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Rahier, Martine
Nom
Rahier, Martine
Affiliation principale
Fonction
Professeure ordinaire
Email
Martine.Rahier@unine.ch
Identifiants
Résultat de la recherche
Voici les éléments 1 - 8 sur 8
- PublicationAccès libreThe phylogeography of an alpine leaf beetle: Divergence within i Oreina elongata spans several ice ages(2010)
;Borer, Matthias ;Alvarez, Nadir ;Buerki, Sven ;Margraf, Nicolas; Naisbit, Russell E - PublicationAccès libreConserved oviposition preferences in alpine leaf beetle populations despite host shifts and isolation(2007)
;Verdon, Aline ;Margraf, Nicolas ;Davison, Anthony C.; Naisbit, Russell. E.1. Choosing the plant on which to lay their eggs is the last act of care that most female herbivorous insects bestow upon their offspring. These decisions play a pivotal role in insect–plant interactions, placing host preference under strong selection and contributing to the diversity of phytophagous insects as one of the first traits to adapt to new hosts.
2. This study presents a test of whether extreme isolation and exposure to different host plants can produce intra-specific divergence in oviposition preference in alpine insects. Geographic variation should impose selection to fine-tune host plant ranking and specificity to the plants normally encountered, to avoid wasting time during the very limited reproductive season experienced at high altitudes.
3. Beetles from five populations of Oreina elongata differing in host availability were offered three natural hosts: Cirsium spinosissimum, Adenostyles alliariae, and Adenostyles glabra. A novel application of a continuation ratio model (logistic regression) was made to sequential no-choice experiments, combined with quasi-likelihood analysis of multiple-choice experiments.
4. The results show little geographic variation in host plant choice: all populations strongly preferred Cirsium in multiple-choice trials, and in no-choice experiments laid around 47% of their remaining eggs during each stage, almost regardless of the host present.
5. Enemy-free space seems to explain the preference for Cirsium, but isolation and exposure to different plants has clearly not caused local adaptation in host plant ranking or specificity. Reasons for this conservatism despite divergence in other characteristics are discussed. - PublicationAccès libreGeographic variation in oviposition choice of a leaf beetle: the relationship between host plant ranking, specificity, and motivation(2007)
;Gotthard, Karl ;Margraf, NicolasThe degree of adaptation of herbivorous insects to their local flora is an important component of the evolutionary processes that lead to host plant specialization in insects. In this study we investigated geographic variations in the oviposition preference of the leaf beetle Oreina elongata Suffrian (Coleoptera: Chrysomelidae: Chrysolini) in relation to differences in host plant specialization, in the field. We focused on the mechanisms of host choice and asked whether potential differences among populations are due to variations in host plant ranking and/or host plant specificity. We performed a combination of simultaneous choice and sequential no-choice experiments with two of the major host plants of the beetle [Cirsium spinosissimum (L.) and Adenostyles alliariae (Gouan) (Asteraceae)]. The results suggested that spatial variation in host plant specialization has resulted in differences between populations in some aspects of the oviposition choice of O. elongata, while other aspects seem unaffected. We found no variation in host plant ranking among populations, as estimated in simultaneous choice tests. In contrast, the sequential no-choice test indicated that host plant specificity was lower in a population that never encountered the highest ranked plant in the field. This finding agreed with our expectations, and we discuss our results in relation to the commonly used hierarchical threshold model. The results suggested that the mechanism for the differences in specificity is the variation among populations in the general motivation to oviposit, rather than quantitative differences in relative preference for the two hosts. We stress that it is essential to establish which of the two mechanisms is most important, as it will affect the probability of evolutionary change in host plant ranking. - PublicationAccès libreGlacial survival and local adaptation in an alpine leaf beetle(2007)
;Margraf, Nicolas ;Verdon, Aline; Naisbit, Russell. E.The challenge in defining conservation units so that they represent evolutionary entities has been to combine both genetic properties and ecological significance. Here we make use of the complexity of the European Alps, with their genetic landscape shaped by geographical barriers and postglacial colonization, to examine the correlation between ecological and genetic divergence. Montane species, because of the fragmentation of their present habitat, constitute extreme cases in which to test if genetically distinct subgroups based on neutral markers are also ecologically differentiated and show local adaptation. In the leaf beetle Oreina elongata, populations show variation in host plant use and a patchy distribution throughout the Alps and Apennines. We demonstrate that despite very strong genetic isolation (FST = 0.381), variation in host plant use has led to differences in larval life-history traits between populations only as a secondary effect of host defence chemistry, and not through physiological adaptation to plant nutritional value. We also establish that populations that are more ecologically different in terms of larval performance are also more genetically divergent. In addition, morphological variation used to define subspecies appears to be mirrored in the population genetics of this species, resulting in almost perfect clustering based on microsatellite data. Finally, we argue from their strong genetic structure and congruent distribution that the subspecies of O. elongata were divided among the same glacial refugia within the Alps that have been proposed for alpine plants. - PublicationAccès libreThe evolution of larval foraging behaviour in response to host plant variation in a leaf beetle(2005)
