假如有一個獎項可以頒發(fā)給自然界最大的精子的話,，那它的獲得者將是一種名為D.  bifurca的小果蠅，它的盤繞的精子拉直開來有2英寸（5.08厘米）那么長,，是平均人類精子長度的1000倍,。

美國紐約州Syracuse大學(xué)的一位博士生亞當(dāng)·比約克說："打個比方，假如人的精子也有那么長,，然后你讓一個6英尺高的人站在一個足球場的球門線上,，那么他的精子就能伸展到球場的40碼線處。"比約克和雪城大學(xué)的生物學(xué)家司各特·皮特尼克一起,，察看了為什么有些動物,，如一只細(xì)小的果蠅，能夠產(chǎn)生如此長的精子,。它們倆人的發(fā)現(xiàn)表明其中的答案是在雌性果蠅的身體上,。他們的研究細(xì)節(jié)被發(fā)表在了Nature雜志上。

在許多物種里面,，雄性產(chǎn)生精子的數(shù)量要遠(yuǎn)遠(yuǎn)多于雌性產(chǎn)生的卵子數(shù)量,。雄性產(chǎn)生的精子個體細(xì)小卻數(shù)量很多，以此來增加它們中的一個與卵子結(jié)合的幾率,。

按照性別選擇理論預(yù)測,，精子的個頭變得更大，那么它與卵子結(jié)合的比率也變的更大,，雄性的競爭也會減少,。

但是當(dāng)研究人員在檢測D.  bifurca這種果蠅雄性之間的競爭時卻發(fā)現(xiàn)，它們的競爭并沒有如所期望的那樣減少,。而要找到其中的答案就必須對雌性果蠅進行解剖。

雌性隱秘選擇

雖然雄性成功的在雌性體內(nèi)播下了種子,，但在受精發(fā)生之前還有很長一段路要走,。因為許多雌性的身體并不是精子的被動競技場，而更像是一堂障礙課,，通過設(shè)置許多障礙和防衛(wèi)來除掉那些弱的精子,。

例如，雌性的生殖管道可以釋放許多對精子有害的化學(xué)物質(zhì),，或者也可能變長,，使得受精比賽變成了一場對精子持久力的測試。

這種后性別障礙被生物學(xué)家稱之為"雌性隱秘選擇",，因為它們不管是對雌性自身或者是對研究人員都變現(xiàn)的不明顯,。

D.  bifurca雌性果蠅生殖道正好比游過它的精子要長一點。比約克說,，這看起來就像一個巨大盤繞物填充在它的腹部,。

研究人員認(rèn)為正是雌性果蠅特別長的生殖道才驅(qū)使雄性果蠅巨大精子的進化。

比約克說："這項研究結(jié)果表明精子之間的競爭和隱秘雌性選者促使了雄性和雌性兩者在生理和行為上的進化,。"

The testes of Drosophila bifurca fruit flies make up 11 percent of the dry body mass of the male. In this image, a male is "surrounded" by an uncoiled testicle dissected from a male of the same size. Credit: Romano Dallai

One sperm cell from Drosphila bifurca. Image by Romano Dallai

Syracuse University Ph.D. student Adam Bjork is a man on a mission: to unlock the mysteries of cryptic female choice. He’s not studying psychology or trying to get a date—he’s a student of biology in SU’s College of Arts and Sciences, and he has discovered a major paradox in the area of evolutionary biology.

A recent study by co-authors Bjork and SU biology associate professor Scott Pitnick, titled “Intensity of sexual selection along the anisogamy-isogamy continuum,” will be published in the June 8 issue of Nature.

In the article, Bjork and Pitnick discuss a paradox they have discovered within the foundations of sexual selection theory as it relates to the sperm and egg production of Drosophila (fruit flies). Previous work in the Pitnick lab has shown that post-copulatory sexual selection on males (the sperm competition that takes place after females mate with more than one male) can lead to decreased sperm quantities by favoring the production of larger sperm. In other words, their production moves toward a “quality, not quantity” mode, because female fruit flies have evolved so that longer sperm have a greater chance of successfully fertilizing eggs. Successful fertilization is important, since individuals that fail to pass their genes on to the next generation are at an evolutionary dead end.

