A male-killing virus has been discovered in insects
Scientists in Japan have identified a virus that selectively kills men – and it happens to be hereditary, creating all women generation after generation.
The discovery, made in caterpillars and described Monday in The Proceedings of the National Academies of Sciences, is “robust” evidence that “more than one virus has evolved to selectively kill male insects,” according to Greg Hurst, a symbiote specialist at the University of Liverpool in England who was not involved in the study. That could one day help control populations of pest insects and disease vectors such as mosquitoes.
“I expect many more cases like this will be discovered in the near future,” he said Daisuke Kageyamaa researcher at the National Agriculture and Food Research Organization in Japan and one of the authors of the study.
The virus was found by chance. Misato Terao, a research technician at Minami Kyushu University, was cleaning out the campus greenhouse when she found unwanted intruders – fat green caterpillars – nibbling on the impatiens. She picked them up and on a whim dropped them off at the laboratory of Yoshinori Shintani, an insect physiologist who is Minami Kyushu’s insect man.
Dr. Shintani decided that the caterpillars – tobacco worms, a voracious pest species and scourge of Asian agriculture – could be useful as food for other insects. “It was almost a miracle” that they didn’t end up in the trash, he said. By the time he remembered them several days later, he had about fifty adult moths, and unexpectedly, they were all females.
On a hunch, he bred the females from the greenhouse with male tobacco moths that he found fluttering around the lamps in his own house. The greenhouse moths only had daughters – and theirs too daughters, and their daughters’ daughters. In thirteen generations of moth offspring, only three had males.
Dr. Shintani and his colleague Dr. Kageyama soon realized they had a “male killer” on their hands.
Scientists have known for decades that microbial hitchhikers, usually bacteria, can take up residence in the gelatinous cytoplasm of insect cells. And through a process that is not yet well understood, those microbes can be passed from mother to offspring.
Sometimes these microbial symbionts tamper with the host’s reproduction. From the symbiont’s perspective, “males are useless” because they can’t help spread the microbe, said Dr. Kageyama. So the symbiote simply eliminates them. The bacterium Wolbachia can prevent male butterflies from being born. Other bacteria kill developing males before they hatch, reducing competition for the females and giving them a strengthening snack: the eggs that housed their brothers.
The team of Dr. Shintani found that antibiotics did not reverse the male-killing effect on the greenhouse moth’s offspring, so bacteria could not be responsible. Genetic analysis revealed telltale signs of a virus, but unlike anything seen before in a male killer. Only two viruses have ever been documented to kill men; the virus found by the Japanese researchers, which they named SlMKV, appears to have evolved separately.
To confirm that the male killer was actually contagious and hereditary, Dr. Shintani squeezing out some tobacco moths. He and his team mixed the bodies of pupae and adult moths with SlMKV and injected the resulting slurry into the bodies of uninfected pupae and moths. That was enough: the next generation greatly preferred women, and in subsequent generations men disappeared altogether.
Further experiments revealed how lucky the researchers were to find this male killer. Although cool weather can be fatal to tobacco worms, SlMKV is vulnerable to heat, and the researchers found that the virus’s effect was reduced and eventually neutralized at higher temperatures. The native range of the tobacco pinworm is in subtropical parts of China and Taiwan.
The scientists suspect that the mild climate in the caterpillar’s home acts like a constant fever, suppressing the lethal effect of the males. It was purely coincidental that mild temperatures in Japan dropped in the “Goldilocks zone” where SlMKV is active, allowing scientists to notice the sex imbalance in the greenhouse.
Outside experts say the team’s discovery is a sign that viral male killers are becoming more common than expected. And the find could have implications for the control of other important agricultural pests to which the tobacco worm is closely related, said Dr. Hurst.
Everything researchers can learn about male killers advances the search for the pest controller’s holy grail: a “female killer,” which could help control invasive pests or disease-carrying species like mosquitoes.
According to Anne Duplouy, an evolutionary biologist at the University of Helsinki who studies microbial symbionts in insects, time is running out for humans to learn from these temperature-sensitive microbes. As the climate changes, she said, “we’re likely to lose a lot of these interactions” before they can be documented