Kristin L. Mercer

Evolutionary response of landraces to climate change in centers of crop diversity.

Landraces cultivated in centers of crop diversity result from past and contemporary patterns of natural and farmer‐mediated evolutionary forces. Successful in situ conservation of crop genetic resources depends on continuity of these evolutionary processes. Climate change is projected to affect agricultural production, yet analyses of impacts on in situ conservation of crop genetic diversity and farmers who conserve it have been absent. How will crop landraces respond to alterations in climate? We review the roles that phenotypic plasticity, evolution, and gene flow might play in sustaining production, although we might expect erosion of genetic diversity if landrace populations or entire races lose productivity. For example, highland maize landraces in southern Mexico do not express the plasticity necessary to sustain productivity under climate change, but may evolve in response to altered conditions. The outcome for any given crop in a given region will depend on the distribution of genetic variation that affects fitness and patterns of climate change. Understanding patterns of neutral and adaptive diversity from the population to the landscape scale is essential to clarify how landraces conserved in situ will continue to evolve and how to minimize genetic erosion of this essential natural resource.

Asymmetrical local adaptation of maize landraces along an altitudinal gradient

Crop landraces are managed populations that evolve in response to gene flow and selection. Cross‐pollination among fields, seed sharing by farmers, and selection by management and environmental conditions play roles in shaping crop characteristics. We used common gardens to explore the local adaptation of maize (Zea mays ssp. mays) landrace populations from Chiapas, Mexico to altitude. We sowed seeds of 21 populations from three altitudinal ranges in two common gardens and measured two characteristics that estimate fitness: likelihood of producing good quality seed and the total mass of good quality seed per plant. The probability of lowland plants producing good quality seed was invariably high regardless of garden, while highland landraces were especially sensitive to altitude. Their likelihood of producing good seed quadrupled in the highland site. The mass of good quality seed showed a different pattern, with lowland landraces producing 25% less seed mass than the other types at high elevations. Combining these two measures of fitness revealed that the highland landraces were clearly adapted to highland sites, while lowland and midland landraces appear more adapted to the midland site. We discuss this asymmetry in local adaptation in light of climate change and in situ conservation of crop genetic resources.

Application of genetic diversity–ecosystem function research to ecological restoration

Summary 1. Three common goals for restoration are (i) rapid plant establishment, (ii) long-term plant persistence and (iii) restoration of functioning ecosystems. Restoration practitioners often use cultivars optimized for rapid plant establishment under highly disturbed conditions to achieve the first goal; locally adapted genotypes are championed for the second because they can be well suited for local environmental conditions. Restoring functioning ecosystems is considered a loftier goal that practitioners struggle to achieve because we lack proven techniques. 2. Similar to the demonstrated benefits of species, functional and phylogenetic diversity for ecosystem functions (EFs), recent genetic diversity (GD)–ecosystem function (EF) experiments have shown that increases in plant GD can positively influence many different EFs. Would the introduction of diverse plant genotypes of a given species into a restoration enhance ecosystem functioning and the evolutionary potential of restored populations? 3. In this review, we first examine three propagule-sourcing approaches: cultivar, local adaptation and GD. Next, we raise questions that if addressed, could help practitioners implement a GD approach in restoration: (i) How might the selection, relatedness and arrangement of genotypes be optimized to restore functioning ecosystems, (ii) How do traits that affect an EF relate to neutral or adaptive diversity, more common measures of GD and (iii) at which spatial and temporal scales does GD influence EFs in restorations? 4. Synthesis and applications. Although each propagule-sourcing approach may be best suited for a particular restoration goal, each approach may simultaneously benefit other goals. Yet cultivars and locally adapted populations that have experienced artificial and/or natural selection may not possess the levels of diversity that will confer expected benefits to different ecosystem functions. Future research should determine the relative value of each approach (or a combination of approaches) for simultaneously achieving multiple restoration goals. Restoration experiments, where plant genetic diversity (GD) is manipulated and monitored over scales relevant to restoration, could reveal the true promise of a GD approach to restoration.

Selection on seedling emergence timing and size in an annual plant, Helianthus annuus (common sunflower, Asteraceae).

