Citrus taxonomy

Citrus taxonomy refers to the botanical classification of the species, varieties, cultivars, and graft hybrids within the genus Citrus and related genera, found in cultivation and in the wild.

Citrus taxonomy is complex and controversial.^[1][2] Cultivated citrus are derived from various citrus species found in the wild. Some are only selections of the original wild types, many others are hybrids between two or more original species, and some are backcrossed hybrids between a hybrid and one of the hybrid's parent species. Citrus plants hybridize easily between species with completely different morphologies, and similar-looking citrus fruits may have quite different ancestries.^[3][4] Some differ only in disease resistance.^[5] Conversely, different-looking varieties may be nearly genetically identical, and differ only by a bud mutation.^[6]

Detailed genomic analysis of wild and domesticated citrus cultivars has suggested that the progenitor of modern citrus species expanded out of the Himalayan foothills in a rapid radiation that has produced at least 11 wild species in South and East Asia and Australia, with more than a half-dozen additional candidates for which either insufficient characterization prevents definitive species designation, or there is a lack of consensus for their placement within the Citrus genus rather than sister genera. Most commercial cultivars are the product of hybridization among these wild species, with most coming from crosses involving citrons, mandarins and pomelos.^[7][8] Many different phylogenies for the non-hybrid citrus have been proposed,^[9] and the phylogeny based on their nuclear genome does not match that derived from their chloroplast DNA, probably a consequence of the rapid initial divergence.^[7] Taxonomic terminology is not yet settled.

Most hybrids express different ancestral traits when planted from seeds (F2 hybrids) and can continue a stable lineage only through vegetative propagation. Some hybrids do reproduce true to type via nucellar seeds in a process called apomixis.^[4] As such, many hybrid species represent the clonal progeny of a single original F1 cross, though others combine fruit with similar characteristics that have arisen from distinct crosses.