A farmer harvests oranges in Pingyuan county, Guangdong province. Zhong Xiaofeng / For China Daily

Chinese scientists have sequenced the genome of the sweet orange (Citrus sinensis), the first time a Chinese research team has independently determined the genome sequence of a fruit.

The achievement opens the door to better quality and higher crop yields, and provides a scientific basis for developing modified genetic breeds that do not have fatal viruses.

The Chinese research team, led by professor Deng Xiuxin, a member of the Chinese Academy of Engineering, and professor Ruan Yijun from Huazhong Agricultural University - along with horticultural, gene and bioinformatic scientists - assembled and annotated the sweet orange genome for a high quality genome.

Citrus fruits are among the most important and widely grown fruit crops in the world, with global production and total area ranking first among all fruit crops. Sweet oranges account for about 60 percent of citrus production, both as a fresh fruit and processed juice.

Citrus is susceptible to a large number of biotic diseases, especially citrus yellow mosaic, which is transmitted by the Asian citrus psyllid, a sap-sucking bug also found in the Middle East and America.

It has not been a good year for sweet orange farmers in Guangdong province, where the citrus yellow mosaic virus has infected many trees.

Currently, there is no effective way of dealing with the virus and the only way to stop the virus spreading is to root out the trees and burn them. Worse, the infected land cannot be used for sweet orange production for years afterward.

It is reported that more than 20 percent of sweet orange trees were infected this year in Guangdong, one of the country's major citrus growing areas.

Deng compares sequencing the genome of the sweet orange to opening the fruit crop's "black box", an achievement that could improve the fruit's color, taste, yield and disease resistance.

Oranges are mostly polyembryonic, meaning two or more embryos develop from a single fertilized egg. These characteristics hinder development, while a complete genome sequence could provide a valuable molecular framework for breeding and improvement.

The genome sequencing was applied to nine chromosomes by genetic markers and FISH (fluorescence in situ hybridization) analysis. Comparative analysis showed that about 81 and 83 percent of sweet orange genes are shared by grapes and strawberries.

Citrus fruits are believed to originate in China and citrus culture here can be traced back 4,000 years.

China has abundant citrus varieties, including mandarins, tangerines, lemons and kumquat. The nation's citrus industry has developed quickly in the past 30 years, and had a total area of 2.21 million hectares and production of 26.45 million tons in 2010, ranking first among all countries.

The world's first human genome sequence map was completed in June 2000. From 2000 to 2009, scientists across the world have drawn whole genome sequence maps for 1,100 species, averaging 118 a year.

Chinese scientists have completed genome sequences for rice, silkworms, chickens, oysters and endangered animals such as the giant panda and Tibetan antelope.

Contact the writers at wangru@chinadaily.com.cn and zhoulihua@chinadaily.com.cn.