Bacteria make male lacewings disappear

It’s the perfect setup for a teen dystopian novel: Men start disappearing from the population as many women fail to give birth to living male babies. But for some insects, it’s real life, and when that plot plays out among green lacewings, bacteria are the masterminds behind it all.

A species of green lacewing, Mallada desjardinsi, can be found under street lamps near trees and bushes on the campus of Chiba University in Japan. While there are males, females far outnumber them. When Masayuki Hayashi of Chiba University and colleagues collected some of the tiny insects in 2011, they found 57 females and a mere seven males.

The researchers brought the insects into the lab; 34 of the females produced offspring. But again, the sex ratio was off. Of the 35 broods produced, 14 contained both males and females and 21 contained only females.

Many of the females that produced offspring, though, were infected with bacteria. The scientists discovered that all 21 that produced only females were infected with a species of Spiroplasma. In these broods, the females did lay eggs that included males, but these males died during the embryonic or larval phases. And treating females with an antibiotic that killed off the bacteria restored the survival of male offspring. The team reports their findings June 15 in PLOS ONE.

Invertebrates, unlike humans and other vertebrates, let some groups of bacteria proliferate within their reproductive cells, and these microbes can pass from mother to offspring (though not from father to child). For such a bacterium, males just aren’t that necessary, and so they’ve evolved to kill off the males. Male-killing Spiroplasma have been found before in moths, butterflies, ladybird beetles and Drosophila flies. But what’s weird about the green lacewings is that their Spiroplasma, a previously unknown species, is more closely related to Sprioplasma species that infect plants than those that infect other insects, the researchers discovered by creating a huge phylogenetic tree of the bacteria. For now, the scientists aren’t sure, but green lacewings may have picked up an ancestor of their Spiroplasma sometime in the past while feeding on flower nectar.

What will happen to a species in which the males are disappearing? It could lead to altered sexual behavior, since males would no longer have to try so hard to find females since they’d be so outnumbered, Hayashi’s team notes. But to find out, they suggest a deeper study of the evolutionary forces acting on this bacteria-insect pairing.

Or we could just wait for someone to write that dystopian novel.