EFFETS DU LIT ET DE LA DENSITÉ DE PLANTATION SUR LA CROISSANCE VÉGÉTATIVE DE L’ANANAS (ANANAS COMOSUS L., VAR. MD2) DANS LA LOCALITÉ DE DABOU EN CÔTE D’IVOIRE

The aim of this study was to identify the best planting bed and density for vegetative growth of Ananas comosus L. var. MD2 plants in the locality of Dabou in Côte d’Ivoire. The effects of three planting beds : flat land, ridge without polyethylene film and ridge covered with polyethylene film ; and two planting densities : 50,000 and 70,000 plants/ha (D50 and D70, respectively) were tested through a split-plot disposition. At the end of the vegetative phase of the cycle, growth parameters were evaluated. The results showed that D70, average heights apical bunches were higher (91.52 cm) than D50 (86.87 cm). The number of fresh leaves varied between 48 and 51. The ridge covered with polyethylene film was distinguished with a number of relatively larger fresh leaves. The follow-up of the D leaves biomass revealed that seven months, the value of 70 g required to trigger floral induction therapy (FIT) had been reached in all plants, whatever planting bed and density. The flat land - density 70,000 plants/ha combinaison was illustrated with the D leaves biomass the highest (80.20 g). The weight of the fruit beingrelated to the plant’s growth during the floral induction therapy, at the end of the vegetative phase, the results could contribute to improving the production of MD2 pineapple in locality of Dabou

Theme and Variations in the Evolutionary Pathways to Virulence of an RNA Plant Virus Species

The diversity of a highly variable RNA plant virus was considered to determine the range of virulence substitutions, the evolutionary pathways to virulence, and whether intraspecific diversity modulates virulence pathways and propensity. In all, 114 isolates representative of the genetic and geographic diversity of Rice yellow mottle virus (RYMV) in Africa were inoculated to several cultivars with eIF(iso)4G-mediated Rymv1-2 resistance. Altogether, 41 virulent variants generated from ten wild isolates were analyzed. Nonconservative amino acid replacements at five positions located within a stretch of 15 codons in the central region of the 79-aa-long protein VPg were associated with virulence. Virulence substitutions were fixed predominantly at codon 48 in most strains, whatever the host genetic background or the experimental conditions. There were one major and two isolate-specific mutational pathways conferring virulence at codon 48. In the prevalent mutational pathway I, arginine (AGA) was successively displaced by glycine (GGA) and glutamic acid (GAA). Substitutions in the other virulence codons were displaced when E48 was fixed. In the isolate-specific mutational pathway II, isoleucine (ATA) emerged and often later coexisted with valine (GTA). In mutational pathway III, arginine, with the specific S2/S3 strain codon usage AGG, was displaced by tryptophane (TGG). Mutational pathway I never arose in the widely spread West African S2/S3 strain because G48 was not infectious in the S2/S3 genetic context. Strain S2/S3 least frequently overcame resistance, whereas two geographically localized variants of the strain S4 had a high propensity to virulence. Codons 49 and 26 of the VPg, under diversifying selection, are candidate positions in modulating the genetic barriers to virulence. The theme and variations in the evolutionary pathways to virulence of RYMV illustrates the extent of parallel evolution within a highly variable RNA plant virus species

Molecular Tools to Infer Resistance-Breaking Abilities of Rice Yellow Mottle Virus Isolates

Rice yellow mottle virus (RYMV) is a major biotic constraint to rice cultivation in Africa. RYMV shows a high genetic diversity. Viral lineages were defined according to the coat protein (CP) phylogeny. Varietal selection is considered as the most efficient way to manage RYMV. Sources of high resistance were identified mostly in accessions of the African rice species, Oryza glaberrima. Emergence of resistance-breaking (RB) genotypes was observed in controlled conditions. The RB ability was highly contrasted, depending on the resistance sources and on the RYMV lineages. A molecular marker linked to the adaptation to susceptible and resistant O. glaberrima was identified in the viral protein genome-linked (VPg). By contrast, as no molecular method was available to identify the hypervirulent lineage able to overcome all known resistance sources, plant inoculation assays were still required. Here, we designed specific RT-PCR primers to infer the RB abilities of RYMV isolates without greenhouse experiments or sequencing steps. These primers were tested and validated on 52 isolates, representative of RYMV genetic diversity. The molecular tools described in this study will contribute to optimizing the deployment strategy of resistant lines, considering the RYMV lineages identified in fields and their potential adaptability

Inferring the Evolutionary History of Rice Yellow Mottle Virus from Genomic, Phylogenetic, and Phylogeographic Studies

Fourteen isolates of Rice yellow mottle virus (RYMV) were selected as representative of the genetic variability of the virus in Africa from a total set of 320 isolates serologically typed or partially sequenced. The 14 isolates were fully sequenced and analyzed together with two previously reported sequences. RYMV had a genomic organization similar to that of Cocksfoot mottle sobemovirus. The average nucleotide diversity among the 16 isolates of RYMV was 7%, and the maximum diversity between any two isolates was 10%. A strong conservative selection was apparent on both synonymous and nonsynonymous substitutions, through the amino acid replacement pattern, on the genome size, and through the limited number of indel events. Furthermore, there was a lack of positive selection on single amino acid sites and no evidence of recombination events. RYMV diversity had a pronounced and characteristic geographic structure. The branching order of the clades correlated with the geographic origin of the isolates along an east-to-west transect across Africa, and there was a marked decrease in nucleotide diversity moving westward across the continent. The insertion-deletion polymorphism was related to virus phylogeny. There was a partial phylogenetic incongruence between the coat protein gene and the rest of the genome. Overall, our results support the hypothesis that RYMV originated in East Africa and then dispersed and differentiated gradually from the east to the west of the continent