Aldous Huxley is most famous for “Brave New World” (1932), but among scientists working on sperm competition and reproductive biology his “Fifth Philosopher’s Song” (1920) is also well-known:
|A million million spermatozoa
All of them alive
Out of their cataclysm but one
Dare hope to survive
And among that billion minus one
In biology texts and lectures, these first two verses are all that are usually quoted, the point being that males must manufacture millions of sperm that compete intensively to fertilize the female’s eggs. They also emphasize that all sperm are different, carrying a random half of the father’s genes. But there is a third, lesser known verse, that after new findings reported this week is sure to get quoted:
|Shame to have ousted your betters thus
Taking ark while the others remained outside
Better for all of us, froward homunculus
If you’d quietly died
Do some sperm die for the sake of another? Do they sacrifice themselves and their opportunity to fertilize an egg in order to increase the chances of one of their sperm brothers? Huxley’s verse may have been truer than he knew. According to a paper in Nature this week, the answer is yes.
The paper’s lead author, Harry Moore, of the section of reproductive and developmental medicine at the University of Sheffield, England, made the discovery by an unusual route.
“I noticed that wood mice have very large testes,” Moore said in an e-mail interview. “Actually, our family cat deposited a dead male on the kitchen floor for me and, being a reproductive biologist, I had a look at its rear!”
Moore and colleagues from the Czech Republic and Australia, found that wood mice have one of the largest testes/body-weight ratios for rodents — the testes account for about 4.5 percent of body weight. Not surprisingly, animals with large testes produce large amounts of sperm and are usually involved in sperm competition — which occurs when a female mates with different males within a short space of time during her reproductive cycle.
“One might expect each sperm to compete for the egg whether from the same male or from different males,” said Moore. “But it was postulated about 25 years ago that sperm from the same male might cooperate to be more successful at fertilization.”
That’s because although all sperm are different, half of the genes they carry are the same — they are sperm brothers. “It may pay for sperm of a particular male to band together to make sure that at least 50 percent of their genes are inherited,” Moore said.
That was the theory, but sperm cooperation had never been observed in mammals, only in some insects and molluscs. The large testes of wood mice, however, prompted Moore and his team to take a closer look at their sperm.
“We were amazed at what we found,” he said. “When we looked at the sperm in culture medium used for in vitro fertilization, it was noticed they formed what we have called ‘sperm trains.’ “
Hundreds, sometimes thousands of sperm link up after ejaculation, forming trains that move 50 percent faster than individual sperm. Moore and his co-workers found that wood mouse sperm have a unique feature that facilitates cooperation. Each sperm bears a long hook which is folded away under the head until after ejaculation, when it is unlatched. The researchers saw that sperm catch hold of the hooks or the tails of other sperm, quickly forming congregations. After five minutes, more than 85 percent of the sperm had formed trains.
A sperm train is an impressive example of cooperation, to be sure, but what about sperm self-sacrifice?
“These trains must break up before fertilization, so many of the component sperm commit genetic hara-kiri by undergoing a premature ‘acrosome reaction,’ ” said Roger Short, a biologist in the department of obstetrics and gynecology at the University of Melbourne, in a commentary in Nature.
The acrosome is a capsule on the head of sperm containing enzymes that are usually released — in what is known as the acrosome reaction — when the sperm reaches the egg. The enzymes breach the egg’s defenses, allowing the sperm nucleus to enter and fertilization to take place. With no acrosome, the sperm has no chance of fertilizing an egg.
“Somewhere on the train — perhaps it’s the locomotive driver up front — there must be one acrosome-intact sperm that has retained its capacity to perform fertilization,” said Short.
“We know that in many mammals a proportion of sperm undergo a premature acrosome reaction in the female tract or the test tube,” said Moore. Most scientists assumed that such sperm were “misfiring” — but there was another suggestion.
“Previously some investigators suggested such [premature acrosome reacting] sperm were killing the sperm of rivals and were designated ‘kamikaze’ sperm,” said Moore.
For all that sperm researchers have a penchant for using Japanese words to describe their ideas, the kamikaze concept — postulated to occur in humans and rats — was not supported by Moore’s own experimental evidence. But his work on the wood mouse has led Moore to speculate that a different form of sperm cooperation might also occur in other mammals, perhaps in humans.
“Perhaps we should evaluate whether sperm undergoing such a reaction facilitate other sperm from the ejaculate in some way to fertilize [the eggs]. Human sperm have been shown to move through the mucus secretions of the cervix in a phalanx formation, with the sperm at the front displaying a different motility to those behind,” Moore said.
“Perhaps this motility pattern represents cooperation in some form or other.”