Pourquoi la résistance polygénique du piment au Potato Virus Y est-elle plus durable ?

Diplôme : DU

Genotypic variability of sugarcane resistance to the aphid #Melanaphis sacchari,# vector of the #Sugarcane yellow leaf virus#

International audienceThe objective of this study was to analyse the genotypic variability of sugarcane resistance to the main aphid vector of the Sugarcane yellow leaf virus (ScYLV) (Polerovirus, Luteoviridae), Melanaphis sacchari. We assessed the incidence of aphids in a field trial comparing 181 sugarcane cultivars. Based on the percentage of infested leaves, aphid incidence was scored every two weeks during three cropping seasons. Analysis of variance revealed highly significant genotype, and genotype 9 year variance, and high broad sense heritability. Using semivariograms, we showed that the alpha lattice design used in the field trial was able to cope with spatial correlation issues caused by the patchy nature of aphid infestations. Twenty-two aphid resistant cultivars were identified. A laboratory study of the development of M. sacchari on four of these 22 resistant cultivars confirmed the resistant status of three of them. We observed modest positive phenotypic and genetic correlations between the aphid incidence and the incidence of ScYLV. The 22 cultivars resistant to M. sacchari showed twofold lower mean virus incidence than the remaining 159 cultivars. (Résumé d'auteur

Host Plant Specialization in the Sugarcane Aphid Melanaphis sacchari.

Most aphids are highly specialized on one or two related plant species and generalist species often include sympatric populations adapted to different host plants. Our aim was to test the hypothesis of the existence of host specialized lineages of the aphid Melanaphis sacchari in Reunion Island. To this end, we investigated the genetic diversity of the aphid and its association with host plants by analyzing the effect of wild sorghum Sorghum bicolor subsp. verticilliflorum or sugarcane as host plants on the genetic structuring of populations and by performing laboratory host transfer experiments to detect trade-offs in host use. Genotyping of 31 samples with 10 microsatellite loci enabled identification of 13 multilocus genotypes (MLG). Three of these, Ms11, Ms16 and Ms15, were the most frequent ones. The genetic structure of the populations was linked to the host plants. Ms11 and Ms16 were significantly more frequently observed on sugarcane, while Ms15 was almost exclusively collected in colonies on wild sorghum. Laboratory transfer experiments demonstrated the existence of fitness trade-offs. An Ms11 isofemale lineage performed better on sugarcane than on sorghum, whereas an Ms15 lineage developed very poorly on sugarcane, and two Ms16 lineages showed no significant difference in performances between both hosts. Both field and laboratory results support the existence of host plant specialization in M. sacchari in Reunion Island, despite low genetic differentiation. This study illustrates the ability of asexual aphid lineages to rapidly undergo adaptive changes including shifting from one host plant to another

Low genetic diversity in #Melanaphis sacchari# Aphid populations at the worldwide scale

International audienceNumerous studies have examined the genetic diversity and genetic structure of invading species, with contrasting results concerning the relative roles of genetic diversity and phenotypic plasticity in the success of introduced populations. Increasing evidence shows that asexual lineages of aphids are able to occupy a wide geographical and ecological range of habitats despite low genetic diversity. The anholocyclic aphid Melanaphis sacchari is a pest of sugarcane and sorghum which originated in the old world, was introduced into the Americas, and is now distributed worldwide. Our purpose was to assess the genetic diversity and structuring of populations of this species according to host and locality. We used 10 microsatellite markers to genotype 1333 individuals (57 samples, 42 localities, 15 countries) collected mainly on sugarcane or sorghum. Five multilocus lineages (MLL) were defined, grouping multilocus genotypes (MLG) differing by only a few mutations or scoring errors. Analysis of a 658 bp sequence of mitochondrial COI gene on 96 individuals revealed five haplotypes, with a mean divergence of only 0.19 %. The distribution of MLL appeared to be strongly influenced by geography but not by host plant. Each of the five MLL grouped individuals from (A) Africa, (B) Australia, (C) South America, the Caribbean and the Indian Ocean including East Africa, (D) USA, and (E) China. The MLL A and C, with a wide geographic distribution, matched the definition of superclone. Among aphids, M. sacchari has one of the lowest known rates of genetic diversity for such a wide geographical distribution. (Résumé d'auteur

Observed microsatellite Multi Locus Genotypes (MLG): allele size (bp) at each locus, distribution by host plant and in each of the five Multi Locus Lineages (MLL) defined with GENCLONE. Within MLL allelic variations are in bold.

<p>Observed microsatellite Multi Locus Genotypes (MLG): allele size (bp) at each locus, distribution by host plant and in each of the five Multi Locus Lineages (MLL) defined with GENCLONE. Within MLL allelic variations are in bold.</p

Laboratory comparison of fitness trade-offs in three MLGs.

<p>Ms11 (circles), Ms15 (squares), or Ms16 (triangles). Host transfer experiments were carried out with four isofemale lineages derived from individuals collected either on sorghum (white symbols) or on sugarcane (grey symbols). The figure represents the mean number of aphids per sugarcane or sorghum plantlet 240 hours after 12 plantlets had each been infested with 10 individuals. The experiment was repeated three times. Vertical lines represent the standard errors of means.</p