Construction of High-quality Linkage Maps in Populus with RADseq Data and their Application in QTL Mapping and Genome Assembly

Abstract

Background: Although great efforts have been made to construct genetic linkage maps in Populus using traditional molecular markers, these maps are typically sparse, and the number of linkage groups does not match the karyotype of Populus . With the advances in high-throughput sequencing technologies and new software packages available for extracting single nucleotide polymorphisms (SNPs) across a mapping population, it is possible to construct high-quality, high-density genetic linkage maps with thousands of SNPs in such outbred species. Results: Two parent-specific linkage maps were constructed with restriction site-associated DNA sequencing (RADseq) data from an F 1 hybrid population from Populus de l toides and Populus simonii and were applied to identify growth trait loci and facilitate genome assembly. The female Populus deltoides map contained 4,018 SNPs, which were divided into 19 linkage groups under a wide range of LOD thresholds from 7 to 55, perfectly matching the karyotype of Populus . The male Populus simonii map showed similar characteristics, consisting of 2,097 SNPs, which also belonged to 19 linkage groups under LOD thresholds of 7 to 29. The SNP genotype data for linkage analysis were confirmed to be of high quality. The SNP order of each linkage group was optimal among different ordering results from several available software platforms. Moreover, the linkage maps allowed the detection of 39 QTLs underlying tree height and 47 for diameter at breast height, some of which were strongly associated with QTLs identified in previous studies. In addition, the linkage maps enabled the anchoring of 671 contigs of a draft genome assembly of Populus simonii to chromosomes, corresponding to approximately 83.6% of the genome and showing a high level of collinearity with the genetic maps. Conclusions: The two parental genetic maps of Populus are of high quality, especially in terms of SNP data quality, the SNP order within linkage groups, and the perfect match between the number of linkage groups and the karyotype of Populus as well as performance in QTL mapping and genome assembly. The adopted approaches for both extracting and ordering SNPs could be applied to other species to construct high-density, high-quality genetic maps.