Temperature plays a pivotal role in defining the distribution of species and the fitness of individuals within species’ ranges. Phenotypic plasticity can allow individuals to cope with varying environmental conditions, including rapid climate change. Populations at range edges experience more variable conditions than core populations and thus are hypothesized to exhibit higher thermal plasticity. However, as the strength of plasticity often varies between individuals, it can also differ among local populations at range edges. We studied the extent of and variation in thermal plasticity for several traits within and between populations of the perennial herb Plantago lanceolata L. (Plantaginaceae) at its northern range edge. We sampled seeds from nine sites within a 50 x 50 km region and grew them under three temperature regimes in a greenhouse. We measured traits related to size, flowering, pathogen responses, and inflorescence pigmentation. We expected to find higher plasticity in traits less strongly connected to fitness and that differences between individuals would outweigh differences between populations in underpinning this variation in plasticity. Our results show thermal plasticity in leaf size and abundance, flowering probability and abundance, and pigmentation. Notably, we also found increased pathogen symptoms and higher infection rates of one of two viruses screened, highlighting the potential for changes in pathogen sensitivity and exposure under climate change. Importantly, in all traits but flower abundance, more variation in plasticity was attributable to differences within populations than between populations. Although this contribution was small in magnitude compared to thermal effects on traits, the higher intra- versus interpopulation variation in plasticity suggests that differences between individuals provide most of the variation in thermal plasticity, which may be driven by small-scale variations in habitat conditions; highlighting the need for conservation strategies that consider microhabitat variation to support short-term adaptive responses to thermal variability.

Integration of community ecology with disease biology is viewed as a promising avenue for uncovering determinants of pathogen diversity, and for predicting disease risks. Plant-infecting viruses represent a vastly underestimated component of biodiversity with potentially important ecological and evolutionary roles. We performed hierarchal spatial analysis of wild plant populations to characterise the diversity and coexistence structure of within-host virus communities, and their predictors. Our results show that these virus communities are characterised by single infections of few, dominating virus taxa as well as diverse, non-random coinfections. Using a novel graphical modelling framework we demonstrate that after accounting for environmental heterogeneity at the level of both individual host plants and populations, most virus co-occurrence patterns can be attributed to virus-virus associations. Moreover, we show that conditioning variables changed virus association networks especially through their indirect effects. This highlights a previously underestimated mechanism of how human-driven environmental change can influence disease risks.

The trade-off between within-host infection load and transmission to new hosts is predicted to constrain pathogen evolution, and to maintain polymorphism in pathogen populations. The life-history stages and their correlations that underpin infection development may change under coinfection with other parasites as they compete for the same limited host resources. Cross-kingdom interactions are common among pathogens in both natural and cultivated systems yet their impact on disease ecology and evolution are rarely studied. Host plant Plantago lanceolata is naturally infected by both Phompopsis subordinaria, a seed killing fungus, as well as Plantago lanceolata latent virus (PlLV) in the Åland Islands, SW Finland. We performed an inoculation assay to test whether coinfection with PlLV affects performance of two P. subordinaria strains, and the correlation between within-host infection load and transmission potential. The strains differed in the measured life-history traits and their correlations. Moreover, we found that under virus coinfection, within-host infection load of P. subordinaria was lower but transmission potential was higher compared to strains under single infection. The negative correlation between within-host infection load and transmission potential detected under single infection became positive under coinfection with PlLV. In wild populations, within-host infection load was positively associated with within-population disease prevalence. Jointly, our results suggest that the trade-off between within-host infection load and transmission may be strain specific, and that the pathogen life-history that underpin epidemics may change depending on the diversity of infection, generating variation in disease dynamics.