Evolution of microsporidia: An extremely successful group of eukaryotic intracellular parasites

OverviewMicrosporidia are obligate intracellular parasites that can infect a wide range of hosts.They cause death and disease in both humans and agriculturally important animals.They have also become a powerful model for understanding the evolution of intracellular parasites.Recent work has explored how microsporidia genomes have evolved, the evolutionary origins of microsporidia, and how microsporidia adapt to interact with their hosts. What do microsporidia have in common?Microsporidia can only exist outside of their host as environmentally resistant, chitin-containing spores.These spores contain a unique infectious apparatus known as the polar tube.During infection, the polar tube is rapidly discharged and can pierce a host cell, depositing the parasite's sporoplasm within the host cell [1].The sporoplasm then proliferates, eventually producing spores that then exit the host to cause subsequent infections.Microsporidia are thought to reproduce mostly asexually, although most are likely diploid based on genomic heterozygosity and conservation of meiotic genes [2-5].Because of the intimate relationship between microsporidia and their hosts, they are heavily dependent on host resources and have undergone extensive genomic reduction [6,7].Analysis of sequenced microsporidia genomes (Fig 1A) has revealed a wide loss of protein families that are present in other eukaryotes, leaving only approximately 800 conserved microsporidia proteins [6].The proteins that have been retained function in essential core cellular processes such as DNA replication, transcription, and translation [6,8,9].Proteins that have been lost include many metabolic enzymes, regulatory pathways, and proteins involved in vesicular transport such as TOR (Target of Rapamycin) and clathrin [8][9][10].In addition, the proteins that are retained are shorter than their fungal orthologs, with Encephalitozoon cuniculi proteins being on average only 85% as long as their yeast counterparts [3,11].One such instance of proteins that have decreased in size are the aminoacyl-tRNA synthetases, which have lost many regulatory regions, including a domain in leucyl-tRNA synthetase (LeuRS) that edits mischarged tRNAs.The microsporidia Vavraia culicis was shown to have mistranslation error rates of 5.9% at positions that coded for leucine-although valine, which has a corresponding tRNA synthetases with an intact editing domain, has a mistranslation rate of 7.5%, suggesting that there are other factors responsible for the high levels of mistranslated proteins [12].Microsporidia also have reduced noncoding RNAs, including the 18s RNA, which is only approximately 2/3 as large as in other eukaryotes and results in the smallest eukaryotic ribosomes [13].Although this reduction has led to several binding sites being eliminated, structural and mass spectrometry analysis of ribosomes from Vairimorpha necatrix demonstrated

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