Sequences from mandarin (Citrus reticulata) cultivars 'Clementine', 'Ponkan' and 'Willowleaf', pummelo (Citrus maxima) cv 'Chandler' and a low acid genotype, sweet orange (Citrus sinensis) cv 'Ridge Pineapple' and the Seville sour orange (Citrus x aurantium) have been released recently, along with analysis and insights into their origin and domestication in the journal Nature Biotechnology. In addition to supplying several high quality genomic sequences, this work also sheds lights on the ancestral history of traditional mandarin, pummelo and sweet oranges. In particular, it reveals a genetic bottleneck in pummelo history, lower than expected diversity among mandarins and apparent admixture with pummelo's in the mandarin accessions sequenced. The results also cement the roles Citrus maxima (pummelo) and citrus reticulata (mandarins) played in the origin of modern cultivated citrus and highlights genetic material that likely originated from a wild mandarin. These insights into the origin and relationsips of sweet orange will help geneticists and breeders approach their work with more precision and and will ultimately enable them to broaden the genetic base of cultivated orange so the crop can better respond to environmental challenges.
Cultivated citrus are selections from, or hybrids of, wild progenitor species whose identities and contributions to citrus domestication remain controversial. Here we sequence and compare citrus genomes—a high-quality reference haploid clementine genome and mandarin, pummelo, sweet-orange and sour-orange genomes—and show that cultivated types derive from two progenitor species. Although cultivated pummelos represent selections from one progenitor species, Citrus maxima, cultivated mandarins are introgressions of C. maxima into the ancestral mandarin species Citrus reticulata. The most widely cultivated citrus, sweet orange, is the offspring of previously admixed individuals, but sour orange is an F1 hybrid of pure C. maxima and C. reticulata parents, thus implying that wild mandarins were part of the early breeding germplasm. A Chinese wild 'mandarin' diverges substantially from C. reticulata, thus suggesting the possibility of other unrecognized wild citrus species. Understanding citrus phylogeny through genome analysis clarifies taxonomic relationships and facilitates sequence-directed genetic improvement.
The open-access article can be read here.