Does urbanisation lead to parallel demographic shifts across the world in a cosmopolitan plant?

Urbanisation is occurring globally, leading to dramatic environmental changes that are altering the ecology and evolution of species. In particular, the expansion of human infrastructure and the loss and fragmentation of natural habitats in cities is predicted to increase genetic drift and reduce gene flow by reducing the size and connectivity of populations. Alternatively, the 'urban facilitation model' suggests that some species will have greater gene flow into and within cities leading to higher diversity and lower differentiation in urban populations. These alternative hypotheses have not been contrasted across multiple cities. Here, we used the genomic data from the GLobal Urban Evolution project (GLUE), to study the effects of urbanisation on non-adaptive evolutionary processes of white clover (Trifolium repens) at a global scale. We found that white clover populations presented high genetic diversity and no evidence of reduced Ne linked to urbanisation. On the contrary, we found that urban populations were less likely to experience a recent decrease in effective population size than rural ones. In addition, we found little genetic structure among populations both globally and between urban and rural populations, which showed extensive gene flow between habitats. Interestingly, white clover displayed overall higher gene flow within urban areas than within rural habitats. Our study provides the largest comprehensive test of the demographic effects of urbanisation. Our results contrast with the common perception that heavily altered and fragmented urban environments will reduce the effective population size and genetic diversity of populations and contribute to their isolation.

Multivariate phenotypic divergence along an urbanization gradient.

Evidence suggests that natural populations can evolve to better tolerate the novel environmental conditions associated with urban areas. Studies of adaptive divergence in urban areas often examine one or a few traits at a time from populations residing only at the most extreme urban and nonurban habitats. Thus, whether urbanization drives divergence in many traits simultaneously in a manner that varies with the degree of urbanization remains unclear. To address this gap, we generated seed families of white clover (Trifolium repens) collected from 27 populations along an urbanization gradient in Toronto, Canada, grew them in a common garden, and measured 14 phenotypic traits. Families from urban sites had evolved later phenology and germination, larger flowers, thinner stolons, reduced cyanogenesis, greater biomass and greater seed set. Pollinator observations revealed near-complete turnover of pollinator morphological groups along the urbanization gradient, which may explain some of the observed divergences in floral traits and phenology. Our results suggest that adaptation to urban environments involves multiple traits.

Herbivores and plant defences affect selection on plant reproductive traits more strongly than pollinators.

Pollinators and herbivores can both affect the evolutionary diversification of plant reproductive traits. However, plant defences frequently alter antagonistic and mutualistic interactions, and therefore, variation in plant defences may alter patterns of herbivore- and pollinator-mediated selection on plant traits. We tested this hypothesis by conducting a common garden field experiment using 50 clonal genotypes of white clover (Trifolium repens) that varied in a Mendelian-inherited chemical antiherbivore defence-the production of hydrogen cyanide (HCN). To evaluate whether plant defences alter herbivore- and/or pollinator-mediated selection, we factorially crossed chemical defence (25 cyanogenic and 25 acyanogenic genotypes), herbivore damage (herbivore suppression) and pollination (hand pollination). We found that herbivores weakened selection for increased inflorescence production, suggesting that large displays are costly in the presence of herbivores. In addition, herbivores weakened selection on flower size but only among acyanogenic plants, suggesting that plant defences reduce the strength of herbivore-mediated selection. Pollinators did not independently affect selection on any trait, although pollinators weakened selection for later flowering among cyanogenic plants. Overall, cyanogenic plant defences consistently increased the strength of positive directional selection on reproductive traits. Herbivores and pollinators both strengthened and weakened the strength of selection on reproductive traits, although herbivores imposed ~2.7× stronger selection than pollinators across all traits. Contrary to the view that pollinators are the most important agents of selection on reproductive traits, our data show that selection on reproductive traits is driven primarily by variation in herbivory and plant defences in this system.

The evolution of city life.

