Why Sequence Sweet Orange?

Citrus fruit image courtesy of Ottillia Bier

Sweet orange (Citrus sinensis L.) is one of the most important fruit crops in the world. Although considerable genetic diversity is available within the genus, diversity within Citrus sinensis is quite limited as all known cultivars have originated by mutation, not by sexual hybridization. Citrus breeding by conventional means, in general, is hampered by a long generation time, high heterozygosity, inability to produce inbred lines, and apomixis in many important cultivars. Sweet orange is a diploid species, with 9 pairs of chromosomes and a small genome size (382 Mb). It is placed in the Sapindales, a sister taxon to the Brassicales which contains Arabidopsis, but is otherwise taxonomically isolated from other plants with genome sequencing projects. Genome sequencing will complement other citrus genomics resources now available or under development, including a considerable EST database, high density microarrays, a BAC physical map, and a high density linkage map. These tools will allow geneticists and breeders to more effectively manipulate various traits in breeding programs. A genome sequence for Citrus will also facilitate comparative genomics research by expanding genome sequence data to a new taxonomic group, and should be particularly valuable for comparative studies with poplar and other trees

Pineapple orange

The initial JGI project will provide low sequence coverage (about 1.2 X) of ‘Ridge Pineapple’ orange, using a combination of plasmid and fosmid clones. JGI will also sequence about 4000 cDNA clones to obtain 3' ends of genes currently lacking this information. To assess the accuracy of sequence assembly in six heterozygous regions, 24 BAC clones that were assigned to haplotypes using SSR markers will be sequenced and assembled. It is hoped that the initial project will provide a foundation for a full genome sequence in the near future

Principal investigators: Mikeal L. Roose and Timothy J. Close (UC Riverside), Randy Niedz (USDA-USHRL), Abhaya Dandekar (UC Davis), and Fred G. Gmitter Jr. (Univ. of Florida).