Males compete all the way to sperm shape

Rams glare, charge and lock horns. Frogs chorus in a deafening battle of the bands. Two walrus tangle in a multi-ton weight war. There are examples of males fighting over females throughout the animal kingdom. This sexual selection is often dramatic, violent and can even be a matter of life or death.

But sexual competition doesn’t end when boy gets girl. It persists down to the microscopic level, as sperm race for the egg. And a new study shows that competition influences the smallest parts of the sexual arms race, right down to changes in proteins that help a sperm get ahead. The results show associations between the proteins that gives sperm its iconic shape and sexual competition. These relationships give scientists a clue where to go next: how to take an association, and prove that sexual competition produces changes in how the very cells in our bodies function.

In many species, a female can mate with more than one male, resulting in sperm competing within the female’s reproductive system. This means that only the best and fastest sperm are going to get the egg. A fast-swimming sperm is one that is efficiently shaped, with a bullet-like head and whipping tail. An efficient shape requires the right protein combinations.

Lena Lüke and her colleagues at the Museo Nacional de Ciencias Naturales in Madrid, Spain, wanted to see if there was a connection between sexual competition at the behavioral level and the proteins that make up sperm. They examined eight different mouse species native to Europe, all with different levels of sexual competition as measured by testicle size. Testicle size in rodents is an indicator of female promiscuity: A sizable pair relative to the mammal’s body size means the female of the species is likely to have many partners, and a relatively small pair means female/male relations are likely to be closer to monogamy.

The researchers were particularly interested in the proteins protamine 1 and protamine 2. These are DNA-binding proteins. DNA binds around the protamines and becomes tightly compacted, allowing for the formation of the tiny sperm head.  If ratios of protamine 1 and 2 were involved sperm competition, the ratio might change depending on the how sexually competitive a species is.

In a paper published in the Proceedings of the Royal Society B, Lüke and her group show that there is an association between the ratio of protamines 1 and 2 and the amounts of sexual competition in mouse species. Specifically, a low level of protamine 2 relative to protamine 1 is associated with a larger testicle size, indicating a challenging sexual contest. They also showed that the ratio of protamine 1 and 2 was associated with differences in the shape of the sperm head.

The ratios of protamine 1 and 2 were not based on differences in the protamine DNA. Instead, the differences appear to be in the promoter regions for these genes, an area of DNA close to the gene that helps control whether it ultimately used to make protein.

W. Steven Ward, a reproductive biologist at the University of Hawaii at Manoa says that the study indicates a new role for protamines. “For years, for all of the time that [protamines] have been studied,” he says, scientists focused on other roles of protamines like DNA compaction, ignoring the final sperm shape. “To my knowledge, no one has focused on their potential role in making the sperm more streamlined.”

While the results suggest a new role for protamines, they only show an association between the protamine ratio and testicle size. A study like this one raises 10 questions for every one that it answers. Protamine ratios are associated with sperm shape. What exactly does this mean? They are associated with sperm competition. How does sexual competition affect protamine ratios? What is the ideal protamine ratio to produce the perfect sperm head shape? How does that head shape vary as a result of sexual competition?

Eduardo Roldan, a reproductive biologist at the Museo Nacional de Ciencias Naturales and an author on the study, has already given a lot of thought to what questions should be addressed next. “One question we need to ask fairly soon,” he says, “is how general this phenomenon is. We are looking at another group of rodents to see if they show this association as well.” He also explains that studies need to be done to determine how protamine ratios affect sperm shape. “We need to verify that with a certain ratio we do get DNA that is more compacted.”

Polly Campbell, an evolutionary biologist at Oklahoma State University in Stillwater and an author on the paper, thinks that it is important to determine what protamine ratios mean for sperm function. “How well do these sperm swim with different shaped heads?” she says. “Do wider or narrow heads swim faster?”

She also explains that it would be interesting to see if altering a mouse’s sexual competition might eventually alter its protamine ratios, proving the importance of protamine ratios in sperm competition. Roldan is also interested in this question, which would involve manipulating how mice mate in an experiment with selective pressure. “We could examine having the animals mate for many generations under conditions of monogamy or promiscuity,” he says, “and [see] if we get lines that would end up with one type of sperm head or another and one type of protamine ratio or another.” An experiment like this could bridge the gap between behavior (promiscuity or monogamy) and the proteins that compact mouse sperm.

With more experiments and questions to ask, scientists can go from what starts out as only an association between a protein and a behavior to direct understanding what causes sperm to end up with a good head.

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