Major qtls for trunk height and correlated agronomic traits provide insights into multiple trait integration in oil palm breeding

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. Superior oil yield is always the top priority of the oil palm industry. Short trunk height (THT) and compactness traits have become increasingly important to improve harvesting efficiency since the industry started to suffer yield losses due to labor shortages. Breeding populations with low THT and short frond length (FL) are actually available, such as Dumpy AVROS pisifera (DAV) and Gunung Melayu dura (GM). However, multiple trait stacking still remains a challenge for oil palm breeding, which usually requires 12–20 years to complete a breeding cycle. In this study, yield and height increment in the GM × GM (GM-3341) and the GM × DAV (GM-DAV-3461) crossing programs were evaluated and palms with good yield and smaller height increment were identified. In the GM-3341 family, non-linear THT growth between THT_2008 (seven years old) and THT_2014 (13 years old) was revealed by a moderate correlation, suggesting that inter-palm competition becomes increasingly important. In total, 19 quantitative trait loci (QTLs) for THT_2008 (8), oil per palm (O/P) (7) and FL (4) were localized on the GM-3341 linkage map, with an average mapping interval of 2.01 cM. Three major QTLs for THT_2008, O/P and FL are co-located on chromosome 11 and reflect the correlation of THT_2008 with O/P and FL. Multiple trait selection for high O/P and low THT (based on the cumulative effects of positive alleles per trait) identified one palm from 100 palms, but with a large starting population of 1000–1500 seedling per cross, this low frequency could be easily compensated for during breeding selection

An improved oil palm genome assembly as a valuable resource for crop improvement and comparative genomics in the Arecoideae subfamily

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. Oil palm (Elaeis guineensis Jacq.) is the most traded crop among the economically important palm species. Here, we report an extended version genome of E. guineensis that is 1.2 Gb in length, an improvement of the physical genome coverage to 79% from the previous 43%. The improvement was made by assigning an additional 1968 originally unplaced scaffolds that were available publicly into the physical genome. By integrating three ultra-dense linkage maps and using them to place genomic scaffolds, the 16 pseudomolecules were extended. As we show, the improved genome has enhanced the mapping resolution for genome-wide association studies (GWAS) and permitted further identification of candidate genes/protein-coding regions (CDSs) and any non-coding RNA that may be associated with them for further studies. We then employed the new physical map in a comparative genomics study against two other agriculturally and economically important palm species—date palm (Phoenix dactylifera L.) and coconut palm (Cocos nucifera L.)—confirming the high level of conserved synteny among these palm species. We also used the improved oil palm genome assembly version as a palm genome reference to extend the date palm physical map. The improved genome of oil palm will enable molecular breeding approaches to expedite crop improvement, especially in the largest subfamily of Arecoideae, which consists of 107 species belonging to Arecaceae

Impact of sterilization and chemical fertilizer on the microbiota of oil palm seedlings

Soil nutrients and microbiota are known as essential components for healthy plant growth and crop productivity. However, limited studies have been conducted on the importance of soil microbiota in the early growth of oil palm seedlings (Elaeis guineensis Jacq.) under the influence of nitrogen, phosphorus and potassium (NPK) compound fertilizer (nitrogen, phosphorus, and potassium). In this study, we analyzed the root microbial community associated with seedlings grown under normal and sterilized soil conditions to ascertain the microbial strains potentially associated with soil, plant health and chemical fertilizer efficiency. Oil palm seedlings were grown under four treatments: (i) fertilized normal soil (+FN), (ii) unfertilized normal soil (−FN), (iii) fertilized sterilized soil (+FS) and (iv) unfertilized sterilized soil (−FS). Our findings revealed that chemical fertilizer promoted the growth of the copiotrophs Pseudomonadota and Bacteroidota in the control +FN, which are known to degrade complex polysaccharides. After autoclaving, the soil macronutrient content did not change, but soil sterilization reduced microbial diversity in the +FS and −FS treatments and altered the soil microbiota composition. Sterilized soil with a depleted microbial population adversely affected crop growth, which was exacerbated by fertilizer use. In the rhizosphere and rhizoplane compartments, a total of 412 and 868 amplicon sequence variances (ASVs) were found depleted in the +FS and −FS treatments, respectively. Several genera were identified in the ASVs with diminished abundance, including Humibacter, Microbacterium, Mycobacterium, 1921-2, HSB OF53-F07, Mucilaginibacter, Bacillus, Paenibacillus, and several unclassified genera, suggesting their possible roles in promoting the plant growth of oil palm seedlings. Soil sterilization might remove these beneficial microbes from the bulk soil pool, affecting the colonization ability in the rhizocompartments as well as their role in nutrient transformation. Therefore, this study provides useful insights concerning the benefits of a soil microbiome survey before making fertilizer recommendations

Characterizing haploinsufficiency of SHELL gene to improve fruit form prediction in introgressive hybrids of oil palm

