University of Arizona evolutionary biologist Nancy Moran, who has extensively studied the co-evolution of insects and their resident bacteria, recruited Eisen to the current project. "My initial interest in sharpshooter symbiosis was in the hope that we could find out exactly how xylem can be used as food," Moran explains. "It's terribly poor in nutrients."
In the study, the team first carried out a painstaking forensic type of DNA analysis known as "metagenomics," in which they sequenced the B. cicadellinicola's genome from material gathered via dissections of hundreds of insects. The scientists were dumbstruck to find no evidence of the biochemical pathways needed to synthesize amino acids. Could the plant be somehow providing amino acids to its insect predator? Unlikely. Was the sharpshooter somehow cranking out its own amino acids? Doubtful. Could there be something else, some other bacteria, adding these essential ingredients?
Yes. Realizing the amino acid pathways might be carried out by other bacteria living inside the insect, the team began picking through their forensic samples of DNA sequences, removing all the sequence reads that matched neither the insect nor its known symbiotic B. cicadellinicola bacteria. A large amount of the leftover DNA mapped to another bacterium, S. muelleri. Sure enough, when they pooled the bits of sample DNA that came from S. muelleri, the team found all the essential amino acid synthesis pathways.
"When doing this type of forensic metagenomics, some scientists suggest you can just analyze the whole system as one unit--a so-called 'black-box' approach--without knowing which piece of DNA came from which organism," Eisen says. "But this black-box ecology just does not work well. To really understand the system, you've got to assign the different bits of DNA to organisms. This study shows why."<
Source:The Institute for Genomic Research