Immune attack makes female flies dump their sperm store


A fruit fly being infected. Watch out little spermies!

Now that we know the Supreme Court’s decision on the healthcare law, we can get back to thinking about the big questions in life. For example: how does having an infection affect the health and vitality of your sperm?

At least in fruit flies, if either the male or female is sick, sperm can suffer, according to research from Preethi Radhakrishnan and Kenneth Fedorka at the University of Central Florida. And if a female gets sperm from an infected male, she’s likely to just dump the sperm altogether.

Making sperm is a sensitive process. Lots of things, including being sick with the flu, can actually lower your sperm count. An infection of the male reproductive tract can even lead to infertility, possibly in part because it impairs the sperm’s ability to swim.

Researchers Radhakrishnan and Fedorka set out to learn what would happen to the sperm of fruit flies if the fly had to mount an immune response to a bacterial infection.

First, they tested the males. They infected male flies with bacteria and then looked the proportion of that male’s sperm that were dead 24 hours later, compared to males they hadn’t been infected. Almost 25% of the sperm in infected males was dead 24 hours after infection, compared to only 5% in the uninfected males. So, the question was: is this because the bacteria actually kill the sperm, or is it a side-effect of mounting an immune response?

To test this, the authors “infected” the males with a compound that mimics bacterial infection (peptidoglycan, or PGN). This would cause the males to ramp up their immune response, even though there aren’t actually any bacteria present. This led to almost the exact same result as with the real bacteria (nearly 20% dead sperm in the “infected males” compared to basically 0% in the controls). This makes the side-effect hypothesis much more likely–something about simply mounting an immune response is dangerous to fly sperm.

That’s all well and good, but what about the ladies?

Fruit fly females store sperm after mating in specialized organs. They can hold onto sperm for almost two weeks, and they use that stockpile to happily fertilize eggs at their own pace. But that means they have to keep those sperm healthy for the whole time they’re in storage. What happens to those sperm if the female gets an infection?

These results were a little less clear. When females were infected with the same bacteria used to infect the males, there was no effect on stored sperm after 24 hours. But, when they were “infected” with PGN, about 20% of the sperm were dead a day later, compared with about 12% dead in females that didn’t get PGN. Not as dramatic an effect as in the males, but still significant. As far as I can tell, the authors make no attempt to explain this–and I have no idea what’s happening there, either.

So, the jury’s still out on whether sick females have trouble maintaining their stored sperm–but wait! What if she gets an STD? How will that affect her ability to keep sperm in storage?

To mimic STDs in flies, the authors dipped the males’ naughty bits in PGN and then let them get in on with some unsuspecting females. They then looked 24 hours later to see how many sperm had died in storage. Perhaps disappointingly for the researchers, there didn’t seem to be any effect on the number of dead vs. live sperm.

But something didn’t look quite right in the STD flies. Females that mated to normal males (that didn’t get their junk dipped in PGN) had lots of sperm still in their storage organs at 1 or 2 days after mating. But about 15% of females that got the fake STD had no sperm in storage at 1 day. 2 days after mating, more than 50% were missing sperm.

To make sure that this wasn’t just because they didn’t receive any sperm to start out with, the authors repeated the experiment and looked for sperm in storage at 75 minutes after mating. STD or not, all the females had sperm in storage. So, it looks like females that get tainted goods are quick to get rid of them.

Why or how this happens is still anyone’s guess. Maybe dumping the sperm–and all the other stuff associated with them–will minimize her chances of getting sick. And if she does get sick, dumping the sperm will help her focus on getting better instead of pumping out eggs. In fact, recent research by Sarah Short, Mariana Wolfner, and Brian Lazzaro at Cornell University has found that sperm and other components of the semen reduce a female fly’s ability to fight off an infection, should she get one. This effect goes away, though, if she can’t make eggs. This supports the idea that there is a tradeoff between fighting infection and making babies (so hard to choose!).

You may now be wondering: how is a lady to know if she’s about to get some bacteria-laced sperm? Is there any way to know in advance? (Hint: get your man tested). But what about, say, if you’re a duck?

Luckily for ducks, females can choose males with the most powerful antibacterial semen simply by examining their bills. Mallard duck females prefer males with more colorful bills–which just so happen to be the males that have semen with “superior bacteria-killing ability”. Sadly, a similar tactic is not available for us.

References:

  • Radhakrishnan P, & Fedorka KM (2012). Immune activation decreases sperm viability in both sexes and influences female sperm storage. Proceedings. Biological sciences / The Royal Society PMID: 22696524
  • Short SM, Wolfner MF, & Lazzaro BP (2012). Female Drosophila melanogaster suffer reduced defense against infection due to seminal fluid components. Journal of insect physiology PMID: 22698822
  • Rowe M, Czirják GÁ, McGraw KJ, & Giraudeau M (2011). Sexual ornamentation reflects antibacterial activity of ejaculates in mallards. Biology letters, 7 (5), 740-2 PMID: 21490006

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