DEMOGRAPHIC SOURCE-SINK DYNAMICS RESTRICT LOCAL ADAPTATION IN ELLIOTT'S BLUEBERRY (VACCINIUM ELLIOTTII)

There and back again; historical perspective and future directions for Vaccinium breeding and research studies

The genus Vaccinium L. (Ericaceae) contains a wide diversity of culturally and economically important berry crop species. Consumer demand and scientific research in blueberry (Vaccinium spp.) and cranberry (Vaccinium macrocarpon) have increased worldwide over the crops' relatively short domestication history (~100 years). Other species, including bilberry (Vaccinium myrtillus), lingonberry (Vaccinium vitis-idaea), and ohelo berry (Vaccinium reticulatum) are largely still harvested from the wild but with crop improvement efforts underway. Here, we present a review article on these Vaccinium berry crops on topics that span taxonomy to genetics and genomics to breeding. We highlight the accomplishments made thus far for each of these crops, along their journey from the wild, and propose research areas and questions that will require investments by the community over the coming decades to guide future crop improvement efforts. New tools and resources are needed to underpin the development of superior cultivars that are not only more resilient to various environmental stresses and higher yielding, but also produce fruit that continue to meet a variety of consumer preferences, including fruit quality and health related traits.

Selection favors adaptive plasticity in a long-term reciprocal transplant experiment

Spatial and temporal environmental variation can favor the evolution of adaptive phenotypic plasticity, such that genotypes alter their phenotypes in response to local conditions to maintain fitness across heterogeneous landscapes. When individuals show greater fitness in one habitat than another, asymmetric migration can restrict adaptation to the lower quality environment. In these cases, selection is predicted to favor traits that enhance fitness in the higher-quality (source) habitat at the expense of fitness in the marginal (sink) habitat. Here, we test whether plasticity is adaptive in a system regulated by demographic source-sink dynamics. Vaccinium elliottii (Ericaceae) occurs in dry upland and flood-prone bottomland forests throughout the southeastern United States, but has larger populations and higher average individual fitness in upland sites. We conducted a multi-year field experiment to evaluate whether plasticity in foliar morphology increases survival and lifespan. Both across and within habitats, selection favored plasticity in specific leaf area, stomatal density, and leaf size. Stabilizing selection acted on plasticity in stomatal density within habitats, suggesting that extreme levels of plasticity are disadvantageous. Thus, even in systems driven by source-sink dynamics, temporal and spatial variation in conditions across the landscape and within habitat types can favor the evolution of plasticity.

Local adaptation in island populations of Plectritis congesta that differ in historic exposure to ungulate browsers

Spatial variation in the occurrence of browsing ungulates can drive local adaptation in plant traits but also lead to trade-offs among traits potentially enhancing competitive ability versus resistance or tolerance to browsing. Plectritis congesta populations co-occurring on islands with and without ungulates offer striking examples of population-level variation in traits, such as plant height and fruit morphology, which may also affect fitness. We monitored split-plot common gardens exposed to and protected from browsing ungulates for 5 yr to test for local adaptation (local vs. foreign comparison) in P. congesta by comparing the survival and fecundity of 4,392 sown fruits from six island populations where ungulates were present ("historically exposed") and six where they were absent ("historically naïve"). Our results indicate that local adaptation to browsing in P. congesta favored rosette formation, delayed flowering, reduced height, and the production of wingless fruits, all of which appeared to enhance survival, fecundity, and population growth in plants from populations historically exposed to ungulate browsers, as compared to plants from historically naïve populations. In contrast, plants from historically naïve populations displayed higher relative fitness in the absence of ungulates, increased in height, flowered earlier, and produced fewer but larger, winged fruits, often in large terminal inflorescences. Our results support the hypothesis that variation in the occurrence of ungulate browsers has led to (1) spatial heterogeneity in natural selection and rapid adaptation in P. congesta populations on islands, and (2) context-dependent trade-offs in the fitness value of traits linked to the resistance or tolerance of browsing versus success in competition for light, pollinators, or other resources. Because patterns of selection in plant communities will vary with the introduction or extirpation of top predators or browsers, we suggest historical context, local adaptation, and the capacity for rapid adaptation should be a focal concern of those aiming to maximize or predict population persistence under environmental change in conservation plans.

