Data from: Size evolution in microorganisms masks trade-offs predicted by the Growth Rate Hypothesis

Adaptation to local resource availability depends on responses in growth rate and nutrient acquisition. The Growth Rate Hypothesis (GRH) suggests that growing fast should impair competitive abilities for phosphorus and nitrogen due to high demand for biosynthesis. However, in microorganisms, size influences both growth and uptake rates, which may mask trade-offs and instead generate a positive relationship between these traits (Size Hypothesis – SH). Here we evolved a gradient of maximum growth rate (μmax) from a single bacterium ancestor to test the relationship between μmax, competitive ability for nutrients, and cell size, while controlling for evolutionary history. We found a strong positive correlation between μmax and competitive ability for phosphorus, associated with a trade-off between μmax and cell size: strains selected for high μmax were smaller and better competitors for phosphorus. Our results strongly support the Size Hypothesis, while the trade-offs expected under GRH were not apparent. Beyond plasticity, unicellular populations can respond rapidly to selection pressure through joint evolution of their size and maximum growth rate. Our study stresses that physiological links between these traits tightly shape the evolution of competitive strategies.

生物对本地资源可获得性的适应，取决于其生长速率与营养获取过程的响应。生长速率假说（Growth Rate Hypothesis, GRH）提出，由于生物合成过程需求较高，快速生长会削弱生物体对磷与氮的竞争能力。不过在微生物中，细胞尺寸会同时影响生长与营养吸收速率，这可能掩盖性状间的权衡关系，反而使这些性状呈现正相关（尺寸假说Size Hypothesis, SH）。本研究以单一细菌祖先为起始菌株，通过定向演化构建了最大生长速率（maximum growth rate, μmax）的梯度系列，在控制演化历史变量的前提下，检验μmax、营养竞争能力与细胞尺寸三者间的关联。研究发现，μmax与磷营养竞争能力间存在显著正相关，且该关联与μmax和细胞尺寸间的权衡关系相契合：被定向选择以获得高μmax的菌株尺寸更小，同时更擅长争夺磷资源。本研究结果有力支持了尺寸假说，而生长速率假说所预期的权衡关系并未显现。除表型可塑性之外，单细胞种群可通过细胞尺寸与最大生长速率的协同演化，快速响应选择压力。本研究强调，这些性状间的生理关联会紧密塑造竞争策略的演化进程。