“重女轻男”细菌扳倒进化平衡【转】.doc

Gender-biased bacteria throw off an evolutionary balanceApril 2011Sweet potato whitefliesThis month, biologists reported that a bacterial infection has run rampant in populations of a major crop pest in the Southwest. The bacterium (called Rickettsia) is a close relative of the species that causes typhus in humans. Its host is the sweet potato whitefly, a tiny bug that can occur in large enough numbers to form visible clouds. Whiteflies suck the sap from plants and spread crop diseases, causing hundreds of millions of dollars of damage in a single season. In just a few years, the percentage of southwestern whiteflies infected with Rickettsia has skyrocketed from 1% to more than 90%. Unfortunately, this is not the boon for local farmers that it might seem. Rickettsia dont sicken the whitefly; instead, the bacterium actively helps the pest spread and increase in numbers. Evolutionary theory accounts for this surprising observation and highlights how we might turn it in our favor.Wheres the evolution?We are most familiar with bacteria that cause illness; however, a whole host of bacterial species are symbionts and provide benefits to their hosts. The advantage that Rickettsia provides to the whitefly is huge: infected whiteflies produce twice as many offspring as Rickettsia-free whiteflies, and these offspring are much more likely to survive to adulthood than the offspring of uninfected individuals. Why would one species be so benevolent towards another? The answer seems to lie in the evolutionary fitness of the bacterium. In this case, whats good for the host also happens to be good for the resident. Rickettsia are transmitted through the whitefly population from mother to offspring not, for example, between bugs that alight on the same leaf. In terms of natural selection, this means that any bacterium that happened to have a gene version that benefitted its hosts survival and reproduction would be passed on to more new hosts. Through many generations, these genetic variants would become more and more common in the Rickettsia population, and eventually, bacteria that provided bigger and bigger benefits to the host could evolve.An evolutionary perspective explains another unusual quirk of this interaction: Rickettsia not only improve their hosts reproductive capacity, they bias it as well towards having girls. Uninfected whiteflies produce about a 50/50 ratio of female to male offspring, while the offspring of infected whiteflies are around 70% female. In terms of the bacteriums fitness, this manipulation also makes sense. More females means that the whitefly population can increase more rapidly, giving a big boost to the Rickettsia population, and since the bacterium is only passed from mother to offspring (not father to offspring), the bias also generates more possible new infections in the future. It might seem far-fetched for a tiny bacterium to influence its hosts reproduction like this, but in fact, nature is full of examples of microorganisms wielding enormous power over their hosts fungi that cause ants to climb to the top of blades of grass (facilitating the funguss dispersal), a protozoan that causes rats to lose their fear of cats (allowing the microorganism to spread up the food chain), and many, even more bizarre cases.Though the human sex ratio is close to 50/50 and our chromosomal system of XX and XY makes that seem natural, sex ratio is not set in stone. Whiteflies produce more females under the influence of Rickettsia, New Zealand parrots produce more