Michel Dollet

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.

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.