RAPD linkage mapping of the shell thickness locus in oil palm (Elaeis guineensis Jacq.)

Shell thickness is an important trait in oil palm breeding programs and is the basis for the classification of the varieties of oil palm into the types dura, tenera and pisifera. This trait seems to be controlled by a single locus, with two alleles (sh ⁺ and sh ⁻) showing codominant expression. Two single-tree linkage maps were constructed for a maternal tenera (sh ⁺ sh ⁻) palm and for a paternal pisifera (sh ⁻ sh ⁻) palm using the pseudo-testcross mapping strategy in combination with RAPD markers through the analysis of an F₁ tenera×pisifera progeny. A total of 308 arbitrary primers were screened in a sample of eight F₁ plants and 121 markers were detected in a testcross configuration. An average of 1.66 polymorphic marker per selected primer were identified in this cross. At LOD 5.0 (with some few exceptions) and θ=0.25 the maternal tenera map included a total of 48 markers distributed in 12 linkage groups or pairs of markers (449.3 cM) while the paternal pisifera map included 42 markers distributed in 15 linkage groups or pairs of markers (399.7 cM). We used RAPD and bulked segregant analysis (BSA) to identify markers more tightly linked to the sh ⁺ locus. A total of 174 new primers not previously used in the linkage analysis were screened using bulks of DNA extracted from plants selected for the contrasting shell-thickness phenotypes. Two RAPD markers (R11–1282 and T19–1046) were identified to be linked on both sides of the sh ⁺ locus on linkage group 4. The estimated map distances from sh ⁺ to R11–1282 and to T19–1046 were 17.5 cM and 23.9 cM, respectively. The results demonstrate the usefulness of RAPD markers and the pseudo-testcross mapping strategy for developing genetic linkage information, and constitute an important step towards early marker-assisted selection for shell thickness in oil palm. Received: 21 February 1999 / Accepted: 29 April 1999