Identification of QTLs associated with callogenesis and embryogenesis in oil palm using genetic linkage maps improved with SSR markers.

Clonal reproduction of oil palm by means of tissue culture is a very inefficient process. Tissue culturability is known to be genotype dependent with some genotypes being more amenable to tissue culture than others. In this study, genetic linkage maps enriched with simple sequence repeat (SSR) markers were developed for dura (ENL48) and pisifera (ML161), the two fruit forms of oil palm, Elaeis guineensis. The SSR markers were mapped onto earlier reported parental maps based on amplified fragment length polymorphism (AFLP) and restriction fragment length polymorphism (RFLP) markers. The new linkage map of ENL48 contains 148 markers (33 AFLPs, 38 RFLPs and 77 SSRs) in 23 linkage groups (LGs), covering a total map length of 798.0 cM. The ML161 map contains 240 markers (50 AFLPs, 71 RFLPs and 119 SSRs) in 24 LGs covering a total of 1,328.1 cM. Using the improved maps, two quantitative trait loci (QTLs) associated with tissue culturability were identified each for callusing rate and embryogenesis rate. A QTL for callogenesis was identified in LGD4b of ENL48 and explained 17.5% of the phenotypic variation. For embryogenesis rate, a QTL was detected on LGP16b in ML161 and explained 20.1% of the variation. This study is the first attempt to identify QTL associated with tissue culture amenity in oil palm which is an important step towards understanding the molecular processes underlying clonal regeneration of oil palm.

SSR mining in oil palm EST database: application in oil palm germplasm diversity studies.

This study reports on the detection of additional expressed sequence tags (EST) derived simple sequence repeat (SSR) markers for the oil palm. A large collection of 19243 Elaeis guineensis ESTs were assembled to give 10258 unique sequences, of which 629 ESTs were found to contain 722 SSRs with a variety of motifs. Dinucleotide repeats formed the largest group (45.6%) consisting of 66.9% AG/CT, 21.9% AT/AT, 10.9% AC/GT and 0.3% CG/CG motifs. This was followed by trinucleotide repeats, which is the second most abundant repeat types (34.5%) consisting of AAG/CTT (23.3%), AGG/CCT (13.7%), CCG/CGG (11.2%), AAT/ATT (10.8%), AGC/GCT (10.0%), ACT/AGT (8.8%), ACG/CGT (7.6%), ACC/GGT (7.2%), AAC/GTT (3.6%) and AGT/ACT (3.6%) motifs. Primer pairs were designed for 405 unique EST-SSRs and 15 of these were used to genotype 105 E. guineensis and 30 E. oleifera accessions. Fourteen SSRs were polymorphic in at least one germplasm revealing a total of 101 alleles. The high percentage (78.0%) of alleles found to be specific for either E. guineensis or E. oleifera has increased the power for discriminating the two species. The estimates of genetic differentiation detected by EST-SSRs were compared to those reported previously. The transferability across palm taxa to two Cocos nucifera and six exotic palms is also presented. The polymerase chain reaction (PCR) products of three primer-pairs detected in E. guineensis, E. oleifera, C. nucifera and Jessinia bataua were cloned and sequenced. Sequence alignments showed mutations within the SSR site and the flanking regions. Phenetic analysis based on the sequence data revealed that C. nucifera is closer to oil palm compared to J. bataua; consistent with the taxanomic classification.

