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Citrus is one of the most important fruit crops in the world, and widely planted in tropic and semi-tropic areas. The citrus growing area reached about 6.66 million hectares all over the world, annual aggregate production climbed to 11 million tones, annual trade value was about $8 billion. It has became the fourth largest agricultural products. In China, Citrus industry boomed as the development of Chinese economy, and citrus industry has already become a supporting industry in the rural areas, especially, in the south of China, which has played an important role in adjusting agriculture and rural economic structure and helping the farmers to throw off poverty.Genetic improvement of cultivars is the key of the healthy development of Chinese citrus industry. Especially, since the entry into WTO, China has been doing the best to improve its citrus cultivars and optimize its citrus industrial structure. However, so far, few excellent cultivars bred by ourselves has been extended in wide areas; additionally, owing to the restrictions of intellectual property rights associated with citrus breeding, few new citrus varieties from other countries can be available to Chinese farmers for satisfying the requirement of rapidly renewing of citrus varieties. Therefore, it is necessary for Chinese citrus breeders to breed new excellent citrus cultivars, which have independent intellectual property rights and can be adapted to the requirement of market. Although considerable progress in citrus genetic breeding had been made in the past several decades, Traditional breeding is becoming inefficient, and can not keep up with the requirement of citrus production.In recent years, the successful application of various molecular breeding technologies (e.g., genetic linkage mapping, marker-assisted selection, QTL, genetic transformation) in many crops provides a novel and promising pathway for citrus breeding. The construction of good molecular genetic map is a crucial basis of the application of the molecular breeding technology in citrus breeding. Besides, a high-density linkage map was also an essential tool for facilitating
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