Urbanization represents a dominant and growing form of disturbance to Earth's natural ecosystems, affecting biodiversity and ecosystem services on a global scale. While decades of research have illuminated the effects of urban environmental change on the structure and function of ecological communities in cities, only recently have researchers begun exploring the effects of urbanization on the evolution of urban populations. The 15 articles in this special feature represent the leading edge of urban evolutionary biology and address existing gaps in our knowledge. These gaps include: (i) the absence of theoretical models examining how multiple evolutionary mechanisms interact to affect evolution in urban environments; (ii) a lack of data on how urbanization affects natural selection and local adaptation; (iii) poor understanding of whether urban areas consistently affect non-adaptive and adaptive evolution in similar ways across multiple cities; (iv) insufficient data on the genetic and especially genomic signatures of urban evolutionary change; and (v) limited understanding of the evolutionary processes underlying the origin of new human commensals. Using theory, observations from natural populations, common gardens, genomic data and cutting-edge population genomic and landscape genetic tools, the papers in this special feature address these gaps and highlight the power of urban evolutionary biology as a globally replicated 'experiment' that provides a powerful approach for understanding how human altered environments affect evolution.

Modern spandrels: the roles of genetic drift, gene flow and natural selection in the evolution of parallel clines.

Urban environments offer the opportunity to study the role of adaptive and non-adaptive evolutionary processes on an unprecedented scale. While the presence of parallel clines in heritable phenotypic traits is often considered strong evidence for the role of natural selection, non-adaptive evolutionary processes can also generate clines, and this may be more likely when traits have a non-additive genetic basis due to epistasis. In this paper, we use spatially explicit simulations modelled according to the cyanogenesis (hydrogen cyanide, HCN) polymorphism in white clover (Trifolium repens) to examine the formation of phenotypic clines along urbanization gradients under varying levels of drift, gene flow and selection. HCN results from an epistatic interaction between two Mendelian-inherited loci. Our results demonstrate that the genetic architecture of this trait makes natural populations susceptible to decreases in HCN frequencies via drift. Gradients in the strength of drift across a landscape resulted in phenotypic clines with lower frequencies of HCN in strongly drifting populations, giving the misleading appearance of deterministic adaptive changes in the phenotype. Studies of heritable phenotypic change in urban populations should generate null models of phenotypic evolution based on the genetic architecture underlying focal traits prior to invoking selection's role in generating adaptive differentiation.

Explaining ecosystem multifunction with evolutionary models.

Ecosystem function is the outcome of species interactions, traits, and niche overlap - all of which are influenced by evolution. However, it is not well understood how the tempo and mode of niche evolution can influence ecosystem function. In evolutionary models where either species differences accumulate through random drift in a single trait or species differences accumulate through divergent selection among close relatives, we should expect that ecosystem function is strongly related to diversity. However, when strong selection causes species to converge on specific niches or when novel traits that directly affect function evolve in some clades but not others, the relationship between diversity and ecosystem function might not be very strong. We test these ideas using a field experiment that established plant mixtures with differing phylogenetic diversities and we measured ten different community functions. We show that some functions were strongly predicted by species richness and mean pairwise phylogenetic distance (MPD, a measure of phylogenetic diversity), including biomass production and the reduction of herbivore and pathogen damage in polyculture, while other functions had weaker (litter production and structural complexity) or nonsignificant relationships (e.g., flower production and arthropod abundance) with MPD and richness. However, these divergent results can be explained by different models of niche evolution. These results show that diversity-ecosystem function relationships are the product of evolution, but that the nature of how evolution influences ecosystem function is complex.

Haplotype-Resolved, Chromosome-Level Assembly of White Clover (Trifolium repens L., Fabaceae).

White clover (Trifolium repens L.; Fabaceae) is an important forage and cover crop in agricultural pastures around the world and is increasingly used in evolutionary ecology and genetics to understand the genetic basis of adaptation. Historically, improvements in white clover breeding practices and assessments of genetic variation in nature have been hampered by a lack of high-quality genomic resources for this species, owing in part to its high heterozygosity and allotetraploid hybrid origin. Here, we use PacBio HiFi and chromosome conformation capture (Omni-C) technologies to generate a chromosome-level, haplotype-resolved genome assembly for white clover totaling 998 Mbp (scaffold N50 = 59.3 Mbp) and 1 Gbp (scaffold N50 = 58.6 Mbp) for haplotypes 1 and 2, respectively, with each haplotype arranged into 16 chromosomes (8 per subgenome). We additionally provide a functionally annotated haploid mapping assembly (968 Mbp, scaffold N50 = 59.9 Mbp), which drastically improves on the existing reference assembly in both contiguity and assembly accuracy. We annotated 78,174 protein-coding genes, resulting in protein BUSCO completeness scores of 99.6% and 99.3% against the embryophyta_odb10 and fabales_odb10 lineage datasets, respectively.

