Fitness consequences of fungal pathogen Melampsora lini on sub-alpine flax Linum lewisii

Abstract

For many decades, Lewis flax (Linum lewisii), a wildflower native to western North America, has been a key study species in plant immunology, climate change, and epidemiological and ecological modeling. In the Rocky Mountain region of Colorado, it is frequently infected by its obligate fungal pathogen, flax rust (Melampsora lini), yet the direct evolutionary fitness consequences of infection are currently unknown. To strengthen the foundations for the use of flax as a predictive and translational study system of plant diseases, I investigated the effects of rust infection on key fitness measures: growth, reproduction, and survival. Over two seasons of field surveys in the Gunnison Valley, I found evidence of an association between infection and lowered likelihood of overwinter mortality, reduced fruit production, and mixed evidence regarding within-season growth. Plant size (height) was a much more robust predictor of all fitness variables than disease status, severity, or duration, especially in seedlings. These findings could also indicate distinct trade-offs between a plant’s investment in proactive immune defense and its investment in growth or overwinter survival. Building a predictive understanding of the system through an integral projection model, I help contextualize the impact of disease dynamics on future population structuring. My model showed that healthy plants do better in overall fitness, but diseased plants are reproducing faster. Further research could explore long-term fitness trade-offs over lifetimes and generations while considering how changing climates may influence future epidemics.