Separating ontogenetic and environmental determination of resistance to herbivory in cottonwood

Vegetative phase change in Populus tremula × alba

Plants transition through juvenile and adult phases of vegetative development in a process known as vegetative phase change (VPC). In poplars (genus Populus) the differences between these stages are subtle, making it difficult to determine when this transition occurs. Previous studies of VPC in poplars have relied on plants propagated in vitro, leaving the natural progression of this process unknown. We examined developmental morphology of seed-grown and in vitro derived Populus tremula × alba (clone 717-1B4), and compared the phenotype of these to transgenics with manipulated miR156 expression, the master regulator of VPC. In seed-grown plants, most traits changed from node-to-node during the first 3 months of development but remained constant after node 25. Many traits remained unchanged in clones over-expressing miR156, or were enhanced when miR156 was lowered, demonstrating their natural progression is regulated by the miR156/SPL pathway. The characteristic leaf fluttering of Populus is one of these miR156-regulated traits. Vegetative development in plants grown from culture mirrored that of seed-grown plants, allowing direct comparison between plants often used in research and those found in nature. These results provide a foundation for further research on the role of VPC in the ecology and evolution of this economically important genus.

Genetics-based interactions of foundation species affect community diversity, stability and network structure

We examined the hypothesis that genetics-based interactions between strongly interacting foundation species, the tree Populus angustifolia and the aphid Pemphigus betae, affect arthropod community diversity, stability and species interaction networks of which little is known. In a 2-year experimental manipulation of the tree and its aphid herbivore four major findings emerged: (i) the interactions of these two species determined the composition of an arthropod community of 139 species; (ii) both tree genotype and aphid presence significantly predicted community diversity; (iii) the presence of aphids on genetically susceptible trees increased the stability of arthropod communities across years; and (iv) the experimental removal of aphids affected community network structure (network degree, modularity and tree genotype contribution to modularity). These findings demonstrate that the interactions of foundation species are genetically based, which in turn significantly contributes to community diversity, stability and species interaction networks. These experiments provide an important step in understanding the evolution of Darwin's 'entangled bank', a metaphor that characterizes the complexity and interconnectedness of communities in the wild.

Outbreak of Drepanopeziza fungus in aspen forests and variation in stand susceptibility: leaf functional traits, compensatory growth and phenology

In the spring of 2015, a severe outbreak of the necrotrophic pathogen Drepanopeziza (also known as Marssonina) spread across large portions of aspen (Populus tremuloides Michx.) forests in the western United States. Among adjacent stands, some were diseased and others were not. Drepanopeziza infection in diseased aspen stands stimulated compensatory growth of second-flush leaves at the top of the canopy. These patterns of infection provided an opportunity to characterize associations of pathogen infection and leaf functional traits. Eight pairs of adjacent healthy and diseased aspen stands were identified across a forest landscape in northern Utah. Average leaf surface area, specific leaf area (SLA), photosynthesis, starch concentration and defense chemistry expression (phenolic glycosides and condensed tannins) were measured on original, first-flush leaves in the lower portion of the tree canopy of healthy and diseased stands and compensatory, second-flush leaves produced in the canopy top of diseased stands. Only first-flush leaves of diseased stands showed high levels of Drepanopeziza infection. Leaf area of second-flush leaves of diseased stands was threefold larger than all other leaf types in healthy or diseased stands. Lower canopy leaves of healthy stands had the highest SLA. Photosynthesis was lowest in infected first-flush leaves, highest in second-flush leaves of diseased stands and intermediate in leaves of healthy stands. Foliar starch concentrations were lower in leaves of diseased stands than leaves from healthy stands. Condensed tannins were greater in second-flush leaves than first-flush leaves in both healthy and diseased stands. Phenolic glycoside concentrations were lowest in infected leaves of diseased stands. Diseased stands leafed out a week earlier in the spring than healthy stands, which may have exposed their emerging leaves to rainy conditions that promote Drepanopeziza infection. Compensatory leaf regrowth of diseased stands appears to offset some of the functional loss (i.e., photosynthetic capacity) of infected leaves.

