Comparative genomics of microsporidia

Abstract

Abstract. Microsporidia have been known for some time to possess among the smallest genomes of any eukaryote. There is now a completely sequenced microsporidian genome, as well as several other large-scale sequencing efforts, so the nature of these genomes is becoming apparent. This paper reviews some of the characteristics of microsporidian genomes in general, and some of the recent discoveries made through comparative genomic analyses. In general, microsporidian genomes are both reduced and compacted. Reduction takes place through gene loss, which is understandable in obligate intracellular parasites that rely on their host for many metabolites. Compaction is a more complex process, and is as yet not fully understood. It is clear from genomes surveyed thus far that the remaining genes are tightly packed and that there is little non-coding sequence, resulting in some extraordinary arrangements, including overlapping genes. Compaction also seems to affect certain aspects of genome evolution, like the frequency of rearrangements. The force behind this compaction is not known, and is especially interesting in light of the fact that surveys of genomes that are significantly different in size yield similar complements of protein-coding genes. There are some interesting exceptions, including catalase, photolyase and some mitochondrial proteins, but the rarity of these raises an interesting question as to what accounts for the significant differences seen in the genome sizes among microsporidia.

Enterocytozoon hepatopenaei sp. nov. (Microsporida: Enterocytozoonidae), a parasite of the black tiger shrimp Penaeus monodon (Decapoda: Penaeidae): Fine structure and phylogenetic relationships

“…This is normal for microsporidia. Many now believe them to constitute a Phylum in the fungal kingdom and to represent reduced forms that have lost their mitochondria and other organelles through adaptation to their intracellular parasitic life-style (Lee et al, 2008;Keeling et al, 2005;Keeling and Fast, 2002;Fischer and Palmer, 2005;James et al, 2006;Hibbett et al, 2007). This is supported not only by gene sequence homology but also by similarity in steps of the meiotic division cycle and in possession of paired nuclei in diplokaryons (microsporidia) and dikaryons (fungi) (Flegel and Pasharawipas, 1995).…”

Intracellular infection of tick cell lines by the entomopathogenic fungus Metarhizium anisopliae

“…Microsporidia are unicellular eukaryotes that are obligate parasites of a variety of animals. Although they are classically not considered part of the fungal Kingdom, recent, though still controversial, phylogenetic analyses derived from genome datasets place microsporidia as a sister to a combined ascomycete+basidiomycete clade (Gill & Fast, 2006;Keeling et al, 2005). Other unique fungal-host relationships in which intracellular residence of fungal cells has been noted include certain types of mycorrhiza and dark septate endophytes (DSE) (Harley, 1992;Jumpponen, 2001;Smith & Read, 1997).…”

Phylogenetic Approach to the Variability of the Microsporidian Enterocytozoon bieneusi and Its Implications for Inter- and Intrahost Transmission

“…The infection occurs after a sophisticated interaction with the host cell (31,65). These organisms are unicellular eukaryotes (included in the group of Fungi) that have lost mitochondria and have an extremely reduced genome (Encephalitozoon cuniculi has about 3 Mbp), although it is organized in several eukaryotic chromosomes (2,30,62). Some microsporidia seem to be host specific (9,17), while others, such as the species of the genera Encephalitozoon and Enterocytozoon, may proliferate in various vertebrate species (4).…”