The origin of spliceosomal introns is one of the most enduring mysteries in molecular biology. In nuclear genomes such as our own, the protein-coding regions of genes (exons) can be separated from one another by hundreds of thousands of base pairs (bp) of intronic (non-coding) DNA, and while they are often considered ‘junk’, introns are increasingly ascribed important regulatory functions [1]. Here we present evidence that an intron in a GTPase superfamily gene in the unicellular alga Bigelowiella natans is derived from — and was created by — the insertion of a fragment of mitochondrial DNA. Organelle-to-nucleus DNA transfer is an increasingly well-understood phenomenon, one that has the potential to greatly influence genome structure 2, 3. Our data suggest that such transfers could represent a hitherto underappreciated source of new spliceosomal introns.
