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DNA methylation and somaclonal variation in oil palm

Rival A., Jaligot E., Beulé T., Verdeil J.L., Tregear J.. 2000. In : Cassells A.C. (ed.), Doyle B.M. (ed.), Curry R.F. (ed.). Proceedings of the international symposium on methods and markers for quality assurance in micropropagation. Leuven : ISHS, p. 447-454. (Acta Horticulturae, 530). International Symposium on Methods and Markers for Quality Assurance in Micropropagation, 1999-08-24/1999-08-27, Cork (Irlande).

Clonal propagation of oil palm (Elaeis guineensis Jacq., Arecaceae) through somatic embryogenesis has led to the production of several million clonal plantlets to date. Evaluation in the field has revealed the occurrence of variant palms (ca 5%) showing an abnormal flower development, called "mantled". Preliminary studies of ploidy level and RAPD polymorphism have not revealed any genomic changes that could be linked with the variant phenotype. These results are consistent with the hypothesis of an epigenetic origin for the abnormality. Levels of global DNA methylation were estimated after enzymatic hydrolysis of genomic DNA to nucleosides and HPLC quantification of 5-Methyl deoxyCytidine. Methylation rates of genomic DNA [(5mdC) / (5mdC+dC)] have been investigated in somatic embryo-derived-oil palms, with the aim of comparing normal and variant regenerants within the same clonal line. Global levels of genomic DNA methylation were found to range from 20 to 25%. DNA methylation has been found to discriminate the variants from normal regenerants at the adult age. In some clonal lines, analyses of leaf genomic DNA have revealed a substantial demethylation in severely "mantled" palms (20.6 % vs 22.2 %). Furthermore, clonal palms originating from Nodular Compact Calli (NCC) have been shown to exhibit the "mantled" variant phenotype at an average level of 5%, whereas this rate has been found to attain 100% in palms derived from Fast Growing Calli (FGC). A significant hypomethylation in FGC (18.7% vs 23.2 %) has been found when compared to NCC from the same clonal line. Evidence for a direct relationship in oil palm between hypomethylation of genomic DNA and the determinism of the "mantled" somaclonal variant phenotype is still yet to be obtained, but our studies on DNA methylation have already opened very promising lines of research.

Mots-clés : elaeis guineensis; clone; variation somaclonale; embryogénèse somatique; micropropagation; cal; feuille; fleur; trouble de différenciation sexuelle; anatomie végétale; hydrolyse enzymatique; adn; cytosine; génome; méthylation

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