Angélique Berger

Fine genetic mapping of a gene required for Rice yellow mottle virus cell-to-cell movement

Abstract. The very high resistance to Rice yellow mottle virus observed in the two rice varieties Gigante (Oryza sativa) and Tog 5681 (O. glaberrima) is monogenic and recessive. Bulked segregant analysis was carried out to identify AFLP markers linked to the resistance gene. Mapping of PCR-specific markers, CAPS and microsatellite markers on 429 individuals of an IR64 × Gigante F2 population pinpointed this resistance gene on the long arm of chromosome 4 in a 3.7-cM interval spanned by PCR markers. These markers also flanked the resistance gene of the O. glaberrima accession Tog 5681 and confirmed previous allelism tests. The rarity of this recessive natural resistance was in line with a resistance mechanism model based on point mutations of a host component required for cell-to-cell movement of the virus. Preliminary data on the genetic divergence between the two cultivated rice species in the vicinity of the resistance locus suggested that two different resistance alleles are present in Gigante and Tog 5681. A large set of recombinants is now available to envisage physical mapping and cloning of the gene.

Characterization of the genetic diversity of the Tall coconut (Cocos nucifera L.) in the Dominican Republic using microsatellite (SSR) markers

The predominant coconut variety cultivated in the Dominican Republic is a local Tall, known as criollo. It was never characterized genetically. The Malayan Dwarf and its hybrid with the local Tall are also present. Thirteen accessions, representing nine localities, are planted in a collection at the Instituto Dominicano de Investigaciones Agropecuarias y Forestales (IDIAF). We explored genetic diversity in 114 individuals from this collection. The main aim was to detect possible relationship with resistant varieties to coconut lethal yellowing (LY) disease. Contrarily to what happened in other Caribbean countries, LY never became an epidemic in the Dominican Republic. Thirteen simple sequence repeats markers from a kit dedicated to coconut diversity were used. In addition to diversity parameters, we used Bayesian assignment tests and cluster analysis to determine its population structure and its relationship with other coconut populations. The criollo coconut proved to be a typical Indo-Atlantic variety and is probably highly susceptible to the usual LY pathogens. Local conditions and the nature of the local phytoplasma strain probably explain the particular epidemiology of LY in the Dominican Republic. As a cross-pollinating variety, the criollo presents polymorphism within a population, but there is little if any variation among populations. The marker study confirmed the hybrid status of each member of two accessions and, thus, the reliability of the sampling.

The Panama Tall and the Maypan hybrid coconut in Jamaica: did genetic contamination cause a loss of resistance to Lethal Yellowing?

We applied Bayesian population assignment methods to assess the trueness to type of four populations of the coconut cultivar Panama Tall (PNT) located in Jamaica and found that two of them presented a high percentage of off-types, while genetic contamination was low in the two others. The PNT is the pollen parent of the MAYPAN hybrid, which used to be planted in Jamaica to control an epidemic disease: Lethal Yellowing. The main source of contamination was the susceptible Jamaica Tall, thus increasing the susceptibility in the resulting MAYPAN progeny. The incidence of genetic contamination seems however to be insufficient to be the only cause of the latest outbreak of the disease. Neither the MAYPAN nor its parents can be said resistant in the present context of Jamaica.

QTL analysis of fruit components in the progeny of a Rennell Island Tall coconut (Cocos nucifera L.) individual

We investigated the genetic factors controlling fruit components in coconut by performing QTL analyses for fruit component weights and ratios in a segregating progeny of a Rennell Island Tall genotype. The underlying linkage map of this population was already established in a previous study, as well as QTL analyses for fruit production, which were used to complement our results. The addition of 53 new markers (mainly SSRs) led to minor amendments in the map. A total of 52 putative QTLs were identified for the 11 traits under study. Thirty-four of them were grouped in six small clusters, which probably correspond to single pleiotropic genes. Some additional QTLs located apart from these clusters also had relatively large effects on the individual traits. The QTLs for fruit component weight, endosperm humidity and fruit production were found at different locations in the genome, suggesting that efficient marker-assisted selection for yield can be achieved by selecting QTLs for the individual components. The detected QTLs descend from a genotype belonging to the “Pacific” coconut group. Based on the known molecular and phenotypic differences between “Pacific” and “Indo-Atlantic” coconuts, we suggest that a large fraction of coconut genetic diversity is still to be investigated by studying populations derived from crosses between these groups.

Recent lethal yellowing outbreak: why is the Malayan Yellow Dwarf Coconut no longer resistant in Jamaica?

In Jamaica, the Maypan, a hybrid of Malayan Yellow Dwarf (MYD) and Panama Tall coconut, previously considered highly resistant, is currently being devastated by an epidemic outbreak of lethal yellowing disease. There are several possible causes for this change. In this study, we checked that affected planting material in Jamaica is genetically the same as the material shown to be resistant. We compared the deoxyribonucleic acid (DNA) of MYD sampled in four locations in Jamaica with a reference DNA of the same cultivar collected in five different countries. The results of our analyses showed more variation at 34 simple sequence repeat loci in Jamaica than in the rest of the world providing clear evidence for the presence of about 16% of alleles that do not match the usual typical MYD genotype. These alleles appear to have already been present in the introduced germplasm. This rules out a possible cause of the new outbreak: The observed heterogeneity may have caused some loss of resistance but is insufficient to explain a massive outbreak of the disease.