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    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

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Development and application of a 20K SNP array in potato
Vos, Peter - \ 2016
Wageningen University. Promotor(en): Richard Visser; Fred van Eeuwijk, co-promotor(en): Herman van Eck. - Wageningen : Wageningen University - ISBN 9789462579569 - 166
solanum tuberosum - potatoes - genotypes - single nucleotide polymorphism - data analysis - plant breeding - linkage disequilibrium - genome analysis - tetraploidy - aardappelen - genotypen - gegevensanalyse - plantenveredeling - verstoord koppelingsevenwicht - genoomanalyse - tetraploïdie

In this thesis the results are described of investigations of various application of genome wide SNP (single nucleotide polymorphism) markers. The set of SNP markers was identified by GBS (genotyping by sequencing) strategy. The resulting dataset of 129,156 SNPs across 83 tetraploid varieties was used directly to map traits, but also as a basis for the development of a 20K SNP array in Potato (Solanum tuberosum L.). Subsequently this array, named SolSTW, was used to collect genotypic data from 569 potato genotypes. This dataset offered insight in the breeding history of potato, population structure, linkage disequilibrium (LD) and the potential of GWAS (genome wide association studies) in potato.

In Chapter 2 we describe to development of the SolSTW 20K Infinium SNP array. One third of the SNPs on this array originate from the well-known SolCAP 8303 SNP array. The other SNPs are a subset from a targeted re-sequencing project of 83 tetraploid potato varieties. Because of the high SNP density in potato only a limited number of SNPs is suitable for assay development on a SNP array. An obvious outcome is that flanking SNPs contribute to assay failure, particularly for assays with SNPs located in introns. We used fitTetra software to cluster the distribution of captured signals of each marker into the expected five genotypic classes (nulliplex, simplex, duplex, triplex, quadruplex), resulting in a dataset with 14,530 SNP markers. Subsequently the genotypic data obtained with the SolSTW array was used to characterize a set of 569 potato varieties, advanced breeding clones and progenitors. This resulted in the identification of several footprints of potato breeding. Firstly SNPs were dated i.e. the year of market release of the first variety showing polymorphism for a SNP locus is an indication of the ancestry of a SNP. In such a way we identified SNPs with an ancestry tracing back to heirloom varieties, and SNPs (post-1945 SNPs) tracing back to wild species used in modern introgression breeding. Secondly, the changes in allele frequency were calculated over time. Most SNPs show a relative stable allele frequency over time, and very limited genetic variation is removed from the gene-pool of potato i.e genetic erosion is almost absent. Therefore we conclude that 100 years of breeding has not been able to get rid of non-beneficial genetic variation. Only a limited number of SNPs show a rapid increased in allele frequency, which can be explained by positive selection for disease resistance by breeders, or the more frequent use of several founders.

Better understanding of the genome wide decay of Linkage Disequilibrium (LD) and population structure offers relevant knowledge to perform and interpret the results of a genome wide association study (GWAS) (Chapter 3). Linkage disequilibrium (LD) is a complex phenomenon, and the influence of the factors shaping LD in tetraploids is hardly studied. Therefore we used simulated data to disentangle and therewith understand often-confounded factors underlying LD-decay. We simulated datasets differing in number of haplotypes in a population, and differing in percentage of haplotype specific SNPs. In these simulations we observed that the choice of an estimator of LD-decay has a major effect on the outcome of an LD-decay estimate, while the true LD-decay remains the same. Based on the simulation we conclude that a 90% percentile and a so-called D1/2 (the distance where 50% of the initial LD is decayed) performed best to estimate and compare LD-decay in potato. To understand the various aspects of LD-decay in the variety panel of 537 varieties, the panel was subdivided in several groups based on the age of a variety and the population structure groups. This resulted in the identification of LD-decay over time, i.e in relatively young varieties the average size of the LD-blocks is smaller. The differences between subpopulations were smaller and are most likely the effect of the population structure. We also observed that there are very long LD-blocks caused by introgression breeding and that different a priori MAF-thresholds also can influence the outcome of LD-decay estimation.

