Analyse spatiale, évaluation et cartographie du risque glissement de terrain
Summary (4 min read)
Introduction
- When introduced into a new environment, plants may be attacked by herbivores.
- Crops are often introduced into multiple geographical areas: each introduction is an additional opportunity for a new pest species to arise, as documented for instance in the pea aphid Acyrthosiphon pisum (Peccoud & Simon 2010).
- They showed that these two species are morphologically identical except for adult males: ABB males can have mid-tibiae of three different types – small, medium or massive – whereas all ECB males have small mid-tibiae (Frolov et al. 2007, 2012).
- Larvae of both species are polyphagous, but they feed on different sets of host plants: ECB larvae are preferentially found on maize and ABB larvae are mostly found on a number of dicotyledon species, notably mugwort (Artemisia vulgaris L.), hop (Humulus lupulus L.) and hemp (Cannabis sativa L.) (review in Ponsard et al.
Ostrinia populations and strains
- Diapausing Ostrinia larvae were collected at several sites in China and in one site in Khabarovsk (Eastern Russia), on two host plant types: maize (Zea mays L.) and dicotyledons [mugwort (Artemisia vulgaris L.), hop (Humulus lupulus L.), cocklebur (Xanthium spp.), goldenrod (Solidago canadensis L.), thistle (Cephalanoplos segetum (Bunge) Kitam), ragweed (Ambrosia trifida L.) and clematis (Clematis vitalba L.)].
- A total of 12 and 16 populations were sampled on maize and dicotyledons, respectively.
- Name, location and details of each sample are given in Table 1 and Fig.
- Some of the larvae collected in 2009 were directly frozen at 20 °C prior to genotyping.
- These strains had been founded with individuals obtained from wild ACB feeding on maize and collected as overwintering larvae in Hangzhou (Zheijiang Province) and Hengshui (Hebei Province), respectively.
Genetic clustering
- Bayesian clustering analyses were conducted using the software STRUCTURE (Pritchard et al. 2000).
- STRUCTURE, version 2.3.1. (Hubisz et al. 2009), offers an extension of the previous STRUCTURE model, making it possible to use available information about ‘sampling locations’ (sensu lato, i.e, geographical location or host plant).
- This information is used as a prior and more prior weight is placed on clustering outcomes that are correlated with this sampling information.
- Following Evanno et al. (2005), the authors computed the statistic DK, which is based on the rate of change in the log posterior probability of the data between successive values of K, to detect the uppermost hierarchical level of structure.
- Finally, the authors used the ‘greedy’ algorithm implemented in the software CLUMPP (Jakobsson & Rosenberg 2007) to detect possible distinct modes among the results of the set of 20 independent runs obtained for any given value of K.
Genetic assignment
- No differentiation was found between the ABB and the ECB at mitochon- drial loci (Martel et al. 2003).
- In contrast, the mitochondrial genomes of the ACB and the ECB and those of the ACB and the ABB are significantly differentiated from each other (Coates et al. 2005).
- Two separate digestions of the PCR products were performed: one with restriction enzyme ClaI and one with VspI.
- These two enzymes were chosen following a preliminary analysis of the amplified COI fragment of two individuals collected in European maize fields (presumably ECB), six individuals collected in Asian.
Chengdu Shanghai
- Maize fields (presumably ACB) and 13 individuals from dicotyledons (presumably ABB) collected worldwide (S. Ponsard, unpublished data).
- This analysis showed that the COI fragment contained a ClaI restriction site shared by all individuals collected on maize in Europe and 85% individuals from dicotyledons.
- Based on these results, the authors defined four ClaI-VspI mitochondrial types: acb-acb, acb-abb/ecb, abb/ ecb-acb and abb/ecb-abb/ecb.
- It then computes for each individual the posterior probability that it belongs to each class by Markov chain Monte Carlo.
- Anderson and Thompson (2002) have noted that NEWHYBRIDS requires the presence of diagnostic alleles to distinguish the respective genotype frequencies of distinct hybrid classes (F1 vs. F2 and backcrosses).
Genetic differentiation and IBD
- For each population, the authors excluded all individuals that were not assigned to a probability >95% to either ACB or ABB according to their genotype at microsatellite loci.
- Over all the ACB and ABB populations, the genetic diversity at each microsatellite locus was calculated using FSTAT 2.8 (Goudet 1999).
- When a significant deviation from HWE was detected at a particular locus, the authors used the program MICRO-CHECKER (van Oosterhout et al. 2004) to determine whether the deviation was likely to be due to the presence of null alleles or to any other genotyping errors.
- This corrected FST estimator will be referred to as FST {ENA}.
- Isolation-by-distance (IBD) patterns were assessed by testing for independence between geographical [ln(geographical distance)] and genetic distances [FST {ENA}/ (1 FST{ENA})] among populations collected on each host plant type – maize vs. dicotyledons – as described by Rousset (1997).
Mating success
- Diapausing larvae collected around Shanghai and Beijing in 2005 were kept at 4 °C for several weeks to allow diapause completion.
- They were then placed in plastic boxes with a moist piece of cotton, maintained at 25 °C under a 16:8 h light/dark (L:D) photoperiod.
