Asynchronous Life Histories Generate Uneven Arms Races and Impact the Maintenance of Mutualisms.

AbstractMutualisms constitute a diverse class of ecologically important interactions, yet their ecological and evolutionary stability remain topics of debate in coevolutionary theory. Recent theoretical and empirical work has suggested that coevolutionary arms races may be involved in the maintenance of mutualistic interactions, sustaining mutually beneficial outcomes for interacting species while producing exaggerated traits. Here we present an individual-based model that evaluates how asynchronous life histories-that is, partners with different average lifespans-change the dynamics of trait coevolution, the expected fitness outcomes for species involved, and the dynamics of selection differentials across time for each species. Results indicate that a longer-lived mutualist will consistently "lose" an otherwise balanced coevolutionary arms race, being outpaced in both the mean trait value and fitness outcome compared with a shorter-lived partner. Furthermore, linear selection differentials on mutualistic traits become increasingly divergent as life histories become increasingly asynchronous, with the longer-lived species experiencing persistent directional selection and the shorter-lived species experiencing weaker, more inconsistent selection. These results suggest that asynchronous life histories can complicate the maintenance of mutualistic interactions via coevolutionary arms races and that detecting coevolution via selection differentials may be difficult when life histories are sufficiently divergent.

Mechanical memory stored through epigenetic remodeling reduces cell therapeutic potential.

Understanding how cells remember previous mechanical environments to influence their fate, or mechanical memory, informs the design of biomaterials and therapies in medicine. Current regeneration therapies, such as cartilage regeneration procedures, require 2D cell expansion processes to achieve large cell populations critical for the repair of damaged tissues. However, the limit of mechanical priming for cartilage regeneration procedures before inducing long-term mechanical memory following expansion processes is unknown, and mechanisms defining how physical environments influence the therapeutic potential of cells remain poorly understood. Here, we identify a threshold to mechanical priming separating reversible and irreversible effects of mechanical memory. After 16 population doublings in 2D culture, expression levels of tissue-identifying genes in primary cartilage cells (chondrocytes) are not recovered when transferred to 3D hydrogels, while expression levels of these genes were recovered for cells only expanded for eight population doublings. Additionally, we show that the loss and recovery of the chondrocyte phenotype correlates with a change in chromatin architecture, as shown by structural remodeling of the trimethylation of H3K9. Efforts to disrupt the chromatin architecture by suppressing or increasing levels of H3K9me3 reveal that only with increased levels of H3K9me3 did the chromatin architecture of the native chondrocyte phenotype partially return, along with increased levels of chondrogenic gene expression. These results further support the connection between the chondrocyte phenotype and chromatin architecture, and also reveal the therapeutic potential of inhibitors of epigenetic modifiers as disruptors of mechanical memory when large numbers of phenotypically suitable cells are required for regeneration procedures.

Spatial Gradients of Quantitative MRI as Biomarkers for Early Detection of Osteoarthritis: Data From Human Explants and the Osteoarthritis Initiative.

BACKGROUND: Healthy articular cartilage presents structural gradients defined by distinct zonal patterns through the thickness, which may be disrupted in the pathogenesis of several disorders. Analysis of textural patterns using quantitative MRI data may identify structural gradients of healthy or degenerating tissue that correlate with early osteoarthritis (OA). PURPOSE: To quantify spatial gradients and patterns in MRI data, and to probe new candidate biomarkers for early severity of OA. STUDY TYPE: Retrospective study. SUBJECTS: Fourteen volunteers receiving total knee replacement surgery (eight males/two females/four unknown, average age ± standard deviation: 68.1 ± 9.6 years) and 10 patients from the OA Initiative (OAI) with radiographic OA onset (two males/eight females, average age ± standard deviation: 57.7 ± 9.4 years; initial Kellgren-Lawrence [KL] grade: 0; final KL grade: 3 over the 10-year study). FIELD STRENGTH/SEQUENCE: 3.0-T and 14.1-T, biomechanics-based displacement-encoded imaging, fast spin echo, multi-slice multi-echo T2 mapping. ASSESSMENT: We studied structure and strain in cartilage explants from volunteers receiving total knee replacement, or structure in cartilage of OAI patients with progressive OA. We calculated spatial gradients of quantitative MRI measures (eg, T2) normal to the cartilage surface to enhance zonal variations. We compared gradient values against histologically OA severity, conventional relaxometry, and/or KL grades. STATISTICAL TESTS: Multiparametric linear regression for evaluation of the relationship between residuals of the mixed effects models and histologically determined OA severity scoring, with a significance threshold at α = 0.05. RESULTS: Gradients of individual relaxometry and biomechanics measures significantly correlated with OA severity, outperforming conventional relaxometry and strain metrics. In human explants, analysis of spatial gradients provided the strongest relationship to OA severity (R2  = 0.627). Spatial gradients of T2 from OAI data identified variations in radiographic (KL Grade 2) OA severity in single subjects, while conventional T2 alone did not. DATA CONCLUSION: Spatial gradients of quantitative MRI data may improve the predictive power of noninvasive imaging for early-stage degeneration. EVIDENCE LEVEL: 1 TECHNICAL EFFICACY: Stage 1.

