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The objective of this study was to apply the QTL mapping by Markers-Assisted Selection approach for early selection on growth and latex yield in rubber breeding. The plant material consisted of 196 progenies derived from the F1 family RRIM 600 x PB 217. A genetic linkage map was built for this family with 229 SSR markers (microsatellites) and 198 AFLP markers. Phenotyping was carried out over a 6 years period on a field trial of 5 hectares, with around 2,400 trees measured individual. The two major QTLs were detected repeatedly. The QTL g16-6 was associated with latex yield near the position of marker a131. This QTL explained up to 66% of the phenotypic variance. It was also associated to other traits that were strongly correlated to production such as inorganic phosphorus and dry rubber content (latex diagnostic), as well as plugging index. This finding indicates the existence of one major gene (or a cluster of genes) located on linkage group g16 and involved in the intensity of metabolic activity of latex cell. A second important QTL associated with girth of the trunk (growth) was detected (QTL g3-60) at the position of marker a312. It explained up to 31% of the phenotypic variance. The discovery of the two major QTLs g3-60 and g16-6 suggest that two major genes act as limiting factors in the genetic determinism of the growth in girth during favorable condition and of rubber production in low-intensive tapping systems. These loci should become privileged targets for the identification of key-genes in rubber. The possible use of their neighboring markers for developing Markers-Assisted Selection (MAS) in the short run appears very reasonable. It should contribute to improve the accuracy of the first selection stage which is the weakest point of the rubber clonal selection scheme.

The objective of this study was to analyse the genetic determinism of growth and latex production in rubber tree by a QTL approach. The plant material consisted of 196 progenies derived from the F1 family RRIM600 x PB217. A genetic linkage map was built for this family with 229 SSR markers (microsatellites) and 198 AFLP markers. Phenotyping was carried out over a 6-year period on a field trial of 5 hectares, with around 2400 trees measured individually. A high broad sense heritability was found for latex production, and a major QTL directly associated to production was discovered (Hbg16a131). This QTL explained from 32% to 59% of the genetic variance of the production traits. It was also associated to other traits that were strongly correlated to production such as inorganic phosphorus and dry rubber content (latex diagnostic), as well as plugging index. This finding indicates the existence of one major gene (or a cluster of genes) located on linkage group 16 and involved in the genetic determinism of latex production. A second important QTL associated to the girth of the trunk (growth) was detected (Hbg3a312). Depending on the age of the trees, it explained from 16% to 28% of the genetic variance of the girth. By contrast, no significant QTL was found for sucrose content of the latex, as estimated independently from the production, so suggesting that sucrose genetic determinism might not include any single gene having by itself some important effect. The application of one ethephon stimulation did not evidence any specific QTL. Those results pave the way for Markers-Assisted Selection (MAS) as applied to clonal selection in rubber. Genotyping the candidates to selection for the 2 QTLs with important effects should allow a much more accurate estimation of genetic values as soon as at the first selection stage.

La Thaïlande est le premier producteur mondial de caoutchouc naturel qui représente 43 % de l'ensemble des élastomères (naturel + synthétique). Les variétés d'hévéa sont des clones hétérozygotes multipliés par greffage sur porte-greffes issus de graines. La première identification de QTLs chez l'hévéa a d'abord été réalisée au Cirad avec des marqueurs RFLP pour l'étude de la tolérance à la maladie sud-américaine des feuilles dans le cadre d'une coopération avec Michelin [1]. On est ensuite passé à l'utilisation de marqueurs SSR (microsatellites) pour la détection de QTLs associés à la croissance et à la production de latex de l'hévéa [2]. Deux QTLs importants ont été trouvés concernant la croissance et la production de latex. Leur utilisation en Sélection Assistée par Marqueurs, lors du screening initial très précoce des génotypes issus de la recombinaison par pollinisation manuelle, permettrait d'améliorer significativement l'efficacité de la sélection clonale.

Starch and total soluble sugar (SS) status in rubber tree were investigated carbohydrate reserve related to tapping system. RR IM 600 clone were studied for three years in Chachoengsao Rubber Research Center (CRRC-RRIT-DOA). Trees were tapped using 1/2S d/2 (control, D/2), 1/2S d/3.ET 2.5%,8/y (D/3) and the DCA system (double-cut alternative tapping system). Bark and wood tissue were collected in October 2002 (period of high rainfall and high latex production) at various positions along trunks and were analyzed using starch enzymatic analysis technique. Together with a higher production of latex, DCA system had higher content in both starch and SS in wood. In bark, SS was also higher in DCA, whereas starch was not different among treatments. According to accumulation of starch on various part of tree, it was found that the starch decreased with height of samples up the trunk. Content in lower part of tree was higher than that in the top or crown part, but there was the decrease of starch in both wood and bark within the bark regeneration area or the area which has been already tapped, confirming the hypothesis that tapping affects the level and distribution pattern of carbohydrate within the trunk.

Distribution patterns of latex sucrose content and concurrent metabolic activity at the trunk level with different tapping systems, namely 1/2S d/2 (a half spiral cut alternate daily), 1/2S d/3 ET 2.5%.8/y (a half spiral cut one day in tapping followed by two days of rest, stimulated with ethephon, eight applications per year) and DCA (Double Cut Alternate, 2x1/2S d/4 (t,t), two half spiral cuts, each cut tapped alternate on every four days) were studied. RRIM600 rubber trees were used to map the latex metabolic status from different heights on both panel A and panel B. Latex was collected by puncturing trunks at various positions and was analyzed using Latex Diagnosis technique (LD). The concurrent comparison of latex sucrose and inorganic phosphorus concentrations for every sampling position allowed estimating the size and shape of the actual latex regeneration area. Regular tapping induced latex sucrose depletion in tapped panel bark. This sucrose depletion was enhanced by the use of DCA, as a consequence of increased production and therefore higher latex regeneration metabolism. DCA also induced a huge sucrose sink effect above tapped cut on both panel A and panel B and outside the metabolically active area.