Having both LD-decay and population structure defined a genome wide association study (GWAS) was conducted (Chapter 4). For this purpose α-solanine and α-chaconine were measured in potato tubers. Subsequently the sum of both (total SGA) and the ratio between the two were used to discover QTLs for these traits in a GWAS. Additionally we used three bi-parental populations to validate the GWAS results. Total SGA content was confounded with population structure and therefore it was difficult to explain all phenotypic variation with SNP markers. Two QTLs (Sgt1.1 and Sgt11.1) were identified which could be validated in one of the segregating populations. The ratio between α-solanine and α-chaconine was not confounded with population structure, resulted in the identification of two major-effect QTLs (Sgr7.1 & Sgr8.1) located near the candidate genes SGT1 and SGT2, which are known for being responsible in the final steps towards either α-solanine or α-chaconine. The QTL Sgr8.1 could be validated, however similar phenotypes were explained by different haplotypes in two populations. We show that population structure, low frequent alleles and genetic heterogeneity may explain to some degree the missing heritability in GWAS in potato.

In Chapter 5 we describe how the method of graphical genotyping, which is widely used in diploid bi-parental populations, can be applied in a variety panel of tetraploid varieties. We show that a few discrete filtering steps in Excel can be used to display patterns that are visual representations of introgression segments and the locations of historical recombination events. Using this method we identified introgression segments from Solanum vernei including the Gpa5 locus on chromosome 5 and Solanum stoloniferum introgression segment including a gene involved in resistance to Potato Virus Y on chromosome 11. This method requires that the haplotypes that cause the phenotypic effect have to be identical by descent (IBD).

In the final chapter 6 the results of chapter 2 to 5 are discussed. We look forward on how our results can be used in future research and applied in marker-assisted breeding. Additionally some new GWAS results are presented for tuber flesh colour, foliage maturity and resistance to Globodera pallida pathotype 3.

Discovery and genotyping of existing and induced DNA sequence variation in potato
Uitdewilligen, J.G.A.M.L. - \ 2012
Wageningen University. Promotor(en): Richard Visser, co-promotor(en): Herman van Eck; Anne-Marie Wolters. - S.l. : s.n. - ISBN 9789461732330 - 165
solanum tuberosum - aardappelen - dna-sequencing - dna - nucleotidenvolgordes - genotyping - plantenveredeling - genotypen - fenotypen - tetraploïdie - potatoes - dna sequencing - nucleotide sequences - plant breeding - genotypes - phenotypes - tetraploidy

