The mystery of the male-killing bacteria has finally been solved

4 May 2018

Image: Tomatito/Shutterstock

Scientists have been mystified as to why male fruit flies of the same species couldn’t make males, until now.

If it were about humans it would be considered the plot of some dystopian fiction. But, for the fruit fly species Drosophila, it is actually the case that two strains of it simply cannot produce male offspring.

The discovery was made in the 1950s with scientists perplexed as to what was preventing the natural 50:50 male to female ratio from being achieved, eventually settling on it being a genetic mutation.

Future Human

Now, however, a team of researchers from the École Polytechnique Fédérale de Lausanne has discovered that it is a hidden bacterium called Spiroplasma poulsonii.

Hidden bacterium

In a paper published to Nature, the research team found that the bacterium lives in the fruit fly’s blood and is passed on to its offspring through the female’s oocytes.

Remaining hidden from its host, it induces the fascinating, but damaging, reproductive manipulation that kills off all male embryos.

Given the nature of bacterium such as this, its efforts to live and thrive is down to the fact that it promotes the long-term persistence of the symbiotic bacteria by increasing the frequency of infected females who then transmit the bacteria to their offspring.

Despite its strangeness, male-killing bacterium is not restricted to Spiroplasma as it is also observed in several endosymbiotic bacteria. But, until now, the molecular mechanism underlying the process had never been clarified.

Big impact for evolution

Originally, the theory was that Spiroplasma produces a toxin known as androcidin which has been shown to kill males, but a battery of tests showed no links between it and killing males.

After much effort, the Swiss-based team found the culprit is a protein dubbed Spaid, after Spiroplasma poulsonii androcidin.

The scientists found that just expressing Spaid in fruit flies was enough to recreate all the phenotypes associated with male killing in the insect, as it binds to the X chromosome of male embryos.

The team also discovered a strain of the Spiroplasma bacterium with a mutation in the Spaid gene, which showed a reduced ability to kill males.

“To our knowledge, Spaid is the first bacterial effector protein identified to date that affects host cellular machinery in a sex-specific manner,” said Dr Toshiyuki Harumoto.

“And it is also, to our knowledge again, the first paper to identify an insect endosymbiont factor causing male killing. As such, we expect that it will have a big impact on the fields of symbiosis, sex determination and evolution.”

Colm Gorey was a senior journalist with Silicon Republic