Development and characterization of microsatellite markers for the oomyceta [i]Aphanomyces euteiches[/i].

International audienceAphanomyces euteiches Drechsler is a serious pathogen of leguminous crops that causes devastating root rot of pea worldwide. Given that A. euteiches is a diploid organism, robust, codominant markers are needed for population genetics studies. We have developed and screened a microsatellite-enriched small-insert genomic library for identification of A. euteiches SSR containing sequences. Fourteen out of the 48 primer pairs designed to amplify SSR, produced unambiguous polymorphic products in our test population of 94 isolates. The number of alleles at each locus ranged from one to four. The identification of new markers would enhance the ability to evaluate the genetic structure of A. euteiches populations, and pathogen evolution

Masculinization of the X Chromosome in the Pea Aphid

International audienceEvolutionary theory predicts that sexually antagonistic mutations accumulate differentially on the X chromosome and autosomes in species with an XY sex-determination system, with effects (masculinization or feminization of the X) depending on the dominance of mutations. Organisms with alternative modes of inheritance of sex chromosomes offer interesting opportunities for studying sexual conflicts and their resolution, because expectations for the preferred genomic location of sexually antagonistic alleles may differ from standard systems. Aphids display an XX/X0 system and combine an unusual inheritance of the X chromosome with the alternation of sexual and asexual reproduction. In this study, we first investigated theoretically the accumulation of sexually antagonistic mutations on the aphid X chromosome. Our results show that i) the X is always more favourable to the spread of male-beneficial alleles than autosomes, and should thus be enriched in sexually antagonistic alleles beneficial for males, ii) sexually antagonistic mutations beneficial for asexual females accumulate preferentially on autosomes, iii) in contrast to predictions for standard systems, these qualitative results are not affected by the dominance of mutations. Under the assumption that sex-biased gene expression evolves to solve conflicts raised by the spread of sexually antagonistic alleles, one expects that male-biased genes should be enriched on the X while asexual female-biased genes should be enriched on autosomes. Using gene expression data (RNA-Seq) in males, sexual females and asexual females of the pea aphid, we confirm these theoretical predictions. Although other mechanisms than the resolution of sexual antagonism may lead to sex-biased gene expression, we argue that they could hardly explain the observed difference between X and autosomes. On top of reporting a strong masculinization of the aphid X chromosome, our study highlights the relevance of organisms displaying an alternative mode of sex chromosome inheritance to understanding the forces shaping chromosome evolution

Spatial and temporal genetic variability in French populations of the peach-potato aphid, Myzus persicae

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Ecological specialization correlates with genotypic differentiation in sympatric host-populations of the pea aphid

International audienceThe pea aphid, Acyrthosiphon pisum, encompasses distinct host races specialized on various Fabaceae species, but the extent of genetic divergence associated with ecological specialization varies greatly depending on plant and geographic origins of aphid populations. Here, we studied the genetic structure of French sympatric pea aphid populations collected on perennial (pea and faba bean) and annual (alfalfa and red clover) hosts using 14 microsatellite loci. Classical and Bayesian population genetics analyses consistently identified genetic clusters mostly related to plant origin: the pea/faba bean cluster was highly divergent from the red clover and the alfalfa ones, indicating they represent different stages along the continuum of genetic differentiation. Some genotypes were assigned to a cluster differing from the one expected from their plant origin while others exhibited intermediate genetic characteristics. These results suggest incomplete barriers to gene flow. Howeve! r, this limited gene flow seems insufficient to prevent ecological specialization and genetic differentiation in sympatr

Bacterial communities associated with host-adapted populations of pea aphids revealed by deep sequencing of 16S ribosomal DNA.

Associations between microbes and animals are ubiquitous and hosts may benefit from harbouring microbial communities through improved resource exploitation or resistance to environmental stress. The pea aphid, Acyrthosiphon pisum, is the host of heritable bacterial symbionts, including the obligate endosymbiont Buchnera aphidicola and several facultative symbionts. While obligate symbionts supply aphids with key nutrients, facultative symbionts influence their hosts in many ways such as protection against natural enemies, heat tolerance, color change and reproduction alteration. The pea aphid also encompasses multiple plant-specialized biotypes, each adapted to one or a few legume species. Facultative symbiont communities differ strongly between biotypes, although bacterial involvement in plant specialization is uncertain. Here, we analyse the diversity of bacterial communities associated with nine biotypes of the pea aphid complex using amplicon pyrosequencing of 16S rRNA genes. Combined clustering and phylogenetic analyses of 16S sequences allowed identifying 21 bacterial OTUs (Operational Taxonomic Unit). More than 98% of the sequencing reads were assigned to known pea aphid symbionts. The presence of Wolbachia was confirmed in A. pisum while Erwinia and Pantoea, two gut associates, were detected in multiple samples. The diversity of bacterial communities harboured by pea aphid biotypes was very low, ranging from 3 to 11 OTUs across samples. Bacterial communities differed more between than within biotypes but this difference did not correlate with the genetic divergence between biotypes. Altogether, these results confirm that the aphid microbiota is dominated by a few heritable symbionts and that plant specialization is an important structuring factor of bacterial communities associated with the pea aphid complex. However, since we examined the microbiota of aphid samples kept a few generations in controlled conditions, it may be that bacterial diversity was underestimated due to the possible loss of environmental or transient taxa

Accelerated evolution of sex chromosomes in aphids, an X0 system.