Evolution in response to climate in the native and introduced ranges of a globally distributed plant.

The extent to which species can adapt to spatiotemporal climatic variation in their native and introduced ranges remains unresolved. To address this, we examined how clines in cyanogenesis (hydrogen cyanide [HCN] production-an antiherbivore defense associated with decreased tolerance to freezing) have shifted in response to climatic variation in space and time over a 60-year period in both the native and introduced ranges of Trifolium repens. HCN production is a polymorphic trait controlled by variation at two Mendelian loci (Ac and Li). Using phenotypic assays, we estimated within-population frequencies of HCN production and dominant alleles at both loci (i.e., Ac and Li) from 10,575 plants sampled from 131 populations on five continents, and then compared these frequencies to those from historical data collected in the 1950s. There were no clear relationships between changes in the frequency of HCN production, Ac, or Li and changes in temperature between contemporary and historical samples. We did detect evidence of continued evolution to temperature gradients in the introduced range, whereby the slope of contemporary clines for HCN and Ac in relation to winter temperature became steeper than historical clines and more similar to native clines. These results suggest that cyanogenesis clines show no clear changes through time in response to global warming, but introduced populations continue to adapt to their contemporary environments.

A global horizon scan for urban evolutionary ecology.

Research on the evolutionary ecology of urban areas reveals how human-induced evolutionary changes affect biodiversity and essential ecosystem services. In a rapidly urbanizing world imposing many selective pressures, a time-sensitive goal is to identify the emergent issues and research priorities that affect the ecology and evolution of species within cities. Here, we report the results of a horizon scan of research questions in urban evolutionary ecology submitted by 100 interdisciplinary scholars. We identified 30 top questions organized into six themes that highlight priorities for future research. These research questions will require methodological advances and interdisciplinary collaborations, with continued revision as the field of urban evolutionary ecology expands with the rapid growth of cities.

Global urban environmental change drives adaptation in white clover.

Urbanization transforms environments in ways that alter biological evolution. We examined whether urban environmental change drives parallel evolution by sampling 110,019 white clover plants from 6169 populations in 160 cities globally. Plants were assayed for a Mendelian antiherbivore defense that also affects tolerance to abiotic stressors. Urban-rural gradients were associated with the evolution of clines in defense in 47% of cities throughout the world. Variation in the strength of clines was explained by environmental changes in drought stress and vegetation cover that varied among cities. Sequencing 2074 genomes from 26 cities revealed that the evolution of urban-rural clines was best explained by adaptive evolution, but the degree of parallel adaptation varied among cities. Our results demonstrate that urbanization leads to adaptation at a global scale.

Predicting the strength of urban-rural clines in a Mendelian polymorphism along a latitudinal gradient.

Cities are emerging as models for addressing the fundamental question of whether populations evolve in parallel to similar environments. Here, we examine the environmental factors that drive the evolution of parallel urban-rural clines in a Mendelian trait-the cyanogenic antiherbivore defense of white clover (Trifolium repens). Previous work suggested urban-rural gradients in frost and snow depth could drive the evolution of reduced hydrogen cyanide (HCN) frequencies in urban populations. Here, we sampled over 700 urban and rural clover populations across 16 cities along a latitudinal transect in eastern North America. In each population, we quantified changes in the frequency of genotypes that produce HCN, and in a subset of the cities we estimated the frequency of the alleles at the two genes (CYP79D15 and Li) that epistatically interact to produce HCN. We then tested the hypothesis that cold climatic conditions are necessary for the evolution of cyanogenesis clines by comparing the strength of clines among cities located along a latitudinal gradient of winter temperature and frost exposure. Overall, half of the cities exhibited urban-rural clines in the frequency of HCN, whereby urban populations evolved lower HCN frequencies. Clines did not evolve in cities with the lowest temperatures and greatest snowfall, supporting the hypothesis that snow buffers plants against winter frost and constrains the formation of clines. By contrast, the strongest clines occurred in the warmest cities where snow and frost are rare, suggesting that alternative selective agents are maintaining clines in warmer cities. Some clines were driven by evolution at only CYP79D15, consistent with stronger and more consistent selection on this locus than on Li. Together, our results demonstrate that urban environments often select for similar phenotypes, but different selective agents and targets underlie the evolutionary response in different cities.

A roadmap for urban evolutionary ecology.

