Effect of cellular nutrient economy on the evolution of genome size in phytoplankton

The origin of genome size variation remains a central question in evolutionary biology. Previous analyses suggest that the energetic costs of small insertions and deletions (indels) influence genome size by altering the cellular energy budget. Here, we present a theoretical framework for genome evolution in phytoplankton—the main producers of the sunlit ocean—whose growth depends on nutrient availability. We derive an expression for the selection coefficient on indels as a function of the minimum amount of nutrient inside the cell. Parameter estimates indicate that 1-bp indels can be counterselected in phytoplankton with minimal nutrient requirements. We then test three predictions of nutrient-driven selection on genome evolution. Altogether, this model provides a quantitative and mathematical basis for the genome streamlining hypothesis.