- Male and female pupae from all populations and strains were kept separately, so that adults were all virgin prior to being used to perform crosses and backcrosses.
- They were paired and allowed to mate during 3 consecutive nights at 25 °C and under a 16:8 h L/D photoperiod.
- Indeed, the sperm and nutritious substances transferred by males during mating form an easily recognizable solidified structure, the spermatophore (one per mating event), that is later used by the female to fertilize her oocytes.
Hybrid fitness
- To explore possible post-zygotic isolation between ABB, ECB and ACB, the authors raised eggs obtained from hybrid and intraspecies crosses to adult stage at 25 °C under a 16:8 h L/D photoperiod.
- Larvae were fed on a standard artificial corn-based diet.
- The authors calculated the average% of hatching, the average number of first instars (L1) and the% of survival at different life stages: from first (L1) to fifth (L5) instar, from L5 to pupation and from pupation to adult emergence.
Results
- When increasing the number of clusters up to K = 6, all populations collected on maize – except Yining – were mostly assigned to a single cluster regardless of geographical origin, and this cluster never included any population collected on dicotyledons (Fig. 2).
- The genetic differentiation was rather low and mostly nonsignificant, both among ACB and among ABB populations (Table 5 and Tables S3 and S4, Supporting information).
- Five loci displayed mean FST {ENA} values around 0.100.
Discussion
- Ostrinia furnacalis and O. scapulalis: two widely distributed species with specific host plants.
- This species can oviposit on maize, as ABB migrants were detected on maize at Shenyang and Beijing (this study) and possibly in Hebei Province (Li et al. 2003).
- As expected, all ACB males displayed small midtibia.
- They described two further dicotyledon-feeding species that differed from the ABB only by male mid-tibia size: O. narynensis (medium mid-tibia) and O. orientalis (small mid-tibia).
- Whether this is merely due to their sampling of two ends of a continuous gradient (Europe and Eastern Asia, leaving out Siberian populations in between) or reflects the existence of 2 distinct taxa separated by a sharp genetic discontinuity (with or without overlapping geographical distribution) is unclear.
O. nubilalis and O. scapulalis
- As in the ECB-ABB species pair (Bethenod et al. 2005; P elozuelo et al. 2007), the authors obtained successful interspecific hybridizations in both directions between ACB and ABB individuals in no-choice laboratory conditions, although at a lower rate than intraspecific crosses in the same conditions.
- Likewise, Y. Thomas and D. Bourguet (unpublished observations) found no obvious postzygotic isolation with respect to survival until adult stage in the ECB-ABB species pair.
- The authors also found introgressed genotypes (F1, F2 and/or backcrosses) in natural populations collected on both maize and dicotyledons.
- It has been shown that some (rare) ACB and ECB males are attracted both to the pheromone of their own species and to that of the opposite species (Roelofs et al. 2002; Linn et al. 2003, 2007).
O. scapulalis
- On maize, all larvae assigned to the ACB by means of microsatellites had an ACB mitochondrial type.
- This suggests that although hybrids are found on both host plants (indicating that there is no drastic genetic and/or ecological selection against them, at least at larval stages), there is an asymmetric gene flow between these species – the introgression going from the ACB towards the ABB.
- More generally, the authors cannot exclude that the ACB and the ECB were already to some degree reproductively isolated from the ABB when maize was introduced into Europe and Asia.
- Unlike many other cases of host races/sibling species diverging through ecological speciation, such premating barriers do not seem to be a by-product of ecological adaptation (except possibly temporal isolation: Thomas et al. 2003 but see Malausa et al. 2005).
Perspectives
- Some important parts of the evolutionary histories of ACB and ECB are still missing for one pair of species (e.g. the asymmetry of introgression and the larval fitness of hybrid in the ACB/ABB pair) or for both (e.g. adult hybrid fitness).
- 2010; however, it infests both the ACB and the ABB in China: P elissi e et al. 2012), (ii) difference in sex pheromones (the ECB and the ABB show polymorphism between the same two female pheromone blends: P elozuelo et al.
- A most intriguing aspect is to understand what may have driven the evolution of strong premating barriers – differences in female and male sex pheromones, short-range mechanisms, courtship ultrasounds – which are unlikely to be involved in host plant adaptation.
- There is no obvious indication that the premating barriers documented so far between the ABB, ECB and ACB might be due to any ‘magic trait’ simultaneously ensuring reproductive isolation and adaptation to host plant (Gavrilets 2004).
Supporting information
- Additional supporting information may be found in the online version of this article.
- Table S2 P-values for departure from heterozygosity expected under Hardy-Weinberg equilibrium.
- Pairwise FST {ENA} values are given below the diagonal.
- Table S5 Genetic differentiation (pairwise FST {ENA}) between ABB populations collected on dicotyledons vs. ACB collected on maize in China and Eastern Russia.
- Fig. S2 Relation between genetic differentiation and geographical distance between Ostrinia populations collected on maize – excluding the population collected at Yining – or on dicotyledons.
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Q2. What is the concept of a spatial analysis?
Le concept fondamental de ces modèles est d’étudier le comportement d'une variable dépendante (localisation des glissements de terrain) à partir d’une combinaison de variables indépendantes prédictives pour des unités géomorphologiques homogènes.