Genetic variation for sensitivity to a thyme monot

Authors: Catrine Grønberg Jensen Bodil Kirstine Ehlers

Publish Date: 2009/11/17

Volume: 162, Issue: 4, Pages: 1017-1025

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Abstract

Recent studies have shown that plant allelochemicals can have profound effects on the performance of associated species such that plants with a history of coexistence 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 coevolved 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 stabilityMany plant species produce secondary compounds with allelopathic effects reviews in Rice 1979 Langenheim 1994 Some of these plants have the ability via leaching of allelochemicals to modify their local environment in a way that puts associated species under selective pressure to adapt to the direct as well as indirect effects of the allelochemicals In trees conifers are known for their high production and diversity of terpenes and phenolics which significantly modify the soil conditions White 1991 Pavolainen et al 1998 Prescott et al 2000 and affect the diversity of the understory vegetation Iason et al 2005 Some invasive species can nearly eradicate entire native plant communities via the modifications that come about from the leaching of their allelochemicals For instance Centaurea maculosa and Centaurea diffusa are invasive species in North America Both of these species leach specific allelochemicals which negatively impact the performance of other plant species and modify the local soil biota creating a positive feedback on themselves The negative effects of the allelochemicals are much greater in North American communities compared to native Eurasian ones eg Callaway and Aschehoug 2000 Callaway et al 2004 see also Blair et al 2008 for discussions on detection and stability of specific allelochemicals in soil Prati and Bossdorf 2004 showed similar responses in their study on the allelopathic effects of the invasive plant Alliaria petiolata The high production of phenolic compounds in Vaccinium myrtillus differentially affects the germination and growth of two spruce species having a more negative effect on the introduced rather than native spruce species Mallik and Pellissier 2000 These studies demonstrate that sensitivity to specific plant allelochemicals can vary with the history of coexistence of the interacting plantsTerpenes are the largest class of secondary compounds in plants Langenheim 1994 Gershenzon and Dudareva 2007 and their ecological role is a key component in both antagonistic and mutualistic interactions Their functions vary from having either inhibitory or stimulating effects on a number of associated organisms such as bacteria fungi other plants and herbivores Langenheim 1994 Gershenzon and Dudareva 2007 In a community context the role of terpenes in soil nutrient cycling may be particularly important It is known that terpenes in the soil can alter the nitrogen cycle by inhibiting the nitrification process thereby decreasing the amount of available nitrate in the soil and increasing that of ammonium White 1991 Pavolainen et al 1998 Hättenschwiler and Vitousek 2000 Castells et al 2003 Presence of terpenes in the soil may thereby affect the competitive interactions between plants in the community by favouring plants that are good at adjusting or adapting to the local changes in nitrogen availabilityOne plant genus famous for its production of terpenes is the aromatic Thymus The common Mediterranean species Thymus vulgaris has a genetically determined polymorphism for the production of monoterpenes constituting the main component of its essential oil A total of at least seven different socalled chemotypes each dominated by a different monoterpene are known from this species Vernet et al 1986 Thyme populations differ with respect to chemotypes produced and individual populations are usually dominated by one to two different types Thompson et al 2003 The grass Bromus erectus shows local adaptation to its home nonphenolic chemotype by germinating and growing better on soil collected under its “home” thyme chemotype compared to soil from sites where T vulgaris produces a phenolic chemotype Ehlers and Thompson 2004 Thyme species found in northern Europe also show variation in their terpene production Sáez and StahlBiskup 2002 Grøndahl and Ehlers 2008 studied four different plant species originating from sites dominated by either Thymus pulegioides producing a phenolic monoterpene or T serpyllum producing sesquiterpenes and showed that plants germinated and grew better on soil treated with their “home” terpene Plants that showed an adaptive response to their local thyme terpene originated from sites where associated plants and thyme showed a significantly positive spatial associationThe above studies demonstrate that plant allelochemicals can act as a strong selective force driving evolution in plant–plant interactions Fundamental for local adaptation to evolve is the presence of genetic variation for an adaptive response upon which selection can operate So far studies of associated plant species’ response to the presence of a “chemical neighbour” have focused on a “populationlevel” response by comparing the performance of species from different origins and history with the plant producing the allelochemical Variation in performance and sensitivity among individual plants to a plant allelochemical has been documented eg Callaway et al 2005a However as far as we know no one has yet examined the presence of genetic variation in associated species for sensitivity to specific plant allelochemicals and asked if such variation is in accordance with selection operating on thisIn this study we looked for the presence of genetic variation for sensitivity to the thyme monoterpene carvacrol a dominant component of the essential oil in T pulegioides We use maternal seed families from three different species originating from sites where they either grow together with T pulegioides experienced or from sites where T pulegioides is absent naïve All three study species are common in plant communities where T pulegioides dominates the ground vegetation We ask the following questions

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