Drivers of insect herbivory resistance and tolerance to plant damage in the Brachypodium distachyon species complex.

premiseInsect herbivory is a major biotic factor shaping plant populations and driving the evolution of defensive traits. Polyploidy (whole-genome duplication) often induces substantial phenotypic and genotypic changes that may affect species interactions, including herbivory. However, natural variation in herbivory responses and the drivers of resistance and tolerance across heteroploid lineages remain poorly understood.

methodsWe conducted a bioassay to quantify variation in plant damage and tolerance to locust herbivory across multiple diploid and allotetraploid populations of the Brachypodium distachyon species complex, a model system comprising two diploid species (B. distachyon and B. stacei) and their allotetraploid derivative (B. hybridum). For each species, we also examined which plant functional traits were associated with resistance and tolerance to herbivory.

resultsHerbivory reduced maternal fitness across the species complex, although its magnitude depended on species and the fitness component considered. Our results do not support enhanced herbivory resistance or tolerance in the allotetraploid lineage: Levels of plant damage in B. hybridum were comparable to those of one diploid parent (B. distachyon), and diploid B. distachyon had higher tolerance than B. hybridum for two of three fitness estimators. Variation in resistance was associated with differences in plant traits, particularly C:N ratio and silica content. In B. distachyon, tolerance was negatively associated with silica and water content, suggesting allocation trade-offs between resistance- and tolerance-related traits.

conclusionsOverall, our findings indicate that variation in herbivory responses across Brachypodium populations is more closely linked to population history and trait differentiation than to polyploid formation per se.