Normalized embryoid cDNA library of oil palm (Elaeis guineensis) / Biblioteca ADNc normalizada de embrioides en aceite de palma (Elaeis guineensis)

A normalized embryoid cDNA library (EON) was constructed based on reassociation kinetics reaction. Results from dot blot hybridization and sequencing of EON cDNA clones clearly indicated that the normalization process reduced the frequency of high abundance transcripts and increased the frequency of low abundance gene transcripts. A total of 553 non-redundant expressed sequence tags (ESTs) were identified, 325 of these were not observed in the standard oil palm cDNA libraries sequenced previously. A total of 10 EON cDNA clones were chosen for expression profiling across samples from different stages of the tissue culture process. Two of the genes exhibited promising expression patterns for predicting the embryogenic potential in callus. Some of these genes were also differentially expressed in the various tissues of oil palm. This study showed that normalization of the existing embryoid library improved the chances of identifying transcripts not captured in the standard libraries, some of which could be associated with embryogenesis. This collection of ESTs is particularly well suited for use as candidate genes for development of an oil palm DNA chip, which can be used to obtain a more comprehensive view of the molecular mechanism associated with oil palm tissue culture.

Mapping quantitative trait loci (QTLs) for fatty acid composition in an interspecific cross of oil palm.

BACKGROUND: Marker Assisted Selection (MAS) is well suited to a perennial crop like oil palm, in which the economic products are not produced until several years after planting. The use of DNA markers for selection in such crops can greatly reduce the number of breeding cycles needed. With the use of DNA markers, informed decisions can be made at the nursery stage, regarding which individuals should be retained as breeding stock, which are satisfactory for agricultural production, and which should be culled. The trait associated with oil quality, measured in terms of its fatty acid composition, is an important agronomic trait that can eventually be tracked using molecular markers. This will speed up the production of new and improved oil palm planting materials. RESULTS: A map was constructed using AFLP, RFLP and SSR markers for an interspecific cross involving a Colombian Elaeis oleifera (UP1026) and a Nigerian E. guinneensis (T128). A framework map was generated for the male parent, T128, using Joinmap ver. 4.0. In the paternal (E. guineensis) map, 252 markers (199 AFLP, 38 RFLP and 15 SSR) could be ordered in 21 linkage groups (1815 cM). Interval mapping and multiple-QTL model (MQM) mapping (also known as composite interval mapping, CIM) were used to detect quantitative trait loci (QTLs) controlling oil quality (measured in terms of iodine value and fatty acid composition). At a 5% genome-wide significance threshold level, QTLs associated with iodine value (IV), myristic acid (C14:0), palmitic acid (C16:0), palmitoleic acid (C16:1), stearic acid (C18:0), oleic acid (C18:1) and linoleic acid (C18:2) content were detected. One genomic region on Group 1 appears to be influencing IV, C14:0, C16:0, C18:0 and C18:1 content. Significant QTL for C14:0, C16:1, C18:0 and C18:1 content was detected around the same locus on Group 15, thus revealing another major locus influencing fatty acid composition in oil palm. Additional QTL for C18:0 was detected on Group 3. A minor QTL for C18:2 was detected on Group 2. CONCLUSION: This study describes the first successful detection of QTLs for fatty acid composition in oil palm. These QTLs constitute useful tools for application in breeding programmes.

Oil palm (Elaeis guineensis Jacq.) tissue culture ESTs: identifying genes associated with callogenesis and embryogenesis.

BACKGROUND: Oil palm (Elaeis guineensis Jacq.) is one of the most important oil bearing crops in the world. However, genetic improvement of oil palm through conventional breeding is extremely slow and costly, as the breeding cycle can take up to 10 years. This has brought about interest in vegetative propagation of oil palm. Since the introduction of oil palm tissue culture in the 1970s, clonal propagation has proven to be useful, not only in producing uniform planting materials, but also in the development of the genetic engineering programme. Despite considerable progress in improving the tissue culture techniques, the callusing and embryogenesis rates from proliferating callus cultures remain very low. Thus, understanding the gene diversity and expression profiles in oil palm tissue culture is critical in increasing the efficiency of these processes. RESULTS: A total of 12 standard cDNA libraries, representing three main developmental stages in oil palm tissue culture, were generated in this study. Random sequencing of clones from these cDNA libraries generated 17,599 expressed sequence tags (ESTs). The ESTs were analysed, annotated and assembled to generate 9,584 putative unigenes distributed in 3,268 consensi and 6,316 singletons. These unigenes were assigned putative functions based on similarity and gene ontology annotations. Cluster analysis, which surveyed the relatedness of each library based on the abundance of ESTs in each consensus, revealed that lipid transfer proteins were highly expressed in embryogenic tissues. A glutathione S-transferase was found to be highly expressed in non-embryogenic callus. Further analysis of the unigenes identified 648 non-redundant simple sequence repeats and 211 putative full-length open reading frames. CONCLUSION: This study has provided an overview of genes expressed during oil palm tissue culture. Candidate genes with expression that are modulated during tissue culture were identified. However, in order to confirm whether these genes are suitable as early markers for embryogenesis, the genes need to be tested on earlier stages of tissue culture and a wider range of genotypes. This collection of ESTs is an important resource for genetic and genome analyses of the oil palm, particularly during tissue culture development.