males when food is plentiful, and buffalos produce more females during the dry season to name just a few exceptions to the 50/50 rule. For Rickettsia and the whitefly, biasing reproduction towards females seems to be a win-win situation. Why then, do whiteflies normally produce a 50/50 sex ratio? After all, wouldnt it be advantageous for them to produce more females all the time even when not infected by Rickettsia? Evolutionary theory explains why most species are comprised of half males and half females and why exceptions occur.To understand why a 50/50 sex ratio is so common, consider a hypothetical population of whiteflies, uninfected with Rickettsia, with a 50/50 sex ratio. Imagine that a mutation occurs that causes an individual to produce more female than male offspring. The next generation would have more females than males, and there would be an increase in the gene versions that favor female offspring. Since this generation has excess females, each male has a higher chance of fathering offspring e.g., each male might father the offspring of 1.5 females on average. At this point, individuals with gene versions that cause them to produce moresons are at an advantage, since sons will likely get to mate more and pass more copies of their genes on to the next generation than daughters will. Now, natural selection will act to increase the frequency of gene versions that favor male offspring and in the next generation, there would be a corresponding increase in the frequency of males. This cycle can continue indefinitely: whenever females are more common in the population, male-producing gene versions are favored, and whenever males are more common, female-producing gene versions are favored. This is an example of frequency dependent selection, in which the advantage or disadvantage of a particular trait depends on how common that trait is in the population. The result of this see-saw of natural selection is that sex ratios tend to converge on 50/50.Extreme endosymbiosisA cell infected withRickettsiaRickettsia probably sounds like an exotic, unfamiliar strain of bacteria; however, you are closer to this particular bacterium than you might think. More than a billion years ago a close relative of Rickettsia invaded another cell (much as modern Rickettsia took up residence inside the whitefly a relationship known as endosymbiosis) and wound up staying forever. That cell lineageevolved into the first eukaryotes, and those ancient relatives of Rickettsia evolved into the first mitochondria, the cellular organelles which fuel cells. Eventually, that eukaryote ancestor (and the mitochondria inside it) evolved into all the plant, animals, and fungi alive today including us! You actually carry the evolutionary cousins of Rickettsia inside every cell of your body in the form of mitochondria.When sex ratios deviate from 50/50, it is for specific reasons. For example, in some organisms (e.g., New Zealands kakapo parrot), males compete for access to females, and males with particular traits (e.g., large size, strength, large horns, colorful plumage, etc) are likely to mate with several females while other males may not mate at all. This means that if a mother doesnt have the resources she needs to invest in offspring that are likely to have those advantageous male traits, she is better off producing daughters than sons. This situation favors the evolution of gene versions that bias offspring towards female when resources are scarce and towards male when resources are plentiful.Of course, the southwestern