The Diving Beetle's Sexy Tower of Sperm

When insect expert Dawn Higginson first squinted an eye and peered down the laboratory microscope at the diving beetle sperm before her, she was bewildered. The gyrating mass (seen here), seemingly animated with a life and logic of its own, made her question whether the slide had accidentally been contaminated.

"It took me a while to figure out what I was even seeing," says Higginson, a postdoctoral research associate at the University of Arizona's Center for Insect Science who recently analyzed sperm from 42 different species of beetle. "We had no expectation that the sperm were gonna behave like this." 

The samples from the female storage sack of a diving beetle--a family of freshwater insects found in lakes, streams, and ponds across the nation--also baffled Higginson's then doctoral adviser, Syracuse University biology professor Scott Pitnick.

After further analysis with DNA stains and electron microscopy, the scientists soon established that the never-before-seen formation, which Higginson says initially resembled "a pile of spaghetti," contained hundreds or even thousands of individual sperm orderly piled one on top of the other "like traffic cones." 

"Their heads have a hood or pocket that a tip of one head can slip into," she explains. "You end up with  these elongated stacks of sperm. We'd never seen anything like that before."

Although scientists had previously documented evidence of sperm pairs, this phenomenon represented a morphology, or conjugation, altogether more complex and concerted.

"How those sperm stack is really regular," Higginson says. "If you just look at the heads, you tend to see really equal spacing. It's actually pretty organized."

If the beetles' sperm acted alone in navigating the narrow maze of the female reproductive tract, they'd be much less likely to successfully anchor themselves in the fertilization duct and fend off competing sperm from other males. 

"When you look at the intricate morphology of the reproductive tracts, you can't help but think that sperm needs Swiss army knives and compasses to make it through there," Pitnick explained in a statement. "The females make it really complicated."

The findings, published this week in the Proceedings of the National Academy of Sciences, indicate that the female beetles' circuitous and compact reproductive route has driven the evolutionary development of these towering, militia-like configurations. The better suited the sperm armies are for the challenge, the more offspring they produce.

"It was really exciting that the dimensions of the sperm were correlated with the dimensions of the female reproductive tract," Higginson says. "We think that conjugation might be an adaptation to maintain these preferred positions within the female reproductive tracts."

As for what's causing dimension shifts in the female? It could be coevolution with sperm or it could be an external pressure, like accommodating room for additional eggs.

Whatever it is, Higginson will likely be the first to unearth another oddity in this insect's sex life.

--Ryan Jacobs/photos courtesy of iStock and Dawn Higginson