This image depicts a pair of spermathecae, a type of sperm storage organ in the fruit fly species Drosophila melanogaster, which shows sperm from different males in red and green. (Courtesy of Dr. Manier)
By Kristen Mitchell
A new paper by Mollie Manier, assistant professor of biology in the Columbian College of Arts and Sciences, finds that the reproductive success of male fruit flies who produce long sperm leads to greater longevity for future generations.
Dr. Manier studies the evolution of extremely long sperm in Drosophila fruit flies. Males have sperm that can be up to 20 times longer than the fly itself, and females store sperm in a specialized organ within the reproductive tract—a long coiled tube called the seminal receptacle (SR). These SRs can be even longer than the sperm they house.
When female fruit flies mate with multiple males, their sperm compete for fertilizations in a process called sperm competition, with longer sperm outcompeting shorter sperm. However, long sperm only win when females also have long SRs. In this way, long SRs are more selective for longer sperm, in the same way a peahen chooses a peacock with the most impressive tail feathers, Dr. Manier said. Unlike the peahen, female fruit flies select for long sperm based on female reproductive tract morphology, not cognitive choice.
But what do females gain from mating with males with long sperm or tail feathers? Often times carrying an extravagant trait comes at a cost in development time or energy expenditure. The idea is that if the male trait is costly and incurs a handicap on a male’s ability to carry it, the trait signals strong genes to potential mates. The evolution of extravagant male traits based on female preference is called Fisherian runaway selection.
Dr. Manier’s paper, recently published in the Journal of Evolutionary Biology, finds that long sperm in males actually leads to longer lifespans in future generations. Thus, there doesn’t seem to be much of a cost to extravagant sperm in fruit flies.
“A big question in biology is, why do females prefer things a certain way? It’s really hard to get at that question because female preference can be difficult to quantify,” she said. “In this system, females seems to gain a genetic benefit from mating with long-sperm males because their sons and daughters get to live longer.”
The paper also examined an idea called intragenomic conflict, the evolutionary phenomenon that occurs when a trait is beneficial for one sex and not beneficial for the other. The study found that female fruit flies, who carry genes for long sperm but do not express them, do not face any evolutionary costs for carrying these genes.
“Both males and females benefit from genomes that produce long sperm,” she said. “On the whole, long sperm and long SR’s tend to increase longevity in both sexes. At the very least, there were no costs associated with bearing these things.”
Dr. Manier’s lab plans to complete a similar study to see if they can duplicate the results, she said. She aims to look at lifetime productive success in a more robust way.
It’s possible, Dr. Manier said, to suggest from this study that sperm quality matters.
“For human sperm, it’s not necessarily length but it could be something else,” she said. “Sperm quality can have direct implications for the quality of the male that produces it.”
