Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 133

Oil palm has one well-characterized Mendelian gene of overwhelming agronomic importance: shell-thickness. Shell thickness is an important trait in oil palm breeding programmes 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. Oil yield of the heterozygous tenera form (sh+/sh−) is at least 30% greater than the thick-shelled homozygous dura form (sh+/sh+), with the shell-less homozygous pisifera form (sh−/sh−) often producing no yield because of female sterility. Almost all commercial material planted today is of the tenera, thin-shelled, phenotype. This necessitates the development of separate pools of pisifera and dura parents to produce hybrid tenera planting material (Mayes et al., 2000). Billotte et al. (2005) constructed a microsatellite-based high-density linkage map for oil palm from a cross between two heterozygous parents, a tenera palm from the La Mé population (LM2T) and a dura palm from the Deli population (DA10D). A set of 390 SSR markers was developed from microsatellite-enriched libraries and evaluated for polymorphism along with 21 coconut SSRs. A dense and genome-wide microsatellite framework as well as saturating amplified fragments length polymorphisms (AFLPs) allowed the construction of a linkage map consisting of 255 microsatellites, 688 AFLPs and the locus of the Sh gene, which controls the presence or absence of a shell in the oil palm fruit. An AFLP marker E-Agg/M-CAA132 was mapped at 4.7 cM from the Sh locus. The 944 genetic markers were distributed on 16 linkage groups (LGs) and covered 1743 cM. This linkage map was the first in oil palm to have 16 independent linkage groups corresponding to the plant’s 16 homologous chromosome pairs. Singh et al. (2013a) described the mapping and identification of the SHELL gene responsible for the different fruit forms in oil palm. Using homozygosity mapping by sequencing, they found two independent mutations in the DNA-binding domain of a homologue of the MADS-box gene SEEDSTICK (STK, also known as AGAMOUS-LIKE 11), which controls ovule identity and seed development in Arabidopsis thaliana. The SHELL gene is responsible for the tenera phenotype in both cultivated and wild palms from sub-Saharan Africa, and Singh et al. (2013a) provided a genetic explanation for the single gene hybrid vigour (or heterosis) attributed to SHELL, via heterodimerization. This gene mutation explains the single most important economic trait in oil palm.