Bodil K. Ehlers

QUALITATIVE AND QUANTITATIVE VARIATION IN MONOTERPENE CO-OCCURRENCE AND COMPOSITION IN THE ESSENTIAL OIL OF THYMUS VULGARIS CHEMOTYPES

Thymus vulgaris has a chemical polymorphism with six different chemotypes that show marked spatial segregation in nature. Although some populations have a single chemotype in majority, many have two or three chemotypes. In this study we analyze the quantitative variation among T. vulgaris populations in the percentage of oil composed of the dominant monoterpene(s) for each chemotype. In general, phenolic chemotypes (thymol and carvacrol), which occur at the end of the biosynthetic chain, have a significantly lower proportion of their oil composed of their dominant monoterpene than nonphenolic chemotypes (geraniol, α-terpineol, and linalool). This is due to the presence of high amounts of precursors (γ-terpinene and paracymene) in the oil of phenolic chemotypes. The essential oil of the nonphenolic thuyanol chemotype has four characteristic monoterpenes that together make up a lower proportion of the oil than the single dominant monoterpene of the other nonphenolic chemotypes. For all chemotypes, the percentage composition of the dominant monoterpene decreased significantly at sites where the chemotype is not the majority type. This decrease is correlated with a significant increase in either the proportion of the two precursors for the thymol chemotype or the monoterpenes characteristic of the other chemotypes at the site. The latter result suggests that a plant with dominant genes is responsible for the production of different monoterpenes can produce several molecules.

Soil microorganisms alleviate the allelochemical effects of a thyme monoterpene on the performance of an associated grass species.

Background Plant allelochemicals released into the soil can significantly impact the performance of associated plant species thereby affecting their competitive ability. Soil microbes can potentially affect the interaction between plant and plant chemicals by degrading the allelochemicals. However, most often plant-plant chemical interactions are studied using filter paper bioassays examining the pair-wise interaction between a plant and a plant chemical, not taking into account the potential role of soil microorganisms. Methodology/Principal findings To explore if the allelopathic effects on a grass by the common thyme monoterpene “carvacrol” are affected by soil microorganisms. Seedlings of the grass Agrostis capillaris originating from 3 different thyme sites were raised in the greenhouse. Seedlings were grown under four different soil treatments in a 2*2 fully factorial experiment. The monoterpene carvacrol was either added to standard greenhouse soil or left out, and soil was either sterilized (no soil microorganisms) or not (soil microorganisms present in soil). The presence of carvacrol in the soil strongly increased mortality of Agrostis plants, and this increase was highest on sterile soil. Plant biomass was reduced on soil amended with carvacrol, but only when the soil was also sterilized. Plants originating from sites where thyme produces essential oils containing mostly carvacrol had higher survival on soil treated with that monoterpene than plants originating from a site where thyme produced different types of terpenes, suggesting an adaptive response to the locally occurring terpene. Conclusions/Significance The study shows that presence of soil microorganisms can alleviate the negative effect of a common thyme monoterpene on the performance of an associated plant species, emphasizing the role of soil microbes in modulating plant-plant chemical interactions.

Genetic variation for sensitivity to a thyme monoterpene in associated plant species

Recent studies have shown that plant allelochemicals can have profound effects on the performance of associated species, such that plants with a history of co-existence with “chemical neighbour” plants perform better in their presence compared to naïve plants. This has cast new light on the complexity of plant–plant interactions and plant communities and has led to debates on whether plant communities are more co-evolved than traditionally thought. In order to determine whether plants may indeed evolve in response to other plants’ allelochemicals it is crucial to determine the presence of genetic variation for performance under the influence of specific allelochemicals and show that natural selection indeed operates on this variation. We studied the effect of the monoterpene carvacrol—a dominant compound in the essential oil of Thymus pulegioides—on three associated plant species originating from sites where thyme is either present or absent. We found the presence of genetic variation in both naïve and experienced populations for performance under the influence of the allelochemical but the response varied among naïve and experienced plant. Plants from experienced populations performed better than naïve plants on carvacrol soil and contained significantly more seed families with an adaptive response to carvacrol than naïve populations. This suggests that the presence of T. pulegioides can act as a selective agent on associated species, by favouring genotypes which perform best in the presence of its allelochemicals. The response to the thyme allelochemical varied from negative to neutral to positive among the species. The different responses within a species suggest that plant–plant interactions can evolve; this has implications for community dynamics and stability.

