The old saying may tell us that it’s a man’s world, but this might not be the case for the green lacewing. A new study has revealed that some green lacewings (Mallada desjardinsi) produce solely female offspring, and the cause of this population bias is a bacterium that kills males.
In order to understand the importance of this research it is first important to understand a little more about the function of the green lacewing in agriculture. The larvae of this species is a prolific consumer of aphids, which makes them extremely useful to farmers who are always happy to be rid of such plant-eating pests.
However, the lacewing caught the attention of Chiba University’s Masayuki Hayashi for more than just their agricultural application. The green lacewing larvae has a clever method for avoiding attack by ants – they carry dead aphids on their backs. Ants don’t eat aphids, instead forming mutualistic relationships with them whereby they protect the aphids from predators in exchange for a sweet honeydew that the aphids excrete. So, by carrying dead aphids on their backs, the lacewing larvae disguise themselves from the ants, who seemingly mistake them for aphids.
Whilst investigating this cunning disguise, Hayashi noticed that there were many more female lacewings than males, and was determined to find out more. He contacted fellow researcher Daisuke Kageyama and a collaboration began.
The first stage of the study was to capture wild lacewings and encourage breeding. From the broods which were consequently produced in the lab, 60% contained solely female offspring. Upon further investigation, all of the mothers to each of these broods were found to be infected with a bacterium called Spiroplasma.
In order to determine if Spiroplasma was truly the cause of such a dramatic population bias the all-female broods were given antibiotic treatment. This quickly restored the sex ratio to the usual 1:1. Hayashi and team have concluded that Spiroplasma is a male-killing bacterium.
Of the remaining broods without such a strong sex-ratio bias, some were also infected with Spiroplasma, but in the presence of another bacterium – Rickettsia. This suggests that there may be other factors that are able to suppress the male-killing affects of Spiroplasma.
Further work would be needed to examine what natural immunity exists to Spiroplasma, and to discover more about the mechanism behind the total elimination of males. One thing is for certain, thanks to Spiroplasma, the female lacewing certainly rules these roosts.
Hayashi M, Watanabe M, Yukuhiro F, Nomura M, Kageyama D (2016) A Nightmare for Males? A Maternally Transmitted Male-Killing Bacterium and Strong Female Bias in a Green Lacewing Population. PLoS ONE 11(6): doi:10.1371/journal.pone.0155794
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