Species with large distributions that span significant environmental gradients have evolved complex mechanisms to adapt to different environments. This adaptation can be achieved both through the direct selection on traits that convey higher fitness under local environmental conditions, or through the evolution of phenotypic plasticity. Under a common-garden experimental setup, we tested for the keystone African tree species Acacia tortilis how the interaction between direct selection on traits and evolution of plasticity has led to its widespread distribution to two contrasting environments: deserts and tropical savannas. In addition, we tested the predictability of evolution by comparing populations in opposing deserts of its distribution, the Sahara and Kalahari deserts. We observed significant differences in how various phenotypes responded to the harsh desert environment compared with the more moderate tropical-savanna environment, with a stronger among-population differentiation expressed under desert conditions. We found an isolation-by-environment signal mainly for phenotypic plasticity, with a strong association to precipitation gradients, suggesting that differentiation across the tropic-desert gradient occurs in this species primarily through selection on trait plasticity rather than trait means. Interestingly, the two desert-edge populations showed similar plastic responses, despite the large geographical distances between them, suggesting convergence in plasticity. Synthesis. Taken together, our results suggest that phenotypic plasticity plays an important role in differentiation across the tropical-savanna to desert gradient in Acacia tortilis, with similar phenotypic responses observed between climatically comparable populations at opposing range edges.

The distribution of ecological and evolutionary forces throughout space bring about the patterning of biodiversity. In large geographical areas, this causes the regionalization of biodiversity into structured units known as bioregions. In order to understand how such patterns emerge, a clear delineation of bioregions is required. We use tree species as model taxa in order to analyze the global distribution of biodiversity and understand how latitudinal gradients of biodiversity, specifically the latitudinal phylogenetic and diveristy gradients are formed. By compiling an extensive dataset of tree species distributions and their phylogenetic relationships, we use a data-driven approach to delineate global bioregions of similar evolutionary histories, termed phyloregions. Our analysis reveals the presence of a region between the tropical and temperate regions, coined 'bridge' phyloregion, with a unique evolutionary composition and characteristically weaker association to climatic and environmental parameters. Through simulations, we show that the presence of latitudinal phylogenetic and diversity gradients are much more likely to emerge in the presence of an independent ecological region between tropical and temperate regions, suggesting that its role as a stepping-stone in colonization of species between distinct climatic zones has shaped latitudinal gradients. This study highlights that accurate delineation of evolutionary structures of biodiversity can reveal previously cryptic regions with fundamental evolutionary roles in the formation of biodiveristy patterns. ### Competing Interest Statement The authors have declared no competing interest.

The presence and distribution of mycorrhizal symbionts can influence plant distribution through specific host-mycorrhiza symbiosis interactions. However, generalist hosts also exist, such as dual-mycorrhizal plants that form symbiotic associations with both ectomycorrhizal fungi (EM) and arbuscular mycorrhizal fungi (AM). Little is known about the effect of dual mycorrhization status on the hosts' global distribution and acclimation to specific environments. This study investigates the potential advantage of dual associations of more than 400 woody genera spread at a global scale. We found that dual-host woody species occupy a broader geographical range and environmental niche space compared to those associating exclusively with either AM or EM. We show that the increased geographic range and expanded environmental niche space are independent of the phylogenetic architecture and evolutionary history of the woody genera. Our results highlight the advantage of generalist host-microbe symbioses between woody species and fungi to expand their range, and their potential role in colonising dry climates.

Phenotypic plasticity enables rapid responses to environmental change, and could facilitate range shifts in response to climate change. What drives the evolution of plasticity at range edges, and the capacity of range-edge individuals to be plastic, remain unclear. Here, we propose that accurately predicting when plasticity itself evolves or mediates adaptive evolution at expanding range edges requires integrating knowledge on the demography and evolution of edge populations. Our synthesis shows that: (i) the demography of edge populations can amplify or attenuate responses to selection for plasticity through diverse pathways, and (ii) demographic effects on plasticity are modified by the stability of range edges. Our spatially explicit synthesis for plasticity has the potential to improve predictions for range shifts with climate change.

