Between plagues, the solitarious desert locust (Schistocerca gregaria) is generally thought to exist as small populations, which are particularly prone to extinction events in arid regions of Africa and Asia. Given the high genetic structuring observed in one geographical area (the Eritrean coast) by former authors, a metapopulation dynamics model involving repeated extinction and colonization events was favoured. In this study, we assessed the validity of a demographic scenario involving temporary populations of the solitarious phase of the desert locust by analysing large-scale population genetic data. We scored 24 microsatellites in 23 solitarious population samples collected over most of the species range during remission. We found very little genetic structuring and little evidence of declining genetic diversity. A Bayesian clustering method distinguished four genetically differentiated units. Three groups were largely consistent with three population samples which had undergone recent bottleneck events. Nevertheless, the last genetically homogeneous unit included all individuals from the remaining 18 population samples and did not show evidence of demographic disequilibrium. An approximate Bayesian computation treatment indicated a large population size for this main genetic group, moderately reduced between plague and remission but still containing tens of thousands of individuals. Our results diverge from the hypothesis of a classical metapopulation dynamics model. They instead support the scenario in which large populations persist in the solitarious phase of the desert locust.

The desert locust (Schistocerca gregaria) has been feared agricultural pest since early civilization, with plagues documented in ancient texts. Population genetic studies of the desert locust are needed to determine genetic variation and movement pattern for efficient control of the pest. In this study, we complemented the limited available microsatellite collection for the desert locust with 34 new polymorphic and multiplexed microsatellite loci. To this aim, we screened an expressed sequence tags library and constructed a partial genomic library enriched for dinucleotide repeats to develop high-throughput and high-quality genotyping assays. We then paid particular attention to quality control and carefully validated 26 of these novel microsatellites and six previously described loci for the absence of null alleles in Western African field populations. This large panel of high-quality microsatellite markers provides new opportunity to infer dispersal rates between populations of the desert locust and help prioritize early monitoring and control. Furthermore, high potential for cross-taxa utility of markers was observed within Schistocerca genus, which includes other locust pest species, with reliable amplification achieved for at least ten of loci per species. Microsatellite markers developed from transcriptome resources were largely devoid of null alleles and were conserved across species compared with those derived from traditional genomic libraries. Accordingly, the number of highly reliable microsatellite markers was greatly improved compared with that of previous studies on Orthopteran species, and this strategy might be broadly applied in other insect species prone to null alleles. (Résumé d'auteur

In insects, extra-molting has been viewed as a compensatory mechanism for nymphal growth that contributes to optimize body weight for successful reproduction. However, little is known on the capacity of extra-molting to evolve in natural populations, which limits our understanding of how selection acts on nymphal growth. We used a multi-generational pedigree, individual monitoring and quantitative genetics models to investigate the evolution of extra-molting and its impact on nymphal growth in a solitarious population of the desert locust, Schistocerca gregaria. Growth compensation via extra-molting was observed for 46% of the females, whose adult weight exceeded by 4% that of other females, at a cost of a 22% longer development time. We found a null heritability for body weight threshold only, and the highest and a strongly female-biased heritability for extra molting. Our genetic estimates show that (1) directional selection can act on growth rate, development time and extra-molting to optimize body weight threshold, the target of stabilizing selection, (2) extra-molting can evolve in natural populations, and (3) a genetic conflict, due to sexually antagonistic selection on extra-molting, might prevent its fixation. Finally, we discuss how antagonistic selection between solitarious and gregarious environments and/or genetic correlations between growth and phase traits might also impact the evolution of extra-molting in locusts.

Landscape analysis and genetics can allow a better understanding of grasshoppers for which ecology is not very well known. We analysed landscape changes between 1990 and 2006 at two areas from southern France where three grasshopper species ( Calliptamus italicus (L., 1758), Calliptamus wattenwylianus Pantel, 1896, and Calliptamus barbarus (Costa, 1836)) occur. We then applied microsatellite markers to the study of 1200 georeferenced samples collected over both areas. We used a recent Bayesian clustering method with correlated allele frequencies to detect weak population genetic structure. We found evidence of breaks in gene flow only in C. wattenwylianus, thought to be sedentary relative to its congenerics. By using different allele frequency models and prior information to different levels of genetic differentiation for our six real data sets, our study also informs on the ability of the newly available Bayesian clustering methods model to detect weak genetic structure in natural field populations.

Abstract. We conducted a comparative population genetic analysis of levels of genetic variation and its geographical structuring in three closely related species of grasshopper that co-occur in the Mediterranean Basin: Calliptamus italicus, C. barbarus and C. wattenwylianus. In the western part of their distributions 5 populations of C. italicus, 13 of C. barbarus and 10 of C. wattenwylianus were sampled. Bootstrap re-sampling of populations and microsatellite loci within each species indicated a lower level of genetic diversity and higher level of genetic differentiation in C. barbarus, which is less of an outbreak pest species than either of the other Calliptamus species studied. This may be due to lower effective sizes of non-outbreak populations and/or lower gene flow among them. 445C. wattenwylianus (Pantel, 1896) are syntopic across the western part of Mediterranean Basin. C. italicus and C. barbarus have the widest distributions, from the Mediterranean Basin to southern Siberia, whereas C. wattenwylianus is recorded only along the Mediterranean coasts of France, Spain, Morocco and Algeria. Morphological discrimination between the juvenile stages and adult females of these three closely related species is ambiguous, whereas adult males can be identified based on the shape of their wings and pallium (Jago, 1963;Harz, 1975). However, the three species can be easily differentiated genetically, with sequence divergences in Cytochrome Oxidase I mtDNA estimated to be over 3% (Blanchet et al., 2010b). These three species of grasshopper damage crops (COPR, 1982) and share some general ecological features, such as, a one-year generation time, a seasonal diapause, feed on grass (Poacea) (Chopard, 1951) and a moderate dispersal ability (Reinhardt et al., 2005). Given that these three species co-occur and are closely related, ecological and evolutionary factors that affect one species are likely to affect the others, therefore a comparison of these species might provide insights into the relative importance of the factors that shape genetic structuring (McCoy et al., 2005;Brouat et al., 2007).Although the three species of Calliptamus studied are closely related, ecologically similar, and syntopic, previous studies report fine-scale differences between them in terms of ecological features and neutral genetic variation (see Fig. 1). Firstly, Blanchet et al. (2012) showed that at a scale of 70 km 2 between sites in Southern France, gene flow was restricted only in C. wattenwylianus, a species thought to be relatively sedentary compared to other congeneric species (Chara, 1987). Indeed, Jago (1963) showed that C. italicus and C. barbarus have a higher wing/body length ratio, which is a reliable proxy of dispersal ability, than C. wattenwylianus. Secondly, the same fine-scale microsatellite study highlighted lower levels of genetic diversity in C. barbarus than in C. italicus and C. wattenwylianus. This result suggests that long-term effective sizes of populations of C. barbarus might be lower than those of the other s...