PREMISE Variation in seedling emergence timing is considered adaptive over the long term in wild populations, but early emergence can result in a fitness advantage. To explore the adaptive significance of seedling emergence timing, it should be studied under realistic conditions and in the context of other traits that influence fitness. METHODS In a common garden, we monitored maternal families from seed to flowering (including over winter) with intra- and interspecific competition. We assessed the effects of emergence timing and plant size on survival to anthesis in different genetic backgrounds and under varying competition. KEY RESULTS We found genetic variation for emergence (probability and timing), size, and survival to anthesis. We also found negative selection, both phenotypic and genetic, on emergence time, such that early emergers (day 8) had almost twice as great a predicted probability of surviving as later emergers (day 28). Size had strong positive effects on survival and, furthermore, the beneficial effects of early emergence may be mediated through size. Maternal family and competitive environment can also affect selection on emergence timing. CONCLUSIONS Our results indicate that early emergence is related to greater survival in wild sunflower, although there may be little direct selection on this trait; rather, its importance may be mediated by its effects on highly adaptive traits associated with size. Also, the effects of early emergence may vary across genetic backgrounds and competitive conditions, facilitating the maintenance of variation for this trait across a diverse landscape.

Vitamin variation in capsicum spp. Provides opportunities to improve nutritional value of human diets

Chile peppers, native to the Americas, have spread around the world and have been integrated into the diets of many cultures. Much like their heat content, nutritional content can vary dramatically between different pepper types. In this study, a diverse set of chile pepper types were examined for nutrient content. Some pepper types were found to have high levels of vitamin A, vitamin C, or folate. Correlations between nutrient content, species, cultivation status, or geographic region were limited. Varietal selection or plant breeding offer tools to augment nutrient content in peppers. Integration of nutrient rich pepper types into diets that already include peppers could help combat nutrient deficiencies by providing a significant portion of recommended daily nutrients.

Fitness of Crop-Wild Hybrid Sunflower under Competitive Conditions: Implications for Crop-to-Wild Introgression

Understanding the likelihood and extent of introgression of novel alleles in hybrid zones requires comparison of lifetime fitness of parents and hybrid progeny. However, fitness differences among cross types can vary depending on biotic conditions, thereby influencing introgression patterns. Based on past work, we predicted that increased competition would enhance introgression between cultivated and wild sunflower (Helianthus annuus) by reducing fitness advantages of wild plants. To test this prediction, we established a factorial field experiment in Kansas, USA where we monitored the fitness of four cross types (Wild, F1, F2, and BCw hybrids) under different levels of interspecific and intraspecific competition. Intraspecific manipulations consisted both of density of competitors and of frequency of crop-wild hybrids. We recorded emergence of overwintered seeds, survival to reproduction, and numbers of seeds produced per reproductive plant. We also calculated two compound fitness measures: seeds produced per emerged seedling and seeds produced per planted seed. Cross type and intraspecific competition affected emergence and survival to reproduction, respectively. Further, cross type interacted with competitive treatments to influence all other fitness traits. More intense competition treatments, especially related to density of intraspecific competitors, repeatedly reduced the fitness advantage of wild plants when considering seeds produced per reproductive plant and per emerged seedling, and F2 plants often became indistinguishable from the wilds. Wild fitness remained superior when seedling emergence was also considered as part of fitness, but the fitness of F2 hybrids relative to wild plants more than quadrupled with the addition of interspecific competitors and high densities of intraspecific competitors. Meanwhile, contrary to prediction, lower hybrid frequency reduced wild fitness advantage. These results emphasize the importance of taking a full life cycle perspective. Additionally, due to effects of exogenous selection, a given hybrid generation may be especially well-suited to hastening introgression under particular environmental conditions.

Evolution of plant materials for ecological restoration: insights from the applied and basic literature

Summary Restoration is normally conducted with the goal of creating plant populations that establish, survive, successfully reproduce, contribute to ecosystem function and persist in the long term. Restoration often relies on revegetation that, on large scales, requires agronomic increase of native plant materials. During this propagation process, restoration populations are subject to genetic sampling as well as natural and artificial selection that could result in adaptation contrasting sharply with that of native populations. Here we draw on insights from the evolutionary and agricultural literature to illustrate how changes in the amount and type of genetic variation in ex situ repositories (source collections and production farms) could affect plant performance in restoration. The consequences of intentional and/or inadvertent evolutionary modification of restoration materials are discussed with respect to population viability and ecosystem function. Synthesis and applications. We conclude that sampling effects and intentional and unintentional selection during collection, propagation and restoration planting have the potential to diminish restored populations. We describe testing for evolutionary change in plant materials using neutral molecular markers and/or field observations. Six practices, multiple collections through time, multiple collections through space, large effective population size, provenance tracking, promoting gene flow and reducing selection comprise ‘evolutionarily enlightened management’ that decreases the potential for unintentional evolution and maladaptation.

