Male fruit flies produce (relatively) gigantic sperm: The animal itself is only a few millimeters long, but his sperm can reach an impressive length of nearly 6 centimeters (2.4 inches). According to new findings published in Nature this week, the males evolved enormous sperm to outcompete the smaller sperm of their lesser rivals. And the female sperm-storing organ is driving this process.
In most animals, sperm is cheaper and smaller than eggs. That difference is what drives intense male-male competition and elaborate male ornamentation – ranging from peacock tail feathers to stag antlers. Male fruit flies – who produce small amounts of unusually large sperm – are one of the most extreme examples of sexual ornamentation. The record holder for this exaggerated sexual trait is Drosophila bifurca: Its sperm is 20 times longer than the male itself and about a thousand times longer than human sperm. It has to be transmitted as tightly coiled balls. But sexual selection typically favors males who produce copious amounts of tiny sperm that require little investment, especially if the females are promiscuous.
To investigate, Syracuse University’s Scott Pitnick and colleagues conducted genetic analyses of interacting sex-specific traits in Drosophila fruit flies, and they also compared the effects that male and female reproductive potential have on health and nutrition across multiple species.
The team found that sperm production is related to the male’s condition: The largest, healthiest males produce the most and biggest sperm. Small males aren’t able to invest as much in sperm production. And since female fruit flies are more likely to successfully reproduce with the highest-quality males, the giant sperm benefits her sons.
Furthermore, sperm length co-evolved with the length of the female's sperm-storing tubule (called the seminal receptacle). This organ of "female sperm choice" is helical and can be nearly 8 centimeters (3 inches) long in Drosophila bifurca. "Crazy sperm forms are evolving because female reproductive tracts are evolving that bias fertilization in favor of these weird, specific traits," Pitnick said in a statement.
Increasing the size of her seminal receptacle and upping mating frequency increases the odds of successful mating – and this drove males to evolve gigantic sperm that can outcompete rivals with smaller sperm. "Longer sperm are really good at displacing their competitors from the female reproductive tract, which gives them an advantage in the competition for fertilization," first author Stefan Lüpold from the University of Zurich said in another statement. And the longer the sperm, the fewer of them can be produced and then transferred.
Detail of female Drosophila bifurca sperm-storage tubule. Scott Pitnick
Image in the text: Drosophila fruit flies mating. Stefan Luepold/University of Zurich