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. While energetic costs have been proposed to influence genome size through selection on insertions and deletions (indels), nutrient availability may be a more relevant constraint in primary producers such as phytoplankton. We derived an expression for the selection coefficient of indels based on the phosphorus and nitrogen costs of nucleotides and the cellular nutrient requirements. Selection coefficient estimates indicate that natural selection dominates over genetic drift and favors the fixation of mutations that reduce genome size in phytoplankton with low nutrient requirements. Model predictions are supported by comparative genomics and metagenomic analyses. Together, this model provides a rigorous quantitative framework for understanding genome size evolution, particularly in small cells and oligotrophic environments, highlighting how nutrient limitation drives genome streamlining.