REVIEW The rise and falls of introns

There has been a lively debate over the evolution of eukaryote introns: at what point in the tree of life did they appear and from where, and what has been their subsequent pattern of loss and gain? A diverse range of recent research papers is relevant to this debate, and it is timely to bring them together. The absence of introns that are not self-splicing in prokaryotes and several other lines of evidence suggest an ancient eukaryotic origin for these introns, and the subse-quent gain and loss of introns appears to be an ongoing process in many organisms. Some introns are now function-ally important and there have been suggestions that invoke natural selection for the ancient and recent gain of introns, but it is also possible that fixation and loss of introns can occur in the absence of positive selection. Heredity advance online publication, 1 February 2006; doi:10.1038/sj.hdy.680079