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Open Access Highly Accessed Research article

Transcriptome analysis of the parasite Encephalitozoon cuniculi: an in-depth examination of pre-mRNA splicing in a reduced eukaryote

Cameron J Grisdale1, Lisa C Bowers23, Elizabeth S Didier23 and Naomi M Fast1*

Author Affiliations

1 Biodiversity Research Centre and Department of Botany, University of British Columbia, Vancouver, British Columbia, Canada

2 Division of Microbiology, Tulane National Primate Research Center, Covington, LA 70433, USA

3 Department of Tropical Medicine, Tulane University School of Public Health and Tropical Medicine, New Orleans, LA 70112, USA

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BMC Genomics 2013, 14:207  doi:10.1186/1471-2164-14-207

Published: 28 March 2013

Abstract

Background

The microsporidian Encephalitozoon cuniculi possesses one of the most reduced and compacted eukaryotic genomes. Reduction in this intracellular parasite has affected major cellular machinery, including the loss of over fifty core spliceosomal components compared to S. cerevisiae. To identify expression changes throughout the parasite’s life cycle and also to assess splicing in the context of this reduced system, we examined the transcriptome of E. cuniculi using Illumina RNA-seq.

Results

We observed that nearly all genes are expressed at three post-infection time-points examined. A large fraction of genes are differentially expressed between the first and second (37.7%) and first and third (43.8%) time-points, while only four genes are differentially expressed between the latter two. Levels of intron splicing are very low, with 81% of junctions spliced at levels below 50%. This is dramatically lower than splicing levels found in two other fungal species examined. We also describe the first case of alternative splicing in a microsporidian, an unexpected complexity given the reduction in spliceosomal components.

Conclusions

Low levels of splicing observed are likely the result of an inefficient spliceosome; however, at least in one case, splicing appears to be playing a functional role. Although several RNA decay genes are encoded in E. cuniculi, the lack of a few key players could be reducing decay levels and therefore increasing the proportion of unspliced transcripts. Significant proportions of genes are differentially expressed in the first forty-eight hours but not after, indicative of genetic changes that precede the intracellular to infective stage transition.