Measuring the fitness of symbiotic rhizobia

The legume-rhizobia symbiosis is an important model system for research on the evolution of cooperation and conflict. A key strength of this system is that the fitness consequences of greater or lesser investment in cooperative behaviors can be measured for each partner. Most empirical studies have characterized the fitness of symbiotic rhizobia exclusively by their numbers within nodules, often estimated using nodule size as a proxy. Here we show that the relationship between nodule size and rhizobial numbers can differ drastically between strains of the same species. We further show that differences in accumulation of the storage polyester poly-3-hydroxybutyrate (PHB), which can support future reproduction, can be large enough that even direct measurements of rhizobial numbers alone can lead to qualitatively incorrect conclusions. Both results come from a comparison of strains differing in production of the ethylene-inhibitor rhizobitoxine (Rtx). A broader study (using three legume-rhizobia species pairs) showed that PHB/cell cannot be reliably estimated from its correlation with rhizobia/nodule or nodule size. Differences in PHB between strains or treatments will not always make major contributions to differences in fitness, but situation-specific data are needed before PHB can safely be neglected.