Genome sequences of nine species of citrus, including oranges, pummelos and mandarins, reveal pathways of domestication and provide resources for breeding.
Cultivated citrus are selections from, or hybrids of, wild progenitor species whose identities and contributions to citrus domestication remain controversial. Researchers now sequenced and compared citrus genomes— a high-quality reference haploid clementine genome and mandarin, pummelo, sweet-orange and sour-orange genomes—and showed 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. Citrus are widely consumed worldwide as juice or fresh fruit, providing important sources of vitamin C and other health-promoting compounds. Global production in 2012 exceeded 86 million metric tons, with an estimated value of $9 billion (http://www.fas.usda.gov/psdonline/circulars/citrus.pdf). The very narrow genetic diversity of cultivated citrus makes them highly vulnerable to disease outbreaks, including citrus greening disease (also known as Huanglongbing or HLB), which is rapidly spreading throughout the world's major citrus-producing regions1. Understanding the population genomics and domestication of citrus will enable strategies for improvements, including resistance to greening and other diseases.