International audienceSex chromosomes play a role in many important biological processes, including sex determination, genomic conflicts, imprinting and speciation. In particular, they exhibit several unusual properties such as inheritance pattern, hemizygosity and reduced recombination, which influence their response to evolutionary factors (e.g. drift, selection, demography). Here, we examine the evolutionary forces driving X chromosome evolution in aphids, an XO system where females are homozygous (XX) and males hemizygous (X0) at sex chromosomes. We show by simulations that the unusual mode of transmission of the X chromosome in aphids coupled to cyclical parthenogenesis results in similar effective population sizes and predicted levels of genetic diversity for X chromosomes and autosomes under neutral evolution. These results contrast with expectations from standard XX/XY or XX/X0 systems (where the effective population size of the X is 3/4 of that of autosomes) and have deep consequences for aphid X chromosome evolution. We then localized 52 microsatellites markers on the X and 351 on autosomes. We genotyped 167 individuals with 356 of these loci and found similar levels of allelic richness on the X and on the autosomes, as predicted by our simulations. In contrast, we detected higher dN and dN/dS ratio for X-linked genes compared to autosomal genes, a pattern compatible with either positive or relaxed selection. Given that both types of chromosomes have similar effective population sizes and that the single copy of the X chromosome of male aphids exposes its recessive genes to selection, some degree of positive selection seems to best explain the higher rates of evolution of X-linked genes. Overall, this study highlights the particular relevance of aphids to study the evolutionary factors driving sex chromosomes and genome evolution

Improvement of Barley yellow dwarf virus-PAV detection in single aphids using a fluorescent real time RT-PCR

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Genomic regions repeatedly involved in divergence among plant-specialized pea aphid biotypes

International audienceUnderstanding the genetic bases of biological diversification is a long-standinggoal in evolutionary biology. Here, we investigate whether replicatedcases of adaptive divergence involve the same genomic regions in the peaaphid, Acyrthosiphon pisum, a large complex of genetically differentiated biotypes,each specialized on different species of legumes. A previous studyidentified genomic regions putatively involved in host-plant adaptation and/or reproductive isolation by performing a hierarchical genome scan in threebiotypes. This led to the identification of 11 FST outliers among 390 polymorphicmicrosatellite markers. In this study, the outlier status of these 11loci was assessed in eight biotypes specialized on other host plants. Four ofthe 11 previously identified outliers showed greater genetic differentiationamong these additional biotypes than expected under the null hypothesis ofneutral evolution (a < 0.01). Whether these hotspots of genomic divergenceresult from adaptive events, intrinsic barriers or reduced recombination isdiscussed

Effects of climate and land use on the occurrence of viruliferous aphids and the epidemiology of barley yellow dwarf disease

International audienceBarley yellow dwarf (BYD) disease is one of the most severe viral diseases in autumn sown cereals. In Western Europe, crop losses are mainly due to the PAV species of BYD viruses transmitted by Rhopalosiphum padi, the most abundant aphid in autumn. The proportion of migrant winged aphids that carry viruses in autumn is considered a major epidemiological factor for determining the disease incidence. In the main French cereal areas, during a 6-week period in autumn 1999-2002, the proportion of viruliferous R. padi assessed using a TAS-ELISA technique was on average of 4.98% (range 2.01-9.91%). Variations according to trap location were correlated with land use at the regional scale, annual variations being correlated with the climate of the year. Implementations of these results to improve BYD disease management program are discussed

Data from: Genomic regions repeatedly involved in divergence among plant-specialized pea aphid biotypes

Understanding the genetic bases of biological diversification is a long-standing goal in evolutionary biology. Here we investigate whether replicated cases of adaptive divergence involve the same genomic regions in the pea aphid, Acyrthosiphon pisum, a large complex of genetically differentiated biotypes, each specialized on different species of legumes. A previous study identified genomic regions putatively involved in host-plant adaptation and/or reproductive isolation by performing a hierarchical genome scan in three biotypes. This led to the identification of 11 FST outliers among 390 polymorphic microsatellite markers. In this study, the outlier status of these 11 loci was assessed in eight biotypes specialized on other host plants. Four of the 11 previously identified outliers showed greater genetic differentiation among these additional biotypes than expected under the null hypothesis of neutral evolution (α<0.01). Whether these hotspots of genomic divergence result from adaptive events, intrinsic barriers or reduced recombination is discussed