Aphids support symbionts with borrowed DNA

Aphids have carried the buddy system to the extreme. To give a symbiotic bacterium living inside them a helping hand, the insects have borrowed two genes from former bacterial residents.

Japanese scientists report online March 10 in BMC Biology that pea aphids carry two genes from bacteria. Those two genes are active in specialized cells called bacteriomes where a different bacterium called Buchnera aphidicola lives. The insects provide a home for the bacterium and the bacterium provides essential nutrients for the insects’ growth.

While scientists have known that bacteria can transfer DNA to their hosts, most of the transferred DNA doesn’t contain genes. The new discovery is the first evidence of the transfer of functional genes from bacteria to host.

“It’s the kind of thing people have speculated about a lot, but actually showing it is another thing,” comments Nancy Moran, an evolutionary biologist at the University of Arizona in Tucson. “That’s what they’ve done in this paper.”

Aphids and Buchnera have lived together for more than 100 million years. The organisms have become so interdependent that the bacterium’s genome is stripped down to the point that it no longer contains many of the genes the bacterium needs to sustain itself.

One of the genes Buchnera lacks is ldcA, which encodes a molecule that helps recycle a component of the bacterium’s cell wall. Atsushi Nakabachi of the RIKEN Advanced Science Institute in Wako, Japan, and Naruo Nikoh of the Open University of Japan in Chiba discovered that aphids contain an ldcA gene similar to one found in Wolbachia bacteria. The aphids make the recycling molecule in cells where Buchnera live, suggesting that the aphids are providing the bacterium with a way to build and repair cell walls.

Wolbachia don’t live in pea aphids or in most aphid species now, though the bacteria are symbionts of some other insects. The transfer most likely happened in an ancestor of several different aphid species, one that was colonized perhaps by both Wolbachia and Buchnera, Nakabachi says.

Pea aphids also contain another gene that came from bacteria, probably from bacteria called Bradyrhizobium. The researchers found that the rlpA gene is found in many species of bacteria, pea aphids and two closely related species of aphids, but not in other animals. The gene is also active in bacteriome cells, but the researchers don’t yet know how it helps Buchnera live in the cells.

Scientists have previously suspected that gene transfers happen frequently between multicellular organisms such as animals and the bacteria they live closely with, but that those transfers were dead ends, says John Archibald, a molecular evolutionist at Dalhousie University in Halifax, Canada. Unless the DNA makes its way into the germ line – the tissues that will produce eggs and sperm – the transferred DNA won’t be passed along to subsequent generations of hosts. It isn’t yet clear how Wolbachia managed to insert their genes into the aphid genome in a heritable way, Archibald says.

“These [findings] indicate that organisms including animals are much more flexible than expected, and can be more easily fused with other organisms,” Nakabachi says.