When aphids have sex, the male often infect the females with beneficial bacteria that gives them useful powers, like the ability to fight off parasites.
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.
Sex is power
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.
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.
Sex, flies and symbionts
If other insects do this, we could use sexually-transmitted bacteria 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.
More on aphids:
Aphids defend themselves with chemical bombs
More about animal sex and reproduction:
Virgin birth by Komodo dragons
Butterflies evolve resistance to male-killing bacteria in record time
When the heat is on, male dragons become females
Chimerism, or How a marmoset’s sperm is really his brother’s
Reference: Moran & Dunbar. 2006. PNAS Epub