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The Ordospora colligata Genome: Evolution of Extreme Reduction in Microsporidia and Host-To-Parasite Horizontal Gene Transfer

Pombert, Jean-Francois, Haag, Karen Luisa, Beidas, Shadi, Ebert, Dieter, Keeling, Patrick J.
mBio 2015 v.6 no.1 pp. e02400-14
Candida albicans, parasites, parasitism, Daphnia, evolution, hosts, Encephalitozoon, horizontal gene transfer, endocytosis, receptors, introns, digestive system, pathogens, fungi
Microsporidia are a group of obligate intracellular parasites that are best known for their unique infection mechanism and their unparalleled levels of genomic reduction and compaction. We sequenced the genome of Ordospora colligata , a gut parasite of the microcrustacean Daphnia sp. and the closest known relative to the microsporidia characterized by the most extreme genomic reduction, the model genus Encephalitozoon . We found that the O. colligata genome is as compact as those of Encephalitozoon spp., featuring few introns and a similar complement of about 2,000 genes, altogether showing that the extreme reduction took place before the origin of Encephalitozoon spp. and their adaptation to vertebrate hosts. We also found that the O. colligata genome has acquired by horizontal transfer from its animal host a septin that is structurally analogous to septin 7, a protein that plays a major role in the endocytosis-based invasion mechanism of the fungal pathogen Candida albicans . Microsporidian invasion is most often characterized by injection through a projectile tube, but microsporidia are also known to invade cells by inducing endocytosis. Given the function of septins in other systems, we hypothesize that the acquired septin could help O. colligata induce its uptake by mimicking host receptors.