“Killer Sperm” Found, Prevents Worms From Cross-Breeding

Talk about tough love: Mating with a male of another species can be lethal, at least if you’re a nematode worm.

Scientists have found that female worms that mate with other nematode species often become infected by “killer sperm,” leading to sterility and an early death. The newfound phenomenon could shed light on the evolutionary puzzle of how certain species remain distinct from one another, according to the study, published July 29 in the journal PLOS Biology.

A mating pair of Caenorhabditis nigoni nematodes. The female (about a millimeter in length) is on the bottom. Photograph courtesy Janice Ting

In the lab, scientists mated different species of worms in the Caenorhabditis genus, which thrive on rotting fruit and vegetables. Using a fluorescent dye to observe what happens when these transparent worms hook up, the study team discovered that the foreign sperm broke through the female’s uterus and prematurely fertilized its eggs before destroying the ovaries. (See “Sperm Tracked in 3-D—A First.)

In some cases, the sperm penetrated farther into the worm’s body, resulting in fatal injury. Females that survived the cross-species mating were often left sterile and unable to breed with their own species.

“We found that the incidence of sterility depends strongly on which species pair is crossed,” co-author Asher Cutter, a professor of ecology and evolutionary biology at the University of Toronto, said in an email.

In the most extreme example—C. briggsae matched with C. nigoni males—95 percent of females were sterilized, the study found.

Sperm Wars

Why unfamiliar worm sperm wreaks such havoc isn’t yet clear, Cutter added.

A closeup shows the body parts of a nematode worm.
A closeup shows the body parts of a nematode worm. Photograph by Gavin Woodruff

One possibility, he said, is that the seminal fluid, or sperm themselves, contain a compound that relaxes muscle cells that normally keep sperm corralled within the female’s reproductive tract. This could result in an uncontrolled “sperm invasion.”

“There’s a lot of evidence that cross-species mating can be harmful, but usually it’s the seminal fluid proteins or things that are transferred along with the ejaculate that cause” problems to the female—not the sperm, noted Maria Servedio, a biology professor at the University of North Carolina at Chapel Hill, who wasn’t involved in the study.

Whatever the mechanism behind killer sperm, the team believes the underlying reason for these ill-fated pairings is the strong evolutionary pressure on animals to adapt to the reproductive ruses of the opposite sex. Such adaptations are usually very specific to each species.

In the case of nematode worms, which have a reputation for promiscuity, sperm from different males compete inside a female to fertilize her eggs. In turn, the female reproductive organs adapt to withstand these sperm wars. (Related: “Sperm Recognize ‘Brothers,’ Team Up for Speed.”)

A worm that’s equipped to deal with the sperm of its own kind may find itself overrun, quite literally, if it mates with another species.

Indeed, the study found evidence that the worms may actively avoid mating with species that have larger, more aggressive sperm, despite their free-loving tendencies.

In behavioral experiments looking at mate choice, some females “would more strongly avoid [mating with] foreign species by crawling away,” Cutter noted.

Most of nature’s mechanisms to prevent successful crossbreeding occur either before mating or after fertilization is completed, Servedio said. For example, a mule, a cross between a horse and a donkey, can’t reproduce.

But this killer sperm is novel in that it acts as a species barrier after mating but prior to fertilization. While such a mechanism had previously been predicted, “there’s not been much evidence for it,” she said.

“This sperm mechanism I’ve never heard of before… it’s crazy—pretty cool.”

More Killer Sperm?

Since killer sperm either prevents or drastically reduces the chances of worms cross-breeding, the discovery may help scientists to understand how closely related species stay separate from one another.

Punishing cross-species mating by sterility or death would represent a powerful evolutionary way to maintain a barrier and prevent hybridizing, the study team argues. (Related: “Pictures: ‘Pizzly’ to Be Joined by More Arctic Hybrids?“)

As to how widespread the newfound phenomenon might be, Cutter thinks it “very plausible” that there are killer sperm in other animals, including certain insects.

“As scientists begin to study these kinds of phenomena in more organisms, I suspect that we will find harmful effects of sperm to be more common than is currently appreciated,” he said.


Meet the Author
James Owen is a journalist and author based in Stockholm, Sweden. After cutting his teeth on the news and features desks of several UK newspapers, he struck out as a freelance writer, specializing in life sciences and natural history. His fish biography 'Trout' (Reaktion Books) was published in 2012.