Genomics of Long- and Short-Term Adaptation in Maize and Teosintes

Maize is an excellent model for the study of plant adaptation. Indeed, post domestication maize quickly adapted to a host of new environments across the globe. And work over the last decade has begun to highlight the role of the wild relatives of maize-the teosintes Zea mays ssp. parviglumis and ssp. mexicana-as excellent models for dissecting long-term local adaptation.Although human-driven selection associated with maize domestication has been extensively studied, the genetic basis of natural variation is still poorly understood. Here we review studies on the genetic basis of adaptation and plasticity in maize and its wild relatives. We highlight a range of different processes that contribute to adaptation and discuss evidence from natural, cultivated, and experimental populations. From an applied perspective, understanding the genetic bases of adaptation and the contribution of plasticity will provide us with new tools to both better understand and mitigate the effect of climate changes on natural and cultivated populations.

Gene flow improves fitness at a range edge under climate change

Populations at the margins of a species' geographic range are often thought to be poorly adapted to their environment. According to theoretical predictions, gene flow can inhibit these range edge populations if it disrupts adaptation to local conditions. Alternatively, if range edge populations are small or isolated, gene flow can provide beneficial genetic variation and may facilitate adaptation to environmental change. We tested these competing predictions in the annual wildflower Clarkia pulchella using greenhouse crosses to simulate gene flow from sources across the geographic range into two populations at the northern range margin. We planted these between-population hybrids in common gardens at the range edge and evaluated how genetic differentiation and climatic differences between edge populations and gene flow sources affected lifetime fitness. During an anomalously warm study year, gene flow from populations occupying historically warm sites improved fitness at the range edge and plants with one or both parents from warm populations performed best. The effects of the temperature provenance of gene flow sources were most apparent at early life history stages, but precipitation provenance also affected reproduction. We also found benefits of gene flow that were independent of climate: after climate was controlled for, plants with parents from different populations performed better at later lifestages than those with parents from the same population, indicating that gene flow may improve fitness via relieving homozygosity. Further supporting this result, we found that increasing genetic differentiation of parental populations had positive effects on fitness of hybrid seeds. Gene flow from warmer populations, when it occurs, is likely to contribute adaptive genetic variation to populations at the northern range edge as the climate warms. On heterogeneous landscapes, climate of origin may be a better predictor of gene flow effects than geographic proximity.

Pervasive migration across rainforest and sandy coastal plain Aechmea nudicaulis (Bromeliaceae) populations despite contrasting environmental conditions

Understanding the colonization of extreme marginal habitats and the relative roles of space and environment in maintaining peripheral populations remains challenging. Here, we leverage a system of pairs of rainforest and sandy coastal plain communities that allow us to decouple spatial and environmental effects in the population structure and migration rates of the bromeliad Aechmea nudicaulis. Structure and gene flow between populations were estimated from Bayesian clustering and coalescent-based migration models applied to chloroplast sequence and nuclear microsatellite data. Contrary to our initial expectation, the sharp environmental gradient between rainforest and sandy plains does not seem to have affected the colonization and migration dynamics in A. nudicaulis. Our analyses uncover pervasive gene flow between neighbouring habitats in both chloroplast and nuclear data despite the striking differences in environmental conditions. This result is consistent with a scenario of repeated colonization of the sandy coastal plains from forest populations through seed dispersal, as well as the maintenance of gene flow between habitats through pollination. We also recovered a broad north/south population structure that has been found in other Atlantic rainforest groups and possibly reflects older phylogeographic dynamics.

