Mathematical Models Uncover the "Churning" Secrets of Super-Sized Sperm

The supersize sperm of fruit flies swim about their storage cavity with elegant churning choreography, and scientists at the Simons Foundation's Flatiron Institute have figured out why.

Within the abdomen of the humble fruit fly (Drosophila melanogaster) are some of the longest sperm the animal kingdom has ever known. Uncoiled, the tails of these swimmers stretch around 2,000 microns long. That's nearly the length of the male fly itself, and 10 times longer than the storage organ that houses sperm by the thousands.

Yet surprisingly, the sperm-filled organ isn't a chaotic mess. The sperm form orderly flows, undulating like waves. "Each one of them is dragging this massive cargo, and they're not getting entangled," says Jasmin Imran Alsous of the Flatiron Institute's Center for Computational Biology (CCB). "That's what really blew my mind." The question became: How could they move around in such tight quarters without getting tangled up in knots?

To find out, Imran Alsous and her collaborators developed mathematical models that could be used to simulate the highly complex biological system. They found that, rather than propelling themselves by creating waves through fluid like human sperm, fruit fly sperm actually push off their brethren to propel themselves forward. When thousands of sperm interact in this manner, the result is a collective churn.

The researchers report this new understanding of how individual fly sperm movements give rise to orderly collective dynamics in a new paper published June 22 in Nature Physics.

The larger ethos behind their work - building collaborations to develop quantitative approaches to reproductive biology - could help scientists answer other fundamental questions in biology, such as how nutrients flow through networks of veins, how organelles self-assemble within cells and how embryos develop.

This is how you're going to reach a truly quantitative understanding of biology."

Mike Shelley, Director, CCB