Take a look at the battle unfolding in this remarkable image. It shows the sperm from two related fruit fly species competing inside a reproductive tract. It’s an evolutionary struggle that, as scientists are now learning, often leads to the introduction of entirely new species.
A good portion of this competition comes before mating, which is referred to as precopulatory selection. Males of many species duke it out between themselves to earn the privilege of mating. Others evolve elaborate physical displays, like the peacock, or perform persuasive mating calls.
But there’s another kind of competition — postcopulatory selection — and it’s the kind that happens after mating. When a female has multiple partners, her reproductive tract becomes a battlefield. Over time this can result in sperm heteromorphism — the simultaneous production of multiple sperm types by a single male. These sperm can differ in shape, size, and even chromosome complement. Some aren’t even capable of fertilization; they’re there to disrupt the sperm of competing males. It’s like a game of flags, but fought on a microscopic playing field.
Females also have some say in the matter, a phenomenon scientists refer to as cryptic female choice. Some females can store and separate sperm from multiple males, allowing them to manipulate paternity by choosing which lucky sperm gets to fertilize their egg. Consequently, females of some species, most notably insects, birds, reptiles, gastropods, and arachnids, can mate multiple times and allocate sperm to their eggs according to autonomous physiological triggers, like paternal phenotype or other characteristics. Basically, cryptic female choice increases her odds of reproducing with the “fittest” mate.
It should come as no surprise, therefore, that precopulatory selection — and all its resulting reproductive strategies for both males and females — can lead to some fairly serious evolutionary changes. And indeed, it can be so substantial that it can give rise to actual speciation — that point in evolution when one species branches off into two.
But for biologists, observing all this action in the reproductive tract has been next to impossible.
“It’s difficult to observe the competition between ejaculates and female discrimination among sperm, given that it takes place inside the female and may involve complex biochemical, physiological and morphological interactions,” said Scott Pitnick in a statement. “Although we have powerful tools for assigning paternity and for quantifying the outcome of sperm competition and cryptic female choice, it might as well be 1871, in terms of understanding the traits and processes of postcopulatory sexual selection."
Pitnick, along with a team of biologists in Syracuse University’s College of Arts and Sciences, recently figured out how to catch a glimpse of postcopulatory selection in action by genetically modifying the sperm of two related fruit fly species so that they glow in the dark. The two species diverged more than 260,000 years ago (which is not long in evolutionary terms) — and this divergence may have been instigated by sperm wars.
Looking at the image above, you can see two different sperm. The turquoise sperm belong to Drosophila simulans and the orangey-red belong to Drosophila mauritiana.
Now, even though the female fruit fly has mated with these two, her body will now invoke cryptic female choice as a way to sway the odds in favor of her ideal reproductive candidate. The researchers can watch, for example, as the female discards sperm by forcefully ejecting it from her reproductive tract. As a result, the scientists can assess the predictability of the mating outcomes, which is based on the new insights gleaned about evolved ejaculate-female incompatibilities.
“The take-away from our study is that postcopulatory sexual selection can quickly generate critical incompatibilities between ejaculates and female reproductive tracts that limit gene flow between isolated populations or species,” noted Pitnick. “Because female promiscuity and, by extension, postcopulatory sexual selection is so ubiquitous, it is likely to be a widespread engine of speciation.”
Read the entire study at Current Biology: “Postcopulatory Sexual Selection Generates Speciation Phenotypes in Drosophila.”