Marguerite Rodier-Goud

A saturated genetic linkage map of rubber tree (Hevea spp.) based on RFLP, AFLP, microsatellite, and isozyme markers

Abstract The first genetic map for Hevea spp. (2n=36) is presented here. It is based on a F1 progeny of 106 individuals allowing the construction of a female, a male, and a synthetic map according to the pseudo-testcross strategy. Progeny were derived from an interspecific cross between PB260, a H. brasiliensis cultivated clone, and RO38, a H. brasiliensis×H. benthamiana interspecific hybrid clone. The disomic inheritance observed for all the codominant markers scattered on the 2n=36 chromosomes revealed that Hevea behaves as diploids. Homologous linkage groups between the two parental maps were merged using bridge loci. A total of 717 loci constituted the synthetic map, including 301 RFLPs, 388 AFLPs, 18 microsatellites, and 10 isozymes. The markers were assembled into 18 linkage groups, thus reflecting the basic chromosome number, and covered a total distance of 2144 cM. Nine markers were found to be unlinked. Segregation distortion was rare (1.4%). Average marker density was 1 per 3 cM. Comparison of the distance between loci in the parental maps revealed significantly less meiotic recombination in the interspecific hybrid male parent than in the female parent. Hevea origin and genome organisation are discussed.

Identification of QTLs involved in the resistance to South American leaf blight (Microcyclus ulei) in the rubber tree

Abstract South American leaf blight (SALB) is a disease of the rubber tree caused by the fungus Microcyclus ulei. Quantitative trait loci (QTLs) for resistance were mapped using 195 F1 progeny individuals derived from the cross between a susceptible cultivated clone, PB260, and a resistant clone, RO38, derived from interspecific hybridization. The resistance level of the progeny individuals was evaluated in controlled conditions. The reaction type (RT) and the lesion diameter (LD) were measured on immature leaves after artificial inoculation of the fungus. Five different strains of the fungus were used, all highly sporulating on PB260. Among those, four did not sporulate and one sporulated partially on RO38. Both pseudo-testcross parental genetic maps and the consensus map were constructed. The search for QTLs was performed using the Kruskal-Wallis marker-by-marker test and the Interval-Mapping method for the three maps. Eight QTLs for resistance were identified on the RO38 map. Only one QTL was detected on the PB260 map. The analysis of the F1 consensus map confirmed results obtained with the parental maps. A common QTL was detected for resistance to the five strains for both RT and LD. Two QTLs were common for complete resistance to four strains, for RT and LD respectively. Resistance determinism for complete and partial resistance, and perspectives for breeding for durable resistance to SALB are discussed.

Molecular mapping of genes conferring field resistance to South American Leaf Blight (Microcyclus ulei) in rubber tree

The South American Leaf Blight (SALB), caused by the fungus Microcyclus ulei, is the major rubber tree disease in all Central and South America. A population of 192 progeny individuals derived from a cross between a resistant clone and a susceptible cultivated clone was planted in a field trial in French Guiana in order to evaluate the resistance parameters under real infestation conditions. The resistance type (RT), the presence of stromata (ST) and the level of attack (AT) were observed 20-times on a 22-months period, and semi-quantitative evaluation of stromata was registered only once. The search for QTLs was performed using the Kruskal-Wallis test, Interval Mapping and the Composite Interval Mapping method. One major QTL located on linkage group g13 was detected on the RO 38 map, responsible for 36 to 89% of the phenotypic variance of resistance. This resistance QTL corresponds to one that had previously been detected under controlled conditions of infestation and we called it M13-1bn. Surprisingly, the effect of this QTL was larger under natural conditions of infestation than under controlled inoculation. Other minor QTLs (four on the RO38 map and one on the PB 260 map) were also detected. The type of resistance brought by M13-1bn, as well as its durability, are discussed. Applications for rubber tree breeding programs are considered.

Homoeologous chromosome pairing between the A and B genomes of Musa spp. revealed by genomic in situ hybridization.

BACKGROUND AND AIMS Most cooking banana and several desert bananas are interspecific triploid hybrids between Musa acuminata (A genome) and Musa balbisiana (B genome). In addition, M. balbisiana has agronomical characteristics such as resistance to biotic and abiotic stresses that could be useful to improve monospecific acuminata cultivars. To develop efficient breeding strategies for improving Musa cultivars, it is therefore important to understand the possibility of chromosome exchange between these two species. METHODS A protocol was developed to prepare chromosome at meiosis metaphase I suitable for genomic in situ hybridization. A series of technical challenges were encountered, the main ones being the hardness of the cell wall and the density of the microsporocytes cytoplasm, which hampers accessibility of the probes to the chromosomes. Key parameters in solving these problems were addition of macerozyme in the enzyme mix, the duration of digestion and temperature during the spreading phase. RESULTS AND CONCLUSIONS This method was applied to analyse chromosome pairing in metaphase from triploid interspecific cultivars, and it was clearly demonstrated that interspecific recombinations between M. acuminata and M. balbisiana chromosomes do occur and may be frequent in triploid hybrids. These results provide new insight into Musa cultivar evolution and have important implications for breeding.

Radiation of the Nod‐independent Aeschynomene relies on multiple allopolyploid speciation events

• The semi-aquatic legumes belonging to the genus Aeschynomene constitute a premium system for investigating the origin and evolution of unusual symbiotic features such as stem nodulation and the presence of a Nod-independent infection process. This latter apparently arose in a single Aeschynomene lineage. But how this unique Nod-independent group then radiated is not yet known. • We have investigated the role of polyploidy in Aeschynomene speciation via a case study of the pantropical A. indica and then extended the analysis to the other Nod-independent species. For this, we combined SSR genotyping, genome characterization through flow cytometry, chromosome counting, FISH and GISH experiments, molecular phylogenies using ITS and single nuclear gene sequences, and artificial hybridizations. • These analyses demonstrate the existence of an A. indica polyploid species complex comprising A. evenia (C. Wright) (2n = 2x = 20), A. indica L. s.s. (2n = 4x = 40) and a new hexaploid form (2n = 6x = 60). This latter contains the two genomes present in the tetraploid (A. evenia and A. scabra) and another unidentified genome. Two other species, A. pratensis and A. virginica, are also shown to be of allopolyploid origin. • This work reveals multiple hybridization/polyploidization events, thus highlighting a prominent role of allopolyploidy in the radiation of the Nod-independent Aeschynomene.