Urban ecosystems are rapidly expanding throughout the world, but how urban growth affects the evolutionary ecology of species living in urban areas remains largely unknown. Urban ecology has advanced our understanding of how the development of cities and towns change environmental conditions and alter ecological processes and patterns. However, despite decades of research in urban ecology, the extent to which urbanization influences evolutionary and eco-evolutionary change has received little attention. The nascent field of urban evolutionary ecology seeks to understand how urbanization affects the evolution of populations, and how those evolutionary changes in turn influence the ecological dynamics of populations, communities, and ecosystems. Following a brief history of this emerging field, this Perspective article provides a research agenda and roadmap for future research aimed at advancing our understanding of the interplay between ecology and evolution of urban-dwelling organisms. We identify six key questions that, if addressed, would significantly increase our understanding of how urbanization influences evolutionary processes. These questions consider how urbanization affects nonadaptive evolution, natural selection, and convergent evolution, in addition to the role of urban environmental heterogeneity on species evolution, and the roles of phenotypic plasticity versus adaptation on species' abundance in cities. Our final question examines the impact of urbanization on evolutionary diversification. For each of these six questions, we suggest avenues for future research that will help advance the field of urban evolutionary ecology. Lastly, we highlight the importance of integrating urban evolutionary ecology into urban planning, conservation practice, pest management, and public engagement.

Tibiotalocalcaneal arthrodesis: a biomechanical comparision of multiplanar external fixation with intramedullary fixation.

BACKGROUND: Methods of achieving tibiotalocalcaneal arthrodesis include intramedullary nailing, crossed lag screws, blade plates, and external fixation. While reports in the orthopaedic literature have compared the biomechanical properties of some of these fixation techniques, to our knowledge none has evaluated multiplanar external fixation. The purpose of this study was to compare the biomechanical properties of intramedullary nail fixation and external ring fixation for tibiotalocalcaneal arthrodesis. MATERIALS AND METHODS: Tibiotalocalcaneal arthrodesis was performed on ten matched pairs of fresh-frozen human cadaveric legs. A ring fixator stabilized the arthrodesis in one leg from each pair and a 10 mm x 150 mm nail inserted retrograde across the subtalar and ankle joint stabilized the arthrodesis in the contralateral leg. The bending stiffness of the resulting constructs was quantified in plantarflexion, dorsiflexion, inversion, and eversion, and torsional stiffness was measured in internal and external rotation. RESULTS: No difference in bending stiffness between the two constructs was identifiable in any of the four bending directions (p > 0.05). Torsional stiffness was approximately two-fold greater in both internal and external rotation in specimens with the ring fixator arthrodesis than in those with the intramedullary nail (p = 0.002). CONCLUSION: The ring fixator provides a stiffer construct than a 10 mm x 150 mm intramedullary nail in torsion, but no difference in bending stiffness was demonstrable. Both techniques can provide satisfactory fixation; however, the ring fixator may better minimize rotational joint motion. CLINICAL RELEVANCE: This study provides a basis for selecting an arthrodesis method that offers optimized fixation.

Subtalar distraction arthrodesis using interpositional frozen structural allograft.

BACKGROUND: Subtalar bone-block distraction arthrodesis using structural autograft carries the risk of donor site morbidity. Recent reports suggest that structural allograft may be an attractive alternative to structural autograft in subtalar arthrodesis. This prospective study analyzes subtalar distraction arthrodesis using interpositional structural allograft. MATERIALS AND METHODS: Between 2000 and 2006, 22 patients (24 feet; mean age, 45.6 years) underwent subtalar arthrodesis with interpositional fresh-frozen femoral head structural allograft. Indications included subtalar arthrosis, loss of heel height, and anterior ankle impingement. Clinical outcome was assessed using the AOFAS ankle-hindfoot scoring system. Time to union was determined by previously reported clinical findings and radiographic evidence for bridging trabeculation between host bone and structural allograft. RESULTS: Mean followup was 35.8 months for 20 patients (21 feet) available for followup evaluation. Union was achieved in 19 of 21 patients (90%) at a mean of 15.5 (range, 11 to 19) weeks. Mean AOFAS hindfoot score improved from 21 to 71 points (p < 0.05). Radiographic analysis suggested significant (p < 0.05) improvement in all measurements. Complications included nonunion (2), varus malalignment (1), persistent subfibular impingement (1), sural neuralgia (1), and prominent hardware (2). Both patients with nonunions had avascular bone at the arthrodesis site and used tobacco products. CONCLUSION: This study supports recent publications that subtalar arthrodesis using interpositional structural allograft can have a favorable outcome. Our clinical and radiographic results suggest that restoration of hindfoot function and dimensions with structural allograft are comparable to results reported for the same procedure using structural autograft. LEVEL OF EVIDENCE: Level IV, prospective case series.

