The Polymorphism Of Citrus And Its Relatives Based On Single-copy Nuclear Gene (SCNGs) DNA Sequences

Citrus is the world’s largest fruit, It is widely cultivated in tropical, subtropical and transition region of middle type climate. The genus Citrus established by Linneaus in1753, it’s the first time which founded modern citrus taxonomy. Then, the researchers all over the world launched a series of research from morphology to genomics on Citrus classification problem. However, the phylogenetic relationships between Citrus and its closely related genera have remained controversial. The original species and number of wild type of Citrus plants is still uncertain. The basic framework of Citrus plants and its system evolution of still need improvement. Due to the high frequency of bud mutations, no reproductive isolation between species, sexual compatibility between Citrus and related genera, the questions of classification, evolution and hybrid origin have been debated.Because of the characteristic of biparental inheritance, carry a lot of information site and orthology, single-copy nuclear gene can better reflect the evolution of the relationship between species compared with the cytoplasmic genome. In the present study, we select the materials analyzed with Murraya paniculata (L.) Jack, Clausina and Severinia buxifolia (Poir.) Tenore as outgroups. Then we sequenced and analysed nine single-copy nuclear genes (SCNG) distributing in nine different chromosomes by clone measure to assess the phylogeny of27primitive accessions from Citrus and its relatives. We also sequenced the chloroplast matK genes as a comparison by PCR-direct sequencing. Using PAUP*(version4.0) and raxML software we constructed the maximum parsimony (MP) tree and maximum likelihood (ML) tree respectively.1The MP and ML trees constructed by SCNG and matK sequences consistently showed:The resources selected in this research, C. tachibana (Mak.) Tanaka is more original than C. reticulata’daoxian’and C. reticulata’Nanfengmiju’Blanco; C. indica Tanaka does not belong to mandarin and has a close relationship with citron; C. reticulata Blanco (round leaf) is a very primitive type in Citrus plants. C. ichangensis Swingle formed a clade with loose skin mandarin.2The MP and ML trees constructed by SCNG sequences consistently showed: Microcitrus is closely relative to citron; there is closely relationships between Poncirus and Fortunella.3The MP and ML trees constructed by chloroplast gene (matK) sequences consistently showed: Poncirus and Fortunella formed a clade respectively, and both them located at the base of the tree; Microcitrus can not form a clade independently, and its female parent is close to papeda and pummelo species; C. ichangensis Swingle is located with loose skin mandarin.Finally, we combined SCNG sequences and matK sequence identified the origin of some hybrid species of Citrus: sour orange is a female parent of lemon, the male parent of lemon could be citron. Mexican Lime is a cross species with the loose skin mandarin as female parent and citron as male parent. The C. ichangensis Swingle have close relationships with loose skin mandarin. We speculated that C. tachibana (Mak.) Tanaka originated in nanling mountain area of our country.These results are of significance in Citrus origin classification and hybrid genetic breeding.