Constraints Imposed by a Natural Landscape Override Offspring Fitness Effects to Shape Oviposition Decisions in Wild Forked Fungus Beetles

Oviposition site decisions often maximize offspring fitness, but costs constraining choice can cause females to oviposit in poor developmental environments. It is unclear whether these constraints cumulatively outweigh offspring fitness to determine oviposition decisions in wild populations. Understanding how constraints shape oviposition in natural landscapes is a critical step toward revealing how maternal behavior influences fundamental phenomena like the evolution of specialization and the use of sink environments. Here, we used a genetic capture-recapture technique to reconstruct the oviposition decisions of individual females in a natural metapopulation of a beetle (Bolitotherus cornutus) that oviposits on three fungus species. We measured larval fitness-related traits (mass, development time, survival) on each fungus and compared the oviposition preferences of females in laboratory versus field tests. Larval fitness differed substantially among fungi, and females preferred a high-quality (high larval fitness) fungus in laboratory trials. However, females frequently laid eggs on the lowest-quality fungus in the wild. They preferred high-quality fungi when moving between oviposition sites, but this preference disappeared as the distance between sites increased and was inconsistent between study plots. Our results suggest that constraints on oviposition preferences in natural landscapes are sufficiently large to drive oviposition in poor developmental environments even when offspring fitness consequences are severe.

Detecting black bear source-sink dynamics using individual-based genetic graphs

Source-sink dynamics affects population connectivity, spatial genetic structure and population viability for many species. We introduce a novel approach that uses individual-based genetic graphs to identify source-sink areas within a continuously distributed population of black bears (Ursus americanus) in the northern lower peninsula (NLP) of Michigan, USA. Black bear harvest samples (n = 569, from 2002, 2006 and 2010) were genotyped at 12 microsatellite loci and locations were compared across years to identify areas of consistent occupancy over time. We compared graph metrics estimated for a genetic model with metrics from 10 ecological models to identify ecological factors that were associated with sources and sinks. We identified 62 source nodes, 16 of which represent important source areas (net flux > 0.7) and 79 sink nodes. Source strength was significantly correlated with bear local harvest density (a proxy for bear density) and habitat suitability. Additionally, resampling simulations showed our approach is robust to potential sampling bias from uneven sample dispersion. Findings demonstrate black bears in the NLP exhibit asymmetric gene flow, and individual-based genetic graphs can characterize source-sink dynamics in continuously distributed species in the absence of discrete habitat patches. Our findings warrant consideration of undetected source-sink dynamics and their implications on harvest management of game species.

Variations in several morphological characteristics and Cd/Pb accumulation capacities among different ecotypes of torpedograss responding to Cd-Pb stresses

Torpedograss (Panicum repens) has been recognized as an useful plant species for phytoremediation of water-level-fluctuation zones, which is a worldwide challenge. In this study, 10 ecotypes collected from tropical zone and flooded habitats (Group A) and subtropical zone and drought habitats (Group B) were used to clarify their responses to Cd-Pb stresses and effects of long-term adaptation on their morphological features and Cd-Pb accumulation capacities. Branch capacity, shoot and root biomasses of Group A under control were smaller than those of Group B, while the opposite results were observed under Cd-Pb stresses. The average plant shoot Cd concentrations of Group A under L-Cd-Pb and H-Cd-Pb were 24.84 and 52.38 mg kg^-1, respectively, significantly lower than those of Group B (36.81 and 67.60 mg kg^-1), while the variation among each group was insignificant, suggesting that habitat isolation and long-term adaptation may have led to differentiation in morphological features and metal uptake capacity. Torpedograss possesses high tolerance to Cd-Pb toxicities, and those ecotypes with larger biomass had higher Cd-Pb accumulation capacities. Torpedograss is a potential plant species for Cd phytoremediation and approximately 16 years would be required to clean soil contained by Cd as high as 10 mg kg^-1 using the selected torpedograss ecotypes.