In this thesis natural and induced DNA sequence diversity in potato (Solanum tuberosum) for use in marker-trait analysis and potato breeding is assessed. The study addresses the challenges of reliable, high-throughput identification and genotyping of sequence variants in existing tetraploid potato cultivar panels using traditional Sanger sequencing and next-generation massively parallel sequencing (MPS), and the application of this knowledge in the form of genetic markers. Furthermore, it explores the efficiency of ethyl methanesulphonate (EMS) mutagenesis combined with high resolution melting (HRM) DNA screening to induce and discover novel sequence variants in potato genotypes.
Discovery and genotyping of sequence diversity in outcrossing autotetraploid species like potato is complex. In autotetraploid species, genotyping implies the quantitative identification of five alternative allele copy number states. In Chapter 1, several methodologies to identify and genotype DNA sequence variants, and the application of these sequence variants is discussed. This chapter provides an introduction to genotyping-by-sequencing (GBS) and the determination of allele copy number.
In Chapter 2 the sequence diversity in three genes of the carotenoid pathway is assessed in diploid and tetraploid potato genotypes using direct Sanger sequencing. To investigate the genetics and molecular biology of orange and yellow flesh colour in potato, association analysis between SNP haplotypes and flesh colour phenotypes was performed, and the inheritance and gene expression of associated alleles was studied. We observed among eleven beta-carotene hydroxylase 2 (CHY2) alleles one dominant allele with a major effect, changing white into yellow flesh colour. In contrast, none of the lycopene epsilon cyclase (LCYe) alleles seemed to have a large effect on flesh colour. Analysis of zeaxanthin epoxidase (ZEP) alleles showed that a recessive allele with a non-LTR retrotransposon sequence in intron 1 reduced the expression level of the ZEP gene and caused accumulation of zeaxanthin. Genotypes combining presence of the dominant CHY2 allele with homozygosity for the recessive ZEP allele produced orange-fleshed tubers that accumulate large amounts of zeaxanthin.
Sanger amplicon sequencing was applied in Chapter 3 to evaluate the sequence diversity in α-Glucan Water Dikinase (StGWD), a candidate gene underlying a QTL involved in potato starch phosphate content. Sanger sequences of two StGWD amplicons from a global collection of 398 commercial cultivars and progenitor lines were used to identify 16 unique haplotypes. By assigning tag SNPs to these haplotypes and by determining the allele copy number of identified sequence variants, we inferred the four-allele genetic composition for almost all cultivars assayed at this locus. This allowed genetic diversity parameters like the average number of different alleles present in a single cultivar (Ai=3.1) and the average intra-individual heterozygosity (Ho=0.765) to be estimated for this locus. Pedigree analysis confirmed that the identified haplotypes are identical by descent (IBD) and offered insight in the breeding history of elite potato germplasm. Haplotype association analysis led to the identification of two StGWD alleles causing altered starch phosphate content, which was further verified in diploid and tetraploid mapping populations containing the relevant alleles. One of these alleles (Allel H) increases the fraction of starch that is phosphorylated, while the other one (Allele A) decreases it.
To scale up the discovery and genotyping of sequence variants, and to make it more whole-genome oriented, Chapter 4 reports on massively parallel sequencing (MPS) of approximately 800 genes scattered over the potato genome and resequenced in 83 tetraploid potato cultivars and a monoploid reference accession. We show that by combining MPS with genome complexity reduction and indexed sequencing, sufficient read depth for GBS can be achieved for reliable discovery and genotyping of sequence variants in individual tetraploid potato genotypes. With a custom designed, SureSelect enrichment library, 1.44 Mb of DNA sequence was targeted. The genes targeted were mainly single-copy genes, selected based on putative gene functions in both primary and secondary metabolic pathways, potato quality traits and biotic and abiotic stresses, and included a large set of conserved orthologous sequence genes (COSII) useful for genetic anchoring and phylogenetic studies. The indexed and enriched DNA libraries were sequenced on a Illumina HiSeq. After filtering and processing the raw sequence data, 12.4 Gb of high-quality sequence data was mapped to the potato genome, covering 2.1 Mb of the genome sequence with a median average read depth of 63× per cultivar. We detected over 129,000 sequence variants in these data and determined allele copy number of the variants in individual potato samples. The accuracy of the sequence-based allele copy number estimates was verified by a low-density SNP genotyping assay. This showed that for reliable genotyping a read count-based genotype quality score is best applied and a read depth of 80× is recommended for determining allele copy number in autotetraploid potato. Average nucleotide diversity (π=10.7×10-3 genome-wide, ≈1 variant/93 bp between two random alleles) varied along the twelve potato chromosomes, and individual genes under selection were identified. As an example for application of GBS for genome-wide association analysis (GWAS), the identified sequence variants and genotype data were tested in a marker-trait association analysis with plant maturity and tuber flesh colour. This led to the identification of alleles accounting for significant phenotypic variation in these traits.
In Chapter 5 we applied the chemical mutagen EMS to diploid potato by two different treatments, a pollen and a seed treatment. We screened the resulting populations for novel mutations using HRM analysis. A pollen treatment with EMS dissolved in a sucrose solution was found to induce mutations only at a low frequency (only one mutation discovered after screening >2.7 Mb of sequence). In planta selection of the most vital mutagenized pollen seems to have lowered the mutation density to a frequency that is not suitable for reverse genetics studies. Treatment of potato seeds with EMS on the other hand provided a high density of novel mutations (1 mutation/65 kb), discovered in the M1 generation. In contrast to most EMS mutagenesis studies, we directly screened the M1 generation of the seed-treated population. In six candidate genes involved in potato starch and frying quality traits, 65 novel sequence variants were discovered. In all six genes, missense mutations that are predicted to damage protein function were discovered, and for four genes five premature stop codon mutations were identified. We attempted to stabilize and transfer 27 putatively interesting mutations to the M2 and M3 generation for further evaluation. Genetically stable M2 and M3 plants have been generated for 10 (37%) of these mutations. The estimated density of M1 mutations that are transferable to the M2 generation (one “accessible” mutation/118-176 kb) is higher than the mutation densities obtained in most other plant species, for which the M2 generation has been screened. The results of this chapter thus demonstrate that screening the M1 generation offers a good alternative to the commonly applied M2 screening for the rapid generation of novel genetic variation at a high density, without too much complication in recovering mutations in the M2 generation.
In the concluding Chapter 6, results of preceding chapters are evaluated, and the prospects of the findings for potato research and breeding are discussed.