Dispersal evolution in temporally variable environments: implications for plant range dynamics.

Dispersal-the movement of an individual from the site of birth to a different site for reproduction-is an ecological and evolutionary driver of species ranges that shapes patterns of colonization, connectivity, gene flow, and adaptation. In plants, the traits that influence dispersal often vary within and among species, are heritable, and evolve in response to the fitness consequences of moving through heterogeneous landscapes. Spatial and temporal variation in the quality and quantity of habitat are important sources of selection on dispersal strategies across species ranges. While recent reviews have evaluated the interactions between spatial variation in habitat and dispersal dynamics, the extent to which geographic variation in temporal variability can also shape range-wide patterns in dispersal traits has not been synthesized. In this paper, we summarize key predictions from metapopulation models that evaluate how dispersal evolves in response to spatial and temporal habitat variability. Next, we compile empirical data that quantify temporal variability in plant demography and patterns of dispersal trait variation across species ranges to evaluate the hypothesis that higher temporal variability favors increased dispersal at plant range limits. We found some suggestive evidence supporting this hypothesis while more generally identifying a major gap in empirical work evaluating plant metapopulation dynamics across species ranges and geographic variation in dispersal traits. To address this gap, we propose several future research directions that would advance our understanding of the interplay between spatiotemporal variability and dispersal trait variation in shaping the dynamics of current and future species ranges.

Range edges in heterogeneous landscapes: Integrating geographic scale and climate complexity into range dynamics.

The impacts of climate change have re-energized interest in understanding the role of climate in setting species geographic range edges. Despite the strong focus on species' distributions in ecology and evolution, defining a species range edge is theoretically and empirically difficult. The challenge of determining a range edge and its relationship to climate is in part driven by the nested nature of geography and the multidimensionality of climate, which together generate complex patterns of both climate and biotic distributions across landscapes. Because range-limiting processes occur in both geographic and climate space, the relationship between these two spaces plays a critical role in setting range limits. With both conceptual and empirical support, we argue that three factors-climate heterogeneity, collinearity among climate variables, and spatial scale-interact to shape the spatial structure of range edges along climate gradients, and we discuss several ways that these factors influence the stability of species range edges with a changing climate. We demonstrate that geographic and climate edges are often not concordant across species ranges. Furthermore, high climate heterogeneity and low climate collinearity across landscapes increase the spectrum of possible relationships between geographic and climatic space, suggesting that geographic range edges and climatic niche limits correspond less frequently than we may expect. More empirical explorations of how the complexity of real landscapes shapes the ecological and evolutionary processes that determine species range edges will advance the development of range limit theory and its applications to biodiversity conservation in the context of changing climate.

The adaptive potential of plant populations in response to extreme climate events.

The frequency and magnitude of extreme climate events are increasing with global change, yet we lack predictions and empirical evidence for the ability of wild populations to persist and adapt in response to these events. Here, we used Fisher's Fundamental Theorem of Natural Selection to evaluate the adaptive potential of Lasthenia fremontii, a herbaceous winter annual that is endemic to seasonally flooded wetlands in California, to alternative flooding regimes that occur during El Niño Southern Oscillation (ENSO) events. The results indicate that populations may exhibit greater adaptive potential in response to dry years than wet years, and that the relative performance of populations will change across climate scenarios. More generally, our findings show that extreme climate events can substantially change the potential for populations to adapt to climate change by modulating the expression of standing genetic variation and mean fitness.

