6 JULY 2006
Sperm: Surely shrimpy?
As it swims toward an egg, a mammal's sperm reminds us of a flying saucer approaching
a planet. It's important. It could change history. But you might not guess that
from the shrimpy size of the approaching vehicle.
Now we hear from Syracuse University about super-size sperm. If you delicately unwound the whip-like flagellum of a sperm from the fruit fly Drosophila bifurca, it would stretch out 6 centimeters -- 20 times as long as the fly-guy that made it. "To put that into perspective," says Adam Bjork, a graduate fellow at Syracuse, "if humans made sperm that long and you took a six-foot man and stood him on the goal line of a football field, his sperm would stretch out to the 40-yard line."
Bjork has long been fascinated with the male gamete: the carrier of the male half of the chromosomes.
We will avoid the temptation to crack sophomoric sperm-quips and get to the point:
theories of sexual selection say giant sperm should not exist. The idea has
been that the male who gets the largest number of active sperm close to the
egg is the evolutionary winner, because that boosts its chances of fertilizing
the egg and passing along its genes to another generation. So a guy who makes
huge numbers of sperm has a higher chance of fertilizing an egg and passing
on his genes. 
During sex, human males send hundreds of millions of sperm toward
one egg And since small sperm are much easier to make, for sperm, small is
beautiful. That's especially true in the many species where females copulate
with multiple males. Chimpanzees, for example, have huge testicles because
females typically mate with several guys, and getting a lot of sperm close
to the egg raises your chance of success.
The testes of Drosophila bifurca fruit flies make up 11 percent of the dry body mass. A male is encircled by uncoiled testes. Courtesy Scott Pitnick
Sperm: Super size me!
But in a series of experiments, Bjork and his graduate advisor, Scott Pitnick, showed that sperm competition can favor evolution toward a few large sperm, rather than many small ones. They looked at four species of fruit flies that make different size sperm, from about 2 millimeters long to those 6-centimeter giants, and found that sexual selection could favor a shift toward super-size sperm.
Drosophila bifurca females store sperm in a long, skinny container, Bjork told us. "The sperm enters the female and straightens out, and the longer sperm have a fertilization advantage over the shorter ones, so the next generation of sperm is slightly longer."
It's Evolution 101: genes that favor successful reproduction become more common in the next generation. It's not clear why the females prefer longer sperm, but the trait may identify males with better genetics, Bjork says.
One sperm cell from Drosophila bifurca. That ball of "hair" is
the flagellum. Courtesy Romano Dallai
Gargantuan gametes
Pitnick first located the 6-centimeter D. bifurcata gametes about 10 years ago, but they seemed more like an evolutionary impossibility than an evolutionary oddity. The beauty of the new study is its demonstration that when sperm get big, females start to favor the evolution of even larger sperm.
Having started to unravel the mystery of the giant sperm, Pitnick's lab has also started to find more examples. A survey of insects around Syracuse, for example, has located other giant gametes of the male persuasion, Bjork says. "There are lots of insects that make sperm that are over one centimeter long. We also see huge variation in morphology (shape). There are sperm without tails or with several tails. Some species make sperm in several sizes."
In biology, a focus on plants, bacteria and bugs instead of just mammals can expand your picture of life's creativity.
But how did sex evolve in the first place? Bacteria reproduce by dividing in half; they have no time for sex. In the ocean, back in the woebegone epoch before the invention of sex, gametes (the reproductive cells) were not sorted by size, Bjork says. There were no guys with tiny gametes or gals with giant ones.
A Drosophila bifurca male produces six sperm during the
time a female needs to produce one egg. Other species of Drosophila make smaller
sperm much faster. Courtesy Romano Dallai
Biological theory suggests that the largest and smallest gametes would have distinct advantages over mid-size models, Bjork explains. Big gametes "would do better because they offer better nutrition for the zygote after fertilization. Smaller ones would have a slight advantage because there are more of them, so they have greater chance of bumping into other gametes." These divergent forces favored the evolution of a few big gametes or many smaller ones. "The ones in the middle size range did not have any advantage, so they disappeared," Bjork says.
Big gametes mark females, Bjork says, and small one mark males. So once the gametes had evolved into distinct sizes, "you had the beginning of the evolution of two sexes."
— David Tenenbaum
Bibliography
• Intensity of Sexual Selection Along
the Anisogamy-Isogamy Continuum, Adam Bjork & Scott Pitnick, Nature,
June 8, 2006.
• Evolution: The
Paradox of Sperm Leviathans, Tommaso Pizzari, Current Biology, 2006.
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