The fundamental trait in selective breeding of oil palm (Eleais guineensis Jacq.) is the shell thickness surrounding the kernel. The monogenic shell thickness is inversely correlated to mesocarp thickness, where the crude palm oil accumulates. Commercial thin-shelled tenera derived from thick-shelled dura × shell-less pisifera generally contain 30% higher oil per bunch. Two mutations, shᴹᴾᴼᴮ (M1) and shAVROS (M2) in the SHELL gene – a type II MADS-box transcription factor mainly present in AVROS and Nigerian origins, were reported to be responsible for different fruit forms. In this study, we have tested 1,339 samples maintained in Sime Darby Plantation using both mutations. Five genotype-phenotype discrepancies and eight controls were then re-tested with all five reported mutations (shAVROS, shᴹᴾᴼᴮ, shᴹᴾᴼᴮ², shᴹᴾᴼᴮ ³ and shᴹᴾᴼᴮ⁴) within the same gene. The integration of genotypic data, pedigree records and shell formation model further explained the haploinsufficiency effect on the SHELL gene with different number of functional copies. Some rare mutations were also identified, suggesting a need to further confirm the existence of cis-compound mutations in the gene. With this, the prediction accuracy of fruit forms can be further improved, especially in introgressive hybrids of oil palm. Understanding causative variant segregation is extremely important, even for monogenic traits such as shell thickness in oil palm

A practical genome-enabled legitimacy assay for oil palm breeding and seed production

Background: Legitimacy in breeding and commercial crop production depends on optimised protocols to ensure purity of crosses and correct field planting of material. In oil palm, the presence of three fruit forms permits these assumptions to be tested, although only after field planting. The presence of incorrect fruit forms in a cross is a clear sign of illegitimacy. Given that tenera forms produce 30% more oil for the same weight of fruit as dura, the presence of low levels of dura contamination can have major effect during the economic lifespan of an oil palm, which is around 25 years. We evaluated two methods for legitimacy test 1) The use of SHELL markers to the gene that determines the shell-thickness trait 2) The use of SNP markers, to determine the legitimacy of the cross

Linkage-based genome assembly improvement of oil palm (Elaeis guineensis)

Meiotic crossovers in outbred species, such as oil palm (Elaeis guineensis Jacq., 2n = 32) contribute to allelic re-assortment in the genome. Such genetic variation is usually exploited in breeding to combine positive alleles for trait superiority. A good quality reference genome is essential for identifying the genetic factors underlying traits of interest through linkage or association studies. At the moment, an AVROS pisifera genome is publicly available for oil palm. Distribution and frequency of crossovers throughout chromosomes in different origins of oil palm are still unclear. Hence, an ultrahigh-density genomic linkage map of a commercial Deli dura x AVROS pisifera family was constructed using the OP200K SNP array, to evaluate the genetic alignment with the genome assembly. A total of 27,890 linked SNP markers generated a total map length of 1,151.7 cM and an average mapping interval of 0.04 cM. Nineteen linkage groups represented 16 pseudo-chromosomes of oil palm, with 61.7% of the mapped SNPs present in the published genome. Meanwhile, the physical map was also successfully extended from 658 Mb to 969 Mb by assigning unplaced scaffolds to the pseudo-chromosomes. A genic linkage map with major representation of sugar and lipid biosynthesis pathways was subsequently built for future studies on oil related quantitative trait loci (QTL). This study improves the current physical genome of the commercial oil palm, and provides important insights into its recombination landscape, eventually unlocking the full potential genome sequence-enabled biology for oil palm

Characterizing haploinsufficiency of SHELL gene to improve fruit form prediction in introgressive hybrids of oil palm

The fundamental trait in selective breeding of oil palm (Eleais guineensis Jacq.) is the shell thickness surrounding the kernel. The monogenic shell thickness is inversely correlated to mesocarp thickness, where the crude palm oil accumulates. Commercial thin-shelled tenera derived from thick-shelled dura × shell-less pisifera generally contain 30% higher oil per bunch. Two mutations, shᴹᴾᴼᴮ (M1) and shAVROS (M2) in the SHELL gene – a type II MADS-box transcription factor mainly present in AVROS and Nigerian origins, were reported to be responsible for different fruit forms. In this study, we have tested 1,339 samples maintained in Sime Darby Plantation using both mutations. Five genotype-phenotype discrepancies and eight controls were then re-tested with all five reported mutations (shAVROS, shᴹᴾᴼᴮ, shᴹᴾᴼᴮ², shᴹᴾᴼᴮ ³ and shᴹᴾᴼᴮ⁴) within the same gene. The integration of genotypic data, pedigree records and shell formation model further explained the haploinsufficiency effect on the SHELL gene with different number of functional copies. Some rare mutations were also identified, suggesting a need to further confirm the existence of cis-compound mutations in the gene. With this, the prediction accuracy of fruit forms can be further improved, especially in introgressive hybrids of oil palm. Understanding causative variant segregation is extremely important, even for monogenic traits such as shell thickness in oil palm

Evaluation of methods and marker systems in genomic selection of oil palm (Elaeis guineensis Jacq.)

