Elhanan Borenstein of the Santa Fe Institute and Stanford University, together with Anat Kreimer, Uri Gophna, and Eytan Ruppin of Tel Aviv University, constructed the metabolic networks of many species of bacteria and measured how much those networks broke into pieces, or modules. Then they looked for factors that correlate.

They found that large networks involving many different enzymes tended to be more modular. Also, bacteria that tend to live in many different environments tend to be more modular. This makes sense, Borenstein says. "If your environment is always the same, then you just do whatever you need to do in the most optimal way. If the environment is constantly changing, you might develop one set of processes to cope with one set of environments and another to cope with another."

Many bacteria transfer genes among themselves, instead of simply handing them from ancestor to descendant. Bacteria that do more of this also tend to be more modular. "If you are going to give away and get parts of different networks, it makes sense for your network to be modular," Borenstein says. That way, the transfers are more likely to be useful. In this case, the greater rate of transfer could easily be both cause and effect of modularity, Borenstein points out.

Finally, the researchers found that modularity tends to decrease over generations. "This is probably because bacteria get more specialized as they evolve," Borenstein says.

Their work appeared May 13 in the Proceedings of the National Academy of Sciences.

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