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International audience; Cassava suffers from bacterial blight attack in all growing regions. Control by resistance is unstable due to high genotype–environment interactions. Identifying genes for resistance to African strains of Xanthomonas axonopodis pv. manihotis can support breeding efforts. Five F1 cassava genotypes deriving from the male parent ‘CM2177-2’ and the female parent ‘TMS30572’ were used to produce 111 individuals by backcrossing to the female parent. In all, 16 genotypes among the mapping population were resistant to stem inoculation by four strains of X. axonopodis pv. manihotis from different locations in Africa, and 19 groups with differential reactions to the four strains were identified, suggesting that the strains represent different pathotypes. Four genotypes were resistant to leaf inoculation, and three were resistant to both stem and leaf inoculations. Genotypes with susceptible, moderately resistant, and resistant reactions after leaf and stem inoculation partly differed in their reactions on leaves and stems. Based on the genetic map of cassava, single-marker regression analysis of disease severity after stem-puncture inoculation was performed. Eleven markers were identified, explaining between 16 and 33.3% of phenotypic variance of area under disease progress curve. Five markers on three and one linkage groups from the female- and malederived framework of family CM8820, respectively, seem to be weakly associated with resistance to four strains of X. axonopodis pv. manihotis. Based on the segregation of alleles from the female of family CM8873, one marker was significantly associated with resistance to two X. axonopodis pv. manihotis strains, GSPB2506 and GSPB2511, whereas five markers were not linked to any linkage group. The quantitative trait loci (QTL) mapping results also suggest that the four African strains belong to four different pathotypes. The identified pathotypes should be useful for screening for resistance, and the QTL and markers will support breeding for resistance.

A direct comparison of the genetic linkage maps of sorghum and rice is proposed. It is based on the mapping of a common set of 123 RFLP probes scattered on the genomes of both species. For each species a composite map was established by merging two individual maps comprising many common loci. This enabled us to confirm the global correspondence scheme that had previously been established between the chromosomes of sorghum and rice. It also provided a more detailed insight into the conservation of synteny and colinearity : 69% of the loci mapped on a given rice chromosome mapped to the corresponding homoeologous chromosome in sorghum ; among them, 84% formed a colinear arrangement between the two species. Local inversions and translocations were detected. (Résumé d'auteur)

DOI: 10.1139/gen-44-4-589; International audience

National audience

Cassava bacterial blight (CBB), caused by #Xanthomonas axonopodis$ pv. #manihotis$ (#Xam$), is a major disease of cassava (#manihot esculenta$ Crantz) in Africa and South America. Planting resistant varieties is the preferred method of disease control. Recent genetic mapping of an F1 cross (TMS 30572 X CM 2177-2) led to the construction of the first molecular genetic map of cassava. To better understand the genetics of resistance to CBB, we evaluated individuals of the F1 cross for CBB resistance by controlled greenhouse inoculations and visually assessed symptoms on days 7, 15, and 30 days after inoculation, using a scale where 0 = no disease and 5 = maximum susceptibility. Five #Xam$ strains were used : CIO-84, CIO-1, CIO-136, CIO-295 and ORST X-27. Area under the disease progress curve (AUDPC) was used as a quantitative measure of resistance in QTL analysis by single-marker regression. Based on the AUDPC values, eight QTLs (Quantitative trait loci), located on linkage groups B, D, L, N, and X of the female-derived framework map, were found to explain 9-20% of the phenotypic variance of the crop's response to the #Xam$ strains. With the male-derived framework map, four QTLs on linkage groups G and C explained 10.27-27.1% of the variance. A scheme to confirm the usefulness of these markers in evaluating segregating populations for resistance to CBB is proposed. (Résumé d'auteur)

