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Organisms


The three citrus organisms with the most genomic and genetic data are C. clementina, C. sinensis and C. trifoliata. These three organisms are the focus of this database for now, but more will be added as more information becomes available.

Clementine (Citrus clementina)

Mandarin, the second largest species in terms of worldwise citrus production, consists of Clementine (C. clementina), Satsuma (C. unshiu Marc.), and other diverse tangerine types. In contrast to sweet orange (C. sinensis), mandarin is often called loose-rind citrus and primarily produced for fresh market, because their peel is generally very easy to remove. Clementine is almost exclusively grown in Spain, Italy, France, and other Mediterranean countries, and Satsuma mostly in Japan, China, and Southern Korea.  The genome size is estimated at 370Mb across 9 haploid chromosomes.



Sweet Orange (Citrus sinensis)

Sweet orange is the No.1 citrus production in the world, accounting for about 70% of the total. Brazil, Flordia (USA), and China are the three largest sweet orange producers. Sweet orange fruits have very tight peel and are classified into the hard-to-peel group. They are often used for juice processing, rather than fresh consumption. Valencia, Navel, Blood, Acidless, and other subtypes are bud mutants of common sweet orange varieties. Sweet orange is considered as an introgression of a natural hybrid of mandarin and pummelo; some estimates shows more mandarin genomic background than pummelo.  The genome size is estimated at 380Mb across 9 haploid chromosomes.



Trifoliate Orange (Poncirus trifoliata)

C. trifoliata is a valuable species within Citrus because it possesses genes conferring many agriculturally important traits not discovered in other Citrus. It is s ometimes classified as a species outside the Citrus genus, Poncirus trifoliata or Poncirus trifoliata (L.) Raf. While other Citrus species are evergreen and u nifoliate, C. trifoliata is deciduous and trifoliate. As a primary rootstock and rootstock breeding parent for agricultural Citrus, it exhibits resistance to citrus tristez a virus (CTV), Phytophthora root rot, and citrus nematode (CN). It also confers tolerance of low temperature, a dwarfing effect, and other beneficial attributes. Most genetic mapping po pulations are derived from crosses with C. trifoliata, because of high interest in its beneficial traits, attempts at comparison of the related genomes, and the great advantage of the dominant trifoliate leaf trait of C. trifoliata over the monofoliate of Citrus that allows zygotic hybrids to be visually distinguished from nucellar seedlings. The genome size is estimated at 380Mb across 9 haploid chromosomes.



General Citrus Info

Citrus can refer to as a common term or a genus name. In general, citrus includes all species and natural hybrids in Citrus and its closely related genera (sexually compatible with each other to certain extent), such as Poncirus (trifoliate orange), Fortunella (kumquat), Microcitrus (Australian wild lime), and Eremocitrus (desert lime). They all belong to the family Rutaceae that is in the order Sapindales, a sister taxon to Brassicales that contains Arabidopsis. The taxonomy of citrus has been very controversial and inconsistent binomial naming are often seen. Even some same species are prefixed with different genus names because some researchers suggested all the above genera to be classified as one genus, Citrus. However, many widely used binomial names, though known not true species, are in popular use, such as sweet orange (Citrus sinensis [L.] Osbeck).

From genomic perspectives, most citrus species are diploid (2n = 2x = 18), with relatively small genomes; for instance, sweet orange has a genome of about 380Mb, approximately three times the size of the 125Mb Arabidopsis genome. So far available genomic resources include more than half millions citrus ESTs, most sweet orange (~90%) and next following by Clementine, trifoliate orange, Satsuma, and other citrus varieties; high density microarrays in several different platforms; several BAC libraries; one sweet orange physical map; and high density linkage maps for sweet orange and Clementine. One haploid Clementine genome are being sequenced using Sanger technology, and one diploid sweet orange genome sequenced using Roche 454 technology. These genome sequence resources and tools will allow geneticists and breeders to more effectively manipulate various traits in breeding programs, and genomicists and bioinformaticians to preform comparative genomic studies on a new taxonomic group.

Citrus includes almost all commercially grown citrus species/cultivar groups, producing diverse fruit types including sweet orange (Citrus sinensis [L.] Osbeck), mandarin (C. reticulata Blanco), grapefruit (C. paradisi Macfad.), lemon (C. limon (L.) Burm.), lime (C. aurantifolia Christ.), pummelo (C. maxima Merril.), and citron (C. medica L.). Only C. reticulata, C. maxima, and C. medica are believed true species, and the others natural hybrids or introgressions, according to some characteristic traits and molecular markers.