Towards efficient improvement of greenhouse grown roses: genetic analysis of vigour and powdery mildew resistance
Yan, Z. - \ 2005
Wageningen University. Promotor(en): P. Stam, co-promotor(en): Oene Dolstra. - [S.l.] : S.n. - ISBN 9085042984 - 90
rosa - rozen - genetische analyse - groeikracht - ziekteresistentie - plantenziekteverwekkende schimmels - podosphaera pannosa - podosphaera - meeldauw - plantenveredeling - diploïdie - tetraploïdie - roses - genetic analysis - vigour - disease resistance - plant pathogenic fungi - mildews - plant breeding - diploidy - tetraploidy
Multidisciplinary approach in estimating genetic diversity of Ethiopian tetraploid wheat (Triticum turgidum L.) landraces = Multidisciplinaire benadering in het schatten van de genetische diversiteit van Ethiopische tetraploïde landrassen van tarwe (Triticum turgidum L.)
Tesfaye, M. - \ 2001
Wageningen University. Promotor(en): Maarten Koornneef; J.H. de Jong; Ronald van den Berg. - S.l. : S.n. - ISBN 9789058084644 - 108
triticum turgidum - genetische diversiteit - tetraploïdie - triticum durum - landrassen - biodiversiteit - chromosoompolymorfie - genetic diversity - tetraploidy - landraces - biodiversity - chromosome polymorphism
</strong><p>This thesis presents a diversity analysis for 26 tetraploid wheat landraces from the central region of Ethiopia and four cultivars using five markers: morphological traits, chromosome portraits, microsatellites, AFLP and storage proteins.</p><p>The aim of the study is:</p><UL><LI>To estimate the magnitude of genetic diversity within and between 26 landraces</LI><LI>To identify the pattern of genetic diversity within and between geographical areas</LI><LI>To detect duplications within and between accessions</LI><LI>To prioritise markers used for estimating genetic diversity in Ethiopian tetraploid wheat landraces</LI></UL><p>The study revealed a great difference of diversity between the landraces and also demonstrated that the high variation is due to the within component of accessions rather than the between landrace component. No strong variation for genetic composition was observed between plants from different altitudes and subregions. An integrated application of morphological, chromosomal, molecular and storage protein markers for assessing biodiversity in these wheat landraces is strongly recommended.</p><p> </p></font>
Tetraploide tulpen, een mogelijkheid voor de verbetering van het tulpensortiment
Zeilinga, A.E. ; Schouten, H.P. - \ 1966
Wageningen : [s.n.] (Mededeling / Instituut voor de veredeling van tuinbouwgewassen no. 256) - 7
bloembollen - tulipa - tetraploïdie - ornamental bulbs - tetraploidy
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