Inside every Drosophila melanogaster (fruit fly) female is a battleground where sperm from two different males jockey with each other for the honor of fertilizing her eggs. By sequentially mating females with males genetically-engineered to produce sperm that glow either green or red (see video below), scientists have been able to directly observe sperm competing in what looks like the fruit fly version of Tron. Color coding the sperm in this way allows researchers to distinguish one male’s sperm from another’s and determine who the winners are.
“Sperm from two different males genetically-engineered to express either green or red fluorescent proteins compete within the female reproductive tract of the fruit fly Drosophila melanogaster.”
Surprisingly, it’s not always the fastest sperm that wins.
Fertilization in Drosophila happens quite differently than it does in humans. When a female is inseminated, the sperm isn’t immediately used to fertilize her eggs. Instead, the sperm that swim up her reproductive tract are shunted into the female’s very own internal sperm bank–special storage organs called the seminal receptacles (SR) and spermathecae (ST). There, the sperm are stored until the female is ready to lay her eggs, which are produced in the ovaries. Once ready to be fertilized, the eggs move out of the ovaries and travel down the oviduct in conveyor belt-like fashion. When an egg reaches the entrance of the sperm bank, sperm is released from the receptacles and spermathacae to fertilize the eggs.
Females, however, are thrifty and won’t blow through their sperm savings before receiving another deposit. As little as three days after mating with the first male–and with sperm still in her bank– Drosophila females can remate to replenish their sperm reserves. Why females remate might have to do with ensuring that she doesn’t put all of her eggs in one basket, so to speak. Remating could be a hedge against the possibility that her first partner was infertile or a genetic deadbeat. Mating a second time increases the odds of mating with a male with desirable traits to pass on to her offspring. Thus, Drosophila females can increase their odds of reproducing successfully by remating.
Drosophila “sloppy seconds”, however, also puts sperm from the first male in the awkward position of meeting sperm from the second male, thus setting the stage for sperm competition. When a Drosophila female remates, sperm from both males will engage in a WWE Royal Rumble-style battle, in which contestants jostle and toss each other out of the female’s sperm bank. As they jockey for position in the seminal receptacles and spermathacae, the sperm left outside will be “eliminated” by being ejected from the reproductive tract. Any victorious sperm, whether it be from the first or second male, that remain in the female’s storage organs make up what is called the “fertilization set” that will be subsequently used to fertilize her remaining eggs.
Of course, competition typically brings up questions about evolution. Could natural selection be picking and choosing traits that make sperm successful–giving one male’s sperm an advantage over another male’s? Identifying these traits was what Dr. Stefan Lüpold and colleagues had in mind when they staged hundreds upon hundreds of fluorescent sperm battles. Using males that were genetically different from each other and with varied sperm traits, Dr. Lüpold and colleagues were able identify attributes that make a winner. Their work showed that the sperm that were the best at pushing other sperm out while staying in the female’s sperm storage organs were the ones that were the longest and, unexpectedly, the slowest.
Long sperm have previously been shown to have a competitive advantage in displacing other sperm while resisting being displaced. Being longer makes sense in that it’s probably easier to push smaller sperm aside. Drosophila sperm, measuring in at about 2mm in length, are unusually long (roughly 30 times the length of human sperm), mostly due to the length of their tails. In fact, the total length of the sperm is longer than the distance it has to travel in the female reproductive tract in order to reach the seminal receptacles and spermathacae. While the reason for why Drosophila sperm are so long isn’t clear, some of its length might be explained by the selective pressure that sperm competition exerts.
In terms of speed, however, you might be thinking that being fast would be a good trait for Drosophila sperm to have. That’s because we typically picture sperm as competing in a race to fertilize an egg. It’s not necessarily a race for Drosophila sperm because, for one, the sperm don’t have to travel very far in the female reproductive tract and, two, competition occurs when one male’s sperm is already in the female’s storage organs. Inside both the seminal receptacles and spermathacae, sperm are constantly swimming forward and backward. The faster the sperm move the more likely it is that they’ll be near the entrance of the storage organs where they can be pushed out. Rather than being an advantage, speed is a handicap since it can increase the odds of a sperm being displaced. In the Drosophila sperm competition game, the winners avoid being shown the door by staying away from it.
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