Browsing by Author "Volf, Martin"

Plants produce an astonishing diversity of specialized metabolites as defences against herbivores, pathogens or detrimental abiotic conditions. Plants growing at different elevations are exposed to different biotic and abiotic conditions and typically show pronounced differences in their chemistry. Understanding how these differences arise through changes in various measures of chemical diversity can inform us concerning factors that contribute to the variety of metabolites found among plants. We focused on elevational changes in concentration, richness and intra- and interspecific variation in specialized chemistry in willows (Salix, Salicaceae) and compare them among metabolite classes with different functions. We aim to show how these various measures of chemical diversity change with elevation to reveal trends contributing to changes in plant chemistry along major ecological gradients. We studied chemistry, herbivory and fungal pathogen damage in an assemblage of seven willow species along an elevational gradient in the Alps (800–2600 m a.s.l.). We examined trends in chemical diversity using untargeted metabolomics, and further quantified trends in three specific classes: proanthocyanidins and salicinoids involved in biotic interactions, and flavonoids involved mainly in abiotic protection. We use measures of willow chemistry that take structural relatedness of metabolites into account to show if the roles of structurally distinct metabolites change with elevation. Willows from low elevations exhibited greater proanthocyanidin concentration and structural richness of flavonoids. In contrast, willows from high elevations showed greater structural richness of salicinoids and greater variation in total metabolite composition at both the intra- and interspecific levels. The trends in salicinoid richness and proanthocyanidin concentration were explained by elevational changes in temperature. Our results show how elevational differences in plant chemistry arise through trends in various aspects of their chemical diversity. Willows at high elevations showed reduced structural richness of metabolites involved in abiotic protection. This may reflect focused investment in metabolites with the highest ecological benefit relative to their concentration in high-elevation willows. At the same time, they possessed greater richness of metabolites involved in biotic interactions, while variation in microhabitat preferences among high-elevation species likely contributed to the high variation in their total metabolite pool.

Abstract Premise The huge diversity of Salix subgenus Chamaetia/Vetrix clade in North America and the lack of phylogenetic resolution within this clade has presented a difficult but fascinating challenge for taxonomists to resolve. Here we tested the existing taxonomic classification with molecular tools. Methods In this study, 132 samples representing 46 species from 22 described sections of shrub willows from the United States and Canada were analyzed and combined with 67 samples from Eurasia. The ploidy levels of the samples were determined using flow cytometry and nQuire. Sequences were produced using a RAD sequencing approach and subsequently analyzed with ipyrad, then used for phylogenetic reconstructions (RAxML, SplitsTree), dating analyses (BEAST, SNAPPER), and character evolution analyses of 14 selected morphological traits (Mesquite). Results The RAD sequencing approach allowed the production of a well‐resolved phylogeny of shrub willows. The resulting tree showed an exclusively North American (NA) clade in sister position to a Eurasian clade, which included some North American endemics. The NA clade began to diversify in the Miocene. Polyploid species appeared in each observed clade. Character evolution analyses revealed that adaptive traits such as habit and adaxial nectaries evolved multiple times independently. Conclusions The diversity in shrub willows was shaped by an evolutionary radiation in North America. Most species were monophyletic, but the existing sectional classification could not be supported by molecular data. Nevertheless, monophyletic lineages share several morphological characters, which might be useful in the revision of the taxonomic classification of shrub willows.

Abstract Specialist and generalist insect herbivores evolved different strategies to process host‐plant metabolites. We explored frass composition in Laothoe populi (L.) caterpillars (Lepidoptera: Sphingidae), a specialist moth species that is closely associated with plants of the Salicaceae (Mirb.) family. We fed the caterpillars with leaves from three willow species ( Salix caprea L., S. purpurea L. and S. viminalis L.), one willow hybrid ( S. x rubens Schrank) and one poplar species ( Populus tremula L.). Using untargeted metabolomics, we compared the chemical composition and variation among leaf and frass samples. We quantified the chemical variation using either a simple index based on the presence and concentration of metabolites (i.e., Bray‐Curtis) or an index that additionally accounts for chemical structural‐compositional similarity (CSCS) among metabolites. Due to the high degree of dietary specialisation, we expected low host‐specific variation in frass composition among the caterpillars. Based on literature, we further hypothesised that ingested salicinoids will be largely modified in the caterpillar gut while flavonoids will be often passively excreted. Finally, we compared chemical variation among the samples when quantified with Bray‐Curtis or CSCS metrics. As expected, we found relatively low host‐specific variation in the chemical composition of caterpillar frass. Our results further suggest that flavonoids were largely passively excreted by the caterpillars, while salicinoids were metabolised in the gut. Finally, we found that chemical composition measures based on Bray‐Curtis overestimated the differences in chemical composition between frass and leaves, suggesting that for these type of samples CSCS measures are better suited to reflect metabolic changes more realistically.

Plastome phylogenomics is used in a broad range of studies where single markers do not bear enough information. Phylogenetic reconstruction in the genus Salix is difficult due to the lack of informative characters and reticulate evolution. Here, we use a genome skimming approach to reconstruct 41 complete plastomes of 32 Eurasian and North American Salix species representing different lineages, different ploidy levels, and separate geographic regions. We combined our plastomes with published data from Genbank to build a comprehensive phylogeny of 61 samples (50 species) using RAxML (Randomized Axelerated Maximum Likelihood). Additionally, haplotype networks for two observed subclades were calculated, and 72 genes were tested to be under selection. The results revealed a highly conserved structure of the observed plastomes. Within the genus, we observed a variation of 1.68%, most of which separated subg. Salix from the subgeneric Chamaetia/Vetrix clade. Our data generally confirm previous plastid phylogenies, however, within Chamaetia/Vetrix phylogenetic results represented neither taxonomical classifications nor geographical regions. Non-coding DNA regions were responsible for most of the observed variation within subclades and 5.6% of the analyzed genes showed signals of diversifying selection. A comparison of nuclear restriction site associated DNA (RAD) sequencing and plastome data on a subset of 10 species showed discrepancies in topology and resolution. We assume that a combination of (i) a very low mutation rate due to efficient mechanisms preventing mutagenesis, (ii) reticulate evolution, including ancient and ongoing hybridization, and (iii) homoplasy has shaped plastome evolution in willows.

Understanding variation in resource specialization is important for progress on issues that include coevolution, community assembly, ecosystem processes, and the latitudinal gradient of species richness. Herbivorous insects are useful models for studying resource specialization, and the interaction between plants and herbivorous insects is one of the most common and consequential ecological associations on the planet. However, uncertainty persists regarding fundamental features of herbivore diet breadth, including its relationship to latitude and plant species richness. Here, we use a global dataset to investigate host range for over 7,500 insect herbivore species covering a wide taxonomic breadth and interacting with more than 2,000 species of plants in 165 families. We ask whether relatively specialized and generalized herbivores represent a dichotomy rather than a continuum from few to many host families and species attacked and whether diet breadth changes with increasing plant species richness toward the tropics. Across geographic regions and taxonomic subsets of the data, we find that the distribution of diet breadth is fit well by a discrete, truncated Pareto power law characterized by the predominance of specialized herbivores and a long, thin tail of more generalized species. Both the taxonomic and phylogenetic distributions of diet breadth shift globally with latitude, consistent with a higher frequency of specialized insects in tropical regions. We also find that more diverse lineages of plants support assemblages of relatively more specialized herbivores and that the global distribution of plant diversity contributes to but does not fully explain the latitudinal gradient in insect herbivore specialization.