A genomic look at hybridization in citrus fruits

I just love this article about hybridization and the origins of different varieties of citrus fruits: “Genomics of the origin and evolution of Citrus”.

Here’s the key figure:

Figure 2b from Wu et al. 2018. Original caption: "Genealogy of major citrus genotypes. The five progenitor species are shown at the top. Blue lines represent simple crosses between two parental genotypes, whereas red lines represent more complex processes involving multiple individuals, generations and/or backcrosses. Whereas type-1 mandarins are pure species, type-2 (early-admixture) mandarins contain a small amount of pummelo admixture that can be traced back to a common pummelo ancestor (with P1 or P2 haplotypes). Later, additional pummelo introgressions into type-2 mandarins gave rise to both type-3 (late-admixture) mandarins and sweet orange. Further breeding between sweet orange and mandarins or within late-admixture mandarins produced additional modern mandarins. Fruit images are not to scale and represent the most popular citrus types."

It’s a great example of human-induced evolution for several reasons. I imagine that most people assume that the different varieties of citrus are something like “species” in the traditional sense. People may have a vague idea that these are propagated clonally today, but may not realize the incredible complexity involved in generating the different phenotypic configurations that have become common in today’s citrus agriculture.

The real history involves wild progenitor species that have become extinct in the face of human activity, and are only visible today through their genetic traces in living farmed citrus trees. The story includes contacts between far-off human populations, and trans-oceanic dispersal of citrus species prior to the origin of humans.

It’s truly epic, and all in the pursuit of flavor.