One of these wild species gave rise to cultivated pummelo, the largest citrus fruit that can often range from two to four pounds. Surprisingly, the small, easily peeled mandarins were, in contrast, found to be genetic mixtures of a second species and pummelo. Sweet orange, the most widely grown citrus variety worldwide, was found to be a complex genetic hybrid of mandarin and pummelo, presumably accounting for its unique qualities. Seville or sour orange, commonly used in marmalade, was found to be an unrelated interspecific hybrid.
Since citrus varieties are reproduced asexually by vegetative propagation, trees producing a specific type of fruit are typically genetically identical. This growing strategy produces a uniform, high-quality fruit, but has the drawback that if one tree is susceptible to disease, they all are. By inferring the past hybridization events that gave rise to these common citrus varieties -- either in the wild populations before domestication, or in early undocumented human-directed breeding efforts -- the team hopes to enable strategies for improving citrus, including resistance to greening and other diseases. "Now that we understand the genetic structure of sweet orange, for example, we can imagine reproducing the unknown early stages of citrus domestication using modern breeding techniques that could draw from a broader pool of natural variation and resistance," Gmitter said.
The genomes presented in the published study included pummelos, oranges and mandarins. One of the sequences was the high-quality reference genome of Clementine mandarin sequenced by an international consortium including Genoscope in France, the Institute for Genomic Applications in Italy, the DOE JGI, and the HudsonAlpha Institute for Biotechnology, with contributions from researchers in Spain and Brazil. Another was the sweet orange genome, produced jointly by research
|Contact: David Gilbert|
DOE/Joint Genome Institute