Figure 1.

Phylogenetic distribution of spliced leader addition to mRNAs. Phylogenetic relationships of only those taxa in which spliced leader addition is known to be present or likely to be absent are shown. The phylogenetic relationships between the taxa are based on multiple molecular studies (42–44). (A) A version of the tree in which spliced leader addition arose in a unicellular eukaryote and was subsequently lost from the various taxa indicated in gray. Minuses indicate points at which loss occurred. (B) A version of the tree in which spliced leader addition arose independently in the various phyla indicated in black. +, points at which origin of spliced leader addition occurred.