This leads to the “big sperm paradox” because the idea that postcopulatory sexual selection could favor the evolution of giant sperm clashes with traditional sexual selection theory, which predicts that the most successful sperm competitors will be the males that produce many, tiny sperm. As males evolve to produce larger—and therefore fewer—sperm, eggs become less rare, and sexual selection should weaken, according to theory. The term “isogamy” refers to the state at which males and females have equal investment per gamete (sex cell) when producing sperm and eggs. In a truly isogamous population, each sperm and each egg would have a chance to participate in a successful fertilization. In such a population, sexual selection would be extremely weak, as there would be little or no competition among males to fertilize eggs.

To investigate this apparent paradox between empirical data and traditional theory, Bjork and Pitnick set out to measure the strength of sexual selection in four Drosophila species of varying sperm length, ranging from the anisogamous D. melanogaster (in which a male produces 30 sperm in the time it takes a female to make one egg) to the nearly isogamous D. bifurca (where just six sperm are produced per egg). They found that, contrary to theoretical predictions, the level of competition among males did not decrease; the strength of sexual selection remained high as sperm size increased. Their results show that, once females evolve a preference for longer sperm, intense sperm competition can actually reverse the trajectory of sperm evolution so that the most successful males are those with the most female-like strategy of producing very few, large gametes.

Bjork and Pitnick’s discovery brings into focus a question often pondered by evolutionary biologists. The animal kingdom is full of sometimes odd, sometimes obvious “advertising” features in male physiology, such as the ostentatious feathers of the peacock, presumably to attract the attention of females. But when all other criteria are equal, and when one female mates with a number of males, what causes females to “choose” whose sperm succeeds in fertilizing her eggs? It’s what Bjork and Pitnick call “cryptic female choice.”
It’s a question that fascinates Bjork. He has been interested in the topic since he was a sophomore at Gustavus Adolphus College in Minnesota. Bjork took a seminar in animal behavior his junior year and read the article “Sperm Wars” in Discover magazine. The article led him to Pitnick and his work in the areas of sexual selection, sexual conflict and speciation. In 1995, Pitnick discovered the longest sperm in nature; that of Drosophila bifurca. It is nearly six centimeters long. Pitnick also found that females in the species had evolved to favor this long sperm because they developed long reproductive tracts in which longer sperm have the greatest chance at competitive fertilization success.

“The sperm of Drosophila bifurca is 20 times longer than the male that produces it,” says Bjork. “To put that into perspective, 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.”

Eager to study with such an authority on the subject, Bjork came to Syracuse University to pursue his Ph.D. and work in Pitnick’s lab. His level of enthusiasm for his research is matched only by his devotion to it. For example, Bjork had to journey into the highlands of central Mexico to collect specimens to breed. After baiting them with fermenting bananas and cactus fruit, Bjork carefully deposited them into plastic vials and kept them cool for the drive back to Syracuse, where he was conducting his research. For the entire trip, Bjork kept his charge safely in air conditioned environments. “I couldn’t even go into a restaurant without bringing them with me,” says Bjork. “They weren’t easy to collect, and I wasn’t going to let them die.”

While it is fascinating, the evolution of giant sperm is puzzling. Says Bjork: “Until recently, it was widely believed that selection generated by sperm competition favors males that manufacture the smallest gametes possible in order to maximize sperm number. In essence, sperm competition is attributed with the evolutionary maintenance of anisogamy. I became interested in understanding whether the very act of sexual selection, by definition, can limit its own ability to act.”

The next step is to investigate the details of the effects of sperm length evolution on the intensity of sexual selection. “For instance, our results indicate that, as sperm get longer across and within species, females actually become more ‘male-like,’” says Bjork. “The intensity of sexual selection on females increases. Traditionally, females have been thought to have the same number of progeny whether they mate once or, say, five times because there are way more than enough sperm to fertilize all of their available eggs. But our data show that, as sperm get longer, females actually benefit, in terms of increased offspring numbers by multiplying mating.”

Source: Syracuse University

另一篇相似報道:

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1283891