Evolutionary origin of SL-addition trans-splicing: still an enigma

Spliced-leader (SL) trans-splicing is an essential step in pre-mRNA maturat ion in a variety of lower eukaryotic organisms. However, this processing pa thway is absent in mammals, insects, yeast and plants. The patchy phylogene tic distribution of SL trans-splicing is consistent with either 'multiple g ain' or 'multiple loss' evolutionary scenarios. Recent studies show that tw o additional metazoan phyla carry out SL trans-splicing, significantly incr easing its phylogenetic range. However, it remains unclear whether this unu sual type of splicing is an ancestral or an acquired trait.