Among sperm dashing to the fertilization line, cooperation rather than competition appears to be the key to success, at least among wood mice, British reproductive researchers report.
In an amazing act of altruism, individual reproductive cells sacrifice their own survival to improve their brethren's odds of winning the race and claiming the grand prize -- fertilization of an egg. Even though it means defeat for all but one, the sperm hook up by the hundreds or thousands to form a long train with much greater speed than -- and a clear advantage over -- a single contender.
In the all-for-one push, the sperm start teaming up a minute or two after ejaculation, using a hook on their microscopic heads to link together. Within half an hour, the train begins to uncouple, disengaging so the act of procreation can proceed, with many of the sperm triggering a reaction in their heads -- usually used to bore into the egg -- that blows their own chances at fertilization.
"These trains must break up before fertilization, so many of the component sperm commit genetic hara-kiri by undergoing a premature 'acrosome reaction.' This involves the release of enzymes that break down cell adhesion molecules, which also makes it impossible for the sperm concerned to fertilize the egg," said Roger Short of the Department of Obstetrics and Gynecology at the University of Melbourne in Victoria, Australia.
"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," Short said.
There can only be one winner in this high-stakes game. But by helping their brothers, even at their own expense, the sperm boost their genes' chances to beat out those of other competitors.
"The study suggests that some sperm are altruistic in that they give up their chance to fertilize the egg but help overall to make sure a sperm from the same male will fertilize," said Harry Moore, professor of reproductive biology at the University of Sheffield in England and lead author of the study published in the July 11 issue of the British journal Nature. "Only a single sperm can fertilize the egg."
This cooperative approach would seem to come in handy for species in which females mate with multiple partners, such as the subject of the research, the common European wood mouse -- Apodemus sylvaticus -- researchers said.
"The (authors) find that hundreds or thousands of sperm link hooked structures on their heads and swim en-masse in a train, which enables them to progress at almost twice the speed of a single sperm," said Short, who analyzed the findings in an accompanying commentary.
Reproductive biologists have studied sperm function in only a handful of species, primarily laboratory and farm animals, so they do not know how widespread the newly described process might be. Although the wood mouse sperm trains that chug along toward their egg encounters appear unusual, some form of cooperation among the male reproductive cells may be more common than scientists realize, Moore said.
"The observation is important for our understanding of evolution. Fertilization by sperm may not be such a random process -- and also for our understanding of sperm function -- some processes we assign to fertilization may also have to do with sperm cooperation and other strategies not directly involved in fertilization," he told United Press International.
Just as males vie for the attention of the objects of their desire, so do their sperm compete for fertilization of eggs. The prevalence of this so-called sperm competition in nature gave rise to expectations of equally rigorous rivalry over the penetration of the egg, whether the semen originated from the same male or from different males. But as they gleaned more molecular secrets of the reproductive process, scientists began to speculate about a more cooperative spirit among sperm of common origin.
The scenarios were inspired by new findings involving a process called meiosis, in which the number of chromosomes containing an organism's unique genetic blueprint is halved during formation of the sex cells. Each sperm shares 50 percent of its genes with all the other sperm. Therefore, scientists have reasoned, if competition by sperm of different males is high, it may pay for the sperm of a particular male to band together to make sure at least half of their genes are inherited.
Direct evidence of this view has been hard to come by, however. Examples of sperm cooperation are scarce, with very few observed in mammals.
"Exactly how rare sperm cooperation is in animals is unclear at present," Moore said.
In marsupial mammals from North and South America, such as the possum, sperm conjugate to form a pair. Just before fertilization, the sperm separate and allow one of the pair to penetrate the egg, said Moore whose previous work showed the benefits of such coupling.
"The (wood mouse) sperm trains seem exceptional, although sperm of other rodents can stack up on each other and swim together, for example, guinea-pig and squirrel sperm," he said. "There again, sperm of many rodent species are hooked-shaped but do not form sperm trains. Perhaps the very long hook on the sperm of wood mice allowed sperm cooperation to develop in this case."
Exemplifying a characteristic of males involved in sperm competition, wood mice are endowed with unusually large testes, which account for a hefty 4.5 percent of their total body weight.
"It might be no accident that the wood mouse is a supreme sexual performer among rodents, with males scrambling to mate polygamously with any and every promiscuous female, and they have relatively large testes to prove the point," Short said.
It was the super-sized semen source that steered the scientists to the wood mouse as a study subject, Moore said. "The relatively large testes of wood mice made us look at their sperm. We were amazed at what we found," he recalled.
"Obviously we had to do many experiments to confirm these findings," Moore added. "Sperm trains can be recovered from the female tract soon after mating, and most sperm in the uterus were in trains rather than single. We therefore believe sperm trains help sperm transport along the female tract."
Such findings raise other questions, however, he continued. "If a lot of (sperm tails) beating together are better than single sperm, why haven't sperm evolved multiple tails to improve motility? In fact, a number of invertebrate species do have sperm with several tails, but this normally doesn't occur in mammals. The reason for this is unclear," Moore said. "Occasionally abnormal sperm with two tails are produced -- most human ejaculates contain the odd sperm with two tails -- but motility is usually compromised."
The study also raises the possibility of more subtle forms of sperm cooperation.
"Human sperm have been shown to move through the mucus secretions of the (female) 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."