De novo assembly of middle-sized genome using MinION and Illumina sequencers

Background: The plastid acquisition by secondary endosymbiosis is a driving force for the algal evolution, and the comparative genomics was required to examine the genomic change of symbiont. Therefore, we established a pipeline of a de novo assembly of middle-sized genomes at a low cost and with high quality using long and short reads. Results: We sequenced symbiotic algae Chlorella variabilis using Oxfofrd Nanopore MinION as the long-read sequencer and Illumina HiSeq 4000 as the short-read sequencer and then assembled the genomes under various conditions. Subsequently, we evaluated these assemblies by the gene model quality and RNA-seq mapping rate. We found that long-read only assembly could not be suitable for the comparative genomics studies, but with short reads, we could obtain the acceptable assembly. On the basis of this result, we established the pipeline of de novo assembly for middle-sized algal genome using MinION. Conclusions: The genomic change during the early stages of plastid acquisition can now be revealed by sequencing and comparing many algal genomes. Moreover, this pipeline offers a solution for the assembly of various middle-sized eukaryotic genomes with high-quality and ease.