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Rahier, Martine

Nom
Rahier, Martine
Affiliation principale
Fonction
Professeure ordinaire
Email
Martine.Rahier@unine.ch
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https://libra.unine.ch/handle/123456789/99

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  • Publication
    Accès libre
    Inferring reciprocal evolutionary histories in associated species of plants and insects in two european pollination systems
    (2010)
    Espíndola, María Anahí
    ;
    Alvarez, Nadir
    ;
    Rahier, Martine 
    ;
    Carstens, Bryan C.
    ;
    Després, Laurence
    ;
    Gibernau, Marc
    ;
    Gugerli, Felix
    ;
    Küpfer, Philippe 
    Coevolution is defined as reciprocal evolutionary changes that might arise at any spatiotemporal scale. Despite every organism on Earth undergoes coevolutionary interactions, cases of one-to-one specific relationships are generally rare. However, because of the reduced number of interacting species they concern, these species-specific associations are interesting to evolutionary biologists because they allow testing hypotheses in simple frameworks. Despite the history and evolution of coevolutionary interactions have been studied in several cases in the last decade, this topic remains difficult to fully circumscribe because of the multiplicity of factors that affect one or the other species concerned. Moreover, a lot is known about coevolution at a small scale, but little has been done at larger and more integrative scales spanning wider spatiotemporal ranges. Phylogeography is a young area of biology that allows understanding the distribution of lineages in space and time. Despite that the idea of parallely studying the history of species involved in specific interactions appears simple, this has rarely been done until now probably because of the technical efforts this would represent. From a theoretical point of view, we could propose that in specific interactions, because of the dependence between the partners involved, we should observe some phylogeographic pattern associated to the type of interaction studied. In this way, while partners of mutualistic interactions should present similar postglacial histories, this should not be true for those associated by antagonistic relationships. In this thesis, we exploit different techniques and approaches to test this general hypothesis. The final aim of this study is thus to understand if it is possible to identify a pattern of comparative phylogeography in relation to the type of interaction, using as case-studies two specific and obligate European interactions: the antagonistic relationship established between Arum maculatum L. (Araceae) and its Psychodid (Diptera) pollinating flies, and the nursery pollination mutualism involving Trollius europaeus L. (Ranunculaceae) and the Chiastocheta (Diptera: Anthomyiidae) species complex. Before testing our comparative phylogeographic hypotheses and because studying the phylogeography of interactions requires a wide knowledge of the environmental, taxonomic and historical frameworks in which these ecological relationships arose, it was first needed to clearly delimitate the identity of species, their distribution and the environmental factors influencing their survival to finally understand their comparative history. We thus took advantage of the potentialities that interdisciplinary approaches provide, applying molecular taxonomy, biological and evolutionary methods, biogeographic inferences, ecological niche models and hindcasting techniques, as well as classical and recently-developed phylogeographic analyses. Our results indicate that the phylogeographic patterns of these specific and obligate antagonistic and mutualistic relationships appear to be related to the type of interaction. Antagonistic partners presented incongruent phylogeographic patterns, what can be notably explained by differences in their life-history traits. Species involved in mutualistic interactions partly showed congruent phylogeographic patterns (particularly in the cases of T. europaeus and C. dentifera). Flies interacting with T. europaeus appear moreover to present different histories, regardless of their important ecological similarities. These results demonstrate that the systems studied appear to be far more complex than initially supposed, with crossed effects of environmental and historical features on the dynamics of the interaction. Because of the high complexity and interdependency of factors affecting one or the other partner, performing investigations in an interdisciplinary framework appears indispensable to disentangle the dynamics of interactions.
  • Publication
    Accès libre
    Host-Plant Use in the Specialist Leaf Beetle Oreina gloriosa : the Role of Linear Furanocoumarins
    (2004)
    Nessi, Luca
    ;
    Rahier, Martine 
    ;
    Tabacchi, Raffaele 
    ;
    Küpfer, Philippe 
    ;
    Städler, Erich
    La chrysomèle alpine Oreina gloriosa (Coleoptera: Chrysomelidae) est hautement spécialisée sur l'ombellifère Peucedanum ostruthium (Apiaceae), l'impératoire. Dans la nature, adultes et larves sont très sélectifs: les uns et les autres se nourrissent exclusivement de feuilles, mais surtout, ils choisissent certaines plantes et en ignorent complètement d'autres. En outre, des études préliminaires ont suggéré que les larves soient actives la nuit. Comme de nombreuses autres apiacées, P. ostruthium synthétise plusieurs furanocoumarines linéaires, métabolites secondaires décrits pour leur activité antiappétante et/ou toxique à l'égard d'insectes phytophages généralistes. Par ailleurs, les furanocoumarines linéaires sont connues pour leur action phototoxique: le rayonnement ultraviolet proche du spectre visible induit chez ces substances une excitation électronique qui peut déterminer différents processus toxiques au niveau cellulaire. Le but de cette étude a été de vérifier l'hypothèse selon laquelle la chimie secondaire de P. ostruthium constituerait un facteur explicatif de l'utilisation de la plante hôte par la strictement monophage O. gloriosa. Toute la recherche a été menée au sein d'une seule population de la plante et de la population du coléoptère qui lui est inféodée.

