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.