Background Genomic selection (GS) uses genome-wide markers as an attempt to accelerate genetic gain in breeding programs of both animals and plants. This approach is particularly useful for perennial crops such as oil palm, which have long breeding cycles, and for which the optimal method for GS is still under debate. In this study, we evaluated the effect of different marker systems and modeling methods for implementing GS in an introgressed dura family derived from a Deli dura x Nigerian dura (Deli x Nigerian) with 112 individuals. This family is an important breeding source for developing new mother palms for superior oil yield and bunch characters. The traits of interest selected for this study were fruit-to-bunch (F/B), shell-to-fruit (S/F), kernel-to-fruit (K/F), mesocarp-to-fruit (M/F), oil per palm (O/P) and oil-to-dry mesocarp (O/DM). The marker systems evaluated were simple sequence repeats (SSRs) and single nucleotide polymorphisms (SNPs). RR-BLUP, Bayesian A, B, Cπ, LASSO, Ridge Regression and two machine learning methods (SVM and Random Forest) were used to evaluate GS accuracy of the traits. Results The kinship coefficient between individuals in this family ranged from 0.35 to 0.62. S/F and O/DM had the highest genomic heritability, whereas F/B and O/P had the lowest. The accuracies using 135 SSRs were low, with accuracies of the traits around 0.20. The average accuracy of machine learning methods was 0.24, as compared to 0.20 achieved by other methods. The trait with the highest mean accuracy was F/B (0.28), while the lowest were both M/F and O/P (0.18). By using whole genomic SNPs, the accuracies for all traits, especially for O/DM (0.43), S/F (0.39) and M/F (0.30) were improved. The average accuracy of machine learning methods was 0.32, compared to 0.31 achieved by other methods. Conclusion Due to high genomic resolution, the use of whole-genome SNPs improved the efficiency of GS dramatically for oil palm and is recommended for dura breeding programs. Machine learning slightly outperformed other methods, but required parameters optimization for GS implementation

Framework for genetic improvement of oil yield and harvestability in oil palm

Oil palm (Elaeis guineensis Jacq.) contributes 35% of global vegetable oil and has become the most important oil crop in the world. It occupies only 7% of total global oilseed-harvested land with 10 times more oil yield per hectare of cropland than any other oil crop. To further improve the superior oil yield, enormous efforts have been put into selective breeding programmes in the last eight decades. Nevertheless, the breeding progress of oil palm, like other perennial crops, is extremely slow (typically 12 years per breeding cycle) and costly. In this study, major QTL (quantitative trait loci) for trunk height (THT) and correlated yield traits were discovered in low trunk increment breeding populations using single nucleotide polymorphism (SNP) and simple sequence repeat (SSR) markers. Further understanding of the correlation between THT and oil yield traits, and the effect of cumulative positive alleles of QTL can significantly increase the success rate of multiple trait integration through MAS to produce high-yielding oil palms with low height increment. This compares to an estimated 1% probability of success using conventional breeding methods. Furthermore, illegitimacy in breeding crosses and commercial seed production can be a common problem, if mismanagement happens in controlled pollination (given that oil palm is a naturally outcrossing species) and seed processing. For perennial crops such as oil palm, the illegitimacy problem can easily badly effect a 12-year breeding cycle and illegitimacy can lead to a 25-year yield reduction in commercial planting. Therefore, a practical genomic SNP-based legitimacy testing method with accuracy >97% was developed as a standard quality control procedure for seed purity in breeding and commercial seed production. The method has also successfully addressed the inaccuracy and inefficiency of the conventional fruit census test for contamination. The overall objectives of this study and findings are reported in the thesis

Framework for genetic improvement of oil yield and harvestability in oil palm

Oil palm (Elaeis guineensis Jacq.) contributes 35% of global vegetable oil and has become the most important oil crop in the world. It occupies only 7% of total global oilseed-harvested land with 10 times more oil yield per hectare of cropland than any other oil crop. To further improve the superior oil yield, enormous efforts have been put into selective breeding programmes in the last eight decades. Nevertheless, the breeding progress of oil palm, like other perennial crops, is extremely slow (typically 12 years per breeding cycle) and costly. In this study, major QTL (quantitative trait loci) for trunk height (THT) and correlated yield traits were discovered in low trunk increment breeding populations using single nucleotide polymorphism (SNP) and simple sequence repeat (SSR) markers. Further understanding of the correlation between THT and oil yield traits, and the effect of cumulative positive alleles of QTL can significantly increase the success rate of multiple trait integration through MAS to produce high-yielding oil palms with low height increment. This compares to an estimated 1% probability of success using conventional breeding methods. Furthermore, illegitimacy in breeding crosses and commercial seed production can be a common problem, if mismanagement happens in controlled pollination (given that oil palm is a naturally outcrossing species) and seed processing. For perennial crops such as oil palm, the illegitimacy problem can easily badly effect a 12-year breeding cycle and illegitimacy can lead to a 25-year yield reduction in commercial planting. Therefore, a practical genomic SNP-based legitimacy testing method with accuracy >97% was developed as a standard quality control procedure for seed purity in breeding and commercial seed production. The method has also successfully addressed the inaccuracy and inefficiency of the conventional fruit census test for contamination. The overall objectives of this study and findings are reported in the thesis