The microsporidia are spore-forming unicellular fungal parasites, causing chronic, debilitating diseases to their animal hosts (human, insects, crustaceans, fish). These highly adapted fungi are characterized by a severe reduction, or even absence, of cellular components typical of eukaryotes such as mitochondria, Golgi apparatus and flagella. Lacking mitochondria and peroxysomes, these unicellular eukaryotes were first considered a deeply branching protist lineage, but they are now affiliated to the Fungi. These features previously recognized as primitive are instead highly derived adaptations to their obligate parasitic lifestyle (Corradi et al., 2009).
In the first issue of Nature Communications, Keeling’s group reports the genomic sequence of the microsporidian Encephalitozoon intestinalis. Its genome is extremely compacted with 2.3 Mbp — a 20% reduction from the already severely reduced 2.9 Mbp genome of E. cuniculi. DNA was isolated from 500 million purified spores and used for Illumina sequencing, from which the entire genome was assembled de novo, resulting in an assembly of 137 scaffolds with an average coverage of 40×. The two species share a conserved gene content (~1,800 versus 2,000 protein-coding genes), order and density over most of their sequences. The majority of the size difference is due to gene loss, the protein-coding capacity of the two genomes is however very similar because most of the genes that are absent in E. intestinalis are duplicates of genes that were retained, or unidentified ORFs. Genome compaction is also reflected by reduced intergenic spacers and by the shortness of most putative proteins relative to their eukaryote orthologues. The exceptions are the subtelomeric regions, where E. intestinalis chromosomes are missing large gene blocks of sequence found in E. cuniculi. In the remaining gene-dense chromosome ‘cores’, the diminutive intergenic sequences and introns are actually more highly conserved than the genes themselves, suggesting that they have reached the limits of reduction for a fully functional genome … ‘getting rid of everything that is not essential for gene function’
Photo: Encephalitozoon intestinalis, © www.microbiologybytes.com