This question on the function of introns in eukaryotic genes made me think: I know that more basal organisms have smaller introns and fewer alternatively spliced exons compared to mammals. But are there eukaryotes whose genomes have no introns at all?
I don't believe so. I have never come across a eukaryote that does not have introns although there are some genes that do not contain introns. Some eukaryotes like ciliates actually contain other non-intronic intergenetic regions that seem to be nonfunctional. These are called Internally Existed Sequences and are removed from the germ-line active micronucleus before going to form the transcriptionally active micronucleus. Interestingly, introns are also present in these organisms and are present in both the micro and macronucleus. These introns are spliced out in textbook fashion. I believe one of the organisms where introns was discovered was in Tetrahymena thermophila with the intron in the rDNA sequence.