Summary• Here, we evaluate the role of pollen limitation and selfing in the maintenance of labile sex expression in subdioecious plant species.• We used a literature survey to explore which factors correlated with a significant occurrence of hermaphrodites in dioecious species. We developed models to explore the selective maintenance of labile sex expression. The models had similar ecological assumptions but differed in the genetic basis of sex lability.• We found that a significant frequency of hermaphrodites was associated with animal pollination, and that hermaphrodites were 'inconstant' males with perfect flowers, suggesting evolution through the gynodioecious pathway. Models showed that a modifier converting pure males into inconstant males could be maintained under a wide range of reduction in both male and female fitness. Pollen limitation and self-fertilization facilitated invasion of the modifier. Depending on the genetics of sex determination, we found pure dioecy, stable subdioecy (trioecy), and situations where inconstant males coexisted with either pure females or pure males. Under selfing and pollen limitation, certain conditions selected for inconstant males which will drive populations to extinction.• We discuss our results in relation to the evolution towards, and the breakdown of, dioecy, and the ecological and evolutionary implications of labile sex expression.

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.

Local modification of the soil environment by individual plants may affect the performance and composition of associated plant species. The aromatic plant Thymus vulgaris has the potential to modify the soil through leaching of water-soluble compounds from leaves and litter decomposition. In southern France, six different thyme chemotypes can be distinguished based on the dominant monoterpene in the essential oil, which is either phenolic or non-phenolic in structure. We examine how soils from within and away from thyme patches in sites dominated by either phenolic or non-phenolic chemotypes affect germination, growth and reproduction of the associated grass species Bromus erectus. To do so, we collected seeds of B. erectus from three phenolic and three non-phenolic sites. Seeds and seedlings were grown on soils from these sites in a reciprocal transplant type experiment in the glasshouse. Brome of non-phenolic origin performed significantly better on its home soil than on soil from a different non-phenolic or a phenolic site. This response to local chemotypes was only observed on soil collected directly underneath thyme plants and not on soil in the same site (<5 m away) but where no thyme plants were present. This is preliminary evidence that brome plants show an adaptive response to soil modifications mediated by the local thyme chemotypes. Reproductive effort was consistently higher in brome of phenolic origin than in brome of non-phenolic origin (on both thyme- and grass-soil), indicating that life-history variation may be related to environmental factors which also contribute to the spatial differentiation of thyme chemotypes. Moreover, we found that brome growing on thyme-soil in general was heavier than when growing on grass-soil, regardless of the origin of the brome plants. This is concordant with thyme-soil containing higher amounts of organic matter and nitrogen than grass-soil. Our results indicate that patterns of genetic differentiation and local adaptation may modify competitive interactions and possible facilitation effects in natural communities.

Using a sample of 1,403 flowering plant species, we tested the hypothesis that flower openness and flower‐visitor generalization level of a plant species correlate positively. The “flower‐visitor generalization level” Ln of a flowering plant species n, here defined as number of flower‐visiting animal species attracted to the flowers of n in a given study site, varied enormously among plant species. Its frequency distribution was extremely skewed. Within a study site, L also increased with number of flower‐visitor species A. In order to correct for this, we expressed L relatively, as the proportion of the total flower‐visitor fauna in a study site that visited a given plant species (relative generalization level, L/A). We listed the top‐10 most generalized species (both according to L and L/A) in the “world”, i.e., out of our sample of 1,403 plant species. Flower openness is defined as accessibility to the interior of the flower. We placed the blossom classes of Fægri & van der Pijl along a gradient, albeit not very well defined, of decreasing flower openness (dish‐bowl, bell‐funnel, head‐brush, tube, gullet, flag) and tested for any relationship to their generalization level. The classes differed slightly but significantly in their level of L/A. Tube, bell‐funnel, and dish‐bowl had the highest generalization level and flag, gullet, and head‐brush the lowest. Thus, flower openness and generalization level were not correlated. We discuss other factors influencing generalization level such as accessibility to pollen and nectar, morphology and behavior of visitor, and species diversity of the different functional types of visitors.

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, alpha-terpineol, and linalool). This is due to the presence of high amounts of precursors (gamma-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.

Summary 1.A plant producing secondary compounds may affect the fitness of other plants in the vicinity, and, likewise, associated plants may evolve adaptation to the presence of their 'chemical neighbour'. Species of the genus Thymus are aromatic plants, well known for their production of aromatic oils whose constitution is dominated by mono-or sesquiterpenes. A polymorphism for the production of the dominant terpene in the oil exists both within and between thyme species. 2. Here we examine the effects of two different terpenes produced by Thymus pulegioides and T. serpyllum on the performance of four associated plant species: Achillea millefolium , Agrostis capillaris , Galium verum and Plantago lanceolata . In a reciprocal transplant experiment we studied how plants naturally occurring together with thyme producing either carvacrol or β -caryophyllene perform on soil treated with these compounds. 3. We found evidence of local adaptation to the 'home' terpene. Plants originating from sites where they grow together with carvacrol-producing thyme plants also perform better on soil treated with carvacrol. One of the associated species ( A. millefolium ) also showed evidence of local adaptation to the sesquiterpene β -caryophyllene . 4. Seed germination and root biomass showed an adaptive response to soil treatment. Vegetation analysis supported the results of the reciprocal transplant experiment. When the associated species performed best on 'home' soil, thyme and the associated species also showed a positive spatial association at natural sites of origin. Moreover, coefficients of variation in plant traits were significantly lower on 'home' soil compared to other soils for both A. capillaris and A. millefolium , but higher for G. verum . 5. Synthesis . Our results show that plant species can adapt to the presence of neighbour plants that produce specific chemical compounds. This supports the idea that local plant communities may be a lot more co-evolved than was previously thought.

Many studies report that intraspecific genetic variation in plants can affect community composition and coexistence. However, less is known about which traits are responsible and the mechanisms by which variation in these traits affect the associated community. Focusing on plant-plant interactions, we review empirical studies exemplifying how intraspecific genetic variation in functional traits impacts plant coexistence. Intraspecific variation in chemical and architectural traits promotes species coexistence, by both increasing habitat heterogeneity and altering competitive hierarchies. Decomposing species interactions into interactions between genotypes shows that genotype  genotype interactions are often intransitive. The outcome of plant-plant interactions varies with local adaptation to the environment and with dominant neighbour genotypes, and some plants can recognize the genetic identity of neighbour plants if they have a common history of coexistence. Taken together, this reveals a very dynamic nature of coexistence. We outline how more traits mediating plantplant interactions may be identified, and how future studies could use population genetic surveys of genotype distribution in nature and methods from trait-based ecology to better quantify the impact of intraspecific genetic variation on plant coexistence.

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