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Rice (Oryza glaberrima) yellow mottle virus (RYMV) causes significant economic damage to rainfed and lowland irrigated rice, Oryza sativa L. in West and East Africa. This study investigated the mode of gene action of resistance to RYMV using generation mean analysis. Crosses were made between a more susceptible line (Tog 7258) and three resistant pure lines to produce the F1, F2, backcrosses and F3 populations necessary to conduct the genetic studies. The seven populations were grown in a screen house at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria in 1992 and 1993. Severity of mottle symptom on plants was classified on a 0 - 9 scale based on the intensity of the disease on leaves under artificial virus inoculation. In general, F1 disease scores were higher than the mid-parental value. Mather's scaling test was applied to the data generated from each cross and the results indicated that generation means depended on additive and dominance gene effects. An epistatic effect was suggested in the generation mean analysis using Hayman's method and the primary effect was assumed to be additive and dominance and their interactions as indicated in the scaling test. Estimates of gene numbers indicated that the two parents were different by 2 to 4 genes for resistance to RYMV. Narrow sense heritability was estimated to be 44-65% and, therefore, a breeder should be able to make progress by selecting in the F2 or F3 generation. Key Words: Additive effect, dominance effect, epistasis, heritability, Oryza glaberrima Resume Le virus de tâche jaune (RYMV) de riz (Oryza glaberrima) cause de degâts economiques significatifs au riz (Oryza sativa L.) de terre faiblement irrigue, a Ouest et l'Est de l'Afrique. Cette etude a examine le mode d'action de gene de resistance au RYMV en utilisant l'analyse moyenne de generation. Les croisements etaient faits entre plus d'une ligne susceptible (Tog 7258) et trois lignes pures resistantes pour produire les F1, F2, precroisemenet et F3, populations necessaires pour conduires l'etude genetique. Les sept populations etaient plantees dans une maison cloisonnee a l'Institut international de l'agriculture Tropicale (IITA), en Ibandan au Nigeria en 1992 et 1993. La severite de symptome de tâche sur les plantes etait classifiee sur une echelle de 0-9 basee sur l'intensite de la maladie sur les feuilles sous inoculation artificielle de virus. En general, les marques de la maladie etaient elevees plus que la valeur intermediaire parentale. Le teste mere d'adjustement etaient applique aux donnees generees a partir de chaque croisement et les rsultats indiquerent que les moyens de generation dependaient des effets additifs et dominants des genes. Un effet epististatique etait suggere dans l'analyse moyenne de generation utilisant la methode de Hayman et l'effet primaire etait assume etre additif et dominant et leurs interactions comme indiquees dans le teste d'adjustement. Les estimations de nombres de genes indiquent que les deux parents etaient differents de 2 a 4 genes pour la resistance au RYMV. Dans le sens etroit, l'heritabilite etait estimee etre 44-65% et ainsi donc un reproducteur devrait etre capable de faire de progres en selectionnant dans les generation F2 ou F3. Mots Cles: Effet additif, effet dominant, epistasis, heritabilite, Oryza glaberrima (Af Crop Sci J 2003 Vol 11 No 3 pp143-150)

Mode of gene action of inheritance for resistance to rice yellow mottle virus