The scale of local adaptation in Mimulus guttatus: comparing life history races, ecotypes, and populations

Fitness trade-offs between environments are central to the evolution of biodiversity. Although transplant studies often document fitness trade-offs consistent with local adaptation (LA), many have also found an advantage of foreign genotypes (foreign advantage (FA)). Understanding the mechanisms driving the magnitude and distribution of fitness variation requires comparative approaches that test the ecological scales at which these different patterns emerge. We used a common garden transplant experiment to compare the relative fitnesses of native vs foreign genotypes at three nested ecological scales within Mimulus guttatus: annual vs perennial life history races, perennial ecotypes across an elevational range, and populations within perennial elevational ecotypes. We integrated fitness across the life-cycle and decomposed LA vs FA into contributions from different fitness components. We found LA, measured as home-site advantage, between annual and perennial races and a trend towards LA among populations within montane habitats. Conversely, we found strong FA of low-elevation perennials in a montane environment. LA between life history races reflects the fitness advantages of adult survival and vegetative growth in a mesic environment. Within the perennial race, recent climate conditions or nonselective processes, such as dispersal limitation or mutational load, could explain FA of low-elevation perennials in a montane environment.

What can local and geographic population limits tell us about distributions?

PREMISE OF THE STUDY

Understanding the evolutionary and ecological factors that determine plant distributions is of primary importance in botanical research. These factors may vary in predictable ways across different spatial scales, and thus, we can leverage scale to reveal the underlying processes limiting plant distributions.

METHODS

We review various research considerations across local and geographic scales, including the investigation of dispersal and habitat limitation, evolutionary factors, abiotic and biotic factors, and research logistics. We also present two case studies, slender monkeyflower (Mimulus leptaleus) and cut-leaf monkeyflower (Mimulus laciniatus), in the California Sierra Nevada.

KEY RESULTS

At a local spatial scale (within 50 m), no seeds were produced from plants sown at sites located just beyond known patches of M. leptaleus, but within the species' geographic range. At a much broader spatial scale (kilometers), at the highest and lowest elevations of the species' range, we found greatly reduced abundance and fecundity in plants sown outside of the geographic range limits of M. laciniatus.

CONCLUSIONS

These cases illustrate two contrasting spatial scales, yet agree in their illustration of strong habitat limitation. We end by discussing future avenues of research and by suggesting ways botanical researchers can frame their studies to maximize information gained on species requirements, distribution limits, and conservation among varying spatial scales.

Protected polymorphisms and evolutionary stability of patch-selection strategies in stochastic environments

We consider a population living in a patchy environment that varies stochastically in space and time. The population is composed of two morphs (that is, individuals of the same species with different genotypes). In terms of survival and reproductive success, the associated phenotypes differ only in their habitat selection strategies. We compute invasion rates corresponding to the rates at which the abundance of an initially rare morph increases in the presence of the other morph established at equilibrium. If both morphs have positive invasion rates when rare, then there is an equilibrium distribution such that the two morphs coexist; that is, there is a protected polymorphism for habitat selection. Alternatively, if one morph has a negative invasion rate when rare, then it is asymptotically displaced by the other morph under all initial conditions where both morphs are present. We refine the characterization of an evolutionary stable strategy for habitat selection from Schreiber (Am Nat 180:17-34, 2012) in a mathematically rigorous manner. We provide a necessary and sufficient condition for the existence of an ESS that uses all patches and determine when using a single patch is an ESS. We also provide an explicit formula for the ESS when there are two habitat types. We show that adding environmental stochasticity results in an ESS that, when compared to the ESS for the corresponding model without stochasticity, spends less time in patches with larger carrying capacities and possibly makes use of sink patches, thereby practicing a spatial form of bet hedging.