Genetic variation in NIN1 and C/VIF1 genes is significantly associated with Populus angustifolia resistance to a galling herbivore, Pemphigus betae

The identification of genes associated with ecologically important traits provides information on the potential genetic mechanisms underlying the responses of an organism to its natural environment. In this study, we investigated the genetic basis of host plant resistance to the gall-inducing aphid, Pemphigus betae, in a natural population of 154 narrowleaf cottonwoods (Populus angustifolia). We surveyed genetic variation in two genes putatively involved in sink-source relations and a phenology gene that co-located in a previously identified quantitative trait locus for resistance to galling. Using a candidate gene approach, three major findings emerged. First, natural variation in tree resistance to galling was repeatable. Sampling of the same tree genotypes 20 years after the initial survey in 1986 show that 80% of the variation in resistance was due to genetic differences among individuals. Second, we identified significant associations at the single nucleotide polymorphism and haplotype levels between the plant neutral invertase gene NIN1 and tree resistance. Invertases are a class of sucrose hydrolyzing enzymes and play an important role in plant responses to biotic stress, including the establishment of nutrient sinks. These associations with NIN1 were driven by a single nucleotide polymorphism (NIN1_664) located in the second intron of the gene and in an orthologous sequence to two known regulatory elements. Third, haplotypes from an inhibitor of invertase (C/VIF1) were significantly associated with tree resistance. The identification of genetic variation in these two genes provides a starting point to understand the possible genetic mechanisms that contribute to tree resistance to gall formation. We also build on previous work demonstrating that genetic differences in sink-source relationships of the host influence the ability of P. betae to manipulate the flow of nutrients and induce a nutrient sink.

Host Genetics and Environment Drive Divergent Responses of Two Resource Sharing Gall-Formers on Norway Spruce: A Common Garden Analysis

A central issue in the field of community genetics is the expectation that trait variation among genotypes play a defining role in structuring associated species and in forming community phenotypes. Quantifying the existence of such community phenotypes in two common garden environments also has important consequences for our understanding of gene-by-environment interactions at the community level. The existence of community phenotypes has not been evaluated in the crowns of boreal forest trees. In this study we address the influence of tree genetics on needle chemistry and genetic x environment interactions on two gall-inducing adelgid aphids (Adelges spp. and Sacchiphantes spp.) that share the same elongating bud/shoot niche. We examine the hypothesis that the canopies of different genotypes of Norway spruce (Picea abies L.) support different community phenotypes. Three patterns emerged. First, the two gallers show clear differences in their response to host genetics and environment. Whereas genetics significantly affected the abundance of Adelges spp. galls, Sacchiphantes spp. was predominately affected by the environment suggesting that the genetic influence is stronger in Adelges spp. Second, the among family variation in genetically controlled resistance was large, i.e. fullsib families differed as much as 10 fold in susceptibility towards Adelges spp. (0.57 to 6.2 galls/branch). Also, the distribution of chemical profiles was continuous, showing both overlap as well as examples of significant differences among fullsib families. Third, despite the predicted effects of host chemistry on galls, principal component analyses using 31 different phenolic substances showed only limited association with galls and a similarity test showed that trees with similar phenolic chemical characteristics, did not host more similar communities of gallers. Nonetheless, the large genetic variation in trait expression and clear differences in how community members respond to host genetics supports our hypothesis that the canopies of Norway spruce differ in their community phenotypes.

Population divergence in the ontogenetic trajectories of foliar terpenes of a Eucalyptus species

BACKGROUND AND AIMS

The development of plant secondary metabolites during early life stages can have significant ecological and evolutionary implications for plant-herbivore interactions. Foliar terpenes influence a broad range of ecological interactions, including plant defence, and their expression may be influenced by ontogenetic and genetic factors. This study investigates the role of these factors in the expression of foliar terpene compounds in Eucalyptus globulus seedlings.

METHODS

Seedlings were sourced from ten families each from three genetically distinct populations, representing relatively high and low chemical resistance to mammalian herbivory. Cotyledon-stage seedlings and consecutive leaf pairs of true leaves were harvested separately across an 8-month period, and analysed for eight monoterpene compounds and six sesquiterpene compounds.

KEY RESULTS

Foliar terpenes showed a series of dynamic changes with ontogenetic trajectories differing between populations and families, as well as between and within the two major terpene classes. Sesquiterpenes changed rapidly through ontogeny and expressed opposing trajectories between compounds, but showed consistency in pattern between populations. Conversely, changed expression in monoterpene trajectories was population- and compound-specific.