Grow Where You Thrive, or Where Only You Can Survive? An Analysis of Performance Curve Evolution in a Clade with Diverse Habitat Affinities.

Performance curves are valuable tools for quantifying the fundamental niches of organisms and testing hypotheses about evolution, life-history trade-offs, and the drivers of variation in species' distribution patterns. Here, we present a novel Bayesian method for characterizing performance curves that facilitates comparisons among species. We then use this model to quantify and compare the hydrological performance curves of 14 different taxa in the genus Lasthenia, an ecologically diverse clade of plants that collectively occupy a variety of habitats with unique hydrological features, including seasonally flooded wetlands called vernal pools. We conducted a growth chamber experiment to measure each taxon's fitness across five hydrological treatments that ranged from severe drought to extended flooding, and we identified differences in hydrological performance curves that explain their associations with vernal pool and terrestrial habitats. Our analysis revealed that the distribution of vernal pool taxa in the field does not reflect their optimal hydrological environments: all taxa, regardless of habitat affinity, have highest fitness under similar hydrological conditions of saturated soil without submergence. We also found that a taxon's relative position across flood gradients within vernal pools is best predicted by the height of its performance curve. These results demonstrate the utility of our approach for generating insights into when and how performance curves evolve among taxa as they diversify into distinct environments. To facilitate its use, the modeling framework has been developed into an R package.

Environmental predictors of dispersal traits across a species' geographic range.

Variation in habitat quality and quantity drive selection on dispersal traits in heterogeneous environments, but the extent to which environmental conditions predict geographic variation in dispersal is rarely evaluated. We assessed dispersal trait variation across the range of Cakile edentula var. lacustris, an annual herb that occupies beaches of the Great Lakes. Cakile edentula has dimorphic fruits that each contain one dispersive and one non-dispersive seed. Previous work showed that plant height, branching density, and dispersive fruit wing-loading can determine the distance that seeds disperse locally by wind, while pericarp thickness influences the distance they disperse by water. We tested if these traits vary predictably with latitude across the species' geographic range, and if variation in dispersal characteristics can be predicted by the quality and quantity of habitat available at a site. We observed that the dispersive fruits from northern and southern populations had thinner pericarps than those from the interior of the species' range, reflecting reduced long-distance dispersal by water at both range limits. Plants at the northern range limit were shorter with less dense branching and lower wing-loading than populations elsewhere in the range, suggesting that these populations have enhanced local wind dispersal. In contrast, southern populations exhibited traits with inconsistent effects on wind dispersal: plants tended to be short, which facilitates wind dispersal in C. edentula, but also had relatively higher branching density and distal segment wing-loading that reduce wind dispersal. Geographic variation in maternal plant height and branching density was partially explained by variation in habitat quality, which declined at the species' range limits. In addition, population differences in branching density, fruit wing-loading, and pericarp thickness were predicted by the abundance and distribution of beach habitat. Finally, a common garden analysis recovered latitudinal patterns for the dispersal traits associated with fruits, but not those associated with maternal architecture. Thus, the geographic patterns of dispersal trait variation that we observed likely reflect responses to past selection by the distribution, abundance, and quality of habitat, strong plasticity in dispersal traits, and the effects dispersal itself has in shaping local adaptation by driving gene flow among populations.

Habitat filtering not dispersal limitation shapes oceanic island floras: species assembly of the Galápagos archipelago.

Remote locations, such as oceanic islands, typically harbour relatively few species, some of which go on to generate endemic radiations. Species colonising these locations tend to be a non-random subset from source communities, which is thought to reflect dispersal limitation. However, non-random colonisation could also result from habitat filtering, whereby only a few continental species can become established. We evaluate the imprints of these processes on the Galápagos flora by analysing a comprehensive regional phylogeny for ~ 39 000 species alongside information on dispersal strategies and climatic suitability. We found that habitat filtering was more important than dispersal limitation in determining species composition. This finding may help explain why adaptive radiation is common on oceanic archipelagoes - because colonising species can be relatively poor dispersers with specific niche requirements. We suggest that the standard assumption that plant communities in remote locations are primarily shaped by dispersal limitation deserves reconsideration.

