图片说明:MIT科学家培养酵母细胞进行蔗糖代谢实验。
(图片来源:Donna Coveney)
合作行为(cooperative behavior)是进化论中一个令人困惑的问题,它给种群内其它成员带来好处,但却会损害个体利益。生物学家对此迷惑不解——如果是最适者生存,那么有益于种群内所有成员的行为的基因就不该长期存在,合作行为应该灭绝。
美国麻省理工学院(MIT)科学家近日利用博弈论,解释了酵母规避这一问题的方法。研究显示,如果一个个体能够从合作行为中获取哪怕是微小的利益,那么即使周围的个体均不合作,它也能够生存下去。相关论文4月6日在线发表于《自然》(Nature)。
研究人员设计了一种实验装置,让酵母进行蔗糖代谢。蔗糖并非酵母的首选食物源,但在没有葡萄糖的情况下,它也会进行蔗糖代谢。不过,它们必须分泌一种蔗糖转化酶,帮助将蔗糖分解成单糖以便吸收。
问题在于,大量分解后的单糖也可被周围的其它酵母细胞利用。这种情况下,分泌转化酶的酵母称作合作者,不分泌、只消耗单糖的酵母称作欺诈者。
如果所有的单糖只是四下扩散,分泌转化酶的酵母没有优先使用权的话,那么更好的选择永远都是成为欺诈者,合作者将会灭绝。
研究实际发现,分泌转化酶的酵母对于它们制造的单糖拥有大约1%的优先使用权。这一获益超过了帮助别人所需的代价,使得它们能够成功地与欺诈者进行竞争。
此外,不论开始的酵母种群数量是多少,最后都会达到一种平衡状态,合作者和欺诈者同时存在。这类似于博弈论中的“铲雪博弈”——你和同伴开一辆车被雪堆挡住去路,每个人都可以选择下车铲雪或原位不动。一个人不铲雪,那么另一个人必须铲雪。
表面上看来,你最好的选择是待在温暖的车里,同伴去铲雪。但有时最坏的情况会发生,即你和同伴均不去铲雪,结果你们永远回不了家。因而,最好的策略是永远选择你对手策略的反面。
论文第一作者、MIT物理系的Jeff Gore表示,之前研究已经显示,在野生状态下,酵母携带的转化酶基因拷贝数存在不同。这种野外的遗传多样性,可能与实验室中观察到的合作者和欺诈者的长期共存相类似。(生物谷Bioon.com)
生物谷推荐原始出处:
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.