whitefly population now deviates from a 50/50 sex ratio not because of the whiteflys evolution, but because of Rickettsias evolution. The symbiont has evolved to manipulate its hosts sex ratio for its own advantage setting up an interesting evolutionary conflict. Rickettsias fitness is maximized by maintaining control of the whitefly sex ratio and biasing it towards females. However, any whitefly that happened to have a genetic variant that would allow it to produce more sons (even while infected with Rickettsia) would have a big advantage! Just as in the see-saw of sex-ratio evolution described above, in a female biased whitefly population, having sons will get a mother or father whitefly more grandkids than having daughters. Because of this, the fitness of individual whiteflies would be maximized by shifting the sex ratio back towards 50/50. The outcome of this evolutionary tug-of-war between the whitefly and its symbiont is still unclear.Biologists are monitoring the impact this interaction may have on agriculture in the Southwest. While farmers have not yet seen a spike in crop damage, the spreading Rickettsia infection (and the boost it gives its hosts reproductive capacity) does seem to have poised the whitefly for a major infestation. However, ongoing research on Rickettsia and its host may yield new strategies to combat pests. Symbionts can exert such a powerful influence over their hosts that scientists have considered using them as an all-natural pest control mechanism. In fact, in 2009, scientists successfully infected mosquitoes with a symbiont strain that halves the lifespan of the mosquito! A better understanding of exactly how Rickettsia affects whitefly fitness and fecundity could help us develop new approaches to controlling the spread of this pest.小红猪“重女轻男”细菌扳倒进化平衡Comments小红猪小分队 发表于 2011-09-22 10:00| Tags 标签：内共生, 立克次氏体, 译文译者：Gorgon译者自我简介/征友启事：Gorgon，（为了不做白大褂女PhD）叛出生物转投医卫，身在美帝心系天朝，一脚文艺一脚科学，兜兜转转已到亟待随手解救之年。诚征有共同理想的男青年共渡。xuluting2009#。校对：人神之间小红花等级：4朵原文链接：Gender-biased bacteria throw off an evolutionary balanceSweet potato whiteflies本月（原文发表于2011年4月），生物学家发现一种细菌感染正在美国西南部最常见的一类农业害虫的种群中肆虐。这种细菌（立克次氏体）是人类斑疹伤寒始作俑者的近亲。它的宿主叫甜薯粉虱 (sweet potato whitefly)，它们体型虽小，却能数以万计地出现、形成人眼可见的“虫云”。粉虱吸取植物汁液、传播作物疾病，仅一季就能造成数百万美元的经济损失。短短几年间， 西南部地区粉虱的立克次氏体感染率已从1%飙升到了90%以上。遗憾的是，这对当地农民来说并非喜讯：立克次氏体非但没有让粉虱生病，反而积极地帮害虫大量繁殖和扩张。进化理论就藏在这一惊人现象的背后，同时也为我们如何扭转局面指出了方向。进化在哪里？我们都熟悉能致病的细菌；然而，除此之外另有一整类细菌属于共生体，它们为宿主提供好处。立克次氏体给粉虱带来的益处是巨大的：受到感染的粉虱能繁殖出比 未受感染者多一倍的后代，这些后代存活至成虫的比例又比未受感染者的后代大得多。为什么一种生物会对另一种生物如此慷慨呢？答案与细菌的进化适应度有关。在这个例子里，对宿主有益的恰好也对寄生物有益。立克次氏体在粉虱种群中以母体-子代的方式传播而不是，例如说，同一片叶子上靠近的虫体互相传染。从自然选择的角度来看，这也就是说，任何一种细菌假如带有对宿主生存和繁殖有利的基因型，就会被传递至更多的宿主。历经许多世代之后，这种基因型在立克次氏体 种群中的份额会越来越大，为宿主提供越来越多好处的细菌得以进化下去。以进化的角度，这重关系里的另一个疑点也可以得到解答：立克次氏体不仅提高了宿主的繁殖力，还造成了性别偏向偏向于生产雌性。未受感染的粉虱后代的性 别比大约是50/50，而受到感染者诞下70%左右都是雌性。如果考虑细菌的适应性，这项策略也有道理。更多的雌性意味着粉虱种群增长得更快，也就带动了立 克次氏体种群的激增。同时，因为细菌感染只在母子间传播（父子则不行），这种性别偏向也就为下一代粉虱的被传染制造了更多机会。一种小小的细菌竟能对其宿主的繁殖有如此影响力，听上去似乎有些牵强。但事实上，自然界中，微生物在宿主体内作威作福的例子俯拾皆是真菌感染令蚂蚁爬到草叶的最高处（有利于真菌孢子的播散）；某些原生虫感染使老鼠不再怕猫（方便寄生虫经由食物链扩散）；还有许多、甚至更为奇异的例子。尽管人类的性别比接近50/50，我们的XX和XY性染色体机制也让这个比例显得十分自然，生物界中的性别比却不都是五五均分。随便举几个50/50规则 的异数立克次氏体影响下的粉虱产下更多雌性；新西兰鹦鹉（New Zealand parrots）在食物充足的时候生雄性较多，野牛在旱季则生雌性较多。对立克次氏体和粉虱而言，使宿主性 别偏向雌性似乎是个双赢局。那么，为什么正常情况下的粉虱会选择50/50性别比呢？不论如何，难道不应该生更多雌性以确保有利条件吗哪怕没有被立克次氏体感染？进化理论同时也能解释，为何多数物种保持一半雄性和一半雌性的比例以及为何会有例外出现。要理解50/50性别比普遍存在的原因，请考虑一个假想的粉虱种群：未受立克次氏体感染，性别比50/50均分。想象基因库出现一个突变，导致个体诞下雌性的几率超过雄性。因此下一代将出现更多的雌性粉虱，同时，这个有利于生雌性的基因型的频率也获得增加。由于这一世代雌性过剩，每个雄性个体成为父亲的几率也增加了比如说，平均每个雄性能够与1.5个雌性产生后代。这时候，情况变得有利于那些生雄性的基因型的所有者，因为雄性后代将拥有更多交配机会，也就更有可能把基因传递下去