Abstract
Key messageWe constructed the first integrated genetic linkage map in a polysomic hexaploid. This enabled us to estimate inheritance of parental haplotypes in the offspring and detect multi-allelic QTL.Construction and use of linkage maps are challenging in hexaploids with polysomic inheritance. Full map integration requires calculations of recombination frequency between markers with complex segregation types. In addition, detection of QTL in hexaploids requires information on all six alleles at one locus for each individual. We describe a method that we used to construct a fully integrated linkage map for chrysanthemum (Chrysanthemum × morifolium, 2n = 6x = 54). A bi-parental F1 population of 406 individuals was genotyped with an 183,000 SNP genotyping array. The resulting linkage map consisted of 30,312 segregating SNP markers of all possible marker dosage types, representing nine chromosomal linkage groups and 107 out of 108 expected homologues. Synteny with lettuce (Lactuca sativa) showed local colinearity. Overall, it was high enough to number the chrysanthemum chromosomal linkage groups according to those in lettuce. We used the integrated and phased linkage map to reconstruct inheritance of parental haplotypes in the F1 population. Estimated probabilities for the parental haplotypes were used for multi-allelic QTL analyses on four traits with different underlying genetic architectures. This resulted in the identification of major QTL that were affected by multiple alleles having a differential effect on the phenotype. The presented linkage map sets a standard for future genetic mapping analyses in chrysanthemum and closely related species. Moreover, the described methods are a major step forward for linkage mapping and QTL analysis in hexaploids.
Highlights
A linkage map is a starting point for localization of genomic regions that are associated with agriculturally important traits
QTL analysis was performed on block-corrected mean phenotypic values using an IBD probability model, as described before for tetraploids (Bourke 2014)
Identification of chromosomal linkage groups (CLG) with simplex × simplex (1 × 1) markers resulted in a network of nine CLG representing all homologue clusters of both parents
Summary
A linkage map is a starting point for localization of genomic regions that are associated with agriculturally important traits. This makes it an important tool for DNA-informed breeding (Peace 2017). DNA-informed breeding has lagged behind compared to diploids, because genotyping co-dominant markers and linkage map construction in polyploids requires specialized methods. Polyploids have multiple conformations of heterozygous genotypes; on a locus with two alleles, a hexaploid can have five different heterozygous genotypes ranging from a dosage of one to a dosage of five. Together with the two homozygous conformations, this adds up to seven different dosage scores
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