CONCLUSIONS

The results suggest that adaptive opportunities exist for changing levels of terpene content through ontogeny, and evolution may exploit the ontogenetic patterns of change in these compounds to create a diverse ontogenetic chemical mosaic with which to defend the plant. It is hypothesized that the observed genetically based patterns in terpene ontogenetic trajectories reflect multiple changes in the regulation of genes throughout different terpene biosynthetic pathways.

Long-term exposure to elevated CO2 and O3 alters aspen foliar chemistry across developmental stages

Anthropogenic activities are altering levels of greenhouse gases to the extent that multiple and diverse ecosystem processes are being affected. Two gases that substantially influence forest health are atmospheric carbon dioxide (CO2 ) and tropospheric ozone (O3 ). Plant chemistry will play an important role in regulating ecosystem processes in future environments, but little information exists about the longitudinal effects of elevated CO2 and O3 on phytochemistry, especially for long-lived species such as trees. To address this need, we analysed foliar chemical data from two genotypes of trembling aspen, Populus tremuloides, collected over 10 years of exposure to levels of CO2 and O3 predicted for the year 2050. Elevated CO2 and O3 altered both primary and secondary chemistry, and the magnitude and direction of the responses varied across developmental stages and between aspen genotypes. Our findings suggest that the effects of CO2 and O3 on phytochemical traits that influence forest processes will vary over tree developmental stages, highlighting the need to continue long-term, experimental atmospheric change research.

Relative importance of genetic, ontogenetic, induction, and seasonal variation in producing a multivariate defense phenotype in a foundation tree species

Plant adaptations for defense against herbivory vary both among species and among genotypes. Moreover, numerous forms of within-plant variation in defense, including ontogeny, induction, and seasonal gradients, allow plants to avoid expending resources on defense when herbivores are absent. We used an 18-year-old cottonwood common garden composed of Populus fremontii, Populus angustifolia, and their naturally occurring F(1) hybrids (collectively referred to as "cross types") to quantify and compare the relative influences of three hierarchical levels of variation (between cross types, among genotypes, and within individual genotypes) on univariate and multivariate phytochemical defense traits. Within genotypes, we evaluated ontogeny, induction (following cottonwood leaf beetle herbivory), and seasonal variation. We compared the effect sizes of each of these sources of variation on the plant defense phenotype. Three major patterns emerged. First, we observed significant differences in concentrations of defense phytochemicals among cross types, and/or among genotypes within cross types. Second, we found significant genetic variation for within-plant differences in phytochemical defenses: (a) based on ontogeny, levels of constitutive phenolic glycosides were nearly three times greater in the mature zone than in the juvenile zone within one cottonwood cross type, but did not significantly differ within another cross type; (b) induced levels of condensed tannins increased up to 65 % following herbivore damage within one cottonwood cross type, but were not significantly altered in another cross type; and (c) concentrations of condensed tannins tended to increase across the season, but did not do so across all cross types. Third, our estimates of effect size demonstrate that the magnitude of within-plant variation in a phytochemical defense can rival the magnitude of differences in defense among genotypes and/or cross types. We conclude that, in cottonwood and likely other plant species, multiple forms of within-individual variation have the potential to substantially influence ecological and evolutionary processes.

Genetic variation in functional traits influences arthropod community composition in aspen (Populus tremula L.)

We conducted a study of natural variation in functional leaf traits and herbivory in 116 clones of European aspen, Populus tremula L., the Swedish Aspen (SwAsp) collection, originating from ten degrees of latitude across Sweden and grown in a common garden. In surveys of phytophagous arthropods over two years, we found the aspen canopy supports nearly 100 morphospecies. We identified significant broad-sense heritability of plant functional traits, basic plant defence chemistry, and arthropod community traits. The majority of arthropods were specialists, those coevolved with P. tremula to tolerate and even utilize leaf defence compounds. Arthropod abundance and richness were more closely related to plant growth rates than general chemical defences and relationships were identified between the arthropod community and stem growth, leaf and petiole morphology, anthocyanins, and condensed tannins. Heritable genetic variation in plant traits in young aspen was found to structure arthropod community; however no single trait drives the preferences of arthropod folivores among young aspen genotypes. The influence of natural variation in plant traits on the arthropod community indicates the importance of maintaining genetic variation in wild trees as keystone species for biodiversity. It further suggests that aspen can be a resource for the study of mechanisms of natural resistance to herbivores.