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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?

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Well yeast has very very few if that counts. See here. I recently checked the number if annotated isoforms for 5592 yeast genes and NONE of them had more than one! –  terdon Apr 3 '13 at 19:56
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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.

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Could you expand with some examples? –  nico Apr 4 '13 at 19:02
@nico: examples of what, eukaryotes with introns or eukaryotic ciliates with non-intronic intergenetic regions? –  Kevin Apr 4 '13 at 19:14
thanks for the edit, I was speaking about the ciliates, as I did not know about that. –  nico Apr 4 '13 at 19:26
@nico, Tetrahymena thermophila's macronucleus contains about 15,000 copies of the gene that encodes for the ribosome. I believe it was in this gene sequence that introns were first discovered. The Nobel Price for the intron discovery was shared with another person as they both discovered it in different organisms about the same time. –  Kevin Apr 4 '13 at 19:34
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