Strong selection genome-wide enhances fitness trade-offs across environments and episodes of selection

Fitness trade-offs across episodes of selection and environments influence life-history evolution and adaptive population divergence. Documenting these trade-offs remains challenging as selection can vary in magnitude and direction through time and space. Here, we evaluate fitness trade-offs at the levels of the whole organism and the quantitative trait locus (QTL) in a multiyear field study of Boechera stricta (Brassicaceae), a genetically tractable mustard native to the Rocky Mountains. Reciprocal local adaptation was pronounced for viability, but not for reproductive components of fitness. Instead, local genomes had a fecundity advantage only in the high latitude garden. By estimating realized selection coefficients from individual-level data on viability and reproductive success and permuting the data to infer significance, we examined the genetic basis of fitness trade-offs. This analytical approach (Conditional Neutrality-Antagonistic Pleiotropy, CNAP) identified genetic trade-offs at a flowering phenology QTL (costs of adaptation) and revealed genetic trade-offs across fitness components (costs of reproduction). These patterns would not have emerged from traditional ANOVA-based QTL mapping. Our analytical framework can be applied to other systems to investigate fitness trade-offs. This task is becoming increasingly important as climate change may alter fitness landscapes, potentially disrupting fitness trade-offs that took many generations to evolve.

Adaptive evolution: evaluating empirical support for theoretical predictions

Adaptive evolution is shaped by the interaction of population genetics, natural selection and underlying network and biochemical constraints. Variation created by mutation, the raw material for evolutionary change, is translated into phenotypes by flux through metabolic pathways and by the topography and dynamics of molecular networks. Finally, the retention of genetic variation and the efficacy of selection depend on population genetics and demographic history. Emergent high-throughput experimental methods and sequencing technologies allow us to gather more evidence and to move beyond the theory in different systems and populations. Here we review the extent to which recent evidence supports long-established theoretical principles of adaptation.

Predicting local adaptation in fragmented plant populations: implications for restoration genetics

Understanding patterns and correlates of local adaptation in heterogeneous landscapes can provide important information in the selection of appropriate seed sources for restoration. We assessed the extent of local adaptation of fitness components in 12 population pairs of the perennial herb Rutidosis leptorrhynchoides (Asteraceae) and examined whether spatial scale (0.7-600 km), environmental distance, quantitative (Q(ST)) and neutral (F(ST)) genetic differentiation, and size of the local and foreign populations could predict patterns of adaptive differentiation. Local adaptation varied among populations and fitness components. Including all population pairs, local adaptation was observed for seedling survival, but not for biomass, while foreign genotype advantage was observed for reproduction (number of inflorescences). Among population pairs, local adaptation increased with Q(ST) and local population size for biomass. Q(ST) was associated with environmental distance, suggesting ecological selection for phenotypic divergence. However, low F(ST) and variation in population structure in small populations demonstrates the interaction of gene flow and drift in constraining local adaptation in R. leptorrhynchoides. Our study indicates that for species in heterogeneous landscapes, collecting seed from large populations from similar environments to candidate sites is likely to provide the most appropriate seed sources for restoration.

The evolution of patch selection in stochastic environments

A null model for habitat patch selection in spatially heterogeneous environments is the ideal free distribution (IFD), which assumes individuals have complete knowledge about the environment and can freely disperse. Under equilibrium conditions, the IFD predicts that local population growth rates are zero in all occupied patches, sink patches are unoccupied, and the fraction of the population selecting a patch is proportional to the patch's carrying capacity. Individuals, however, often experience stochastic fluctuations in environmental conditions and cannot respond to these fluctuations instantaneously. An evolutionary stability analysis for fixed patch-selection strategies reveals that environmental uncertainty disrupts the classical IFD predictions: individuals playing the evolutionarily stable strategy may occupy sink patches, local growth rates are negative and typically unequal in all patches, and individuals prefer higher-quality patches less than predicted by their carrying capacities. Spatial correlations in environmental fluctuations can enhance or marginalize these trends. The analysis predicts that continually increasing environmental variation first selects for range expansion, then selects for persisting coupled sink populations, and ultimately leads to regional extinction. In contrast, continually increasing habitat degradation first selects for range contraction and may select for persisting coupled sink populations before regional extinction. These results highlight the combined roles of spatial and temporal heterogeneity on the evolution of habitat selection.