Somaclonal variation in micropropagated oil palm. Characterization of two novel genes with enhanced expression in epigenetically abnormal cell lines and in response to auxin.

In vitro micropropagation based on somatic embryogenesis provides an efficient means to multiply selected genotypes of oil palm (Elaeis guineensis Jacq.). Despite its considerable potential, somatic embryogenesis can yield plants bearing a homeotic flowering abnormality known as mantled. Because the mantled abnormality is epigenetic, it cannot be detected with conventional structural molecular markers. Thus, to develop a means of discriminating among callus cultures carrying or lacking the mantled abnormality, we used a gene expression approach. We describe two novel oil palm genes, EgM39A and EgIAA1, both of which display increased transcript accumulation in epigenetically abnormal calli. EgIAA1 codes for an oil palm relative of the Arabidopsis thaliana (L.) Heynh. AXR3/IAA17 protein involved in early auxin response and EgM39A codes for a protein of unknown function sharing sequence similarities with asparagine synthetases. In addition to their enhanced expression in somaclonal variant callus lines, both genes displayed increased transcript accumulation in response to auxin treatment. Normal seed-derived zygotic embryos germinated in the presence of auxin accumulated increased amounts of EgIAA1 transcripts after a few hours of treatment, suggesting a role in auxin response similar to that demonstrated for IAA genes in other species. The EgM39A gene also displayed enhanced transcript accumulation in auxin-treated zygotic embryos. Although only a small increase was seen after 24 h, greater changes were observed after 15 days. Both genes show potential as early markers of clonal conformity and may help to elucidate the nature of the epigenetic changes causing the mantled abnormality.

Characterization of a defensin gene expressed in oil palm inflorescences: induction during tissue culture and possible association with epigenetic somaclonal variation events.

From differential display studies performed on oil palm (Elaeis guineensis Jacq.) tissue cultures bearing or lacking an epigenetic homeotic flowering abnormality, known as mantled, EGAD1, a gene coding for a putative plant defensin, has been identified and characterized. In whole plants, transcripts of the EGAD1 gene were detected only in inflorescences. The closest characterized relative of the oil palm EGAD1 gene is the Petunia PPT gene, which is expressed principally in the pistil of the flower. The 77 amino acid polypeptide encoded by the EGAD1 gene displays strong similarities with a number of plant defensin proteins, which are thought to play a protective role and which have been shown in some cases to possess antifungal properties. Oil palm tissue cultures exhibit a generally strong induction of accumulation of EGAD1 transcripts, which were detected to differing extents at all stages of the tissue culture regeneration process. The 5' flanking region of the EGAD1 gene was found to contain two different types of potential cis-acting DNA element previously identified in the promoters of plant defence-related genes, which may explain the observed expression in tissue cultures. At the callus stage of the in vitro regeneration procedure, a differential accumulation of EGAD1 transcripts was observed which correlated with the presence or absence of the mantled flowering abnormality. EGAD1 gene expression may therefore be a marker of epigenetic somaclonal variation events.