Positive Genetic Correlation Between Yield and Symbiotic Nitrogen Fixation in Pea

Crop improvement can enhance food security, but side effects, such as trade-offs between valuable agronomic traits, are common. Likewise, fertilisation helps ensure high yields but can devalue mutualisms with soil microbes that would otherwise be essential for nutrient acquisition. If the need for nutritional mutualisms is reduced in crops, mutualisms could be disrupted by selection relaxation or allocation trade-offs. Thus, crops could achieve high yields despite or because of, disruption of nutritional mutualisms. Alternatively, the highest-yielding plants might flourish because they maximise nutrient acquisition from both symbionts and the soil. Here, enhanced mutualism could evolve over the course of agricultural crop improvement. To investigate whether high yields in cultivars and wild accessions are negatively or positively genetically correlated with outcomes in the legume-rhizobia mutualism, we measured whether yield and symbiosis traits trade off or are positively genetically correlated among cultivars and wild accessions. We measured genetic correlations between yield and mutualism traits in 87 domesticated pea (Pisum sativum) accessions released before or after 1950 in a common garden agricultural field across 3 years. Seed yield and N₂ fixation were positively genetically correlated in both pre- and post-1950 accessions. Thus, despite recent selection for pea cultivars with high yields in fertile agricultural conditions, this positive genetic correlation has been maintained. Our findings predict that breeding efforts have the potential to increase both yield and symbiotic nitrogen fixation in pea, enhancing symbiotic mutualism and reducing the proportion of N supplied by fertilisation.