Spatiotemporal heterogeneity in precipitation patterns explain population-level germination strategies in an edaphic specialist.

BACKGROUND AND AIMS: Many locally endemic species in biodiversity hotspots are restricted to edaphic conditions that are fixed in the landscape, limiting their potential to track climate change through dispersal. Instead, such species experience strong selection for germination strategies that can track suitable conditions through time. Germination strategies were compared among populations across the geographic range of a California vernal pool annual, Lasthenia fremontii Local germination strategies were tested to determine the associations with geographic variation in precipitation patterns. METHODS: This study evaluated patterns of seed germination, dormancy and mortality in response to simulated variation in the timing, amount and duration of the first autumn precipitation event using seeds from six populations that span a geographic gradient in precipitation. Next, it was tested whether the germination strategies of different populations can be predicted by historical precipitation patterns that characterize each site. KEY RESULTS: A significant positive relationship was observed between the historical variability in autumn precipitation and the extent of dormancy in a population. Marginal populations, with histories of the most extreme but constant autumn precipitation levels, expressed the lowest dormancy levels. Populations from sites with historically higher levels of autumn precipitation tended to germinate faster, but this tendency was not statistically significant. CONCLUSIONS: Germination in L. fremontii is cued by the onset of the first rains that characterize the beginning of winter in California's Great Central Valley. However, populations differ in how fast they germinate and the fraction of seeds that remain dormant when germination cues occur. The results suggest that seed dormancy may be a key trait for populations to track increasingly drier climates predicted by climate change models. However, the low dormancy and high mortality levels observed among seeds of the southernmost, driest populations make them most vulnerable to local extinction.

Population differentiation and countergradient variation throughout the geographic range in the fern gametophyte Vittaria appalachiana.

PREMISE OF THE STUDY: Theory predicts that limited gene flow between populations will promote population differentiation, and experimental studies have found that differentiation is often explained by local adaptation in sexually reproducing angiosperms. However, few experiments have examined the drivers of differentiation among populations in asexual land plants with limited dispersal potential. Here, we evaluated the role of temperature in driving population differentiation in an asexual, obligate gametophyte fern species. METHODS: We reciprocally transplanted Vittaria appalachiana gametophytes among six populations that spanned the species' geographic range in the Appalachian Mountains and Plateau. Temperature, survival, and senescence rates were measured for 1 year. KEY RESULTS: Populations had significantly different fitness responses to different sites, consistent with the hypothesis that populations have differentiated across the species' range. There was some evidence for local adaptation in marginal populations and for countergradient selection favoring particularly robust genotypes at the northern range edge. Most populations had relatively high fitness at the site with the most stable temperature conditions and were negatively affected by decreasing minimum temperatures. CONCLUSIONS: Populations of Vittaria appalachiana exhibit highly variable responses to transplantation across the species' range, and only a small subset of these responses are due to local adaptation. Differences in daily minimum temperature explain some variation in fitness, but other site-specific factors also have significant impacts on transplant fitness. These results indicate that asexual, patchily distributed species with limited dispersal may exhibit population-specific responses to global climate change that have not been elucidated by empirical work focused on sexually reproducing angiosperms.

The joint evolution of traits and habitat: ontogenetic shifts in leaf morphology and wetland specialization in Lasthenia.

The interplay between functional traits and habitat associations drives species' evolutionary responses to environmental heterogeneity, including processes such as adaptation, ecological speciation, and niche evolution. Seasonal variation is an aspect of the environment that varies across habitats, and could result in adaptive shifts in trait values across the life cycle of a plant. Here, we use phylogenetic comparative methods to evaluate the joint evolution of plant traits and habitat associations in Lasthenia (Asteraceae), a small clade of predominantly annual plants that have differentiated into an ecologically diverse range of habitats, including seasonal ephemeral wetlands known as vernal pools. Our results support the hypothesis that there is a link between the evolution of leaf morphology and the ecohydrological niche in Lasthenia, and, in the formation of aerenchyma (air space), differentiation between vernal pool and terrestrial taxa is fine-tuned to specific stages of plant ontogeny that reflects the evolution of heterophylly. Our findings demonstrate how the relationships between traits and habitat type can vary across the development of an organism, while highlighting a carefully considered comparative approach for examining correlated trait and niche evolution in a recently diversified and ecologically diverse plant clade.

