Microsporidian Introns Retained against a Background of Genome Reduction: Characterization of an Unusual Set of Introns

Spliceosomal introns are ubiquitous features of eukaryotic genomes, but the mechanisms responsible for their loss and gain are difficult to identify. Microsporidia are obligate intracellular parasites that have significantly reduced genomes and, as a result, have lost many if not all of their introns. In the microsporidian Encephalitozoon cuniculi, a relatively long intron was identified and was spliced at higher levels than the remaining introns. This long intron is part of a set of unique introns in two unrelated genes that show high levels of sequence conservation across diverse microsporidia. The introns possess a unique internal conserved region, which overlaps with a shared, predicted stem–loop structure. The unusual similarity and retention of these long introns in reduced microsporidian genomes could indicate that these introns function similarly, are homologous, or both. Regardless, the significant genome reduction in microsporidia provides a rare opportunity to understand intron evolution.