RH mapping by sequencing: chromosome-scale assembly of the duck genome

Like many other species, the duck genome has been sequenced thanks to the technological breakthrough provided by the emergence of Next Generation Sequencing (NGS). The resulting de novo assemblies are however made of thousands of scattered scaffolds. To achieve chromosome-scale contiguity, long-range intermediate genome maps remain indispensable. Radiation Hybrid (RH) maps have been used to assist the generation of chromosome-scale genome assemblies by taking advantage of the high density SNP chips that provide a large number of markers that can be efficiently genotyped on the panel. In the absence of such a resource in duck, we sequenced 100 hybrid clones of a duck RH panel enabling direct genotyping of the assembly scaffolds on the panel. The rationale is to use scaffolds as markers and to genotype the scaffolds by sequencing the clones: the presence/absence of a scaffold in a particular sequenced hybrid is attested by the presence/absence of reads mapping specifically to this scaffold. The detection of scaffolds exhibiting a chromosomal breakage resulting from the irradiation process revealed itself to be a critical issue of this genotyping by sequencing process. This process resulted in the construction of RH vectors for 2,027 scaffolds, representing a total of about 1 Gb of sequences (95% of the current Duck genome assembly). The subsequent linkage analysis enabled the construction of RH maps and therefore to organize, i.e. order and orient, the scaffolds into pseudomolecules associated to the corresponding duck chromosomes. We describe here the whole mapping process, from sequence-based genotyping to the construction of comparative maps, as well as few examples of intra-chromosomal rearrangements that have been identified by the comparison with the chicken, turkey and zebra finch genomes and subsequently confirmed by FISH. We describe a method to order and orient sequence scaffolds into super-scaffolds spanning entire chromosomes. The method, which requires a pre-existing RH panel and sequence scaffolds from an NGS assembly, relies on a shallow sequencing of the RH clones. This approach was applied to the duck genome and produced chromosome-scale scaffolds for 29 out of the 41 duck chromosomes.