Ecological release exposes genetically based niche variation.

The evolutionary trajectories of ecological niches have profound impacts on community, population and speciation dynamics, yet the underlying causes of niche lability vs. stasis are poorly understood. Here, we conducted a field experiment to quantify the effects of competition and, conversely, competitive release on the microevolutionary processes driving microhabitat niche evolution in an annual plant population restricted to California vernal pool wetlands. Removing competitors generated a strong increase in mean fitness, the exposure of genetically based niche variation and directional selection for niche evolution in the experimental population. In contrast, genetic variation in the microhabitat niche and directional selection for niche evolution were not detected in individuals growing with competitors. These results indicate that ecological opportunity (here, the removal of competitors) can trigger the immediate expression of latent, heritable niche variation that is necessary for rapid evolutionary responses; conversely, competitors may restrict niche evolution, contributing to niche conservatism in saturated communities.

Noninvasive assessment of osteoarthritis severity in human explants by multicontrast MRI.

PURPOSE: Medical imaging has the potential to noninvasively diagnose early disease onset and monitor the success of repair therapies. Unfortunately, few reliable imaging biomarkers exist to detect cartilage diseases before advanced degeneration in the tissue. METHOD: In this study, we quantified the ability to detect osteoarthritis (OA) severity in human cartilage explants using a multicontrast magnetic resonance imaging (MRI) approach, inclusive of novel displacements under applied loading by MRI, relaxivity measures, and standard MRI. RESULTS: Displacements under applied loading by MRI measures, which characterized the spatial micromechanical environment by 2D finite and Von Mises strains, were strong predictors of histologically assessed OA severity, both before and after controlling for factors, e.g., patient, joint region, and morphology. Relaxivity measures, sensitive to local macromolecular weight and composition, including T1ρ, but not T1 or T2, were predictors of OA severity. A combined multicontrast approach that exploited spatial variations in tissue biomechanics and extracellular matrix structure yielded the strongest relationships to OA severity. CONCLUSION: Our results indicate that combining multiple MRI-based biomarkers has high potential for the noninvasive measurement of OA severity and the evaluation of potential therapeutic agents used in the treatment of early OA in animal and human trials.

Comparative analysis of complete chloroplast genome sequence and inversion variation in Lasthenia burkei (Madieae, Asteraceae).

PREMISE OF THE STUDY: Complete chloroplast genome studies can help resolve relationships among large, complex plant lineages such as Asteraceae. We present the first whole plastome from the Madieae tribe and compare its sequence variation to other chloroplast genomes in Asteraceae. METHODS: We used high throughput sequencing to obtain the Lasthenia burkei chloroplast genome. We compared sequence structure and rates of molecular evolution in the small single copy (SSC), large single copy (LSC), and inverted repeat (IR) regions to those for eight Asteraceae accessions and one Solanaceae accession. KEY RESULTS: The chloroplast sequence of L. burkei is 150 746 bp and contains 81 unique protein coding genes and 4 coding ribosomal RNA sequences. We identified three major inversions in the L. burkei chloroplast, all of which have been found in other Asteraceae lineages, and a previously unreported inversion in Lactuca sativa. Regions flanking inversions contained tRNA sequences, but did not have particularly high G + C content. Substitution rates varied among the SSC, LSC, and IR regions, and rates of evolution within each region varied among species. Some observed differences in rates of molecular evolution may be explained by the relative proportion of coding to noncoding sequence within regions. CONCLUSIONS: Rates of molecular evolution vary substantially within and among chloroplast genomes, and major inversion events may be promoted by the presence of tRNAs. Collectively, these results provide insight into different mechanisms that may promote intramolecular recombination and the inversion of large genomic regions in the plastome.

Evolution is more repeatable in the introduction than range expansion phase of colonization.