L'analyse du polymorphisme de longueur des fragments de restriction d'ADN amplifie (PCR-RFLP) presente de nombreux avantages pour la caracterisation de la diversite microbienne: relative rapidite, faible cout, possibilite d'application a l'ensemlble ries taxons bacteriens, bonne reproductibilite, possibilite d'analyse simultanee d'un grand nombre de souches, interet au niveau taxonomique. Afin d'optimiser sort usage, nous avons cherche a determiner combien d'enzymes et lesquels etaient le plus approprie, en effectuant une analyse systematique par ordinateur de la capacite de 25 enzymes de restriction differentes a generer par PCR-RFLP, seules ou en combinaison, (les profils de digestion permettant la distinction d'especes proches taxonomiquement. La sous-unite 16S de l'ARN ribosomal (rrs; pour ribosomal RNA small subunit) a ete choisie pour simuler ces PCR-RFLP: elle correspond a la region la plus frequemment etudiee et permet en general une bonne discrimination au niveau de l'espece; de plus, de nombreuses sequences rrs sont disponibles dans les banques de donnees. L'etude a ete focalisee sur les rhizobia qui representent un bon modele de genres differents, entremeles avec d'autres genres tres proches taxonomiquement, et parce que leur importance en agriculture induit la necessite d'outils performants pour la caracterisation de grands nombres de nouveaux isolats. La digestion de 24 sequences rrs correspondant aux souches-types de differentes especes a ete simulee par ordinateur. Les profils de digestion obtenus pour les differentes sequences ont ete compares deux par deux afin de determiner le nombre de profils distincts generes par chaque enzyme individuellement. ou par les combinaisons systematiques de 2, 3, 4, 5, 6 ou 7 enzymes. Differentes combinaisons de 3, 4 ou 5 enzymes permettent la distinction entre les especes. La frequence a laquelle les differentes enzymes sont trouvees dans les i:ounfiinaisons discrimimantes ne correspond pas completement a leur pouvoir de discrimation individuel, mais a la complementarite qu'elles presentent lorsqu'ellas sont analysees ensemble. La combination des profils obtenus avec six enzymes, choisies parmi les sept puis frequentes dans les combinaisons discriminantes, permet en moyenne de distinguer plus de 99,5%, des sequences etudiees, quelle que soit la similarite entre ces sequences. L'utilisation d'au moins six de ces sept enzymes est suggeree afin d'assurer une bonne distinction des especes de rhizobia, connues et inconnues.

The biodiversity of nitrogen-fixing organisms is huge. Taxonomic and phylogenetic research is needed to structure this diversity, to facilitate communication among scientists, and to increase our understanding of the evolution and biology of diazotrophs. Molecular tools for taxonomic and biodiversity studies of diazotrophic rhizobia, frankiae, cyanobacteria and bacilli are presented in sections 2 to 5. Sections 6 to 9 focus on problems with genus and species assignment.

A sample of African #Clarias$ catfishes from the Senegal River was studied using morphometry, allozyme variation, microsatellites and RFLPs of mitochondrial DNA. They all confirmed the presence of two species, #C. gariepinus$ and #C. anguillaris$. The two species were closely related genetically and no diagnostic loci were found in allozymes and microsatellites studies. Two of the 11 haplotypes of mtDNA observed were shared by both species. Three of the four assays (morphometry, allozymes and microsatellites) allowed a precise characterization of both. One specimen occupied an intermediate position in the analysis of the data ; it was considered an F1 hybrid whose possible origin is discussed. (Résumé d'auteur)

*INRA, Unite Centrale de Documentation, Jouy-en-Josas (FRA) Diffusion du document : INRA, Unite Centrale de Documentation, Jouy-en-Josas (FRA) Diplôme : BTS

Plasmids were recovered by an alkaline procedure from six symbiotically effective strains of Rhizobium meliloti of diverse geographical origin, reported to harbour only one middle-size large plasmid (ranging from 89 to 143 Megadaltons). Each purified plasmid was digested with eight restriction endonucleases; cleavage patterns were very complex: only KpnI and XbaI gave a limited number of bands. Fingerprints were very different, whatever the restriction enzyme or the geographical origin of the strains. However, Southern DNA-DNA hybridizations revealed that the plasmids showed homologous sequences having a high thermal stability. We gave evidence that some of these sequences are common to all the plasmids of R. meliloti. The biological function of these common sequences is unknown. Hybridization with cloned nitrogen fixation (nif) genes from Klebsiella pneumoniae had demonstrated that nif genes were not located on the middle -- size plasmids of R. meliloti studied in this paper.