Understanding the causes of the arrest of species distributions has been a fundamental question in ecology and evolution. These questions are of particular interest for trees owing to their long lifespan and sessile nature. A surge in data availability evokes a macro-ecological analysis to determine the underlying forces limiting distributions. Here we analyse the spatial distribution of >3,600 major tree species to determine geographical areas of range-edge hotspots and find drivers for their arrest. We confirmed biome edges to be strong delineators of distributions. Importantly, we identified a stronger contribution of temperate than tropical biomes to range edges, adding strength to the notion that tropical areas are centres of radiation. We subsequently identified a strong association of range-edge hotspots with steep spatial climatic gradients. We linked spatial and temporal homogeneity and high potential evapotranspiration in the tropics as the strongest predictors of this phenomenon. We propose that the poleward migration of species in light of climate change might be hindered because of steep climatic gradients.

PREMISE:Tree growth is a fundamental biological process that is essential to ecosystem functioning and water and element cycling. Climate exerts a major impact on tree growth, with tree species often requiring a unique set of conditions to initiate and maintain growth throughout the growing season. Still, little is known about the specific climatic factors that enable tree growth in savannah and desert tree species. Among the global tree species, Acacia tortilis occupies one of the largest distribution ranges (crossing 6500 km and 54 latitudes), spanning large parts of Africa and into the Middle East and Asia.METHODS:Here we collected climate data and monitored Acacia tortilis tree growth (continuous measurements of stem circumference) in its southern and northern range edges in South Africa (SA) and Israel (IL), respectively, to elucidate whether the growth-climate interactions were similar in both edges.RESULTS:Growth occurred during the summer (between December and March) in SA and in IL during early summer and autumn (April-June and October-November, respectively). Surprisingly, annual growth was 40% higher in IL than in SA. Within the wide distribution range of Acacia tortilis, our statistical model showed that climatic drivers of tree growth differed between the two sites.CONCLUSIONS:High temperatures facilitated growth at the hot and arid IL site, while high humidity permitted growth at the more humid SA site. Our results confer an additional understanding of tree growth adaptation to extreme conditions in Acacia's world range edges, a major point of interest with ongoing climate change.

Abstract Understanding the causes of the arrest of species distributions has been a fundamental question in ecology and evolution. For trees, these questions are of particular interest due to their long lifespan and sessile nature. A surge in data-availability evokes for a macro-ecological analysis to determine the underlying forces limiting distributions. Here we analyze the spatial distribution of 6,000 major tree species to determine geographical areas of range-edge hotspots and find drivers for their arrest. We confirmed biome edges to be strong delineators of distributions. Importantly, we identified a stronger contribution of temperate than tropical biomes to range edges, adding strength to the notion that tropical areas are centers of radiation. We subsequently identified a strong association of range-edge hotspots with low levels of precipitation and steep climatic gradients. We linked temperature homogeneity and high precipitation in the tropics as the strongest predictors for this phenomenon. We propose that the poleward migration of species in light of climate change might be hindered due to high climatic heterogeneity and increasing drought events.

Mutation rate balances the need to protect genome integrity with the advantage of evolutionary innovations. Microorganisms increase their mutation rate when stressed, perhaps addressing the growing need for evolutionary innovation. Such a strategy, however, is only beneficial under moderate stresses that allow cells to divide and realize their mutagenic potential. In contrast, severe stresses rapidly kill the majority of the population with the exception of a small minority of cells that are in a phenotypically distinct state termed persistence. Although persisters were discovered many decades ago, the stochastic event triggering persistence is poorly understood. We report that spontaneous DNA damage triggers persistence in Saccharomyces cerevisiae by activating the general stress response, providing protection against a range of harsh stress and drug environments. We further show that the persister subpopulation carries an increased load of genetic variants in the form of insertions, deletions or large structural variations, which are unrelated to their stress survival. This coupling of DNA damage to phenotypic persistence may increase genetic diversity specifically in severe stress conditions, where diversity is beneficial but the ability to generate de novo mutations is limited.