Citrus species are important fruit crops. They are the result of a unique model of historical breeding. Ancient hybridization of a few initial species gave rise to several crop species, each one consisting of a single genotype.
IGA participated in an international Citrus sequencing project with GenoScope (France) and the Joint Genome Institute (US). The consortium has sequenced a haploid clementine “mandarin” and assembled a reference sequence for the genus Citrus. A 300-Mbp reference genome was generated from 7× Sanger shotgun sequencing. The quality of the assembly is high for sequence contiguity and scaffolding.
IGA has resequenced three ancestral species of Citrus (mandarin, pummelo, citron) and cultivated forms of the derived species sweet orange, clementine, lemon and grapefruit. All cultivated Citrus were originally derived from different crossing combinations of the ancestral species. Sour orange is an interspecific hybrid of C. maxima and C. reticulata. Sweet oranges, the citrus type of highest economic interest, have more complex ancestry with admixture of pummelos and wild mandarins.
All cultivated Citrus showed a narrow genetic diversity. Cultivar groups are derived from single seedlings that arose by interspecific hybridization and/or successive introgressive hybridizations of wild ancestral species. Diversity within cultivar groups was generated only by somatic mutations, without sexual recombination, either as limb sports on trees or as variants among apomictic seedling progeny.
Wu et al (2014) Sequencing of diverse mandarin, pummelo and orange genomes reveals complex history of admixture during citrus domestication. Nature Biotechnology 32(7):656-662
Funded by the Italian Ministry of Agriculture (MIPAAF)
Status: Citrustart project, completed