| Phylogentic relationships among Citrus and its relativesCitrus is one of major fruit trees in the world, with plenty of species and varieties. Citrus phylogeny is confusing and controversial due to asexual seed production, sexual compatibility between Citrus and related genera, the high frequency of bud mutations and the long history of cultivation and wide dispersion. To better understanding of the citrus genetic background, we investigated the phylogenetic relationships among Citrus and its relatives, including 29 accessions belonging to Citrus, Poncirus, Fortunella, Microcitrus, Eremocitrus, Atlantia and Severinia, and probed the origin of some important citrus species using molecular markers, namely, AFLP, SSR and chloroplast DNA analysis. The main results are as follows:1. 4 pairs of universal cpDNA primers generated one monomorphic fragment each. 142 restriction sites were generated after digestion of which 52 were polymorphic. Digestion of amplification products using 2 pairs of universal mitochondrial DNA primers gave 43 restriction sites. Except that Poncirus possesses 3 unique sites, all other accessions have the same mtDNA restriction pattern.2. 7 pairs of SSR primers generated 114 alleles, all of that were polymorphic, with an average of 16.3 alleles per primer. The average polymorphic information content was 0.648.3. 130 EcoRI/MseI primer combinations were tested and 19 of them with best performance were selected for future usage in Citrus research. A subset of six primer combinations generated 571 polymorphic bands out of 599 bands, 79 of which were unique to specific accessions. The polymorphic rate of AFLP markers was 95.33% and average polymorphic information content was 0.253.4. Cluster analysis was conducted respectively using neighbour-joining(NJ) method based on the three kinds of molecular markers. Little correlation exists among the genetic distance generated by the the three kinds of molecular markers. Based on the results of cluster analysis, following conclusions were reached.1) Microcitrus is very close to Citrus and so is C. halimii to Fortunella, however theyare not so close to citrus as reported before. The present work demonstrated that C.halimii could be classfied into Fortunella, not supporting C.halimii as basic species of modern cultivated citrus.2) Organeller analysis showed that P. polyandra should not be the hybrid between trifoliate orange and Citrus biotype and it could be accepted as a new species of Poncirus. The high frequency of the homozygous SSR locus and great number of unique AFLP markers also support its species status.3) That Poncirus is distant from Citrus proves Poncirus could not be the descendant of Citius.4) Sepsration of neither Papeda and Citrus nor Archicitrus and Metacitrus was clear, yet the result seems to support Tanka's assumpation that citrus originated from papeda. C. ichangesis is a distinct species very different from other Citrus accessions, and the results showd it is improper to ck.ssify it into subgenus Metacitrus.5) The present work supported citron, pummelo and mandarin as basic species of cultivated citrus since they can be placed into three distinct clusters.6) Based on the similarity of digesiton pattern of organeller DNA and cluster analysis, the origin of some biotypes were assumed. Grapefruit was assumed as the hybrid of pummelo and sweet orange, probably as the selection of a backcross to pummelo. Citron, pummelo and microcitrus may involve in the origin of one of the limes analyzed. The other lime was assigned as Key lime, and its origin relates to papeda and citron. Rough lemon is the hybrid of citron and mandarin. The origin of red lime relates to mandarin and rough lemon. Sour orange and sweet orange may be the hybrid of pummelo and mandarin.Genetic diversity of Poncirus as revealed by AFLP markersPoncirus is known to originate in China, and there exists numerous variants, many of them have yet not been found and characterized. Due to lack of funds, natural disasters...
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