Data from: Resource and competitive dynamics shape the benefits of public goods cooperation in a plant pathogen

Cooperative benefits depend on a variety of ecological factors. Many cooperative bacteria increase the population size of their groups by making a public good available. Increased local population size can alleviate the constraints of kin competition on the evolution of cooperation by enhancing the between-group fitness of cooperators. The cooperative pathogenesis of Agrobacterium tumefaciens causes infected plants to exude opines—resources that provide a nearly exclusive source of nutrient for the pathogen. We experimentally demonstrate that opines provide cooperative A. tumefaciens cells a within-group fitness advantage over saprophytic agrobacteria. Our results are congruent with a resource-consumer competition model, which predicts that cooperative, virulent agrobacteria are at a competitive disadvantage when opines are unavailable, but have an advantage when opines are available at sufficient levels. This model also predicts that freeloading agrobacteria that catabolize opines but cannot infect plants competitively displace the cooperative pathogen from all environments. However, we show that these cooperative public goods also promote increased local population size. A model built from the Price Equation shows that this effect on group size can contribute to the persistence of cooperative pathogenesis despite inherent kin competition for the benefits of pathogenesis.

合作收益的实现依赖于多种生态因子。许多合作型细菌可通过分泌公共物品来扩大其种群群体的规模。局部种群规模的提升可通过增强合作者的群体间适合度，缓解亲缘竞争对合作演化的制约。根癌农杆菌（Agrobacterium tumefaciens）的合作致病过程会使受侵染植株分泌冠瘿碱（opines）——这类资源几乎是该病原菌专属的营养来源。本研究通过实验证实，冠瘿碱可使合作型根癌农杆菌细胞相较于腐生农杆菌获得群体内适合度优势。本研究结果与资源-消费者竞争模型的预测一致：当冠瘿碱缺失时，合作型致病农杆菌会处于竞争劣势；而当冠瘿碱供应充足时，此类菌株则具备竞争优势。该模型还预测，仅能分解代谢冠瘿碱却无法侵染植物的搭便车农杆菌，可在所有环境中竞争性排挤合作型病原菌。然而，本研究表明，这类合作型公共物品同样可促进局部种群规模的扩大。基于普莱斯方程（Price Equation）构建的模型显示，尽管合作致病过程的收益存在固有亲缘竞争，但这种对群体规模的影响可助力合作致病策略得以维持。