Maternal effects and embryo genetics: germination and dormancy of crop–wild sunflower hybrids

Gene flow between agricultural crops and related wild plant populations can produce hybrids which differ significantly from their wild counterparts in many life history traits, including seed traits. Seeds from wild annuals often possess significant dormancy, while cultivated varieties have been selected to germinate immediately under favourable conditions. Consequently, the dormancy characteristics of crop–wild hybrids could influence their survival, seed-bank dynamics and, ultimately, the extent to which crop genes persist in wild populations. Dormancy characteristics can be influenced by both maternal effects, as well as the genetics of a seeds embryo, which are the result of contributions from both parents. Here we focus on how maternal effects and embryo genetics affect seed dormancy in crop–wild hybrid sunflowers (Helianthus annuus). Using three laboratory experiments, we quantified the germination and dormancy of 15 crop–wild hybrid sunflower cross types, while also identifying achene characteristics that may influence the differential germination observed. We found that increased frequencies of crop alleles and the maternal effects imparted by hybridization can reduce dormancy, though the effect of increased frequencies of crop alleles was more pronounced in wild- and F₁-produced than in the crop-produced achenes. The more open pericarp of the crop-produced achenes and the shorter relative distance that their radicles had to travel to germinate may explain some of the observed maternal effects. Finally, we generated hypotheses about how these results could affect survival and crop gene introgression in the field.

Maternal Environment Influences Propagule Pressure of an Invasive Plant, Raphanus raphanistrum (Brassicaceae)

Premise of research. Weedy species that invade new areas may experience shifts in environmental conditions across generations. Since maternal environments can significantly influence embryo development, shifts in maternal environments could alter the ability of offspring to colonize. Methodology. Here, we report the results of a set of field experiments that study adaptive transgenerational plasticity across three generations using the agricultural, annual weed, Raphanus raphanistrum. We grew replicate lineages across a gradient of experimentally manipulated soil moisture conditions (control rain [×2], no rain, double normal precipitation) for two generations (maternal and offspring) and transplanted individuals of each population to manipulated or unmanipulated soil moisture conditions. We then measured the consequences of the maternal and offspring soil moisture manipulations on traits critical for weediness in the second generation of plants and third (grandchild) generation of seeds. Pivotal results. Maternal moisture environments significantly influenced offspring development. Offspring of parents from relatively dry environments were significantly smaller (reduced seed biomass, floral displays, and size at reproduction) and less fecund, while offspring of parents from relatively wet environments were significantly larger and more fecund compared with related offspring whose parents had been grown under control moisture conditions. The relative differences among lines grown under various maternal environments were intensified when they were grown in a common environment. Conclusions. Weediness is a product of the population genetics of colonists and environmental characteristics of the invaded environment. Plastic responses to abiotic variation experienced by the maternal parent or offspring may also influence the outcome of dispersal, potentially increasing the relative rate of movement or propagule pressure from relatively wet maternal habitats to dry recipient habitats. Possible implications of these environmentally induced phenotypes are discussed with respect to ecological distribution, persistence under novel environments, and evolution in natural populations.

The political ecology of a highway through Belize's forested borderlands

This paper examines how a new highway in Belize will change a tropical forest landscape. Since the end of British colonialism (1981), the Maya communities in Belizes Toledo District have struggled with the state for control of the lands they have customarily used to produce a livelihood. We were approached by some Maya leaders, who recognize that the paving of a new highway through their villages could transform Maya agricultural practices and thus land use and cover change (LUCC), and asked to produce an analysis that could assist them in managing their lands. We braid together historical, social, political, economic, and satellite data to answer two questions: (1) How has land cover changed since 1975 in areas with Maya customary versus noncustomary land-use practices? (2) What will be the consequences of paving the new highway through this region for the landscape and the Maya communities? Using a multifaceted LUCC/political ecology analysis we found that forest clearing is greatest where noncustomary farming practices are employed. Noncustomary practices are spatially concentrated along the Belize-Guatemala border; the shift to noncustomary practices resulted from inmigration by war refugees who maintained close ties to Guatemalan markets. The paving of the highway promises to reduce the functional distance to Guatemalas markets, which could change land use in other villages to the detriment of the forests because of the likely diffusion of noncustomary farm practices. Forest change will be shaped by the regions complex political geography.