Trans splicing in trypanosomes — archaism or adaptation?

In trypanosomes, a single transcription unit usually covers several protein-coding genes. The primary transcript is cut up by trans-splicing and polyadenylation machineries to generate individual mature mRNAs. All nuclear mRNAs acquire the same capped 39 nucleotide sequence at their 5′ end as a consequence of the trans-splicing event. Trans splicing is used in the synthesis of some mRNAs in nematodes and chloroplats. These unusual systems are clearly related to cis-splicing systems, but it remains an intriguing question whether they are merely exotic offshoots of cis splicing or archaic remnants of cis-splicing progenitors.