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Immune System Protects Female Bedbugs From Traumatic Sex

TÜBINGEN, GERMANY—Pity the poor bedbug. It spends most of its life hiding between mattresses and behind headboards, and it feeds only once a week, when it takes a frenzied gulp of human blood while trying to avoid being crushed by its slumbering host. Female bedbugs have it even worse. Once they've finished their meal, males attack with bacteria-covered penises, jabbing them straight into a female's abdomen rather than into her reproductive tract. A new study, presented here yesterday at the 13th Congress of the European Society for Evolutionary Biology reveals that females prepare in advance for this onslaught to avoid getting sick.

Researchers already knew that female bedbugs had evolved one feature to cope with the male mating assault: a sack of immune cells that lies underneath a groove in their tough outer skeleton, where the male can more easily get through the abdomen to deposit his sperm. But scientists didn't know what difference the sack of cells made. "Females need to protect [themselves] but not kill the sperm," says Michael Siva-Jothy, an entomologist at the University of Sheffield in the United Kingdom.

Siva-Jothy and colleagues knew that sperm spend their first 2 hours in this sack before working their way out into the body, where they swim through the blood and then drill into the reproductive tract to fertilize the eggs. That 2 hours was time well spent, as the team's experiments showed that if the sack was bypassed, the females were more likely to die, possibly from bacterial infections.

But bedbug mating only occurs once a week, so Siva-Jothy wondered whether the females only primed their immune response right before coitus, much the way dogs salivate in anticipation of food. To test this notion, he and his colleagues jabbed a group of female bedbugs with a bacteria-laden needle once a week to simulate mating. He assumed that the females would quickly learn to anticipate the jabbing, ramping up their immune response a day or so before the jab. But when Siva-Jothy measured the activity of proteins in the sacks called lysozymes, which destroy the cell walls of bacteria, he found that his jabbed females and another group of control female bedbugs ramped up lysozyme production at the same time every week.

It turns out that feeding sets up the rhythm of immune activity, Siva-Jothy reported at the meeting. He and his colleagues determined this by altering the feeding schedule of bedbugs. Some were fed blood on every seventh day; others were fed irregularly, say, on day five or day nine or day seven, averaging the same weekly food intake as the bedbugs on the regular schedule. Thus a female in this group could not predict when she would get food. The immune sacks of the bedbugs on the regular schedule began revving up lysozyme production on a weekly basis, a day or so before meals. But the sacks on the other bedbugs responded only after they were jabbed. The pattern was protective, as the bedbugs fed irregularly were more likely to die when jabbed with bacteria-laden needles than were bedbugs fed on a regular schedule, Siva-Jothy told conference attendees.

"This discovery is indeed very novel and unexpected," says Hinrich Schulenburg, an evolutionary biologist at Christian-Albrechts-University in Germany, who was not involved with the study. The work "shows that even very simple organisms can evolve an optimal answer [to infection threats]—they increase their immune response not too late and not too early," adds Manfred Milinski, an evolutionary biologist at the Max Planck Institute for Evolutionary Biology in Plön, Germany.

Siva-Jothy and colleagues are now trying to figure out how the rhythm of the immune response is set up and whether other aspects of bedbug immunity are similarly regulated. "We want to understand how the females manage their [overall] immune systems," he says. Such understanding might lead to better scheduling of bedbug exterminations, as they could be timed to periods when the insects are most vulnerable. "What our work shows is that there may be value in adopting control strategies at particular times, particularly if a parasite is used."