Experimental evolution of the grain of metabolic specialization in yeast

Abstract

Adaptation to any given environment may be accompanied by a cost in terms of reduced growth in the ancestral or some alternative environment. Ecologists explain the cost of adaptation through the concept of a trade‐off, by which gaining a new trait involves losing another trait. Two mechanisms have been invoked to explain the evolution of trade‐offs in ecological systems, mutational degradation, and functional interference. Mutational degradation occurs when a gene coding a specific trait is not under selection in the resident environment; therefore, it may be degraded through the accumulation of mutations that are neutral in the resident environment but deleterious in an alternative environment. Functional interference evolves if the gene or a set of genes have antagonistic effects in two or more ecologically different traits. Both mechanisms pertain to a situation where the selection and the alternative environments are ecologically different. To test this hypothesis, we conducted an experiment in which 12 experimental populations of wild yeast were each grown in a minimal medium supplemented with a single substrate. We chose 12 different carbon substrates that were metabolized through similar and different pathways in order to represent a wide range of ecological conditions. We found no evidence for trade‐offs between substrates on the same pathway. The indirect response of substrates on other pathways, however, was consistently negative, with little correlation between the direct and indirect responses. We conclude that the grain of specialization in this case is the metabolic pathway and that specialization appears to evolve through mutational degradation.

Superfluous glutamine synthetase activity in Chinese Hamster Ovary cells selected under glutamine limitation is growth limiting in glutamine‐replete conditions and can be inhibited by serine

“…Excess enzyme capacities may sometimes be due to system design for metabolic robustness and/or responsiveness and not only for increased flux . Studies from microbial cells show that cells selected under particular nutritional status may have suboptimal metabolism for growth under other nutritional status . Escherichia coli cells selected under inorganic phosphate limitation showed better fitness in phosphate‐limited medium but lower fitness under glucose‐limited medium .…”

“…Selection under minimal medium containing glucose reduced the ability of E. coli to utilize a majority of alternate nutrients like serine, malate, succinate, and others, compared to the ancestral cells . Yeast cells were selected in minimal medium containing one of 12 substrates, resulting in increased fitness when grown on the substrate used for selection . However, selected cells showed reduced fitness on other substrates except for a few substrates, which were downstream in the metabolic pathway of the substrate used for selection.…”

“…18 Studies from microbial cells show that cells selected under particular nutritional status may have suboptimal metabolism for growth under other nutritional status. [19][20][21] Escherichia coli cells selected under inorganic phosphate limitation showed better fitness in phosphatelimited medium but lower fitness under glucose-limited medium. 20 Selection under minimal medium containing glucose reduced the ability of E. coli to utilize a majority of alternate nutrients like serine, malate, succinate, and others, compared to the ancestral cells.…”

“…19 Yeast cells were selected in minimal medium containing one of 12 substrates, resulting in increased fitness when grown on the substrate used for selection. 21 However, selected cells showed reduced fitness on other substrates except for a few substrates, which were downstream in the metabolic pathway of the substrate used for selection. Such negative effects, commonly known as the cost of adaptation, allow microbial cells to specialize on the substrate used for selection at the cost of decreasing the breadth of utilizable substrates.…”

“…Such negative effects, commonly known as the cost of adaptation, allow microbial cells to specialize on the substrate used for selection at the cost of decreasing the breadth of utilizable substrates. 21 Nevertheless, selection under a particular nutrient status resulting in gain of fitness in a majority of alternate substrates tested along with loss of fitness observed on a few substrates has also been reported. 22,23 Such studies have however not been commonly reported in mammalian cells.…”