A quantitative comparison of surgical approaches for posterolateral osteochondral lesions of the talus.

BACKGROUND: Exposure of the posterolateral talar dome for osteochondral autograft transfer can be challenging. The purpose of this study is to compare surgical exposures for perpendicular access to the posterolateral talar dome using osteochondral transfer instrumentation. MATERIALS AND METHODS: Five surgical approaches were performed on each of eight cadaveric ankles. The sequence was: (1) Anterolateral arthrotomy with ATFL release, (2) Anterolateral tibial osteotomy, (3) Fibular osteotomy with ATFL intact, (4) Fibular osteotomy with ATFL release, and (5) Fibular osteotomy with ATFL/CFL release. (ATFL repaired between 1 and 2). While maintaining the orientation of the harvester perpendicular to the talar dome articular surface, osteochondral plugs were harvested as far posteriorly as possible using a 6-mm harvester. Distances from the anterior talar articular surface to the posterior aspect of the recipient site were measured. Statistical analysis used ANOVA and Fisher post hoc tests. RESULTS: Average AP exposure (mm) and percentage of AP talar dome dimensions exposed: (1) Anterolateral arthrotomy with ATFL release: 21.2 mm (43.3%), (2) Anterolateral tibial osteotomy: 33.7 mm (68.5%), (3) Fibular osteotomy(ATFL intact): 43.2 mm (87.8%), (4) Fibular osteotomy with ATFL release: 44.9 mm (91.2%), and (5) Fibular osteotomy with ATFL/CFL release: 46.6 mm (94.6%). All osteotomies provided greater exposure than anterolateral arthrotomy with ATFL release (p < 0.0001). A significant difference was obtained between each of the fibular osteotomies and tibial osteotomy (p < 0.0001). Differences between the fibular osteotomy approaches (3 to 5) were not significant. CONCLUSION: Fibular osteotomy provides the greatest perpendicular exposure to the posterolateral talar dome. Anterolateral tibial osteotomy provides greater exposure than arthrotomy alone. CLINICAL RELEVANCE: This study provides a guide for surgical exposures to the posterolateral talar dome for osteochondral autograft transfer.

MRI appearance of surgically proven abnormal accessory anterior-inferior tibiofibular ligament (Bassett's ligament).

OBJECTIVE: A thickened accessory anterior-inferior tibiofibular ligament (Bassett's ligament) of the ankle can be a cause of ankle impingement. Its imaging appearance is not well described. The purpose of this study was to determine if the ligament could be identified on magnetic resonance imaging (MRI), to determine associated abnormalities, and to determine if MRI could be used to differentiate normal from abnormal. MATERIALS AND METHODS: Eighteen patients with a preoperative ankle MRI and an abnormal Bassett's ligament reported at surgery were found retrospectively. A separate cohort of 18 patients was selected as a control population. The presence of Bassett's ligament and its thickness were noted. The integrity and appearance of the lateral ankle ligaments, talar dome cartilage, and anterolateral gutter were also noted. RESULTS: In 34 of the 36 cases (94%), Bassett's ligament was identified on MRI. The ligament was seen in all three imaging planes and most frequently in the axial plane. The mean thickness of the ligament in the surgically abnormal cases was 2.37 mm, compared with 1.87 mm in the control with a p value=0.015 (t test). Nine of the 18 abnormal cases (50%) had talar dome cartilage lesions as a result of contact with the ligament at surgery, with only 3 cases of high-grade defects seen on MRI. Fourteen of the 18 abnormal cases (78%) had of synovitis or scarring in the lateral gutter at surgery, with only 5 cases with scarring seen on MRI. The anterior-inferior tibiofibular ligament was abnormal or torn in 8 of the 18 abnormal cases (44%) by MRI and confirmed in only 3 cases at surgery. DISCUSSION: Bassett's ligament can be routinely identified on MRI and was significantly thicker in patients who had it resected at surgery. An abnormal Bassett's ligament is often present in the setting of a normal anterior-inferior tibiofibular ligament. The cartilage abnormalities and synovitis associated with an abnormal Bassett's ligament are poorly detected by conventional MRI.