Chromosome scale assembly of allopolyploid genome of the diatom Fistulifera solaris

Abstract

Microalgae including diatoms are of interest for environmentally-friendly manufacturing such as biofuel production. However, only a very few of their genomes have been elucidated owing to their diversified and complex evolutionary history. The genome of the marine oleaginous diatom Fistulifera solaris, an allopolyploid diatom possessing two subgenomes, has been analyzed previously by pyrosequencing. However, many unsolved regions and unconnected scaffolds remained. Here we report the entire chromosomal structure of the genome of F. solaris strain JPCC DA0580 using a long-read nanopore sequencing platform. From just one single run using a MinION flow-cell, the chromosome scale assembly with telomere-to-telomere resolution was achieved for 41 out of 44 chromosomes. Centromere regions were also predicted from the chromosomes, and we discovered conserved motifs in the predicted regions. The function of the motifs was experimentally confirmed by successful transformation of the diatom via bacterial conjugation. This discovery provides insights into chromosome replication, facilitating the rational design of artificial chromosomes for large-scale metabolic engineering of diatoms. The chromosome scale assembly also suggests the potential existence of multi-copy mini-chromosomes and tandemly repeated lipogenesis genes related to the oleaginous phenotype of F. solaris. The nanopore sequencing also solved the sequential arrangement of the repeat region in the F. solaris mitochondrial genome. Findings of this study will be useful to understand and further engineer the oleaginous phenotype of F. solaris.