Dawn Higginson thought it was strange when she learned that some diving beetles produce sperm that fuse together at the head like Siamese twins. But when the postdoctoral researcher from the University of Arizona began asking why such conjugate gametes form, things only got even stranger. The sperm of the diving beetle, which gets its name from its ability to swim underwater, occur in many shapes and forms. Whereas a few species make standard tadpole-shaped swimmers, others generate sperm that stack together like traffic cones to form long, many-tailed filaments. Some species even generate two different types of sperm that work together to navigate through the female's fertilization duct.
View the sperm diversity slide show.
"We're trying to understand why on Earth would you do this?" Higginson says. "It requires a lot of energy and specialized machinery to make these conjugates."
In a study published February 7 in Proceedings of the National Academy of Sciences, Higginson and her colleagues hypothesized that female choice might actually be playing a role in the evolution of the strange sperm morphologies. The reproductive tracts of female diving beetles come in several different tortuous sizes and shapes, Higginson says. Much as the preferences of peahens caused peacocks to develop elaborate tails, Higginson thinks that the complex twists and turns of female water beetles' reproductive tracts act like an obstacle course that only the best-adapted sperm can navigate.
So she and her team analyzed whether there were correlations between female reproductive form and sperm morphology across 42 species of diving beetles.
They found that, in species where the female reproductive tract is short and wide, sperm are more likely to work together in conjugate forms. In species where the female reproductive tract is long and thin, the sperm of that species tend to travel alone. Higginson hypothesizes that this difference may occur because as solo sperm swim through slender reproductive tracts, their flagella may bump into the walls, allowing them to anchor or push off the walls and reach the unfertilized egg faster than a clump of sperm might. In wider reproductive tracts, long sperm conjugates have an easier time propelling themselves by pushing off of the more distant wall.
Previous studies have found evidence that the shape of the female reproductive tract may drive simple traits in sperm, such as tail length or head shape, but Higginson's research is the first to look at how females might influence the cooperation between sperm, says Tommaso Pizzari, a zoologist who studies sexual selection at the University of Oxford in England.
"The fact that the sperm have to join forces to get to the egg creates an evolutionary conundrum," he says, because the evolutionary interests of the male and the interests of his sperm are not always in alignment. A male is equally related to all of his sperm, so his evolutionary fitness is enhanced if any of them is able to fertilize the egg. His sperm however, share only about 50 percent of their genes with each other, so each sperm competes fiercely to be the one that fertilizes an egg. "It may be that as the risk that another male will inseminate the female increases, the interests of the male and the sperm become more aligned with each other," because the sperm of one male are more related to each other than to those of another male, Pizzari says. He hopes that future studies will investigate whether there is a correlation between mating system type and sperm conjugation across diving-beetle species.
Sperm competition and conflicts between male and female interests—as well as random chance—have led to a host of outlandish sperm adaptations in other animals as well. View the slide show to find out more.