Evidence of mRNA-Mediated Intron Loss in the Human-Pathogenic Fungus Cryptococcus neoformans

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Abstract
Introns are a defining feature of eukaryotic genomes, though the mechanism of intron gain or loss is not well understood. Reverse transcription of mRNA followed by homologous recombination with the genome has been posited as a mechanism of intron loss, though little direct evidence of recent loss events has been described to support this model. We find supporting evidence for an mRNA-mediated mechanism of loss through comparative genome analyses that revealed a recent loss of 10 adjacent introns in a 22-exon gene in the human-pathogenic fungus Cryptococcus neoformans . We surveyed the gene structures of the entire genomes of Cryptococcus gattii , which diverged from the C. neoformans lineage 37 million years ago (Mya), and C. neoformans var. grubii and var. neoformans , which diverged 18 Mya. Our comparison revealed greater than 99.9% intron conservation, with evidence from 20 genes showing evidence of intron loss, but no convincing evidence of intron gain. Our findings confirm that Cryptococcus introns have been quite stable over recent evolutionary time, with occasional mRNA-mediated intron loss events.