Researchers analyzing genome sequences from loud, sapsucking bugs found that a well known three-way symbiosis is now actually a busy foursome. One of the two helpful bacteria living inside the insect split into two separate, but interdependent species. The findings are published in Cell this month.
Cicadas eat a simple diet of only plant sap. For protein, these insects rely on a symbiotic relationship with two bacteria species that live within the cells of their body. The two bacteria -- Hodgkinia and Sulcia -- produce essential amino acids, and in exchange, they get to live comfortably inside the cicada, snug as bug in a rug. All three organisms divvy up the nutritional roles, so each depend on the others to survive. They’ve been doing this for 10 million years.
While examining the genomes of bacterial symbionts living inside the South American cicada Tettigades undata, a team led by John McCutcheon from University of Montana, Missoula, was surprised to find not two, but three species: Sulcia together with two different kinds of Hodgkinia acting as one. The three-member community has become a four-way assemblage.
Hodgkinia subtly became more complex because of a speciation event about five million years ago, when the original lineage split into two. “When we looked at the genes, they were clearly closely related to each other,” McCutcheon explains in a university statement. “If there was a broken gene in one version of Hodgkinia, it would be complete and functional on the other and visa-versa. So, the functional genes in each, when working together, seem to operate as one.” They’re only complete when they work as a team.
At one time, the cicada and Hodgkinia might have been able to survive without each other, but as they evolved together, the bacterium shed genes that were no longer needed, leaving it with a very small genome of less than 200 genes. The two Hodgkinia genomes were similar at first, Science explains, each having all the same genes and functioning independently. But over time, different genes in the two species disintegrated, the two started occupying different types of cells, and now they rely on each other as well as on Sulcia and the cicada for survival. “This is an obligate symbiosis,” McCutcheon adds, “all of the organisms in there need each other.” Many genes have already been lost, and the losses continue.
He thinks this development is a result of “slop and chance,” like other acts of accidental evolution that made organisms more sophisticated over time. And while the extra symbiont seems to make little difference to the cicada's life, it leaves the insect reliant on more species to create the same nutrients that used to require fewer species to make. The genetic split, the team argues, was non-adaptive: It happened because of chance and had no clear benefit to the organism. As McCutcheon adds in a CIFAR news release: “For the insect, it is probably easier to deal with two symbionts than three.”
Images: Juan Emilio Cucumides Carreño
