The genus Populus is currently the main model system for genetic, genomic, and physiological research in trees. Phenotypic variation in aspen (Populus tremula) populations growing in different environments across Sweden is expected to reflect genetic variation that is important for local adaptation. To analyze such natural phenotypic and genetic variation, the Swedish Aspen (SwAsp) Collection was established. Trees were taken from 12 different populations across Sweden, from 56°to 66°l atitude north and planted in two common gardens in Ekebo (55.9°N) and Sävar (63.4°N). Data related to phenological and growth traits were collected during the second year of growth. Some traits like the date of bud set and leaf area duration showed strong clinal variation patterns with latitude in both field trials, but the date of bud flush did not change along a latitudinal cline. The phenological traits showed moderate within-populations heritabilities, although growth traits showed weaker clinal patterns and lower heritabilities than the phenological traits. This research forms the starting point for the development of the SwAsp collection, a resource facilitating analysis of the natural genetic variation in aspen, the elucidation of the structure and dynamics of aspen populations, and the future identification of the genes controlling adaptive traits using association mapping of selected candidate genes.

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.

Plant-herbivore interactions vary across the landscape and have been hypothesised to promote local adaption in plants to the prevailing herbivore regime. Herbivores that feed on European aspen (Populus tremula) change across regional scales and selection on host defence genes may thus change at comparable scales. We have previously observed strong population differentiation in a set of inducible defence genes in Swedish P. tremula. Here, we study the geographic patterns of abundance and diversity of herbivorous insects, the untargeted metabolome of the foliage and genetic variation in a set of wound-induced genes and show that the geographic structure co-occurs in all three data sets. In response to this structure, we observe local maladaptation of herbivores, with fewer herbivores on local trees than on trees originated from more distant localities. Finally, we also identify 28 significant associations between single nucleotide polymorphisms SNPs from defence genes and a number of the herbivore traits and metabolic profiles.

Fungal endophytes are found in most seed plants, but their ecological function mainly remains elusive, except in pooid (or clavicipitalean) systems. The diversity and dynamics of endophytes in non clavicipitalean plants make studies of their ecological function challenging. This paper describes the advantage of using molecular techniques to survey the ecological function of endophytes in Populus tremula clones. About 1,000 endophyte isolates were distinguished using traditional methods; these isolates represented approximately 100 morphologically distinct groups. We generated 73 DNA-sequences (18S and ITS rDNA) from these groups and determined 33 distinct taxa. They represented the Basidiomycota and Ascomycota, including diverse Sordariomycetes and Dothideomycetes, and three sequences that were identified, by their ITS sequences, as a species of Epicoccum (a genus of supposedly unknown relation within the Ascomycota) were placed within the Pleosporales, in the 18S phylogeny. Primer pairs were designed for eleven of the fungi. Of these, three primers produced bands for a subset of Aspen samples. The primer pairs allowed endophytes in field samples to be readily identified, with a detection limit of 0.15 percent fungal DNA. The presence of fungi in Aspen clones was related to field damage by herbivores and the pathogen Venturia tremula. A negative association was found in two separate surveys between Aureobasidium sp. and herbivore damage, but we found no evidence that endophyte presence was related to a history of Venturia symptoms. This approach promises to enhance greatly the scope for qualitative and quantitative detection of endophyte communities, and to improve our ability to elucidate the ecological function of non clavicipitalean endophytes.

Associations of salicylate-like phenolic glycosides (PGs) with biological activity have been reported in Salix and Populus trees, but only for a few compounds, and in relation to a limited number of herbivores. By considering the full diversity of PGs, we may improve our ability to recognize genotypes or chemotype groups and enhance our understanding of their ecological function. Here, we present a fast and efficient general method for salicylate determination in leaves of Eurasian aspen that uses ultra-high performance liquid chromatography-electrospray ionization/time-of-flight mass spectrometry (UHPLC-ESI/TOFMS). The time required for the liquid chromatography separations was 13.5 min per sample, compared to around 60 min per sample for most HPLC protocols. In leaf samples from identical P. tremula genotypes with diverse propagation and treatment histories, we identified nine PGs. We found the compound-specific mass chromatograms to be more informative than the UV-visible chromatograms for compound identification and when quantitating samples with large variability in PG content. Signature compounds previously reported for P. tremoloides (tremulacin, tremuloidin, salicin, and salicortin) always were present, and five PGs (2'-O-cinnamoyl-salicortin, 2'-O-acetyl-salicortin, 2'-O-acetyl-salicin, acetyl-tremulacin, and salicyloyl-salicin) were detected for the first time in P. tremula. By using information about the formic acid adduct that appeared for PGs in the LTQ-Orbitrap MS environment, novel compounds like acetyl-tremulacin could be tentatively identified without the use of standards. The novel PGs were consistently either present in genotypes regardless of propagation and damage treatment or were not detectable. In some genotypes, concentrations of 2'-O-acetyl-salicortin and 2'-O-cinnamoyl-salicortin were similar to levels of biologically active PGs in other Salicaceous trees. Our study suggests that we may expect a wide variation in PG content in aspen populations which is of interest both for studies of interactions with herbivores and for mapping population structure.Electronic supplementary materialThe online version of this article (doi:10.1007/s10886-011-9991-7) contains supplementary material, which is available to authorized users.

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Statistical distributions like the negative binomial distribution are commonly used to describe aggregation patterns in animals. However, recently it has been suggested that truncated power laws (TPLs) may also be used for this kind of analysis. A TPL consists of two power functions separated by a cut‐off size (C*). The cut‐off size and the slope of power function one (β1) for the smallest group sizes have been suggested to have a biological explanatory value. We applied TPLs to aggregation data of tephritid seed predators on a composite plant, aphids on willows and grey seals on a haulout site. β1 varied between 0.60 and and −0.72, which is higher than predicted. In addition, resource distribution and animal density influenced β1 and C*. This indicates that environmental dimensionality suggested to affect β1 is masked by ecological factors. We conclude that TPLs are useful due to their simplicity and, in comparison with traditional methods, provide additional biologically relevant information. Truncated power laws can therefore prove to be useful in studies of animal behaviour and population dynamics.

The role of phloem proteins in plant resistance to aphids is still largely elusive. By genome-wide association mapping of aphid behavior on 350 natural accessions, we identified the small heat shock-like (). Detailed behavioral studies on near-isogenic and knockout lines showed that SLI1 impairs phloem feeding. Depending on the haplotype, aphids displayed a different duration of salivation in the phloem. On mutants, aphids prolonged their feeding sessions and ingested phloem at a higher rate than on wild-type plants. The largest phenotypic effects were observed at 26°C, when expression is upregulated. At this moderately high temperature, mutants suffered from retarded elongation of the inflorescence and impaired silique development. Fluorescent reporter fusions showed that SLI1 is confined to the margins of sieve elements where it lines the parietal layer and colocalizes in spherical bodies around mitochondria. This localization pattern is reminiscent of the clamp-like structures observed in previous ultrastructural studies of the phloem and shows that the parietal phloem layer plays an important role in plant resistance to aphids and heat stress.