When Masayuki Hayashi and colleagues from Chiba University and National Institute of Agrobiological Sciences, Japan, bred wild-caught Japanese green lacewings in the lab, they were surprised to find that the vast majority of hatched insects were female. Instead of the usual fifty-fifty male-female split, barely 10% of the population was male.
Hayashi speculated that a male-killing bacterium might be the culprit behind this mysteriously female-biased ratio.
Some bacteria are only transmitted maternally; that is, down the female line. These bacteria can gain a reproductive advantage by inducing a female-biased sex ratio in their hosts, increasing the proportion of infected female offspring.
Hayashi and colleagues analyzed their lab-bred lacewings to figure out if they had been infected by a male-killing bacterium.
The researchers report in a PLOS ONE study that 21 of their 35 broods were entirely female. When these broods were treated with an antibiotic, brood mortality declined and the sex ratio was restored to the expected 1:1, indicating that an infectious bacterium was indeed the culprit.
Microscopy and DNA sequencing revealed that the lacewings were infected with both endosymbiotic Rickettsia and Spiroplasma bacteria, but only the Spiroplasma species was associated with the imbalanced sex ratio.
This Spiroplasma species identified in this study is most closely related to two Spiroplasma that infect plants rather than animals. The authors speculate that this bacterium may have originally infected a plant, then jumped hosts when adult lacewings fed on the plants’ nectar.
Daisuke Kageyama, a senior researcher in the Insect-Microbe Research Unit at the Institute of Agrobiological Sciences and one of the study authors, said, “A bacterium – a close relative of phytopathogenic Spiroplasma – causes male-specific lethality, leading to a strongly female-biased sex ratio in a green lacewing population.”
However, some broods infected with Spiroplasma in the study did not have distorted sex ratios. Further research is needed to uncover lacewing suppressor mechanisms that may prevent male killing in lacewings.
Research Article: 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): e0155794. doi:10.1371/journal.pone.0155794
Image Credit: By Mathias Krumbholz – Own work, CC BY-SA 3.0 via WikiMedia.