Jon Ågren

Microbiome homeostasis is crucial for host health and ecosystem function, yet the molecular and ecological mechanisms underlying community assembly and stability remain elusive. Here we uncover a conserved yeast–oomycete association that promotes their coexistence in the leaf microbiome. Using a continental-scale microbiome survey, we identified an asymmetric mutualistic interaction between two eukaryotic hub microbes: the yeast Dioszegia hungarica and the obligate oomycete Albugo laibachii . We show that Dioszegia facilitates Albugo colonization by supplying thiamine via a dedicated membrane permease, alleviating Albugo ’s auxotrophy. Genomic and transcriptomic analyses indicate that natural selection has acted on thiamine production in Dioszegia , shaping this metabolic complementation. In planta assays further suggest that Albugo presence supports Dioszegia persistence under glasshouse conditions. Our study illustrates how the evolution of nutrient cross-feeding mediates microbial coexistence and microbiome stability. Targeting microbial nutrient flows offers new strategies for engineering microbiomes and enhancing plant resilience in natural and agricultural systems.

Flower colour is recognized as a key trait influencing pollinator visitation and behaviour, evolved to match the sensory system of a particular pollinator group. However, some flowers combine colours associated with different pollinators, suggesting bimodal adaptation. The perennial herb Cyrtanthus obliquus produces red corollas with yellow tips and is pollinated by birds and bees. To assess the role of the two pollinator categories in shaping selection on the bicoloured corollas, we examined bee choices in arrays where the colour signals of plants had been manipulated, and we quantified total as well as bird-mediated phenotypic selection on flower colour in an unmanipulated natural population, using bird and bee visual models. Bees preferred flowers with a yellow signal over all-red flowers. The analysis of phenotypic selection indicated that both birds and bees influence net selection on flower colour, resulting in conflicting selection on the colour contrast between corolla and corolla tip. Our findings show that the optimal phenotype should depend on the relative importance of the two categories of pollinators in a given population and are consistent with bimodal adaptation. This study fills an important gap by showing that secondary pollinators can contribute to selection on complex colour signals in angiosperm flowers.

Effects of climate change on phenological timing, like flowering onset, are crucial for population fitness and community dynamics. Recent research has focused on plastic responses to earlier springs, but the optimal phenological timing should depend also on the growing season duration, within which entire annual life cycles must unfold. Optimal energy allocation theory can address life‐history scheduling when this critical time window expands. Extending Iwasa and Cohen's (1989) framework, we predict a nonlinear relationship between growing season length and optimal flowering time of deciduous perennial plants measured from spring onset. Common‐garden experiments with purple loosestrife ( Lythrum salicaria ) and European goldenrod ( Solidago virgaurea ) along Swedish latitudinal gradients strongly supported this a priori prediction. As climate change alters both start and duration of growing seasons, our finding suggests that optimal flowering time expressed as calendar day could stall before accelerating its advancement in response to climate warming at current high‐latitude range margins.

Microbial homeostasis is crucial for host health and ecosystem function, yet the molecular and ecological mechanisms underlying community assembly and stability remain elusive. Here, we uncover a conserved yeast-oomycete metabolic mutualism that promotes their coexistence in the leaf microbiome. Using a continental-scale microbiome survey, we identified a mutualistic interaction between two eukaryotic hub microbes: the yeast Dioszegia hungarica and the obligate oomycete Albugo laibachii. We show that Dioszegia facilitates Albugo colonization by supplying thiamine via a dedicated membrane permease, alleviating Albugo’s auxotrophy. Genomic and transcriptomic analyses reveal that natural selection has acted on thiamine production in D. hungarica, shaping this mutualistic interaction. In planta assays further demonstrate that cross-feeding enhances Albugo colonization and promotes Dioszegia persistence. Our study illustrates how the evolution of nutrient cross-feeding mediates microbial coexistence and microbiome stability. Targeting microbial nutrient flows offers new strategies for engineering microbiomes and enhancing plant resilience in natural and agricultural systems.

Herbivores can affect plant population growth both directly through the damage they inflict to a focal species, and indirectly by moderating conditions for plant recruitment, nutrient cycling, competition and other biotic interactions, for example, through trampling, defecation, and grazing on surrounding vegetation. Still, the relative importance of direct versus indirect effects of herbivores on plant vital rates (establishment, survival, fecundity), size, and population growth rate is poorly known. We quantified direct and indirect effects of ungulate grazers on population growth rate of the short‐lived perennial herb Primula farinosa, using integral projection models based on demographic data collected over seven years in exclosures and open control plots in nine grassland populations in southern Sweden. Grazers had negative direct effects on P. farinosa population growth rate, but these were on average more than balanced by positive indirect effects. As predicted, the strength of the direct negative effect tended to increase with grazing intensity. The positive indirect effect was mainly linked to improved conditions for plant recruitment, and was strongest in populations where vegetation height differed most between exclosure and control. Simulations indicated that indirect effects of ungulate grazers on population growth rate via interactions with pollinators, seed predators, and small herbivores were weak. Our study illustrates how both the overall, direct, and indirect effects of grazing on plant population growth rate can be identified and quantified, and thereby provide a more complete understanding of how grazers influence plant fitness, abundance, and distribution. Such insight will be crucial for predictions of the effects of environmental change on population viability and the management of declining species.