;Gotthard, Karl ;Margraf, Nicolas; The evolutionary causes of variation in host specialization among phytophagous insects are still not well understood and identifying them is a central task in insect–host plant biology. Here we examine host utilization of the chrysomelid beetle Oreina elongata that shows interpopulation variation in the degree of specialization. We focus on larval behaviour and on what selection pressures may favour the use of two different larval host plants (Adenostyles alliariae and Cirsium spinosissimum) in one population as opposed to specialization onto one of them as is seen in other populations. The results suggest that the degree of exploratory foraging behaviour is higher in larvae from the two-host population than in single host populations, and a field survey of the two-host population also indicated that larvae do move between host species. A field experiment indicated that predation rates on O. elongata larvae in the two-host population are higher on one of the host species, A. alliariae, than on the alternative C. spinosissimum. In combination with earlier results this finding suggest that larvae move between hosts to obtain better food on one host, and to get better protection from predators on the other. It appears that in this two-host situation a single plant species does not provide the most beneficial conditions in all parts of O. elongata life cycle and individuals may obtain different plant-specific benefits by moving between host species. This heterogeneous host situation appears to have selected for the explorative larval foraging strategy seen in the in the two-host population. In general, the results support the notion that to understand patterns of host plant use in insects it is often vital to consider a range of host related selection pressures whose relative importance may vary between life stages of the insect. - PublicationAccès libreIsolation and characterization of microsatellite loci in the Alpine leaf beetle, Oreina elongata(2005)
;Margraf, Nicolas ;Gautschi, B.For a study of local adaptations in the Alpine leaf beetle, Oreina elongata, we developed six microsatellite loci and screened them in 305 individuals from 13 populations. All markers were polymorphic with three to 15 alleles per locus. Average observed and expected heterozygosity values were 0.14 and 0.62, respectively. Four markers showed heterozygote deficiency and deviated significantly from Hardy–Weinberg expectations, indicating the presence of null alleles. - PublicationAccès libreThe growth strategy of an alpine beetle: maximization or individual growth adjustment in relation to seasonal time horizons ?(2003)
;Margraf, Nicolas ;Gotthard, Karl1. Life-history theory typically predicts that juvenile growth rate should be maximized and consequently juvenile period minimized. However, in several examples it has been shown that insect larvae do not always grow as fast as they can and this has been explained by costs of high growth rate, typically higher juvenile mortality rate. Hence, some insect larvae have the ability to adaptively adjust growth rate to catch up if development is delayed.
2. The presence of such ability was tested for in the alpine beetle Oreina elongata Suffrian. In this species, the favourable period for development is relatively short and of unpredictable length, and individuals are chemically defended against predation; factors that could affect the balance between the benefits and costs of high growth rate.
3. Here it is shown that when time stressed, O. elongata larvae were able to increase growth rate, accelerate development and reach the normal final weight.
4. Hence, individual growth rate adjustment was present in a situation where its adaptive value appeared to be relatively weak, which supports the notion that flexible growth strategies are a common phenomenon in temperate insects. - PublicationAccès libreLocal adaptations in an alpine leaf beetle(2003)
;Margraf, NicolasL'adaptation locale ainsi que la spécialisation sur un hôte ont souvent été considérés comme les fruits de l'action combinée de trois forces : la sélection naturelle, la dérive génétique et le flux génique. Chez les insectes phytophages, la relation entre l'herbivore et sa plante hôte est si intime que les pressions de sélection entraînant la spécialisation de l'insecte, ne sont souvent envisagées qu'au travers de la plante. Dans notre étude, nous avons examiné les adaptations locales chez la Chrysomèle alpine Oreina elongata dont les populations présentent un sélection variable des quatre plantes hôtes connues chez ce coléoptère. Nous avons testé plusieurs caractères susceptible de révéler des adaptations locales liées à plantes hôtes et qui participent au processus de spécialisation. Nous n'avons pas mis en évidence de différence entre les populations quant aux performances larvaires, ni aux préférences d'oviposition qui puissent être attribuées à l'utilisation d'un certain type de plante en nature. Cependant, nous avons mis en lumière des adaptations comportementales des populations présentant plusieurs type d'hôtes qui permettent aux larves de ces dernières de bénéficier des avantages liés à l'utilisation de plusieurs hôtes. Dans une étude génétique comprenant 13 populations et mettant en œuvre des marqueurs microsatellites, nous avons montré la forte structuration génétique des populations d'O. elongata, ce qui nous a permis de désigner la dérive génétique comme facteur principal de différentiation génétique entre populations. L'affiliation géographique s'est révélée être bien plus fortement corrélée avec les différences génétiques que l'utilisation d'un type de plante. Les groupes formés par les populations se sont démontrés monophylétique et correspondent aux sous-espèces confirmant ainsi la différentiation morphologique en sous-groupe. Nous avons pu rejeter l'idée d'une spécialisation liée à la plante hôte chez O. elongata et nous avons conclu que les facteurs qui sont susceptibles de conduire cette espèce dans un processus de spéciation sont des forces de sélection indépendantes des plantes hôtes mais qui comprennent un phénomène de dérive génétique souligné par un flux génique faible., Local adaptation and host specialization is often seen as the result of the balance between three major forces: natural selection, genetic drift and gene flow. In phytophagous insects, the relationship between the insect and its host plant is so intimate that the selective pressures leading to local adaptation are often seen as being related to host plant use only. In this study, we investigate local adaptation in populations of the alpine leaf-beetle Oreina elongata that differ in the availability of the four host plant of the species. We tested several traits that are susceptible to adapt according to the host plant locally available and that are known mechanisms of specialization. We found no differences among populations in larval performance and oviposition preferences that were attributable to differences in host plant use in the field. However, we revealed a behavioural adaptation of a two host population that allows individuals to take advantage of the presence of both plants at this location. A genetic survey using microsatellite markers on 13 populations of the species showed a highly genetically structured distribution and designated genetic drift as the main factor in population genetic differentiation. Instead of host plant use, regional affiliation turned out to be strongly correlated to genetic differences. Populations perfectly clustered according to their subspecies, therefore confirming previous morphological work. We can then dismiss the idea of host plant specialization in O. elongata and conclude that the factors that may ultimately drive O. elongata towards a speciation event are selective forces independent of host plant use but genetic drift supported by very weak gene flow.