Spliceosomal introns interrupt protein coding genes in all characterized eukaryotic nuclear genomes and are removed by a large RNA-protein complex termed the spliceosome. Diplomonads are diverse unicellular eukaryotes that display compact genomes with few spliceosomal introns. My thesis objectives were to explore spliceosomal intron and spliceosome diversity as well as RNA processing mechanisms in the diplomonads Giardia lamblia and Spironucleus spp. Surprisingly, G. lamblia was found to contain a proportionally large number of fragmented spliceosomal introns that are spliced in trans from separate pre-mRNA molecules. Next, both evolutionarily divergent and conventional spliceosomal small nuclear RNAs were identified in G. lamblia and Spironucleus spp. and an RNA 3ʹ end motif was determined to be involved in processing of both non-coding RNAs and trans-introns in G. lamblia. These findings shed light on spliceosome and spliceosomal intron evolution in eukaryotes undergoing severe genomic reduction and potentially complete loss of their spliceosomal introns.University of Lethbridge (SGS Graduate Fellowship), Natural Sciences and Engineering Research Council of Canada (NSERC) (Discovery Grant and Alexander Graham Bell Canada Graduate Scholarships)