Sex inheritance in gynodioecious species: a polygenic view

Gynodioecy is defined as the coexistence of two different sexual morphs in a population: females and hermaphrodites. This breeding system is found among many different families of angiosperms and is usually under nucleo-cytoplasmic inheritance, with maternally inherited genes causing male sterility and nuclear factors restoring male fertility. Numerous theoretical models have investigated the conditions for the stable coexistence of females and hermaphrodites. To date, all models rest on the assumption that restoration of a given male sterile genotype is controlled by a single Mendelian factor. Here, we review data bearing on the genetic determinism of sex inheritance in three gynodiecious plant species. We suggest that restoration of male fertility is probably best viewed as a quantitative trait controlled by many loci. We develop a threshold model that accommodates an underlying polygenic trait, which is resolved at the phenotypic level in discrete sexual morphs. We use this model to reanalyse data in Thymus vulgaris, Silene vulgaris and Plantago coronopus. A simple Mendelian inheritance of sex determinism is unlikely in all three species. We discuss how our model can shed additional light on the genetics of restoration and point towards future efforts in the modelling of gynodioecy.

The openness of a flower and its number of flower-visitor species

We present a phylogeny for Mesembryanthemoideae (Aizoaceae) based on sampling of nearly all species and subspecies of the subfamily and analysis ofcptrnL-F, rbcL-atpB, rps16, nrITSl and morphology. The larger genera Phyllobolus and Mesembryanthemum are not monophyletic. Although some clades can be circumscribed with morphological (often homoplasious) synapomorphies, several clades are impossible to characterise morphologically. We recognise a single genus, Mesembryanthemum, in Mesembryanthemoideae. The genera Aptenia, Aridaria, Aspazoma, Brownanthus, Caulipsolon, Dactylopsis, Phyllobolus, Prenia, Psilocaulon, Sceletium, and Synaptophyllum are reduced to synonymy. Mesembryanthemum, which now consists of 101 species without recognised sections, can be distinguished by several uniquely derived morphological characters. Mesembryanthemum longipapillosum, which had recently been reduced to synonymy, is reinstated.

ONGOING ADAPTATION TO MEDITERRANEAN CLIMATE EXTREMES IN A CHEMICALLY POLYMORPHIC PLANT

Although extreme climatic events may have profound effects on ecological systems, there is a marked lack of information on adaptation to such events. In this study, we employed reciprocal transplantation on both a geographic scale (experimental sites 200 km apart in different parts of the range of the study species) and a local landscape scale (reciprocal populations separated by 2-8 km) to study the performance of different chemical forms of Thymus vulgaris which naturally occur in different climatic environments. Survival and growth were analyzed in relation to long-term and contemporary climate data in natural populations and our experimental sites. The reciprocal transplants involved a period of six years for clones transplanted in experimental field sites on a geographic scale and three years for seedlings transplanted among natural populations at the local landscape level. Cloned transplants on a geographic scale produced evidence for local adaptation to either summer drought, primarily following the extreme summer drought of 2003, or severe early-winter freezing. Chemotypes that show high survival after intense summer drought showed poor survival after intense early- winter freezing and vice versa, results which directly accord with climate data for their original sites. On the local landscape scale, we found further evidence for local adaptation to summer drought but not to winter freezing (probably due to the absence of extreme freezing during the three years of this study). Future modifications to the occurrence and frequency of extreme climate events may have a profound influence on the spatial distribution of thyme chemotypes.

Flower production in relation to individual plant age and leaf production among different patches of Corydalis intermedia

The life history of an organism can be viewed as the combination of allocations made to maintenance, growth, and reproduction. Allocation to these functions are constrained by trade-offs as increased investment to one function may happen at the expense of another. Moreover, because fecundity and survival probabilities are affected by both the state of an individual and by its surrounding environment, optimal allocation to reproduction and growth may vary with both individual size/age and with the habitat in which it lives. In this study we aim to describe how flower production varies with individual plant age and leaf production among different patches of the perennial herb Corydalis intermedia. We take advantage of the construction of the underground storage organ to estimate the age of individual plants which allows us tacitly to relate flower and leaf production to individual age and successional status of the patch. We sampled all individuals present in nine patches from the same forest and estimated their age, flower production and total leaf area. The age distributions showed that each patch was most often dominated by a few and consecutive age classes. In patches where individuals had the oldest mean age, very few or no juvenile age classes were found suggesting that recruitment had ceased. Based on the age distribution of the patches we propose that the dynamics may best be described as metapopulational with colonization of newly formed open forest gaps and a successionally determined extinction as the patch gradually becomes too shaded for recruitment. Both mean flower production, leaf area and age varied significantly among patches. Flower production increased with both increasing age and leaf area. We found no indication of a trade off between reproduction and vegetative growth since flower production showed a positive relation with leaf production even after removing the effect of age. Number of flowers produced by plants of the same age but growing in different patches did not vary indicating that the difference among patches mainly was due to a difference in age distribution. No individuals produced flowers before they reached an estimated age of three years. Production of flowers followed a power function with increasing age. Our data suggests that C. intermedia plants change their allocation strategy with age investing a relatively large amount of energy in flower production immediately after the immature growth phase when recruitment in their patch may be high. Production of flowers then reaches a plateau around the age of 11 years after which number of flowers produced stays constant.

