , a hybrid of the key lime and , further complicate the ternary representation.
The chart is not a representation of chronology; that’s what makes it interesting. It reveals visually information that would be latent in a typical chronological representation. Clustering shows how various combinations of bitterness and sweetness complement human taste, how historical happenstance, like the development of the - an accidental dot in the ternary plot - dictates the varieties we may see in the supermarket: what we consider to be normal. The crowded defines a hidden directionality within otherwise directionless data. Citrus is made in the image of the ternary plot - the ternary plot, in the image of man.
Sources
Wu et al. (2018) Nature 554:311-316
Langgut (2017) HortScience 52:814-822
Velasco & Licciardello (2014) Nat. Biotechnol. 32:640-642
Curk et al. (2016) Ann. Bot.
Barkley et al. (2006) Genome
Ollitrault et al. (2012) Ann. Bot.
Curk et al. (2015) Mol. Ecol.