    Dans un premier temps, nous avons étudié le rythme d'activité de la chrysomèle sur les 24 heures dans son mileu. Ce travail a révélé que les larves se nourrissent exclusivement pendant la nuit et s'enfoncent dans le sol pendant le reste de la journée. Certaines jeunes larves se retirent dans les gaines ou les bourgeons. Au contraire, les adultes ont été observés sur les plantes pendant les 24 heures. Nous avons ensuite vérifié si la croissance larvaire était affectée par la photoactivation des furanocoumarines. Pour cela, les larves ont été placées dans deux environnements lumineux, avec et sans rayonnement ultraviolet. La mortalité a été plus grande dans le deuxième traitement: nous n'avons néanmoins pas pu tirer de conclusions définitives sur l'effet de la photoactivation des furanocoumarines. Dans un travail de terrain, nous avons montré que les invertébrés antagonistes de O. gloriosa sont essentiellement actifs pendant la nuit. Au vu du chevauchement entre les périodes d'activité de la chrysomèle et de ses prédateurs, nous suggérons que les ennemis naturels ne constituent pas la raison d'être du comportement des larves. Nous proposons que l'activité nocturne représente une contre-adaptation à la photoactivité des furanocoumarines. Il s'agirait alors d'un rare exemple d'adaptation comportementale chez un insecte spécialisé sur une plante hôte avec furanocoumarines linéaires. En effet, les insectes avec un régime alimentaire strictement lié à ces métabolites secondaires, disposent en général de moyens de détoxification métaboliques. La différence comportementale entre larves et adultes est probablement liée à l'épaisseur et à la pigmentation de la cuticule imaginale, vraisemblablement opaque à la lumière ultraviolette.

    Nous avons également vérifié l'hypothèse selon laquelle O. gloriosa dans la nature sélectionnerait les plantes en fonction de leur attributs chimiques. Nous avons échantillonné des plantes fortement consommées et des plantes libres de toute attaque d'O. gloriosa. Les feuilles ainsi récoltées ont été extraites et analysées par chromatographie liquide pour déterminer la composition qualitative et quantitative en furanocoumarines linéaires. La chimie secondaire a permis de bien discriminer plantes consommées et non consommées. O. gloriosa est probablement à même de reconnaître certaines de ces substances comme signaux ou elle est en mesure de mieux en tolérer certaines plutôt que d'autres dans leur action toxique. De toute façon, la chimie secondaire explique, du moins en partie, l'utilisation de la plante hôte par ce coléoptère strictement monophage au sein d'une population de sa plante nourricière. Cet exemple révèle la nécessité d'approfondir l'investigation, trop souvent négligée, du rôle de la variation chimique intrapopulationnelle dans les études concernant la convenance de l'hôte pour des insectes spécialistes., The alpine leaf beetle Oreina gloriosa (Coleoptera: Chrysomelidae) is highly specialized on the umbelliferous plant Peucedanum ostruthium (Apiaceae). Adults and larvae feed on leaves and clearly discriminate between individual plants in nature. Preliminary studies suggested that the larvae are active at night. Like many other apiaceous plants, P. ostruthium produces several linear furanocoumarins, secondary metabolites shown to be deterrent and/or toxic to generalist herbivorous insects. Moreover, linear furanocoumarins are well known as phototoxins: when irradiated with long-wave ultraviolet light, these compounds enter an excited state which can mediate different within cell toxic processes. The aim of the present thesis was to verify the hypothesis that the secondary chemistry of P. ostruthium is a factor explaining the actual host-plant use by the strictly monophagous O. gloriosa. All of our work concentrated on a single plant population and its affiliated beetle population.

    We firstly investigated the daily activity rhythm of the beetles in the field. Larvae have been shown to feed exclusively at night and to burrow into the soil for the remainder of the day. Some young larvae hid within sheaths or buds during daylight. In contrast, adults were found on the plants over the 24 hours. We further verified if the performance of the larvae was affected by the photoactivation of the linear furanocoumarins. Larvae were forced to remain in two different light environments, with and without ultraviolet radiation. O. gloriosa was shown to suffer higher mortality when forced under light with ultraviolet radiation. Nonetheless, we were not able to draw definitive conclusions on the effect of the furanocoumarin photoactivation. In a field study, we showed that invertebrate natural enemies of O. gloriosa are mainly active at night. Because of the congruence in time between the beetle and the predators activities we suggest that natural enemies should not be considered as a driving force for the larval night behaviour. We propose that night activity counteracts the photoactive furanocoumarins and should be considered a rare example of behavioural adaptation in a linear furanocoumarin-specialized insect. In fact, specialists are reported to rely on metabolic detoxification. The difference in the behaviour between larvae and adults may be related to the thick and pigmented cuticle of the latter, probably opaque to ultraviolet light.

