Improving Nelumbo nucifera genome assemblies using high-resolution genetic maps and BioNano genome mapping reveals ancient chromosome rearrangements

Genetic and physical maps are powerful tools to anchor fragmented draft genome assemblies generated from next-generation sequencing. Currently, two draft assemblies of Nelumbo nucifera, the genomes of 'China Antique' and 'Chinese Tai-zi', have been released. However, there is presently no information on how the sequences are assembled into chromosomes in N. nucifera. The lack of physical maps and inadequate resolution of available genetic maps hindered the assembly of N. nucifera chromosomes. Here, a linkage map of N. nucifera containing 2371 bin markers [217 577 single nucleotide polymorphisms (SNPs)] was constructed using restriction-site associated DNA sequencing data of 181 F₂ individuals and validated by adding 197 simple sequence repeat (SSR) markers. Additionally, a BioNano optical map covering 86.20% of the 'Chinese Tai-zi' genome was constructed. The draft assembly of 'Chinese Tai-zi' was improved based on the BioNano optical map, showing an increase of the scaffold N50 from 0.989 to 1.48 Mb. Using a combination of multiple maps, 97.9% of the scaffolds in the 'Chinese Tai-zi' draft assembly and 97.6% of the scaffolds in the 'China Antique' draft assembly were anchored into pseudo-chromosomes, and the centromere regions along the pseudo-chromosomes were identified. An evolutionary scenario was proposed to reach the modern N. nucifera karyotype from the seven ancestral eudicot chromosomes. The present study provides the highest-resolution linkage map, the optical map and chromosome level genome assemblies for N. nucifera, which are valuable for the breeding and cultivation of N. nucifera and future studies of comparative and evolutionary genomics in angiosperms.