As far as humans are concerned, sexually-transmitted infections are things to avoid. But imagine if these infections didn't cause death and disease, but gave you superpowers instead. It may sound like a bizarre fantasy, but it's just part of life for aphids.

Aphids mostly reproduce without sex, giving rise to many all-female generations that are exact copies (clones) of their parents. They only have sex once in autumn, the only time when mothers give birth to males. Asexual reproduction makes sense for aphid mothers since they pass on all of their genes to their daughters. If they reproduced sexually, their offspring would only inherit half of their genes, diminishing their legacy. Why then would a female aphid choose to have sex at all?

Nancy Moran and Helen Dunbar at the University of Tuscon a surprising answer. They may be trying to receive sexually-transmitted infections from other aphids. Aphids carry various strains of bacteria inside their bodies. These 'symbionts', far from causing disease, actually provide the aphids with useful abilities. Some strains allow them to feed off a greater variety of plants, while others give them the ability to withstand higher temperatures. Some can even save their lives.

Sex, flies and symbionts

Aphids are commonly targeted by parasitic wasps. These grisly creatures lay their eggs inside the aphids' bodies and the developing grubs eat their hosts from the inside out. But aphids that carry the symbiont Hamiltona defensa avoid this cruel fate, because their bacterial partners destroy the developing wasp grubs. Clearly, these are friends worth having.

Mothers pass on the helpful symbionts to their children but they can also be transferred between unrelated individuals through sex. In fact, the only way for a female to get some symbionts in the first place, or to add to an existing collection, is to have sex with an infected male.

After this article was first written, Moran and Dunbar found that the number of aphids infected with Hamiltona defensashoots up dramatically when they are attacked by parasitic wasps. This suggests that the presence of the wasps creates an evolutionary pressure that drives the spread of the symbiont in the aphid population. However, when wasps were absent, the presence of the bacteria also declined. To Moran and Dunbar, this implies that the symbionts may be protective but they also exact some hidden penalties on the aphids that limits their use when the threat from wasps is low.

The duo have also since discovered that another symbiontBuchnera aphidicola bestows aphids with the ability to withstand higher temperatures. A single mutation in the bacterium's genetic code - the equivalent of erasing one letter in a book - has massive consequences for the aphids by shortening a gene called ibpA.

Aphids carrying mutant symbionts with the shortened gene become almost infertile after a brief exposure to high temperatures. However, at cooler climes, these individuals do very well and give birth to more young at an earlier age than their peers who carry the normal bacteria. Both versions of the gene can be beneficial to the bacteria and the aphids, depending on the prevailing environmental conditions.

If other insects trade in sexually-transmitted bacteria, we could potentially use these exchanges to our advantage. For example, the African tsetse fly, carrier of sleeping sickness, also harbours a symbiont that resembles the species found in aphids. Moran and Dunbar suggest that we could infect flies with a genetically-engineered symbiont that disables or kills the parasite that causes sleeping sickness. Infected males would pass this killer symbiont through the population and reduce the spread of sleeping sickness.

Reference:Moran, N.A. (2006). Sexual acquisition of beneficial symbionts in aphids. Proceedings of the National Academy of Sciences, 103(34), 12803-12806. DOI: 10.1073/pnas.0605772103

Dunbar, H.E., Wilson, A.C., Ferguson, N.R., Moran, N.A. (2007). Aphid Thermal Tolerance Is Governed by a Point Mutation in Bacterial Symbionts . PLoS Biology, 5(5), e96. DOI: 10.1371/journal.pbio.0050096

Oliver, K.M., Campos, J., Moran, N.A., Hunter, M.S. (2007). Population dynamics of defensive symbionts in aphids. Proceedings of the Royal Society B: Biological Sciences, 275(1632), 293-299. DOI: 10.1098/rspb.2007.1192