De novo Assembly of a 40 Mb Eukaryotic Genome from Short Sequence Reads: Sordaria macrospora, a Model Organism for Fungal Morphogenesis

摘要

Filamentous fungi are of great importance in ecology, agriculture, medicine, and biotechnology. Thus, it is not surprising that genomes for more than 100 filamentous fungi have been sequenced, most of them by Sanger sequencing. While next-generation sequencing techniques have revolutionized genome resequencing, e.g. for strain comparisons, genetic mapping, or transcriptome and ChIP analyses, de novo assembly of eukaryotic genomes still presents significant hurdles, because of their large size and stretches of repetitive sequences. Filamentous fungi contain few repetitive regions in their 30–90 Mb genomes and thus are suitable candidates to test de novo genome assembly from short sequence reads. Here, we present a high-quality draft sequence of the Sordaria macrospora genome that was obtained by a combination of Illumina/Solexa and Roche/454 sequencing. Paired-end Solexa sequencing of genomic DNA to 85-fold coverage and an additional 10-fold coverage by single-end 454 sequencing resulted in ～4 Gb of DNA sequence. Reads were assembled to a 40 Mb draft version (N50 of 117 kb) with the Velvet assembler. Comparative analysis with Neurospora genomes increased the N50 to 498 kb. The S. macrospora genome contains even fewer repeat regions than its closest sequenced relative, Neurospora crassa . Comparison with genomes of other fungi showed that S. macrospora , a model organism for morphogenesis and meiosis, harbors duplications of several genes involved in selfonself-recognition. Furthermore, S. macrospora contains more polyketide biosynthesis genes than N. crassa . Phylogenetic analyses suggest that some of these genes may have been acquired by horizontal gene transfer from a distantly related ascomycete group. Our study shows that, for typical filamentous fungi, de novo assembly of genomes from short sequence reads alone is feasible, that a mixture of Solexa and 454 sequencing substantially improves the assembly, and that the resulting data can be used for comparative studies to address basic questions of fungal biology. Author Summary Fungi have immense impacts on ecosystems and affect many aspects of society. They are used as convenient organisms for fundamental research because their typically haploid genetics enable straightforward phenotyping of mutations and because most fungal cells can differentiate the entire organism. Fungi have compact genomes with few repetitive sequences, and their genomes should be much easier to assemble from short sequence reads than genomes of mammals or higher plants. To test this idea, we used Solexa and 454 sequencing to generate ～4 Gb of raw sequence data from the filamentous fungus Sordaria macrospora . De novo assembly yielded 5,097 contigs. This assembly was improved by comparison with reference genomes of three closely related Neurospora species, resulting in placement of ～40 Mb of genome sequence in 152 scaffolds. From comparisons of predicted proteins we conclude that S. macrospora carries a conserved set of genes for signaling and development, which should encourage its further use as a model organism for morphogenesis and meiosis. We demonstrate that de novo assembly of fungal genomes from short reads is cheap and efficient. Species that are not traditionally considered “model organisms” but await genome sequencing for comparative and functional genomics analyses are at last amenable to in-depth genome-wide analyses.