How repeatable is evolution at genomic and phenotypic scales? We studied the repeatability of evolution during 8 generations of colonization using replicated microcosm experiments with the red flour beetle, Tribolium castaneum. Based on the patterns of shared allele frequency changes that occurred in populations from the same generation or experimental location, we found adaptive evolution to be more repeatable in the introduction and establishment phases of colonization than in the spread phase, when populations expand their range. Lastly, by studying changes in allele frequencies at conserved loci, we found evidence for the theoretical prediction that range expansion reduces the efficiency of selection to purge deleterious alleles. Overall, our results increase our understanding of adaptive evolution during colonization, demonstrating that evolution can be highly repeatable while also showing that stochasticity still plays an important role.

Integrative cartilage repair using acellular allografts for engineered structure and surface lubrication in vivo.

The repair of articular cartilage after damage is challenging, and decellularized tissue offers a possible treatment option to promote regeneration. Here, we show that acellular osteochondral allografts improve integrative cartilage repair compared to untreated defects after 6 months in an ovine model. Functional measures of intratissue strain/structure assessed by MRI demonstrate similar biomechanics of implants and native cartilage. Compared to native tissue and defects, the structure, composition, and tribology of acellular allografts preserve surface roughness and lubrication, material properties under compression and relaxation, compositional ratios of collagen:glycosaminoglycan and collagen:phosphate, and relative composition of types I/II collagen. While high cellularity was observed in bone regions and integration zones between cartilage-allografts, recellularization of chondral implants was inconsistent, with cell migration typically less than ~750 µm into the dense decellularized tissue, possibly limiting long-term cellular maintenance. Our results demonstrate the structural and biomechanical efficacy of acellular allografts for at least six months in vivo.

Evolving cartilage strain with pain progression and gait: a longitudinal study post-ACL reconstruction at six and twelve months.

Background: Anterior cruciate ligament (ACL) injuries are prevalent musculoskeletal conditions often resulting in long-term degenerative outcomes such as osteoarthritis (OA). Despite surgical advances in ACL reconstruction, a significant number of patients develop OA within ten years post-surgery, providing a patient population that may present early markers of cartilage degeneration detectable using noninvasive imaging. Purpose: This study aims to investigate the temporal evolution of cartilage strain and relaxometry post-ACL reconstruction using displacement under applied loading MRI and quantitative MRI. Specifically, we examined the correlations between MRI metrics and pain, as well as knee loading patterns during gait, to identify early candidate markers of cartilage degeneration. Materials and Methods: Twenty-five participants (female/male = 15/10; average age = 25.6 yrs) undergoing ACL reconstruction were enrolled in a prospective longitudinal cohort study between 2022 and 2023. MRI scans were conducted at 6- and 12-months post-surgery, assessing T2, T2*, and T1ρ relaxometry values, and intratissue cartilage strain. Changes in pain were evaluated using standard outcome scores, and gait analysis assessed the knee adduction moment (KAM). Regressions were performed to evaluate relationships between MRI metrics in cartilage contact regions, patient-reported pain, and knee loading metrics. Results: Increases in axial and transverse strains in the tibial cartilage were significantly correlated with increased pain, while decreases in shear strain were associated with increased pain. Changes in strain metrics were also significantly related to KAM at12 months. Conclusions: Changes in cartilage strain and relaxometry are related to heightened pain and altered knee loading patterns, indicating potential early markers of osteoarthritis progression. These findings underscore the importance of using advanced MRI for early monitoring in ACL-reconstructed patients to optimize treatment outcomes, while also highlighting KAM as a modifiable intervention through gait retraining that may positively impact the evolution of cartilage health and patient pain. Key Results: Increased axial and transverse strains in the tibial cartilage from 6 to 12 months post-ACL reconstruction were significantly correlated with increased pain, suggesting evolving changes in cartilage biomechanical properties over time.Decreases in shear strain in inner femoral and central tibial cartilage regions were linked to increased pain, indicating alterations in joint loading patterns.Decreases in shear strain in the inner femoral cartilage were significantly associated with decreased 12-month knee adduction moment (KAM), a surrogate for medial cartilage knee loading during walking.

MRI-derived Articular Cartilage Strains Predict Patient-Reported Outcomes Six Months Post Anterior Cruciate Ligament Reconstruction.

