Domestication of Citrines

A study on citrines investigated the centre of origin of citrus species. Analyzing hybrids and admixed genomes they could find information on the domestication of mandarin, sweet oranges and other citrus. Wu et al. (2018) analyzed 60 accessions from diverse citrus germplasms and studied their relationship by performing whole genome sequencings. They found that mandarins contain pummelo admixture. The process of admixture is the result of interbreeding lineages from distinct sources and further introduction within a population (Keller and Taylor, 2010). This admixture was found to be corelated with fruit size and acidity, suggesting pummelo introgression as taking part in the selection of palatable mandarins during domestication, which requires reduced acidity (Baldwin, 1993). Palatable mandarins demonstrated pummelo admixture in part of the genome, opposite to acidic mandarins, proving the previous statement since, in mandarins, palatability is linked to pummelo introgression at a locus in the start of chromosome 8, and, that 8 locus was found to have a gene (Ciclev10028714) encoding mitochondrial NAD+-dependent isocitrate dehydrogenase (IDH) which regulates citric-acid synthesis.

Wu et al. (2018) concluded that the southeast foothills of the Himalayas was the center of origin, since it was found a fossil of Citrus linczangensis from the late Miocene with traits that provide evidence of a common ancestor of all the citrus group. Although it was difficult to identify a pure or wild progenitor species, due to interspecific hybridization resulting in several clonally propagated and cultivated accessions, they assumed that an ancestral citrus migrated to other regions and underwent sudden speciation, being human selection process of desirable traits and clonal propagation, by grafting or other methods, possibly a great influence in their relatedness.

Figure 1- Possible centers of origin of citrus and migration directions of the ancestral species. From: Wu et al., 2018.

Bibliography 

Baldwin, E.A. (1993). Citrus fruit. In Biochemistry of Fruit Ripening, pp. 107–149.

Keller, S. and Taylor, D. (2010). Genomic admixture increases fitness during a biological invasion. Journal of Evolutionary Biology, 23(8), pp.1720-1731.

Wu, G., Terol, J., Ibanez, V., López-García, A., Pérez-Román, E., Borredá, C., Domingo, C., Tadeo, F., Carbonell-Caballero, J., Alonso, R., Curk, F., Du, D., Ollitrault, P., Roose, M., Dopazo, J., Gmitter, F., Rokhsar, D. and Talon, M. (2018). Genomics of the origin and evolution of Citrus. Nature, 554(7692), pp.311-316.