Scientists in Australia have been hard at work watching fruit flies have sex. Why? To figure out how fly sex affects your food. It is a fact of life that the facts of life can cause problems, especially when fruit flies lay eggs (after doing their business) in the fruits and veggies we grow for food.
Now, just to clarify, these “fruit flies” are not the adorable, lovable Drosophila melanogaster that haunt your wine glass and your genetics exams (yes, I may be biased in favor of Drosophila). What I’m talking about are Queensland fruit flies (or “Q-flies”) that destroy crops in Australia. Researchers Samuel Collins, Diana Pérez-Staples, and Phillip Taylor at Macquarie University in Sydney recently set out to see just how effective a new pest control technique might be in the wild. What they found was that how long flies spend getting it on can affect how well the technique works.
Insect pest species can be devastating for farmers, so there is a constant drive to improve pest control. Since most people don’t want to eat fruits and vegetables doused in pesticides—as evidenced by the popularity of organic versions of these foods—more “natural” methods are in demand. One such method for controlling crop pests (and insects that transmit diseases, like mosquitoes), is called the “sterile insect technique”, or SIT.
The technique is simple enough: take a bunch of male Q-flies and hit them with gamma irradiation. The flies will be fine, but their sperm won’t be. The idea is that these males would then be released into the wild to mate with females. Those females would lay eggs, but most of the eggs won’t hatch, because of the defective sperm. This reduces the population of Q-flies in the next generation, in theory.
This would work great if female Q-flies only mated once in their lives, but that isn’t the case. Even though male Q-flies (like males of many other insect species) transmit molecules in their ejaculate that lower the female’s libido, she still usually finds enough motivation to hook up at least once more. And unlike us, Q-fly females can store sperm from multiple males and use it many days, or even weeks, later, even after mating with a second male.
The researchers were curious how effective SIT is when females mate twice. They might first mate with a normal male and then a male treated with radiation (“sterile male”), or vice-versa. They could also mate twice to the same kind of male. The questions, for each case, were: how likely is a female to remate after mating to each kind of male? How much sperm does the female receive and store? How many of her eggs actually hatch? And how does her fertility correlate with how long she spends having sex with each male?
The first experiment was to find out whether females are more or less likely to mate to another male after first mating to a sterile male (as compared to a normal male). It turns out that whether the guy is shooting blanks or not, a female Q-fly has the same (low) likelihood of feeling in the mood when another male comes along. So, whatever the molecules are that reduce her libido, they aren’t affected when the male is irradiated.
Next, the researchers wanted to know how many sperm females store in their storage organs (called spermathecae) after a single mating. After mating to a normal male, the females in this experiment stored a median of 346 sperm. For those that mated to a sterile male, this number was only 92. They compared these numbers to how long the flies spent mating, but there was no correlation.
Okay, so those were the preliminaries. What about the real question: what happens when the female Q-fly mates twice? Remember, female flies can store sperm. After the second mating, the sperm from both males can fight it out in a process called sperm competition to determine which male’s sperm gets to fertilize the most eggs. In many species, this works like a raffle: the more tickets you buy, the better your chances. In sperm competition, this translates to a better chance at fertilizing the most eggs if the female stored more of your sperm than the other guy. So far, things don’t seem to be in favor of the sterile male, since he seems to have far fewer raffle tickets.
To figure out how the different mating sequences affected the number of fertile eggs (i.e.: eggs that hatched), the researchers counted the number of eggs laid by each female for 40 days after the second mating.
The number of fertile eggs from females that mated twice to normal males was the highest, as expected. Females mated to two sterile males laid nearly no fertile eggs. Females that mated to one type and then the other were somewhere in between.
Here’s where things get tricky. When females mated to either a sterile or normal male twice, how long she spent having sex with either male had no effect on how many offspring she ended up with (same with two sterile males). But for the other treatments, there was a correlation. For females that mated first to a sterile male, the longer she stayed in copula with the second male (the normal one), the more fertile eggs she produced. The opposite was true for females that mated first to a normal male—the longer she stayed doing it with the second, sterile male, the fewer fertile eggs she produced.
The researchers also timed how long the first mating lasted. This had no effect on fertile egg production if the first male was sterile. But if a female was enamored with a normal male for a longer amount of time, and if he was her first, she would end up with more babies.
Why does sexual stamina influence the number of offspring in Q-flies? The authors argue that, since the duration of sex didn’t affect the number of sperm stored in their preliminary experiments, the cause must be something else besides sperm. This could be true: males transfer lots of other stuff (proteins, lipids, carbohydrates, the whole food pyramid, really) in their semen, besides just sperm. Some of those other things could important for determining how many sperm fertilize eggs. Basically, something in the male’s semen might make his raffle tickets worth more than the other guy’s, and the effectiveness is increased when the flies have sex for longer.
However, I’m a little annoyed that they didn’t count how many sperm were stored after the second mating. It has been documented in other insects that males can adjust the composition of their ejaculates in response to social cues. In this case, the cue might be that the female has previously mated. It is possible that the second male noticed his mate had already been with another male and therefore increased his odds by buying more raffle tickets (in other words, transferring more sperm).
This of course doesn’t explain the fact that females who mated longer to a normal male first had more offspring, since that male couldn’t have known that he would have competition, but it might explain the results of the second mating. I don’t deny, though, that other semen components could also play a big role (as you may have noticed from my other posts, I’m a big believer in that), but comparing the sperm stored in a single-pair mating experiment to the results of an experiment where the female mates twice seems to be a less-than-optimal comparison.
But, whatever the reason, Q-fly mating time does impact how many more Q-flies we’ll have to deal with in the future. In order to put that knowledge to practical use in the field, we will need more information about how this happens. The bottom line: a fly’s sex life has important consequences for your life, too.
Collins SR, Pérez-Staples D, & Taylor PW (2012). A role for copula duration in fertility of Queensland fruit fly females mated by irradiated and unirradiated males. Journal of insect physiology PMID: 22906778