Key terms: Multicontrast and Multiparametric, Magnetic Resonance Imaging, Osteoarthritis, Functional Biomechanical Imaging, Knee Joint Degeneration What is known about the subject: dualMRI has been used to quantify strains in a healthy human population in vivo and in cartilage explant models. Previously, OA severity, as determined by histology, has been positively correlated to increased shear and transverse strains in cartilage explants. What this study adds to existing knowledge: This is the first in vivo use of dualMRI in a participant demographic post-ACL reconstruction and at risk for developing osteoarthritis. This study shows that dualMRI-derived strains are more significantly correlated with patient-reported outcomes than any MRI relaxometry metric. Background: Anterior cruciate ligament (ACL) injuries lead to an increased risk of osteoarthritis, characterized by altered cartilage tissue structure and function. Displacements under applied loading by magnetic resonance imaging (dualMRI) is a novel MRI technique that can be used to quantify mechanical strain in cartilage while undergoing a physiological load. Purpose: To determine if strains derived by dualMRI and relaxometry measures correlate with patient-reported outcomes at six months post unilateral ACL reconstruction. Study Design: Cohort study. Methods: Quantitative MRI (T2, T2*, T1ρ) measurements and transverse, axial, and shear strains were quantified in the medial articular tibiofemoral cartilage of 35 participants at six-months post unilateral ACL reconstruction. The relationships between patient-reported outcomes (WOMAC, KOOS, MARS) and all qMRI relaxation times were quantified using general linear mixed-effects models. A combined best-fit multicontrast MRI model was then developed using backwards regression to determine the patient features and MRI metrics that are most predictive of patient-reported outcome scores. Results: Higher femoral strains were significantly correlated with worse patient-reported functional outcomes. Femoral shear and transverse strains were positively correlated with six-month KOOS and WOMAC scores, after controlling for covariates. No relaxometry measures were correlated with patient-reported outcome scores. We identified the best-fit model for predicting WOMAC score using multiple MRI measures and patient-specific information, including sex, age, graft type, femoral transverse strain, femoral axial strain, and femoral shear strain. The best-fit model significantly predicted WOMAC score (p<0.001) better than any one individual MRI metric alone. When we regressed the model-predicted WOMAC scores against the patient-reported WOMAC scores, we found that our model achieved a goodness of fit exceeding 0.52. Conclusions: This work presents the first use of dualMRI in vivo in a cohort of participants at risk for developing osteoarthritis. Our results indicate that both shear and transverse strains are highly correlated with patient-reported outcome severity could serve as novel imaging biomarkers to predict the development of osteoarthritis.

Demographic responses of hybridizing cinquefoils to changing climate in the Colorado Rocky Mountains.

Hybridization between taxa generates new pools of genetic variation that can lead to different environmental responses and demographic trajectories over time than seen in parental lineages. The potential for hybrids to have novel environmental tolerances may be increasingly important in mountainous regions, which are rapidly warming and drying due to climate change. Demographic analysis makes it possible to quantify within- and among-species responses to variation in climate and to predict population growth rates as those conditions change. We estimated vital rates and population growth in 13 natural populations of two cinquefoil taxa (Potentilla hippiana and P. pulcherrima) and their hybrid across elevation gradients in the Southern Rockies. Using three consecutive years of environmental and demographic data, we compared the demographic responses of hybrid and parental taxa to environmental variation across space and time. All three taxa had lower predicted population growth rates under warm, dry conditions. However, the magnitude of these responses varied among taxa and populations. Hybrids had consistently lower predicted population growth rates than P. hippiana. In contrast, hybrid performance relative to P. pulcherrima varied with population and climate, with the hybrid maintaining relatively stable growth rates while populations of P. pulcherrima shrank under warm, dry conditions. Our findings demonstrate that hybrids in this system are neither intrinsically unfit nor universally more vigorous than parents, suggesting that the demographic consequences of hybridization are context-dependent. Our results also imply that shifts to warmer and drier conditions could have particularly negative repercussions for P. pulcherrima, which is currently the most abundant taxon in the study area, possibly as a legacy of more favorable historical climates. More broadly, the distributions of these long-lived taxa are lagging behind their demographic trajectories, such that the currently less common P. hippiana could become the most abundant of the Potentilla taxa as this region continues to warm and dry.