Bacteria that live in the guts of cicadas have split into many separate, but interdependent, species in a strange evolutionary phenomenon that leaves them reliant on a bloated genome, according to a new paper by Canadian Institute for Advanced Research (CIFAR) Fellow Dr. John McCutcheon's laboratory at the University of Montana. Cicadas subsist on tree sap, which does not provide them all the nutrients they need to live. Bacteria in the cicada gut, including one called Hodgkinia, turn the sap into amino acids that sustain the cicadas during their unusual lives. Cicadas spend most of their lives underground before emerging in droves, singing loudly, mating for weeks, and then dying off en masse. Dr. McCutcheon studied the evolution of Hodgkinia in Magicicada tredicim, a type of cicada that burrows underground for 13 years. He dissected the insects and removed the bacteria, then sequenced the bacterial DNA. What he found, shortly after setting up his own lab several years ago, perplexed him so much that he thought there was a technical mistake. "I could not make heads nor tails of it," Dr. McCutcheon says. "It looked so, so broken." Hodgkinia's genome was a fragmented and overlapping mess that seemed to contain many copies of its DNA with slight variations. Dr. McCutcheon set it aside for a while, until last year, when he found that, in the gut of a South American cicada with a much shorter life span, Hodgkinia had split into two separate species about five million years ago. The split left the insect reliant on double the number of species to provide the same nutrients that required only one species to make before.
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