Systems Bioinformatics/Center for Integrative Bioinformatics Vrije Universiteit, Amsterdam Institute for Molecules Medicines and Systems, Vrije Universiteit Amsterdam, 1081HV Amsterdam, The Netherlands.
Proc Natl Acad Sci U S A. 2013 Aug 27;110(35):14302-7. doi: 10.1073/pnas.1308523110. Epub 2013 Aug 12.
Tradeoffs provide a rationale for the outcome of natural selection. A prominent example is the negative correlation between the growth rate and the biomass yield in unicellular organisms. This tradeoff leads to a dilemma, where the optimization of growth rate is advantageous for an individual, whereas the optimization of the biomass yield would be advantageous for a population. High-rate strategies are observed in a broad variety of organisms such as Escherichia coli, yeast, and cancer cells. Growth in suspension cultures favors fast-growing organisms, whereas spatial structure is of importance for the evolution of high-yield strategies. Despite this realization, experimental methods to directly select for increased yield are lacking. We here show that the serial propagation of a microbial population in a water-in-oil emulsion allows selection of strains with increased biomass yield. The propagation in emulsion creates a spatially structured environment where the growth-limiting substrate is privatized for populations founded by individual cells. Experimental evolution of several isogenic Lactococcus lactis strains demonstrated the existence of a tradeoff between growth rate and biomass yield as an apparent Pareto front. The underlying mutations altered glucose transport and led to major shifts between homofermentative and heterofermentative metabolism, accounting for the changes in metabolic efficiency. The results demonstrated the impact of privatizing a public good on the evolutionary outcome between competing metabolic strategies. The presented approach allows the investigation of fundamental questions in biology such as the evolution of cooperation, cell-cell interactions, and the relationships between environmental and metabolic constraints.
权衡取舍为自然选择的结果提供了一个基本原理。一个突出的例子是单细胞生物的增长率与生物量产量之间的负相关。这种权衡取舍导致了一个困境，即增长率的优化对个体有利，而生物量产量的优化对种群有利。在大肠杆菌、酵母和癌细胞等多种生物中都观察到了高速率策略。悬浮培养中的生长有利于快速生长的生物体，而空间结构对于高产策略的进化很重要。尽管已经意识到了这一点，但缺乏直接选择增加产量的实验方法。我们在这里表明，在油包水乳液中对微生物种群进行连续繁殖可以选择具有增加生物量产量的菌株。在乳液中进行繁殖会创建一个空间结构的环境，其中生长受限的基质被个体细胞建立的种群私有化。对几个同源乳球菌（Lactococcus lactis）菌株的实验进化表明，生长速率和生物量产量之间存在权衡取舍，这是一个明显的帕累托前沿。潜在的突变改变了葡萄糖的运输，导致同型发酵和异型发酵代谢之间的重大转变，从而解释了代谢效率的变化。结果表明，将公共物品私有化对竞争代谢策略之间的进化结果产生了影响。所提出的方法可以研究生物学中的基本问题，例如合作的进化、细胞间相互作用以及环境和代谢限制之间的关系。
