圖片說(shuō)明：MIT科學(xué)家培養(yǎng)酵母細(xì)胞進(jìn)行蔗糖代謝實(shí)驗(yàn),。
（圖片來(lái)源：Donna Coveney）
合作行為（cooperative behavior）是進(jìn)化論中一個(gè)令人困惑的問(wèn)題，它給種群內(nèi)其它成員帶來(lái)好處,，但卻會(huì)損害個(gè)體利益,。生物學(xué)家對(duì)此迷惑不解——如果是最適者生存，那么有益于種群內(nèi)所有成員的行為的基因就不該長(zhǎng)期存在,，合作行為應(yīng)該滅絕,。
美國(guó)麻省理工學(xué)院（MIT）科學(xué)家近日利用博弈論，解釋了酵母規(guī)避這一問(wèn)題的方法,。研究顯示,，如果一個(gè)個(gè)體能夠從合作行為中獲取哪怕是微小的利益，那么即使周?chē)膫€(gè)體均不合作,，它也能夠生存下去,。相關(guān)論文4月6日在線(xiàn)發(fā)表于《自然》（Nature）,。
研究人員設(shè)計(jì)了一種實(shí)驗(yàn)裝置,，讓酵母進(jìn)行蔗糖代謝。蔗糖并非酵母的首選食物源,，但在沒(méi)有葡萄糖的情況下,，它也會(huì)進(jìn)行蔗糖代謝。不過(guò),，它們必須分泌一種蔗糖轉(zhuǎn)化酶,，幫助將蔗糖分解成單糖以便吸收。
問(wèn)題在于，大量分解后的單糖也可被周?chē)钠渌湍讣?xì)胞利用,。這種情況下,，分泌轉(zhuǎn)化酶的酵母稱(chēng)作合作者，不分泌,、只消耗單糖的酵母稱(chēng)作欺詐者,。
如果所有的單糖只是四下擴(kuò)散，分泌轉(zhuǎn)化酶的酵母沒(méi)有優(yōu)先使用權(quán)的話(huà),，那么更好的選擇永遠(yuǎn)都是成為欺詐者,，合作者將會(huì)滅絕。
研究實(shí)際發(fā)現(xiàn),，分泌轉(zhuǎn)化酶的酵母對(duì)于它們制造的單糖擁有大約1%的優(yōu)先使用權(quán),。這一獲益超過(guò)了幫助別人所需的代價(jià)，使得它們能夠成功地與欺詐者進(jìn)行競(jìng)爭(zhēng),。
此外,，不論開(kāi)始的酵母種群數(shù)量是多少，最后都會(huì)達(dá)到一種平衡狀態(tài),，合作者和欺詐者同時(shí)存在,。這類(lèi)似于博弈論中的“鏟雪博弈”——你和同伴開(kāi)一輛車(chē)被雪堆擋住去路，每個(gè)人都可以選擇下車(chē)鏟雪或原位不動(dòng),。一個(gè)人不鏟雪,，那么另一個(gè)人必須鏟雪。
表面上看來(lái),，你最好的選擇是待在溫暖的車(chē)?yán)?，同伴去鏟雪。但有時(shí)最壞的情況會(huì)發(fā)生,，即你和同伴均不去鏟雪,，結(jié)果你們永遠(yuǎn)回不了家。因而,，最好的策略是永遠(yuǎn)選擇你對(duì)手策略的反面,。
論文第一作者、MIT物理系的Jeff Gore表示,，之前研究已經(jīng)顯示,，在野生狀態(tài)下，酵母攜帶的轉(zhuǎn)化酶基因拷貝數(shù)存在不同,。這種野外的遺傳多樣性,，可能與實(shí)驗(yàn)室中觀察到的合作者和欺詐者的長(zhǎng)期共存相類(lèi)似。（生物谷Bioon.com）
生物谷推薦原始出處：
Nature advance online publication 6 April 2009 | doi:10.1038/nature07921
Snowdrift game dynamics and facultative cheating in yeast
Jeff Gore1, Hyun Youk1 & Alexander van Oudenaarden1
Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
The origin of cooperation is a central challenge to our understanding of evolution1, 2, 3. The fact that microbial interactions can be manipulated in ways that animal interactions cannot has led to a growing interest in microbial models of cooperation4, 5, 6, 7, 8, 9, 10 and competition11, 12. For the budding yeast Saccharomyces cerevisiae to grow on sucrose, the disaccharide must first be hydrolysed by the enzyme invertase13, 14. This hydrolysis reaction is performed outside the cytoplasm in the periplasmic space between the plasma membrane and the cell wall. Here we demonstrate that the vast majority (99 per cent) of the monosaccharides created by sucrose hydrolysis diffuse away before they can be imported into the cell, serving to make invertase production and secretion a cooperative behaviour15, 16. A mutant cheater strain that does not produce invertase is able to take advantage of and invade a population of wild-type cooperator cells. However, over a wide range of conditions, the wild-type cooperator can also invade a population of cheater cells. Therefore, we observe steady-state coexistence between the two strains in well-mixed culture resulting from the fact that rare strategies outperform common strategies—the defining features of what game theorists call the snowdrift game17. A model of the cooperative interaction incorporating nonlinear benefits explains the origin of this coexistence. We are able to alter the outcome of the competition by varying either the cost of cooperation or the glucose concentration in the media. Finally, we note that glucose repression of invertase expression in wild-type cells produces a strategy that is optimal for the snowdrift game—wild-type cells cooperate only when competing against cheater cells.