An allelopathic plant facilitates species richness in the Mediterranean garrigue

Summary Positive plant–plant interactions are known to increase species richness in stressful and poor habitats that are often species poor, but the role of facilitative interactions in species-rich communities is less well understood. It has been proposed that allelopathic plants may create non-transitive species interactions, which increase species coexistence, and that such indirect facilitation may be important in species-rich communities. We examined species richness in 12 different plant communities all dominated by the aromatic Thymus vulgaris that produces monoterpenes known to inhibit germination and growth of other plants. We found consistently, and across communities, higher species richness in microsites with thyme than without. Species richness in microsites with, respectively without, two other perennial plants did not differ, suggesting that increased species richness in thyme microsites is due to the presence of thyme. We found a more similar species composition among thyme microsites and positive estimates of thyme on landscape richness enhancement, indicating that thyme also affect richness at the community level. However, across communities, we did not find species consistently confined to thyme microsites, albeit within communities some plants were exclusive to thyme. Abundance of a dominant grass was reduced in microsites where thyme produces the monoterpene carvacrol, suggesting that one mechanism by which thyme facilitates species richness was by the repression of a superior competitor that could allow other species to persist. However, this does not explain higher species richness in microsites of thyme producing other monoterpenes. We discuss how chemical variation in thyme and adaptation of associated species to local monoterpenes may affect richness and community diversity. Synthesis. Allelopathic plants are generally believed to negatively impact upon the performance of associated species. However, allelopathic plants may be important determinants of species richness at the community level by creating microenvironments where species-specific interactions differ. Our finding shows that thyme increases species richness both locally and at the community level by creating a mosaic of thyme-modified and unmodified microsites differing in richness and composition. We suggest that this may also apply to other aromatic plants common in Mediterranean vegetation.

A New cis-Sabinene Hydrate Chemotype Detected in Large Thyme (Thymus pulegioides L.) Growing Wild in Denmark

Abstract Foliage was collected from flowering individuals of large thyme (Thymus pulegioides L.) at seven localities in southern Denmark. The leaf material was soaked in 96% ethanol and subsequently analyzed for terpenes using GC and GC/MS. Five chemotypes were detected: carvacrol, linalool, geraniol and thymol, all of which are known to appear in large thyme. In addition, individuals containing the monoterpene cis-sabinene hydrate as the dominant component of their essential oil were found. A sabinene hydrate chemotype has not previously been detected in large thyme, although it does occur in other thyme species.

Covariation and phenotypic integration in chemical communication displays: biosynthetic constraints and eco-evolutionary implications

Chemical communication is ubiquitous. The identification of conserved structural elements in visual and acoustic communication is well established, but comparable information on chemical communication displays (CCDs) is lacking. We assessed the phenotypic integration of CCDs in a meta-analysis to characterize patterns of covariation in CCDs and identified functional or biosynthetically constrained modules. Poorly integrated plant CCDs (i.e. low covariation between scent compounds) support the notion that plants often utilize one or few key compounds to repel antagonists or to attract pollinators and enemies of herbivores. Animal CCDs (mostly insect pheromones) were usually more integrated than those of plants (i.e. stronger covariation), suggesting that animals communicate via fixed proportions among compounds. Both plant and animal CCDs were composed of modules, which are groups of strongly covarying compounds. Biosynthetic similarity of compounds revealed biosynthetic constraints in the covariation patterns of plant CCDs. We provide a novel perspective on chemical communication and a basis for future investigations on structural properties of CCDs. This will facilitate identifying modules and biosynthetic constraints that may affect the outcome of selection and thus provide a predictive framework for evolutionary trajectories of CCDs in plants and animals.