Theoretical perspectives on the statics and dynamics of species' borders in patchy environments

Understanding range limits is a fundamental problem in ecology and evolutionary biology. In 1963, Mayr argued that "contaminating" gene flow from central populations constrained adaptation in marginal populations, preventing range expansion, while in 1984, Bradshaw suggested that absence of genetic variation prevented species from occurring everywhere. Understanding stability of range boundaries requires unraveling the interplay of demography, gene flow, and evolution of populations in concrete landscape settings. We walk through a set of interrelated spatial scenarios that illustrate interesting complexities of this interplay. To motivate our individual-based model results, we consider a hypothetical zooplankter in a landscape of discrete water bodies coupled by dispersal. We examine how patterns of dispersal influence adaptation in sink habitats where conditions are outside the species' niche. The likelihood of observing niche evolution (and thus range expansion) over any given timescale depends on (1) the degree of initial maladaptation; (2) pattern (pulsed vs. continuous, uni- vs. bidirectional), timing (juvenile vs. adult), and rate of dispersal (and hence population size); (3) mutation rate; (4) sexuality; and (5) the degree of heterogeneity in the occupied range. We also show how the genetic architecture of polygenic adaptation is influenced by the interplay of selection and dispersal in heterogeneous landscapes.

Fitness variation and local distribution limits in an annual plant population

Understanding how genetic variation shapes species' distributions involves examining how variation is distributed across a species' range as well as how it responds to underlying environmental heterogeneity. We examined patterns of fitness variation across the local distribution of an annual composite (Lasthenia fremontii) spanning a small-scale inundation gradient in a California vernal pool wetland. Using seeds collected from the center and edge of a population, paternal half-sib families were generated and transplanted back to the center and edge of the original population. All transplants were adapted to the conditions at the center of the population. The effect of the environment on the opportunity for selection depended on the model of selection assumed. Under a model of hard selection, variance in absolute fitness was lower among transplants at the edge of the population than at the center. Under a model of soft selection, the variance in relative fitness was similar between center and edge microhabitats. Given that this population is likely well-mixed, differences in habitat quality between center and edge microhabitats will likely cause selection at the center of the population to dominate the evolutionary trajectory of this population.

Phenotypic plasticity despite source-sink population dynamics in a long-lived perennial plant

• Species that exhibit adaptive plasticity alter their phenotypes in response to environmental conditions, thereby maximizing fitness in heterogeneous landscapes. However, under demographic source-sink dynamics, selection should favor traits that enhance fitness in the source habitat at the expense of fitness in the marginal habitat. Consistent with source-sink dynamics, the perennial blueberry, Vaccinium elliottii (Ericaceae), shows substantially higher fitness and population sizes in dry upland forests than in flood-prone bottomland forests, and asymmetrical gene flow occurs from upland populations into bottomland populations. Here, we examined whether this species expresses plasticity to these distinct environments despite source-sink dynamics. • We assessed phenotypic responses to a complex environmental gradient in the field and to water stress in the glasshouse. • Contrary to expectations, V. elliottii exhibited a high degree of plasticity in foliar and root traits (specific leaf area, carbon isotope ratios, foliar nitrogen content, root : shoot ratio, root porosity and root architecture). • We propose that plasticity can be maintained in source-sink systems if it is favored within the source habitat and/or a phylogenetic artifact that is not costly. Additionally, plasticity could be advantageous if habitat-based differences in fitness result from incipient niche expansion. Our results illuminate the importance of evaluating phenotypic traits and fitness components across heterogeneous landscapes.