    Finally we tested the hypothesis that individual plants remain free from O. gloriosa attack due to their chemical attributes. Samples of plants hosting actively feeding O. gloriosa (strongly consumed leaves) and plants exhibiting no damage (unattacked leaves) were collected in the field and examined by HPLC for their qualitative and quantitative linear furanocoumarin composition. The secondary chemistry allowed us to clearly segregate attacked and unattacked plants. We suggest that plant chemistry, relative to the ability of O. gloriosa to tolerate or recognise plant chemicals, can to some extent explain the host use of this monophagous leaf beetle within a population of its host plant. This shows the need for further work on the neglected impact, for specialists, of intrapopulation variation in secondary chemistry on host suitability., Il crisomelide alpino Oreina gloriosa (Coleoptera: Chrysomelidae) è altamente specializzato sull'ombellifera Peucedanum ostruthium (Apiaceae), l'imperatoria vera o erba rena. Adulti e larve in natura sono estremamente selettivi: gli uni e le altre si nutrono solo di foglie, ma distinguono tra le singole piante cibandosi di alcune e trascurandone completamente altre. Inoltre indagini preliminari hanno suggerito che le larve siano attive di notte. Analogamente ad altre ombellifere, P. ostruthium contiene varie furanocumarine lineari, metaboliti secondari con effetto deterrente e/o tossico per insetti fitofagi generalisti. Le furanocumarine lineari sono pure note per la loro fototossicità: la luce ultravioletta di lunghezza d'onda vicina allo spettro visibile induce in queste sostanze un'eccitazione elettronica che può provocare vari processi tossici a livello cellulare. La nostra tesi aveva lo scopo di verificare l'ipotesi secondo cui la chimica secondaria di P. ostruthium spiegherebbe l'utilizzazione della pianta ospite - così come l'osserviamo oggi - da parte dello specialista O. gloriosa. Per l'insieme della ricerca ci siamo concentrati su una singola popolazione della pianta e sulla popolazione dell'insetto ad essa legata.

    Per prima cosa abbiamo studiato il ritmo di attività dei coleotteri in natura. Abbiamo mostrato che le larve si nutrono esclusivamente di notte e passano il resto della giornata nel suolo. Alcune giovani larve si ritirano dentro le guaine o i germogli. Gli adulti invece sono presenti sulle piante durante tutte le 24 ore. In una ricerca successiva abbiamo verificato se la crescita larvale sia compromessa dalla fotoattivazione delle furanocumarine lineari. Abbiamo tenuto le larve in due distinti ambienti luminosi, con e senza radiazione ultravioletta. Quest'ultima ha determinato una maggiore mortalità di O. gloriosa, tuttavia non siamo stati in grado di formulare conclusioni definitive sull'effetto della fotoattivazione. Un lavoro sul campo ha mostrato che gli invertebrati antagonisti del crisomelide sono essenzialmente attivi di notte. Dato che i periodi di attività di O. gloriosa e dei suoi predatori si sovrappongono, suggeriamo che i nemici naturali non costituiscano il fattore selettivo all'origine del comportamento notturno delle larve. Proponiamo che questa limitazione notturna nell'attività trofica rappresenti un'acquisizione adattativa contro la fotoattività delle furanocumarine. Si tratterebbe allora di un raro esempio di adattamento comportamentale in un insetto specializzato su una pianta contenente furanocumarine lineari. In effetti, in generale gli insetti con un regime alimentare strettamente legato a queste sostanze dispongono di strumenti di detossificazione metabolici. La differenza comportamentale tra larve e adulti potrebbe essere ricondotta al tegumento spesso, colorato e probabilmente opaco alla luce ultravioletta di questi ultimi.

    Abbiamo infine verificato l'ipotesi secondo cui la chimica secondaria determinerebbe l'idoneità delle singole piante per la crescita di O. gloriosa. Abbiamo proceduto ad un campionamento sul campo di piante fortemente consumate e di piante prive di qualsiasi segno d'attacco del crisomelide. Le foglie così raccolte sono state estratte e analizzate con la cromatografia liquida per determinare la composizione qualitativa e quantitativa delle furanocumarine. La chimica secondaria ha permesso di ben caratterizzare le piante consumate rispetto a quelle non utilizzate. O. gloriosa è probabilmente in grado di riconoscere talune di queste sostanze quali segnali o di tollerarne alcune piuttosto di altre nella loro azione di tossine. In ogni caso, la chimica secondaria permette di spiegare perlomeno in parte l'utilizzazione della pianta in seno ad una popolazione da parte di questo coleottero altamente specializzato. Questo risultato rivela l'interesse di ulteriori ricerche su un aspetto spesso ignorato quale è l'impatto, per artropodi specialisti, della variazione intrapopolazionale nella chimica secondaria sull'idoneità della pianta ospite.