University of Florida researchers have analyzed genome sequences of 10 species of citrus fruit for the first time in history, leading to a call for genetic diversity.

Mandarins, sweet and sour oranges and some pummelo types of the fruit were investigated during the study, which lasted a decade. 

Oranges have a long history of more than five million years of evolution. The tasty fruits began from a pair of wild citrus species in Southeast Asia. 

"Citrus has incestuous genes - nothing is pure. Now that we understand the genetic structure of sweet orange, for example, we can imagine reproducing early citrus domestication using modern breeding techniques that could draw from a broader pool of natural variation and resistance," Fred Gmitter of Institute of Food and Agricultural Sciences at the University of Florida, said. 

Citrus trees in the modern age are usually produced by grafting one type of tree to another. This allows the production of more-consistent fruit, in a shorter time than waiting for nature. However, that also means that if one tree in such a field is susceptible to a disease or parasite, every tree with the same genes could be affected. 

"What we've found is that we're planting very similar trees to what were planted 4,000 years ago. These same genotypes do not provide much diversity to resist diseases, and there is very little wild citrus that can be used to easily add diversity to modern citrus," Jeremy Schmutz, one of the lead investigators, told the press. 

Researchers hope their study can assist biotechnology, scientists develop new methods of combating citrus greening, a bacterial infection that is affecting vast swaths of citrus trees in North America. 

The Asian citrus psyllid is part of this danger to the health of citrus crops. The tiny bug is devastating large numbers of trees by itself, leaving behind the bacteria that causes greening. 

Huanglongbing (HLB) is believed to be caused by the bacterium Candidatus Liberibacter asiaticus. The disease has killed millions of trees in Asia, India, the Middle East and Africa. In 2004, it was discovered in the Western hemisphere for the first time, after crossing the ocean into Brazil. One year later, the bacteria made its way into the United States.

"Current control methods include removing and destroying infected trees, controlling the psyllid, and providing additional nutrition in an attempt to keep infected trees productive," researchers stated in a press release. 

Study of the evolution of citrus fruits was published in the journal Nature Biotechnology. 

Citrus species are the most popular type of fruit in the world. More than 75,000 people in Florida are employed producing, processing and shipping the tasty treats to locations around the world.