假如有一个奖项可以颁发给自然界最大的精子的话，那它的获得者将是一种名为D. bifurca的小果蝇，它的盘绕的精子拉直开来有2英寸（5.08厘米）那么长，是平均人类精子长度的1000倍。
美国纽约州Syracuse大学的一位博士生亚当·比约克说："打个比方，假如人的精子也有那么长，然后你让一个6英尺高的人站在一个足球场的球门线上，那么他的精子就能伸展到球场的40码线处。"比约克和雪城大学的生物学家司各特·皮特尼克一起，察看了为什么有些动物，如一只细小的果蝇，能够产生如此长的精子。它们俩人的发现表明其中的答案是在雌性果蝇的身体上。他们的研究细节被发表在了Nature杂志上。
在许多物种里面，雄性产生精子的数量要远远多于雌性产生的卵子数量。雄性产生的精子个体细小却数量很多，以此来增加它们中的一个与卵子结合的几率。
按照性别选择理论预测，精子的个头变得更大，那么它与卵子结合的比率也变的更大，雄性的竞争也会减少。
但是当研究人员在检测D. bifurca这种果蝇雄性之间的竞争时却发现，它们的竞争并没有如所期望的那样减少。而要找到其中的答案就必须对雌性果蝇进行解剖。
雌性隐秘选择
虽然雄性成功的在雌性体内播下了种子，但在受精发生之前还有很长一段路要走。因为许多雌性的身体并不是精子的被动竞技场，而更像是一堂障碍课，通过设置许多障碍和防卫来除掉那些弱的精子。
例如，雌性的生殖管道可以释放许多对精子有害的化学物质，或者也可能变长，使得受精比赛变成了一场对精子持久力的测试。
这种后性别障碍被生物学家称之为"雌性隐秘选择"，因为它们不管是对雌性自身或者是对研究人员都变现的不明显。
D. bifurca雌性果蝇生殖道正好比游过它的精子要长一点。比约克说，这看起来就像一个巨大盘绕物填充在它的腹部。
研究人员认为正是雌性果蝇特别长的生殖道才驱使雄性果蝇巨大精子的进化。
比约克说："这项研究结果表明精子之间的竞争和隐秘雌性选者促使了雄性和雌性两者在生理和行为上的进化。"
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
另一篇相似报道:
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1283891