Global patterns of population genetic variation through time offer a window into evolutionary processes that maintain diversity. Over time, lineages may expand or contract their distribution, causing turnover in population genetic composition. At individual loci, migration, drift and selection (among other processes) may affect allele frequencies. Museum specimens of widely distributed species offer a unique window into the genetics of understudied populations and changes over time. Here, we sequenced genomes of 130 herbarium specimens and 91 new field collections of Arabidopsis thaliana and combined these with published genomes. We sought a broader view of genomic diversity across the species and to test if population genomic composition is changing through time. We documented extensive and previously uncharacterised diversity in a range of populations in Africa, populations that are under threat from anthropogenic climate change. Through time, we did not find dramatic changes in genomic composition of populations. Instead, we found a pattern of genetic change every 100 years of the same magnitude seen when comparing Eurasian populations that are 185 km apart, potentially due to a combination of drift and changing selection. We found only mixed signals of polygenic adaptation at phenology and physiology QTL. We did find that genes conserved across eudicots show altered levels of directional allele frequency change, potentially due to variable purifying and background selection. Our study highlights how museum specimens can reveal new dimensions of population diversity and show how wild populations are evolving in recent history.

Flower colour is recognized as a key trait influencing pollinator visitation and behaviour, evolved to match the sensory system of a particular pollinator group. However, some flowers combine colours associated with different pollinators, suggesting bimodal adaptation. The perennial herb Cyrtanthus obliquus produces red corollas with yellow tips and is pollinated by birds, but also by bees. To assess the role of the two pollinator categories in shaping selection on the bi-coloured corollas, we examined bee choices in arrays in which the colour signals of plants had been manipulated, and we quantified total as well as bird-mediated phenotypic selection on flower colour using both bird and bee visual models. Bees preferred flowers with a yellow signal over all-red flowers. The analysis of phenotypic selection indicated that both birds and bees influence net selection on flower colour, resulting in conflicting selection on the colour contrast between corolla and corolla tip. Our findings show that the optimal phenotype should depend on the relative importance of the two categories of pollinators in a given population, and are consistent with bimodal adaptation. This study fills an important gap by showing that secondary pollinators can contribute to selection on complex colour signals in angiosperm flowers.

Understanding the effects of climate change on the evolution of phenological timing, such as start of flowering, is of major interest because phenology is critical for fitness of populations, and underpins many ecological dynamics. Much recent research has focused on the correlation between phenological timing and the arrival of spring. However, the evolutionarily optimal seasonal timing should depend also on the duration of the growing season, within which entire annual life cycles must unfold. Optimal energy allocation theory can explicitly address life-history scheduling in a seasonal environment, and be used to predict how scheduling should adaptively respond to seasonality shifts. Here we extend a seminal theoretical framework for perennial plant scheduling by Iwasa & Cohen (1989), and predict a specific nonlinear relationship between growing season length and optimal flowering time expressed as number of days after the start of the growing season. We tested and found strong support for this a priori prediction in two independent common-garden experiments with purple loosestrife ( Lythrum salicaria ) and European goldenrod ( Solidago virgaurea ) populations sampled along latitudinal gradients in Sweden. Climate warming is commonly associated with changes in both the start and the duration of the growing season. Considering both effects, our findings suggest that as springs start earlier and growing seasons lengthen, shifts in optimal flowering time expressed as calendar date may initially stall before accelerating, potentially explaining observed variation in phenological shifts across systems. More broadly, we show how mechanistic life history theory can advance understanding of phenological change beyond correlative conclusions. SIGNIFICANCE STATEMENT: The optimal timing for a plant to flower depends both on when spring begins and on the total length of the growing season—both shifting with climate change. However, the adaptation of flowering time to changes in growing season length is much less explored than to an advancing spring. Building on existing theory, we predict a nonlinear relationship between growing season length and optimal flowering time. Data on two species (purple loosestrife and European goldenrod) grown in common gardens strongly support this prediction. Our results demonstrate why species can vary in their observed phenological responses to climate change. More broadly, we highlight the power of mechanistic life history theory for explaining and predicting phenological shifts.

Many Lepidoptera species are declining to the point that active measures such as reinforcements and reintroductions are necessary to maintain viable populations. To maximise success of such efforts, we need to understand the factors influencing habitat preferences for oviposition. In a Swedish island population of the red‐listed chequered blue butterfly (Scolitantides orion), we characterised oviposition preference by documenting whether the presence and abundance of eggs (i) increased with size and local density of its host plant, orpine (Hylotelephium telephium), and (ii) was associated with nectar plant abundance, degree of shading, and ground cover composition within small and large circular plots (1 m vs. 5 m radius) around focal plants. To reinforce a tiny population on a neighbouring island, we released 127 pupae and 46 imagos and recorded oviposition for two seasons. The probability of receiving eggs increased with focal host plant size, and also with mean size of other host plants and cover of lichens within the large plots. The abundance of eggs within large plots increased with host plant density and amount of lichen cover. Habitat characteristics of the large plots predicted oviposition better than did those of small plots, and both outperformed models including host plant size only. In the reinforcement population, all released pupae eclosed, and we found a total of 507 eggs on 272 host plants during the release season. Eggs were present on 5% of examined host plants, and median distance to a release spot was 19.3 m. In the following year, we found 420 eggs on 209 plants, and median distance to a release spot increased to 35.7 m, indicating diffusion through the habitat. Practical implication. The chequered blue butterfly prefers to oviposit on large host plants in patches with high host density and high cover of lichens. To identify suitable release habitats, environmental conditions should be considered in an area of ca 80 m². The reinforcement shows that small releases can be successful and suggests that similar low‐effort approaches can be effective to boost local populations of many threatened butterflies, provided suitable habitats are available.