Lukas Schärer
Some hermaphroditic Macrostomum lignano flatworms copulate, then suck out unwanted sperm. These worms' sperm have bristles to help them stay in place. The sperm of related worms, which don't suck out sperm, have no bristles. In fact, the bristles seem to have disappeared twice, independently, when worms adopted the alternative mating strategy. This led researchers to conclude that sex shapes sperm, which are the most diverse type of animal cell.
updated 1/20/2011 6:21:37 PM ET 2011-01-20T23:21:37

Watching hours upon hours of worms having sex (yes — essentially worm porn), has helped scientists figure out why some flatworms have simple, thin, squiggly sperm, while others sport larger sex cells with bristles and a feeler in the front.

The new findings shed light on the evolution of all animal sperm, the researchers said.

In the study, a team that included Lukas Schärer and his wife, Dita Vizoso, both of the University of Basel in Switzerland, linked the evolutionary loss of the bristles — which they think keep the sperm lodged in the female reproductive tract — as well as the feelers, to the adoption of a new mating strategy that renders the bristles useless.

Schärer described the methodology that led to their discovery: "You take time-lapse movies and sit there and look at worms mating for some hours," Schärer said.

Some were more obliging than others, mating all the time, while other healthy specimens didn't get down to business when placed together, he said.

The team also examined the worms' sperm, the stylet (the organ used to deliver sperm), the antrum (the female sperm-receiving organ) and the evolutionary relationships among the worms. (These worms are equipped simultaneously with both male and female genitalia.)

Sperm 101
The implications of this research go beyond just the worms, and help explain why animal sperm come in so many shapes, according to Scott Pitnick, an evolutionary biologist at Syracuse University, who was not involved in the study.

"When most people think of sperm, they think of a tadpole-looking thing," Pitnick said. "There is no such thing as a typical sperm cell when you look at sperm diversity throughout the animal kingdom. It is just unbelievable, the outrageous, dramatic forms that have evolved."

Sperm, which carry a genetic payload that unites with an egg's during fertilization, are the most diverse type of animal cell. Some are hairy, some have tails they use to propel themselves, some look like amoebas and crawl with pseudopodia, and some, the so-called giant sperm, can be many times longer than the body of the male who produces them. This is the case for a type of fruit fly Pitnick has studied. It has sperm 20 times longer than its body.

"All of this variation has been described, but we don't know what any of it means," he said. "We don't understand the adaptive significance of any of the sperm forms."

Two types of flatworm sex
The flatworm species studied were members of the Macrostomum genus, which live in marine, brackish and fresh water. They also have transparent bodies, making them ideal for research involving internal fertilization.

In one strategy, called reciprocal mating, two worms line up and simultaneously inject sperm from their male genitalia, called stylets, into the other's female antrum, or sperm-receiving organ.

But as hermaphrodites, a worm must deal with conflicting interests between its male and female parts. Evolutionary biologists believe that sex tends to follow the "eager male, choosy female" paradigm, that is, females must fend off eager suitors. This can happen both before and after copulation.

In one mating strategy, some of the hermaphroditic worms deal with this conflict by copulating prolifically to meet their male needs, then afterward, bending around and sucking out unwanted sperm, which satisfies female pickiness, according to the researchers. These worms had sperm that resembled squiggly lines with bristles coming out of them, which the researchers believe interfere with the worm's ability to suck the unwanted sperm out of its female part.

In the second, more brusque mating behavior, dubbed "hypodermic insemination," one worm uses its needlelike stylet to inject sperm through the skin of its partner. Although, in a few cases, the researchers saw sperm injected into worms' heads, they observed that the sperm were generally in the area of the eggs. This technique appears to allow the male interest to overcome the female, since the worms do not suck out unwanted sperm.

"We have no idea how the sperm are moving through the tissue," Schärer said.

These sperm are smaller and do not carry bristles or feelers like those of their reciprocal relatives.

Sexual history
The researchers used genetic information to map out the evolutionary relationships among 16 species, and they found evidence that hypodermic insemination twice evolved separately. The three species that clearly used this strategy all had very similar sperm, stylets and antrums.

Reciprocal mating, meanwhile, appeared to be the ancestral system, and the seven worms that used this system had a greater variation in these structures, they found.

Not all of the species could be clearly classified. Researchers were not able to observe the mating behavior of all of them, and one species, in particular, appeared to be in a transition state between the two strategies. This species' bristles were short, and its stylet pointed and rigid, but without the sharp hook associated with this strategy.

This shift in mating behavior associated with injecting sperm alters the playing field on which sperm compete, and as a result, may lead to a drastic change in sperm design, they write.

The research was published in the Jan. 10 issue of the journal Proceedings of the National Academy of Sciences. As part of their work, the researchers created an online repository for information about the flatworms.

You can follow LiveScience writer Wynne Parry on Twitter @Wynne_Parry.

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